[Rear Suspension & Geometry Fundamentals]

Rear Suspension & Geometry Fundamentals

Welcome,
I promise to post advice only when I have significant knowledge & experience on the topic. Please don't be offended if you ask me to speculate & I decline. I don't like to guess, wing it or BS on things I don't know. I figure you can wing it without my input, so no reason for me to wing it for you.

A few guidelines I'm asking for this thread:
1. I don't enjoy debating the merits of tuning strategies with anyone that thinks it should be set-up or tuned another way. It's not fun or valuable for me, so I simply don't do it. Please don't get mad if I won't debate with you.

2. If we see it different ... let's just agree to disagree & go run 'em on the track. Arguing on an internet forum just makes us all look stupid. Besides, that's why they make race tracks, have competitions & then declare winners & losers.

3. To my engineering friends ... I promise to use the wrong terms ... or the right terms the wrong way. Please don't have a cow.

4. To my car guy friends ... I promise to communicate as clear as I can in "car guy" terms. Some stuff is just complex or very involved. If I'm not clear ... call me on it.

5. I type so much, so fast, I often misspell or leave out words. Ignore the mistakes if it makes sense. But please bring it up if it doesn't.

6. I want people to ask questions. That's why I'm starting this thread ... so we can discuss & learn. There are no stupid questions, so please don't be embarrassed to ask about anything within the scope of the thread.

7. If I think your questions ... and the answers to them will be valuable to others ... I want to leave it on this thread for all of us to learn from. If your questions get too specific to your car only & I think the conversation won't be of value to others ... I may ask you to start a separate thread where you & I can discuss your car more in-depth.

8. Some people ask me things like "what should I do?" ... and I can't answer that. It's your hot rod. I can tell you what doing "X" or "Y" will do and you can decide what makes sense for you.

9. It's fun for me to share my knowledge & help people improve their cars. It's fun for me to learn stuff. Let's keep this thread fun.

10. As we go along, I may re-read what I wrote ... fix typos ... and occasionally, fix or improve how I stated something. When I do this, I will color that statement red, so it stands out if you re-skim this thread at some time too.

---------------------------------------------------------------------------------------------------------------------------------------- 

Let's Clarify the Cars We're Discussing:
We're going to keep the conversation to typical full bodied Track & Road Race cars ... front engine, rear wheel drive ... with a ride height requirement of at least 1.5" or higher. They can be tube chassis or oem bodied cars ... straight axle or IRS ... with or without aero ... and for any purpose that involves road courses or autocross. 

But if the conversation bleeds over into other types of cars too much ... I may suggest we table that conversation. The reason is simple, setting up & tuning these different types of cars ... are well ... different. There are genres of race cars that have such different needs, they don't help the conversation here.

In fact, they cloud the issue many times. If I hear one more time how F1 does XYZ ... in a conversation about full bodied track/race cars with a X" of ride height ... I may shoot someone. Just kidding. I'll have it done. LOL

Singular purpose designed race cars like Formula 1-2-3-4, Formula Ford, F1600, F2000, etc, Indy Cars, IMSA Prototypes, Open Wheel Midgets & Sprint Cars. First, none of them have a body that originated as a production car. Second, they have no ride height rule, so they run almost on the ground & do not travel the suspension very far. Formula 1-2-3-4, Formula Ford, F1600, F2000, etc, Indy Cars, IMSA Prototypes are rear engine. The Open Wheel Midgets & Sprint Cars are front engine & run straight axles in front.

I have a lot of experience with these cars & their suspension & geometry needs are VERY different than full bodied track & road race cars with a significant ride height. All of them have around 60% rear weight bias. That changes the game completely. With these cars we're always hunting for more REAR grip, due to the around 60%+/- rear weight bias.

In all my full bodied track & road race cars experience ... Stock Cars, Road Race GT cars, TA/GT1, etc.  ... with somewhere in the 50%-58% FRONT bias ... we know we can't go any faster through the corners than the front end has grip. So, what we need to do, compared to Formula 1-2-3-4, Formula Ford, F1600, F2000, etc, Indy Cars, IMSA Prototypes, Open Wheel Midgets & Sprint Cars, is very different.

---------------------------------------------------------------------------------------------------------------------------------------- 

Before we get started, let's get on the same page with terms & critical concepts.

Shorthand Acronyms
IFT = Inside Front Tire
IRT = Inside Rear Tire
OFT = Outside Front Tire
ORT = Outside Rear Tire
*Inside means the tire on the inside of the corner, regardless of corner direction.
Outside is the tire on the outside of the corner.

LF = Left Front
RF = Right Front
LR = Left Rear
RR = Right Rear
ARB = Anti-Roll Bar (Sway Bar)
FLLD = Front Lateral Load Distribution
RLLD = Rear Lateral Load Distribution
TRS = Total Roll Stiffness
LT = Load Transfer
RA = Roll Angle
RC = Roll Center
CG = Center of Gravity
CL = Centerline
FACL = Front Axle Centerline
RACL = Rear Axle Centerline
UCA = Upper Control Arm
LCA = Lower Control Arm
LBJ = Lower Ball Joint
UBJ = Upper Ball Joint
BJC = Ball Joint Center
IC = Instant Center is the pivot point of a suspension assembly or "Swing Arm"
CL-CL = Distance from centerline of one object to the centerline of the other
KPI = King Pin Inclination, an older term for the angle of the ball joints in relation to the spindle
SAI = Steering Angle Inclination, a modern term for the angle of the ball joints in relation to the spindle


TERMS:
Roll Centers = Cars have two Roll Centers ... one as part of the front suspension & one as part of the rear suspension, that act as pivot points. When the car experiences body roll during cornering ... everything above that pivot point rotates towards the outside of the corner ... and everything below the pivot point rotates the opposite direction, towards the inside of the corner.

Center of Gravity = Calculation of the car's mass to determine where the center is in all 3 planes. When a car is cornering ... the forces that act on the car to make it roll ... act upon the car's Center of Gravity (CG). With typical production cars & "most" race cars, the CG is above the Roll Center ... acting like a lever. The distance between the height of the CG & the height of each Roll Center is called the "Moment Arm." Think of it a lever. The farther apart the CG & Roll Center are ... the more leverage the CG has over the Roll Center to make the car roll.

Instant Center is the point where a real pivot point is, or two theoretical suspension lines come together, creating a pivot arc or swing arm.

Swing Arm is the length of the theoretical arc of a suspension assembly, created by the Instant Center.

Static Camber is the tire angle (as viewed from the front) as the car sits at ride height. Straight up, 90 degrees to the road would be zero Camber. Positive Camber would have the top of tire leaned outward, away from the car. Negative Camber would have the top of tire leaned inward, towards the center of the car.

Camber Gain specifically refers to increasing negative Camber (top of wheel & tire leaning inward, towards the center of the car) as the suspension compresses under braking & cornering.

Total Camber is the combination of Static Camber & Camber Gain ... under braking, in dive with no roll & no steering, as well as the Dynamic Camber with chassis roll & steering.

Dynamic Camber refers to actual angle of the wheel & tire (top relative to bottom) ... compared to the track surface ... whit the suspension in dive, with full chassis roll & a measure of steering. In others, dynamically in the corner entry. For our purposes, we are assuming the car is being driven hard, at its limits, so the suspension compression & chassis/body roll are at their maximum.

Static Caster is the spindle angle (viewed from the side with the wheel off). Straight up, 90 degrees to the road would be zero Caster. Positive Caster would have the top of spindle leaned back toward to cockpit. Negative Caster would have the top of spindle leaned forward towards the front bumper.

Caster Gain is when the Caster angle of the spindle increases (to the positive) as the suspension is compressed, by the upper ball joint migrating backwards and/or the lower ball joint migrating forward ... as the control arms pivot up. This happens when the upper and/or lower control arms are mounted to create Anti-dive. If there is no Anti-dive, there is no Caster Gain. If there is Pro-Dive, there is actually Caster loss.

Anti-Dive is the mechanical leverage to resist or slow compression of the front suspension (to a degree) under braking forces. Anti-dive can be achieved by mounting the upper control arms higher in the front & lower in the rear creating an angled travel. Anti-dive can also be achieved by mounting the lower control arms lower in the front & higher in the rear, creating an angled travel. If both upper & lower control arms were level & parallel, the car would have zero Anti-dive.

Pro-Dive is the opposite of Anti-dive. It is the mechanical leverage to assist or speed up compression of the front suspension (to a degree) under braking forces. Provide is achieved by mounting the upper control arms lower in the front & higher in the rear, creating the opposite angled travel as Anti-Dive. Pro-dive can also be achieved by mounting the lower control arms higher in the front & lower in the rear, creating the opposite angled travel as Anti-Dive.

Split is the measurement difference in two related items. We would say the panhard bar has a 1" split if one side was 10" & the other side 11". If we had 1° of Pro-Dive on one control arm & 2° of Anti-Dive on the other, we would call that a 3° split. If we have 8° of Caster on one side & 8.75° on the other, that is a .75° split.

Scrub Radius = A car's Scrub Radius is the distance from the steering axis line to tread centerline at ground level. It starts by drawing a line through our upper & lower ball joints, to the ground, that is our car's steering axis line. The dimension, at ground level, to the tire tread centerline, is the Scrub Radius. The tire's contact patch farthest from the steering axis loses grip earliest & most during steering. This reduces the tire's grip on tight corners. The largest the Scrub Radius, the more pronounced the loss of grip is on tight corners. Reducing the Scrub Radius during design increases front tire grip on tight corners.

Baseline Target is the package of information about the car, like ride height, dive travel, Roll Angle, CG height, weight, weight bias, tires & wheel specifications, track width, engine power level, estimated downforce, estimated  max corner g-force, etc. We call it "Baseline" ... because it's where we're starting at & "Target" because these key points are the targets we're aiming to achieve. We need to work this package of information prior to chassis & suspension design, or we have no target.

Total Roll Stiffness (aka TRS) is the mathematical calculation of the "roll resistance" built into the car with springs, Sway Bars, Track Width & Roll Centers. Stiffer springs, bigger Sway Bars, higher Roll Centers & wider Track Widths make this number go UP & the Roll Angle of the car to be less. "Total Roll Stiffness" is expressed in foot-pounds per degree of Roll Angle ... and it does guide us on how much the car will roll.

Front Lateral Load Distribution & Rear Lateral Load Distribution (aka FLLD & RLLD):
FLLD/RLLD are stated in percentages, not pounds. The two always add up to 100% as they are comparing front to rear roll resistance split. Knowing the percentages alone, will not provide clarity as to how much the car will roll ... just how the front & rear roll in comparison to each other. If the FLLD % is higher than the RLLD % ... that means the front suspension has a higher resistance to roll than the rear suspension ... and therefore the front of the car runs flatter than the rear of the suspension ... which is the goal.

Roll is the car chassis and body "rolling" on its Roll Axis (side-to-side) in cornering.

Roll Angle is the amount the car "rolls" on its Roll Axis (side-to-side) in cornering, usually expressed in degrees.

Dive is the front suspension compressing under braking forces.

Full Dive is the front suspension compressing to a preset travel target, typically under threshold braking. It is NOT how far it can compress.

Rise = Can refer to either end of the car rising up.

Squat = Refers to the car planting the rear end on launch or under acceleration.

Pitch = Fore & aft body rotation. As when the front end dives & back end rises under braking or when the front end rises & the back end squats under acceleration.

Pitch Angle is the amount the car "rotates" fore & aft under braking or acceleration, usually expressed by engineers in degrees & in inches of rise or dive by Racers.

Diagonal Roll is the combination of pitch & roll. It is a dynamic condition. On corner entry, when the Driver is both braking & turning, front is in dive, the rear may, or may not, have rise & the body/chassis are rolled to the outside of the corner. In this dynamic state the outside front of the car is lowest point & the inside rear of the car is the highest point. 

Track Width is the measurement center to center of the tires' tread, measuring both front or rear tires.

Tread Width is the measurement outside to outside of the tires' tread. (Not sidewall to sidewall)

Tire Width is the measurement outside to outside of the sidewalls. A lot of people get these confused & our conversations get sidelined.

Floating typically means one component is re-engineered into two components that connect, but mount separate.  In rear ends, a "Floater" has hubs that mount & ride on the axle tube ends, but is separate from the axle itself. They connect via couplers.  In brakes, a floating caliper or rotor means it is attached in a way it can still move to some degree.

Decoupled typically means one component is re-engineered into two components that connect, but ACT separately. In suspensions, it typically means one of the two new components perform one function, while the second component performs a different function. 

Spring Rate = Pounds of linear force to compress the spring 1". If a spring is rated at 500# ... it takes 500# to compress it 1"

Spring Force = Total amount of force (weight and/or load transfer) on the spring. If that same 500# spring was compressed 1.5" it would have 750# of force on it.

Sway Bar, Anti-Sway Bar, Anti Roll Bar = All mean the same thing. Kind of like "slim chance" & "fat chance."

Sway Bar Rate = Pounds of torsional force to twist the Sway Bar 1 inch at the link mount on the control arm.

Rate = The rating of a device often expressed in pounds vs distance. A 450# spring takes 900# to compress 2".

Rate = The speed at which something happens, often expressed in time vs distance. 3" per second. 85 mph. * Yup, dual meanings.

Corner Weight = What each, or a particular, corner of the race car weighs when we scale the car with 4 scales. One under each tire.

Weight Bias = Typically compares the front & rear weight bias of the race car on scales. If the front of the car weighs 1650# & the rear weighs 1350# (3000# total) we would say the car has a 55%/45% front bias. Bias can also apply to side to side weights, but not cross weight. If the left side of the car weighs 1560# & the right 1440#, we would say the car has a 52/42 left side bias.

Cross Weight = Sometimes called "cross" for short or wedge in oval track racing. This refers to the comparison of the RF & LR corner weights to the LF & RR corner weights. If the RF & LR corner scale numbers add up to the same as the LF & RR corners, we would say the car has a 50/50 cross weight. In oval track circles, they may say we have zero wedge in the car. If the RF & LR corner scale numbers add up to 1650# & the LF & RR corners add up to 1350#, we would say the car has a 55/45 cross weight. In oval track circles, they may say we have 5% wedge in the car, or refer to the total & say we have 55% wedge in the car.

Grip & Bite = Are my slang terms for tire traction.

Push = Oval track slang for understeer, meaning the front tires have lost grip and the car is going towards the outside of the corner nose first.

Loose = Oval track slang for oversteer meaning the rear tires have lost grip and the car is going towards the outside of the corner tail first.

Tight is the condition before push, when the steering wheel feels "heavy" ... is harder to turn ... but the front tires have not lost grip yet.

Free is the condition before loose, when the steering in the corner is easier because the car has "help" turning with the rear tires in a slight "glide" condition.

Good Grip is another term for "balanced" or "neutral" handling condition ... meaning both the front & rear tires have good traction, neither end is over powering the other & the car is turning well.

Mean = My slang term for a car that is bad fast, suspension is on kill, handling & grip turned up to 11, etc., etc.

Greedy is when we get too mean with something on the car, too aggressive in our setup & it causes problems.

Steering Turn-In is when the Driver initiates steering input turning into the corner.   

Steering Unwind is when the Driver initiates steering input out of the corner.

Steering Set is when the Driver holds the steering steady during cornering. This is in between Steering Turn-In & Steering Unwind.

Roll Thru Zone = The section of a corner, typically prior to apex, where the Driver is off the brakes & throttle. The car is just rolling. The start of the Roll Thru Zone is when the Driver releases the brakes 100%. The end of the Roll Thru Zone is when the Driver starts throttle roll on.

TRO/Throttle Roll On is the process of the Driver rolling the throttle open at a controlled rate.

Trail Braking is the process of the Driver braking while turning into the corner. Typically, at the weight & size of the cars we're discussing here ... the Driver starts braking before Steering Turn-In ... and the braking after that is considered Trail Braking. This is the only fast strategy. Driver's that can't or won't trail brake are back markers.

Threshold Braking = The Driver braking as hard as possible without locking any tires, to slow the car as quickly as possible to the target speed for the Roll Thru Zone. Typically done with very late, deep braking to produce the quickest lap times.



---------------------------------------------------------------------------------------------------------------------------------------- 

16 CRITICAL RACE CAR DESIGN CONCEPTS:

A.    One of the most important design factors is utilizing all four tires on the track surface for maximum possible adhesion. Shaker Rigs (6, 7 & 8 post rigs) exist to help race car designers, teams & engineers maximize tire contact & loading to the track surface. As a general rule, anything that reduces contact patch and/or tire loading is our enemy & anything that increases contact patch (up to optimum) and/or achieves optimum loading of all four tires is our friend.

B.    Weight is our enemy. Lighter race cars do everything better. They turn better. They accelerate better. They decelerate better. They even crash better (safer). They stress all the components less. Building a lighter car allows us to run less heavy duty, lighter suspension components reducing unsprung & rotating mass ... leading to an even lighter, faster race car. Every ounce matters if you are serious about winning.

C.    Center of Gravity (aka CG ... aka weight mass) matters ... a lot. The mantra of oval track race car builders when it comes to race car weight is "low, light & left." (More left side weight helps cars turning left.) For road race cars it is "low, light & centered." The goal is to have the lightest race car ... weigh the exact same on all four scales ... with the majority of the mass (CG) low in the car & centered in the cockpit.

When the race series or class has a weight minimum, the Racers that build the car as light as practical, then places weight (lead, steel, tungsten) near the center of the car, down low ... will produce a faster, better handling, safer race car.

D.    When we design a race car with a lower center of gravity, it is much easier to drive & can be much faster. A lower center of gravity allows the race car to run flatter through the corners, working all four tires better ... more grip ... more corner speed. A lower center of gravity allows the race car to pitch less (dive & rise) under braking & acceleration, working all four tires better ... more grip ... more corner entry & exit speed. A lower center of gravity makes the race car more stable, higher grip & easier to drive.

E.    If we carelessly design a race car with excessive weight (mass) outside the axle centerlines, we're asking for a scary, ill handling, even dangerous handling, race car. Excess weight ahead of the front axle centerline will make the front end of the car swing out when we exceed total tire grip. Big push (understeer) & then nose hard into the outside barrier.

Similar in the rear. Excess weight behind of the rear axle centerline will make the rear end of the car swing out when we exceed total tire grip. Hard loose condition (oversteer) & then back hard into the outside barrier. Designing the race car with as much of its needed mass inside the axle centerlines is critical.

F.    Track width is CRITICAL. Racing sanctioning bodies know this & enforce track width rules diligently, because all knowledgeable Racers know that even a small increase in track width can provide a significant advantage. Very similar to having a lower center of gravity ... having a wider track width allows the race car to run flatter through the corners, working all four tires better ... more grip ... more corner speed. A wider track width makes the race car more stable, higher grip & easier to drive.

With exception for tight, narrow autocross courses, designing the race car with the widest track width possible is the goal. Widening the car body, or building a wider car body, to achieve the maximum track width is an advantage. A wider track width makes the race car more stable, higher grip, with more corner speed & easier to drive.

G.    With the lowest CG possible, the roll centers also need to be low. Ideally the front roll center is at 0" ground level in full dive at threshold braking. The rear roll center needs to be as low as is practical, while producing a roll axis that is optimum for the particular car to have neutral, balanced, high grip handling through all corners of the course.

H.    Unsprung weight is everything not supported by the springs. In the front this includes half the control arms, tie rods & shocks & all of the tires, wheels, lugs, brake rotors, calipers, mounts, brake shrouds, uprights & hubs ... plus a portion of the brake cooling ducting. If we have IRS in the rear, the list is the same. If we run a straight axle rear, the list includes half the suspension links & shocks & all of the tires, wheels, lugs, brake rotors, caliper, mounts, brake shrouds, rear axle & hubs ... plus a portion of any brake cooling ducting.

Lighter unsprung weight allows the suspension to react & respond quicker to irregular track surface input, providing a higher % of loaded tire contact & grip. Lighter unsprung weight allows the suspension to react & respond quicker to Driver inputs & increases what the Driver feels in the race car.

I.    Of the unsprung weight, the tires, wheels, lugs, brake rotors & hubs are ROTATING WEIGHT. Reducing rotating mass is even more critical than reducing unsprung weight. Accelerating & decelerating a heavier rotating mass take much more time. Said another way, lightening the rotating mass makes the car accelerate & decelerate quicker, producing quicker lap times.

J.    The design structure of every component affects how well that component handles the forces inflicted upon it. The challenge is building lightweight chassis & components without having failures, or reduction in grip due to flex.

K.    The degree of chassis rigidity ... and where it is ... needs to be designed into the car from the start. If a race car chassis is too flexible, the race car will have less grip, be less responsive to tuning changes & have a wider tuning sweet spot. If a race car chassis is too rigid, the race car will have more grip, be more responsive to tuning changes & have a narrower tuning sweet spot.

Race cars that are heavier, more powerful and/or capable of higher cornering g forces ... require more rigidity for optimum track performance. Race cars that are lighter, less powerful and/or capable of lower cornering g forces ... require less rigidity for optimum track performance.

L.    Where the rigidity is designed into the chassis matters as well. Drag cars load the rear suspension significantly more than the front, so the rear of the chassis needs the majority of the chassis rigidity. Road race & oval tracks race cars load the front suspension significantly more than the rear, so the front of the chassis needs the majority of the chassis rigidity. Chassis rigidity designed into the car, needs to be tailored to the direction & location of forces seen dynamically.

M.    Aero drag matters in road racing, but less than you may think. In high powered race cars on road courses, aero downforce is way more important than how much aero drag the race car has. The road race car that has more aero downforce, even with a bit more drag, will be the superior performer. With that said, we don't want unnecessary aero drag.

We want to eliminate & reduce all the aero drag possible, just not to the point of sacrificing aero downforce or track width. Yes, a wider track width & wider front end will create more frontal area & aero drag. The performance advantage of track width trumps aero drag on road courses.

The exception, to these aero rules, is super low powered cars where aero drag is more of a hinderance.

N.    The suspension strategy that includes the target ride height, dive travel (under braking) & roll angle (when cornering) needs to be decided BEFORE the chassis & suspension are designed. There are a variety of reasons why, but the simple ones are ground clearance & camber gain.

If we design an optimal high travel suspension, for example 3"-4" of dive, we may utilize long control arms to slow the camber during dive. This way we can run optimal static camber & achieve the optimal camber gain. If we later decide to run a low travel suspension, for example 1"-1.5" of dive, the long control arms reduce the amount of camber gain can achieve. This problem would require us to run significantly more than optimal static camber, to arrive at the optimal camber. Conversely, we'll have the reverse problem if we start with a low travel design of shorter control arms & decide to run a high travel strategy ... too much camber gain.

On a different note, we may start with a low travel strategy, for example 1"-1.5" of dive, with a 2.5" ride height & later decide we want to run a high travel strategy, for example 3" of dive. The 2.5" ride height back at the firewall isn't the problem. The 2.25" height we designed the FACL crossmember on the front clip is the problem. If we knew we're going higher travel to start, we would raise the front clip (so to speak) in the design phase, so the FACL crossmember allows that travel.

O.    Stiction & friction choices are not often thought about during the design process. But those decisions are often made during design & can be hard to change. Suspension bushings for example. If the chassis & control arms are designed for conventional wide bushings, deciding later to reduce stiction & friction with rod ends can be troublesome. Same with ball joints. Decide this early on.

P.    Safety is often thought of as cage design, seat configuration, harnesses, suits, helmets, HANS & nets. These are all good to decide on beforehand as well, for increased protection in a crash. But spindle, hub & bearing failure cause more crashes than any other part on the car.

Race cars that are heavier, more powerful and/or capable of higher cornering g forces ... create higher load on the critical spindle, hub & bearing components. To PREVENT crashes in the first place, work out the load ratings of our spindle, hub & bearing with a safety factor built in. 

20 CRITICAL HANDLING CONCEPTS:
1.    A car with heavier front weight bias, can go no faster through a corner than the front tires can grip. Balancing the rear tire grip to the front ... for balanced neutral handling ... is relatively easy ... compared to the complexities of optimizing front tire grip.

2.    What we do WITH & TO the TIRES ... are the key to performance. Contact patch is the highest priority, with how we load the tires a close second.

3.    Geometry design, settings & changes to need to focus on how the tires contact the road dynamically & are loaded.

4.    Tires are the only thing that connect the race car to the track surface. Tires play the largest role in race car performance. Rubber in tires hardens rapidly from the day they come out of the mold. Don't run old tires ... unless you want to learn how much it costs to repair race cars. The absolute best performance gain we can make to any race car is fresh, matched tires.

Matching the tires in rubber cure rate, durometer, sizing & sidewall spring rate is key to eliminating handling gremlins that make no sense. The grip level tires are capable of are based on these factors, regardless of tread depth! If the front and/or rear tires aren't matched, we will have different handling issues turning left & right.

5.    After the car is built, tires are selected & the geometry is optimum ... most chassis fine tuning is to control the degree of load transfer to achieve the traction goal & handling balance. Dynamic force (load & load transfer) applied to a tire adds grip to that tire. With the exception of aerodynamics, load transfer from tire(s) to tire(s) is the primary force we have to work with.

6.    The car's Center of Gravity (CG) acts as a lever on the Roll Center ... to load the tires ... separately front & rear. Higher CG's and/or lower RC's increases Roll Angle, but loads the tires more. Lower CG's and/or higher RC's decrease Roll Angle, but load the tires less. Getting the front & rear of the car to roll on an optimum roll axis is desired. Getting them to roll exactly the same is not the goal, because ...

7.    Perpetual goal is to achieve maximum grip & neutral, balanced handling simultaneously through all the corners of the course. To do requires reducing the loading on the inside rear tire (to a degree) ... then increasing the loading of the inside rear tire (to a higher degree) for maximum forward bite on exit. So, on entry & mid-corner, the car needs to roll slightly less in the front to keep both front tires engaged for optimum front end grip, while allowing the car to roll slightly more in the rear to disengage the inside rear tire, to a small degree, to turn better.

For optimal corner exit, the car will have more roll in the front & less in the rear to re-engage the inside rear tire to a higher degree than it was on entry & exit, for maximum forward bite (traction) on exit. This difference is called diagonal roll. This amount differs as speeds & g-forces differ.

8.    Modern day tuners do not use the RC height as the primary means of controlling Roll Angle. We use the suspension tuning items as our priority tools to control Roll Angle. We use the RC priority to load the tires optimally. So, to achieve the optimum balance of Roll Angle & working all four tires optimally ... this all has to work with our suspension ... springs, anti-roll bars & shocks ... and track width ... to end up at the optimum Roll Angle for our car & track application.

9.    Sway Bars primarily control how far the front or rear suspension (and therefore chassis) "rolls" under force, and only secondarily influences the rate of roll. Softer bars allow increased Roll Angle & more load transfer from the inside tires to the outside tires. Stiffer bars reduce Roll Angle, keeping the car flatter & less load transfer from the inside tires to the outside tires.

10.    Springs primarily control how far a suspension corner travels under force, and only secondarily influences the rate of travel. Shocks primarily control the rate of suspension corner travel under force, and only secondarily have influence on how far.

11.    Springs, shocks & sway bars need to work together "as a team." Our springs' primary role is controlling dive & rise, also contribute significantly to the car's roll resistance. Our anti-roll bars (sway bars) primary role is controlling roll, but do contribute minutely to dive & rise. Our shocks primarily role is controlling the RATE of these changes, primarily during race car transitions from Driver input, such as braking throttle & steering. They all affect each other, but choose the right tool for the job & we create a harmonious team.

12.    The front tires need force, from load transfer on corner entry, to provide front tire GRIP. Too little & the car pushes ... too much & the car is loose on entry. The rear tires need force, from load transfer on corner exit, to provide rear tire GRIP. Too little & the car is loose ... too much & the car pushes on exit.

13.    Springs & Sway Bars are agents to load the tires with the force needed to produce maximum grip. Stiffer springs produce the needed force with less travel, whereas softer springs produce the needed force with more travel. Stiffer Sway Bars produce the needed force with less chassis roll, whereas softer Sway Bars produce the needed force with more chassis roll. The tire doesn't care which tool provides the loading force. Ultimately, they combine to produce a wheel load. Our role is to package the right combination for the target dive travel, chassis roll angle & wheel loading we need.

14.    Softer front springs allow more compression travel in dive from braking & therefore a lower CG, more front grip & less rear grip. Stiffer front springs reduce compression travel in dive from braking & therefore a higher CG, less front grip & more rear grip. There are pros, cons & exceptions to these rules.

15.    Too much Roll Angle overworks the outside tires in corners & underworks the inside tires. Too little Roll Angle underworks the outside tires in a corner. Excessive Roll Angle works the outside tires too much ... may provide an "ok" short run set-up ... but will be "knife edgy" to drive on long runs. The tires heat up quicker & go away quicker. If it has way too much Roll Angle ... the car loses grip as the inside tires are not being properly utilized.

16.    Too little Roll Angle produces less than optimum grip. The car feels "skatey" to drive ... like it's "on top of the track." The outside tires are not getting worked enough, therefore not gripping enough. Tires heat up slower & car gets better very slowly over a long run as tires Gain heat.

17.    A lower chassis Roll Angle works both sides of the car's tires "closer to even" ... within the optimum tire heat range ... providing a consistent long run set-up & optimum cornering traction, providing the fastest, most drivable race car.

18.    Higher Roll Angles work better in tight corners but suffer in high speed corners. Lower Roll Angles work better in high speed corners but suffer in tight corners. The goal on a road course with various tight & high speed corners ... is to find the best balance & compromise that produces the quickest lap times. Smart Tuners use Roll Centers & Aero to achieve this.

19.    Tuning is NOT linear two directions with stops at the ends. A car can be loose because it has too little Roll Angle in the rear & is not properly working the outside rear tire. A car can be loose because it has too much Roll Angle in the rear & is not properly working the inside rear tire.

A race car can be pushy because it has too little Roll Angle in the front & is not properly working the outside front tire. A race car can be pushy because it has too much Roll Angle in the front & is not properly working the inside front tire.

20.    Don't forget the role & effects the engine, gears, brakes, Driver & track conditions each have on handling.

[OKAY ... Let's Talk Rear Suspensions]

OKAY ... Let's Talk Rear Suspensions

First, all 20 of the Critical tuning Concepts above matter here, but the red ones matter more as we discuss rear suspensions & geometry. We're going to start with Critical Tuning Concept #2, the rear tire Contact Patch loading dynamically.

What do we mean by "dynamically?" We are of course, talking about the car running on track.  It is almost never in a static state. Due to acceleration, deceleration, braking forces, aero forces, steering forces & g-forces, the car's "state" is constantly changing.  Where do we care about rear end grip the most?  When we're in the dynamic state of braking & turning into the corner ... and accelerating while unwinding the steering out of the corner. The car is in pitch & roll, so diagonal roll, with the outside front corner loaded most ... and the inside rear tire loaded the least.

You can see that clearly if you watch the rear tires of these Porsche race cars. I picked them, due to their heavier rear weight boas, it is more obvious to see in a video. But all cars diagonal roll & unload the inside rear tire to some degree ... or at least need to. Watch: 

Frankly, I base all setup decisions based on the key "dynamic" situations of the race car. We don't care what the setting is in your garage or race shop. We only care what it is when your butt's puckered at threshold braking, turning into turn #6 at WeBeFast raceway ... and then accelerating while unwinding the steering out of the corner. 

I use software to check the Dynamic Camber & Rear Roll Steer ... with the car in the dynamic state of braking & turning into the corner ... and accelerating while unwinding the steering out of the corner.

Of course you'll need to know how much your car will dive (front) & roll (front & rear) ... based on your strategy. Utilizing the software, I "can" calculate how much someone's car will dive & roll with their current setup. But I usually do it the other way around. I "decide" how much dive & roll I want, then work into the spring rates, sway bar rates & Roll Centers to achieve my target dive & roll angle numbers.

---------------------------------------------------------------------------------------------------------------------------------------- 

Rear Suspension Types
There are nine common types of rear suspensions utilized in Track & Road Race Cars today ... Leaf Spring, 3-Link, Parallel 4-Link, Triangulated 4-Link, Torque Arm, Truck Arm, Single Wishbone & Strut, Double Wishbone IRS & Multi-Link IRS.

Ladder bars should not be seriously considered for any corner carving car, as they go into instant bind with body roll & offer practically no articulation. Utilizing leaf spring rear suspensions in road racing & track cars is typical due to rules or budget. I will share how we make them optimum, but I prefer not to run them if rules & budget allow.





Leaf Spring rear suspension has been around way before cars. Horse drawn wagons & carriages utilized leaf springs. In modern day automobiles manufacturing it is rare to see leaf springs. But they still exist on muscle cars built for track days, class racing & autocross. They are simple single arc'd flat spring or multi-leaf clamped together. The reason for their use by manufacturers is obvious ... simple & cheap.

They bolt & pivot on a front chassis mount & pivot on shackles in the rear. The major cons of leaf springs are they are heavy, produce harmonics through out & have side-to-side twist resistance that affects the spring rate. In stock form, they do not utilize a lateral locator. The firm bushings in the leaf springs keep the rear axle centered, more or less. I have raced leaf spring cars & there are methods to improve the handling & tunability with leaf springs, which we'll discuss later.

Moving on to linkage style suspensions ... assuming each type of rear suspension is set-up correctly, rod ends spaced away from brackets properly with high misalignment bushings & clocked correctly ... the 3-Link & Torque Arm suspensions allow the rear axle to articulate more (roll angle in relation to frame) than the 4-Links. They all will bind at some point of articulation (think body/chassis roll). The Parallel 4-Link allows the least articulation before bind ... the Triangulated 4-Link allows a little more articulation before bind ... and the 3-Link & Torque Arm offer quite a bit more articulation before bind ... all things being equal. The Truck Arm lives in bind. They only "articulate" due to torsional weakness.



A Triangulated 4-Link is simple, and fairly common as a factory style rear suspension in production cars. There a 147 bajillion of these suspensions under GM A & G bodies, as well as Fords & Mopars. You see a lot of them in sportsman level drag racing, muscle car autocross & track events. They work "fine" (kind of like when your wife says "it's fine").

If you were going to hard core drag race a car, a Parallel 4-Link would be a better choice. Ideally, you want the push & pull forces going through the links to be parallel with the chassis ... not angled within the chassis ... especially with super high powered cars.

The Triangulated 4-Link does not utilize a lateral locator. The firm bushings in the links keep the rear axle centered, more or less. It will bind with high articulation & for the most part the Roll Center & Instant Center are not adjustable (unless you modify then to be). So, there are better choices for road courses & autocross competition.



A Parallel 4-Link is also simple and fairly common as a factory style rear suspension in production cars. It will handle more torque under hard launches than Triangulated 4-Links. When the links are all parallel with the chassis, the forces are pushing & pulling inline with the chassis, making them more effective & capable of handling large power.

They do require a lateral locator like a panhard bar or watts link. Race versions make the Rear Roll Center easy to tune. In drag racing, it's common to take the panhard bar and mount it diagonally from the front of one lower link to the rear of the other lower link. This is called a diagonal link. What works better in Drag Racing applications is a wishbone style of locator with a sliding snout.

Parallel 4-Links have been very successful in road racing & autocross. The image above shows it with a Panhard bar, but Watts Links work well too.  Racing versions of Parallel 4-Links are adjustable if designed & installed with multiple mounting points. But they can not be as fine tuned as the 3-Link. Since we need both upper links to mirror each other ... exactly & precisely ... you don't see dual jack screw adjusters on the top links of 4-Links.

If you were to have them a little off from each other, the rear axle would go out of square with body roll. So, on all commercially available 4-Link packages, if the upper links are adjustable, they utilize identical brackets with holes. The holes are what make each change much coarser than we can achieve with a jack screw style 3-Link.



3-Link rear suspensions are very common in road racing, especially in full body cars like GT1 & the Trans Am series, because they allow for the most articulation, can be highly adjustable & tunable for track conditions. You also see them a lot on top Autocross racers. They do require a lateral locator like a panhard bar or watts link. Race versions make the Rear Roll Center easy to tune.

The image above shows a centered top link. But there are race versions with the top link offset to the passenger side to counteract torque steer. Also, in various forms of racing, we occasionally see decoupled 3-Links. More on Offset & decoupled 3-Links later in this thread.

It's common to see 3-Links with adjustable upper links. Most utilize brackets with adjustment holes in the axle bracket and/or chassis bracket. Like a 4-Link, these adjustments are somewhat coarse, as moving the top link 1" from one hole to the other can be a 20% Anti-Squat change. If the top link is mounted to the chassis with a "Jack Screw" style adjuster, like the steel version we offer at RSRT, the Anti-Squat can be fined tuned to the ninth degree. 



A Torque Arm is a great low adjustability rear suspension & offers good articulation. Torque Arm suspensions are also common as a factory style rear suspension in some cars. They are the simplest of the designs, allow a high degree of rear end articulation & can take high shock loads from hard launches. They do require a lateral locator like a panhard bar or watts link. Race versions make the Rear Roll Center easy to tune.

They can be made "a little" adjustable with holes in the front mount, but typically offer less adjustability than the other rear suspension designs, as far as controlling the front Instant Center & Anti-Squat. If designed well & installed as instructed, these make a great all around suspension for the person that doesn't want to tune much.



Truck Arm rear suspensions started on half ton pickups in the 60's. They do require a lateral locator like a panhard bar or watts link. Race versions make the Rear Roll Center easy to tune.

Some early Moonshiner built their race car with Truck Arms back in the day ... and some dumbass NASCAR rule writer made them a rule. These were literally ran in NASCAR Cup cars up until the New Gen 7 car. NASCAR Trucks & Xfinity/O'Reilly Cars STILL run them, per the rules.

It's no secret I despise them. Yes they are simple. Simply hard to make the rear do what you want. There are many tricks to making them work ... including designing custom torsional rigidity to tune roll ... but the best trick of all is to not run them. Torque Arms are better. 4-Links better than that & 3-Links best of all.

---------------------------------------------------------------------------------------------------------------------------------------- 

Suspension Type Summary:

Triangulated 4-Links can be made somewhat Anti-Squat adjustable if installed with multiple mounting points. The forces do not pull parallel to the chassis. They do not utilize a lateral locator, so the rear roll center is not adjustable.  In my experience with other crew chiefs & chassis tuners, having the rear roll center height adjustable with an adjustable panhard bar or watts link is critical. I know many crew chiefs & chassis tuners that is it their number one tuning tool to dial in balanced handling at the track.

I also rely on adjustable rear roll center height as a key tuning tool. It is simple & predictable in the handling change. Not every tuning change is. For these reasons, I drop them from my recommendations & won't be covering them further, unless someone asks questions about them.

Truck Arms are barely Anti-Squat adjustable with front chassis holes & do not articulate much, except for flex. They do use a lateral locator & are the standard in old NASCAR Cup & modern NASCAR O'Reilly & Truck racers. They are so limiting as a rear suspension I told myself when I would never work with them again. For this sole reason, I drop them from my recommendations & won't be covering them further, unless someone asks questions about them. Then I will shoot that person. 

Parallel 4-Links with a lateral locator ... to adjust the Rear Roll Center ... work well in road course & autocross applications. The cons are the coarse Anti-Squat adjustability, the fact we can't offset links to remove torque steer & they can't be Decoupled. For these reasons, I drop them from my recommendations & won't be covering them further, unless someone asks questions about them.

Leaf Spring rear suspensions, when stock, do not utilize a lateral locator, so the rear roll center is not adjustable, nor is the Anti-Squat adjustable. Other major cons are weight, harmonics & the side-to-side twist resistance that affects the spring rate. With modification & add-ons, most of these cons can be fixed. Because some race series & classes require the car to stay with its OEM type suspension ... even though they are not my favorite ... I will be expanding on how to fix the issues & make these very racy.

Torque Arm rear suspensions with a lateral locator ... to adjust the Rear Roll Center ... work well in road course & autocross applications. They can be decoupled, which is a huge plus. Their Anti-Squat adjustability is minimal & they can't be offset to remove torque steer so, they're not my favorite. They offer a few pros like being simple, very complex to understand. And, they fit under the car, not intruding into the interior, which is important to some. So, I will be expanding on both the standard & Decoupled versions later in this thread.

3-Link rear suspensions with a lateral locator ... to adjust the Rear Roll Center ... work the best in road course & autocross applications. They can be Decoupled & Offset to eliminate torque steer. With a Jack Screw style top link chassis mounted adjuster, these are quick & easy to dial in the rear grip perfectly. 

The Anti-Squat can be fine tuned to a gnat's eye, which is a huge advantage if we're racing or competing for fast lap times. Frankly, their only con is they can not handle as much power for Drag Racing as a Parallel 4-Link. 3-Links are my favorite rear suspension for straight axles.  So, I will be expanding on both the standard "offset" & Decoupled versions later in this thread.

I wanted to keep this first section focused on straight axle rear suspensions, so I have not shown Independent Rear Suspensions yet. They will get their own section later in this thread. I have a love/hate relationship with IRS.

My aversion for Independent Rear Suspensions is based on they're heavy & not easily Anti-Squat or Rear Roll Center adjustable. We can't offset anything to eliminate Torque Steer, nor can we Decouple them in any way. So, we deal with a compromise Anti-Squat on corner entry & exit. I am a "Tuner."  I love to dial in the total suspension at the track to be perfectly optimum ... in other words ... faster than all our competitors. IRS puts me in a box with less tuning tools at the track. A few different ones, but nothing that allows me to dial in Anti-Squat or Rear Roll Center.   

My love for IRS is based on the infinite range of roll steer & dynamic toe adjustability. We can make each side steer in or out to our heart's content.  If you're not familiar, low to zero ride height rule race cars, struggle to turn well due to little or no pitch angle change. In other words, if the ride height starts at 1/4", we're obviously not high travel diving the front end.

That lack of pitch change, CG migration & load transfer make it harder for these cars to turn well in tight corners. The solution & savior is IRS, because we can dial in Rear Roll Steer. With that, the rear of the car is helping the front of the car turn better. In my racing experience, cars with 2" & higher static ride heights don't "need" IRS. They can benefit from IRS if the front suspension is not up to par. But an offset, decoupled 3-Link will provide greater grip on entry & exit than any IRS. 

More love for IRS is based on the fact that ... well ... each side is independent. We can obviously keep the tires better loaded & following the irregularities of the track than a straight axle. The smoother the track, the less advantage this is. The rougher the track, the more of an advantage this is. Lastly, if we are running a rear engine configuration, IRS is our only choice. Later on in this thread, when I dive deeper into IRS setups, I'll show you why I love the Multi-Link IRS over conventional control arm IRS.

[OKAY ... Let's Talk RACING Rear Suspensions!]

OKAY ... Let's Talk RACING Rear Suspensions!

Leaf Spring suspension has been around way before cars. Horse drawn wagons & carriages utilized leaf springs. In modern day automobiles manufacturing it is rare to see leaf springs. But they still exist on muscle cars built for track days, class racing & autocross. They are simple single arc'd flat spring or multi-leaf clamped together. The reason for their use by manufacturers is obvious ... simple & cheap.

They bolt & pivot on a front chassis mount & pivot on shackles in the rear. You should know the major cons of leaf springs are the weight, harmonics & side-to-side twist resistance. I have raced leaf spring cars & there are methods to improve the handling with leaf springs, which we'll discuss later.

Leaf Spring Race Suspensions

I'm not a big fan of leaf springs in race cars. They can be made to work well in you go that route, either to fit rules or some preference. The cons with leaf springs are many.
1.   With regular bushings, the torsional twist that happens when cornering increases the spring rate.
2.   Some say that additional spring rate is 20-25%. Frankly you have to run it to see where you are.
3.   Changing spring rates are hard work. You have to swap out the rear axle & carry a bunch of leaf springs with you. That's that very practical.
4.   The leaf spring is being asked to do three jobs at once: locate the axle side-to-side, play the role of a suspension link & act as a spring.
5.   Steel multi-leaf springs are F'n heavy.
6.   Steel leaf springs transmit a LOT of harmonics throughout the rear suspension & rear axle.



There are several things you can, and should, do if you're racing with leaf springs.
A.   Switch to Composite Leaf Springs & you'll save a 100#+ & reduce the harmonics 90%.
B.   Install (spherical bearing) monoball bushings front & rear.
C.   Install some version of a roller bushing in the top shackle mount.
D.   Install a Watts link to locate & center the rear axle & to make the Rear Roll Center adjustable.
E.   Buy the best Coil-Over shock you can afford & simply don't run the springs ... if you're happy with the rate of the composite leaf.
F.   Another strategy is to run coil-over springs on the shocks WITH the leaf springs. For example, buy 175# composite springs & add 75# coil-over springs, to get to approximately a 250# spring rate equivalent. Now you can change the coils-over springs to achieve the rate you want.

I have won several Street Stock races with this setup & it works well for production body based track & autocross cars. But, ask yourself, how much did I spend to make this work?  A basic 3-Link or Torque Arm with Coil-Overs & either a Panhard Bar or Watts link won't be much more.

Advanced Leaf Spring Strategy

[/b]

If you are in a series that rules require your car to run leaf springs ... you can still add a 3-Link or Torque Arm if you want that advantage. You would do the same A thru F steps above ... and:
G.   Cut off the leaf spring pads & throw away the U-Bolts
H.   Buy & install a drag racing style leaf spring housing floater with rollers. This allows the rear axle to attach to the leaf ... be able to slide fore & aft on the leaf so the 3-Link or Torque Arm don't bind ... and the rear axle (think pinion side view) can rotate with the 3-Link or Torque Arm, without binding. Drag racers do this with Ladder Bars as well, but we don't want those on a road course. They will side bind.



Figuring out the Anti-Squat of a Leaf Spring suspension is easy. You simply draw a line from the ground at the RACL up through the front leaf spring pivot bolt & on past the CG. The front leaf spring pivot bolt IS THE INSTANT CENTER. The distance froward from the RACL to the front leaf spring pivot bolt, is your swing arm length. You will find this to be very short ... often 2' or less.



We ran a "Stock Class" race car years ago on leaf springs & a 3-Link. In our racing experience, this short swing arm & high Anti-Squat provided good initial grip, but got loose before we finished unwinding the steering out of the corner, which creates an "oh shit" moment.

We changed out the bracket to something like the one shown below & lowered the front spring eye bolt. This moves the IC down ward, lowering the Anti-Squat enough we could drive all the way out of the corners. Worked pretty well. Still, if rules allow, ditching the leaf springs & going coil-over makes more sense.



----------------------------------------------------------------------------------------------------------------------------------------

Torque Arm Race Suspensions

The illustration below can be a little deceiving. But the goal is to show you that wherever the pickup point of the Torque Arm is underneath the race car, the weight bias of the car ... and the length of the Torque Arm itself ... will define how much force it loads the rear tires with.

•   The top illustration shows the length of the Torque Arm is 20% behind the CG of the Race Car.
•   The middle illustration shows the length of the Torque Arm is right under the CG of the Race Car.
•   The bottom illustration shows the length of the Torque Arm is 20% ahead of the CG of the Car.



If you're not familiar with how to calculate where your race car's CG is, follow along.

A. Let's assume we have a sample race car with 108" wheelbase. When you scale this example race car, for conversation sake, the rear scale numbers add up to 1350# & the fronts add up to 1650#, making 3000# the total car weight with fuel, fluids & your fat ass in the Driver seat. 1650# is 55% of the total 3000#. Take your wheelbase & multiply by the FRONT weight percentage. 108" x 55% = 59.4". If you measure 59.4" from your Rear Axle Center, going forward, that is where your CG is in this sample race car. Let's do a couple more.

B. If we have a 2800# Track Warrior car, with 105" WB & all 4 scales read the same ... we have a 50/50 perfectly balanced race car. 50% of 105" = 52.5". This car's CG is 52.5" forward from the RACL.

C. If we have a 2400# RX7 race car, with 95.5" & the front scale numbers add up to 1150#, we have a 47.9% front weight bias (and therefore 52.1% rear weight bias). 47.9% x 95.5" = 45.75". The Cg for this lightweight rocket is 45-3/4" forward of the RACL.

Now ... if you're running a 3-Link or 4-Link, just imagine you ran lines through the upper & lower links (going forward) until they intersect. Most of you know that is called the Instant Center. Think of the distance from your RACL to the Instant Center, as your "Swing Arm" length. Basically the same as Torque Arm length.

For Road Course race cars I like to start with the Instant Center pretty close under the CG & tune from there. For Race Car A that would be 59.4", Car B 52.5" & Car C 45.75". If we were running a 3-Link or 4-Link, I start by running the lower links level (so zero roll steer) & adjust the angle to the top link(s) to hit these targets. Pretty easy to be dead nuts precise in a jack screw adjustable 3-Link. A little harder with a 4-Link as the bracket holes make coarser changes.

For a Torque Arm, you'd need to buy/build it that length & leave it. That's why I don't like Torque Arms for road course cars. I want to be able to tune it for the track & daily conditions. E can change that instant center 0.5% or 20% ... whatever we need for optum handling. With the 3-Link, 4-Link & Torque Arm, if we shorten the "Swing Arm" (move the Instant Center closer to the RACL), we will increase grip at initial throttle roll on, but decrease the grip further up on corner exit. This is why you see some cars grip well right away & then the rear end steps out before they get the steering fully unwound.

If we lengthen the "Swing Arm" (move the Instant Center closer to the RACL), we will decrease grip at initial throttle roll on, but increase the grip further up on corner exit. This is why you see some cars get loose right away & then be ok once they catch it. As always, compromises in racing. I'd rather have this problem & a Driver with good throttle control than the other way around.

Remember, standard (non-decoupled) Torque Arms ... like all rear suspensions (non-decoupled) the rear anti-squat percentage is the same on corner entry & exit ... and that is a compromise. Unfortunately with a Torque Arm, there is very little, if any tunability. So, we have to get it right the first time.

Why do we link Torque Arms for Autocross? Tuning takes time & autocross Racers typically have less time for tuning between rounds. Another reason is the corner exit speeds for autocross do not vary as much as road courses. For autocross, if we pick the right torque arm length, we have pretty good rear grip on all corner exits. There isn't much lap time to be gained by tuning on it. A pro of the Torque Arm suspensions is that they are simple & easy to set up.
A good autocross rule of thumb is for the 3-Link Instant Center or Torque Arm pick up point to be about 10% BEHIND the car's CG ... around 45"-47" in most 108" WB cars.

Decoupled Torque Arm Race Suspensions

Several things to know about designing and/or running a Decoupled Torque Arm:
1.   You can make it a lot shorter to increase corner exit , with no compromise on corner entry. We start with a 45.75" version from Chassisworks for ours.
2.   The Torque Arm itself sees a lot of abuse. So, it needs to be very well built. Heavy even. Heavy is OK because it doesn't move much. About 1" up & down in the front. Light Torque Arms won't survive in decoupled applications.
3.   The Decel link works the same as Decoupled 3-Links. You can use Poly Bump Stops or Air Spring Bump Stops on a Slider (like shown below) or a shock designed for this job.
4.   The challenge you face if you're trying to fit this all under the floor pan & trunk, is packaging the Decel link. You have more flexibility in Tube Chassis cars. But in Production cars with the OEM floor & trunk, you may want to utilize a curved Decel link & mount it low & to the rear of the axle tube. In some cases, this is the only way to make the Decel link long enough & to achieve the Pro-Squat angle we're looking for.



5.   In the image below, you can see the front Torque Arm mount is slotted. This is dual purpose. First & foremost, it has to have the ability for the front pivot shaft to move up & down. Second, it allows us to insert a slug in place of the rollers & make it act as a normal "coupled" Torque Arm. This does require removing the Decel link.



Setup & Adjustment of the Decoupled Torque Arm:
A.   At rest, you adjust two settings.
B.   As long as the coil-overs are BEHIND the rear axle, the Torque Arm pivot shaft will be resting on the Bump Stop at the top of the slider in the front chassis mount.
C.   Adjust the Pinion Angle to achieve a 1°-1.5° DIFFERENCE in angle relative to the driveshaft.
D.   At rest, adjust the Decel link, with Bump Stop shims, until the Poly Bump Stop has just enough preload to move the Torque Arm pivot shaft OFF the Bump Stop at the top of the slider in the front chassis mount. To start, you want a .125" clearance in between the top of the pivot shaft & the Bump Stop, at rest.

Basic Operation of the Decoupled Torque Arm:
•   On acceleration, the rear axle rotates the pinion & the Torque Arm up in front. The Torque Arm's front pivot shaft moves up the slider (chassis bracket) & contacts the Poly Bump Stop to soften the stop. The Torque Arm is now Active & we're utilizing the Anti-Squat of the Torque Arm during all acceleration.
•   At the point above, under hard acceleration, the Decel link now has GAP in the Poly Bump Stop. So, it is "Decoupled" & has no affect on the rear geometry.
•   On hard deceleration, off throttle & on the brakes, the rear axles rotates the pinion & the Torque Arm down in front. The Torque Arm's front pivot shaft moves down the slider. The front pivot shaft should NOT contact the bottom of the slot in the chassis bracket. The front pivot shaft is basically floating or hanging in the slot. At this point the Torque Arm is "Decoupled" & has no affect on the rear geometry.
•   At the point above, under hard deceleration, the Decel link has closed the gap & compressed the Poly Bump Stops about 1/4". We are utilizing the Pro-Squat during all deceleration.
•   All in all, the Decel link moves about 3/8" & the Torque Arm moves about one inch. It's a pretty tight window. So, you need to make sure the shims in the Decel link achieve the proper preloading.



A Decoupled Torque Arm in a Tube Chassis:
1.   In an ideal installation, we're mounting the adjustable (slotted or holes) Decel link chassis mount to a chassis crossmember (blue circle) & bracing it to the roll cage as shown.
2.   You may want to utilize a curved Decel link & mount it low & to the rear of the axle tube ... to achieve a better upward angle, going forward on the Decel link.
3.   The Decel link below is on an 8° angle. That's good. It creates about 50% Pro-Squat. But if we could mount the Decel link around a 15° angle, we'd hit the magic 100% Pro-Squat.
4.   The other option to achieve 100% Pro-Squat would be to make the Decel link chassis mount, higher up. Then build the sheet metal floor higher to cover it up.
5.   Building an access door in the sheet metal floor makes it MUCH easier to access the Decel link chassis mount, bushings & adjustments.

[Offset 3-Link Race Suspensions]

Offset 3-Link Race Suspensions

Under load from the driveshaft, the rear end housing wants to rotate the same direction the driveshaft is ... counter clockwise from the rear view, clockwise from a front view. So as torque is applied the left rear tire is loaded more & the right rear tire is loaded less. This makes the car want to "drive" to the right, a small amount, under hard acceleration. As you make left hand turns the car has more "forward bite" during corner exit ... than right hand turns, which have less "forward bite" during corner exit.

The difference isn't huge, but it exists. If it isn't counteracted ... the effect amplifies with increased power output. For 3-Links, the upper link can be offset to the passenger side to help counteract this torque on acceleration. The formulas I've seen other people use involve rear steer, which makes no sense for handling cars.



Very few people can tell you accurately how far to offset it, because it changes with gear ratio & friction within your rear end. A rule of thumb is 8-15% of track width. I have my own proprietary formulas I use, based on my knowledge of where the force comes from & high tech testing of dynamic loads with load cells. This allows me to calculate the amount of force difference from the left rear to right rear tire & offset the top link precisely to zero out any torque steer. I don't share this formula publicly, but I do offer this information to clients running my offset 3-Links.

In many race applications, it makes sense to make the top link mounts wide, so you can adjust the top link side to side to dial in the optimum torque steer cancelatio. Sometimes in the real world, packaging challenges play a role & prevent this.


 

Offset 3-Links with Torque Absorbers

Most tire spin is from the initial "shock" the tire gets when the driver cracks (slams?) the throttle to get out of the corner. Once the tire is spinning, it's hard to get it to stop. You need to lift & roll on the throttle again. Big loss of lap time. Torque Absorbers can be added to 3-Links & Torque Arm rear suspensions. They are usually a poly (polyurethane) or hard rubber Bump Stop that has been tested for spring/load rate.



Most common Torque Absorbers have 1 Poly Bump Stop on the Decel Side & 2 on the Accel Side. It's not uncommon to use more than 2 Poly Bump Stops on the acceleration side of the Torque Absorber & 1 or 2 hard Poly Bump Stop on the Decel Side. Some torque absorbers use springs or a combination of springs & Poly Bump Stops. No spring is as progressive as Poly Bump Stops when they get near their max. See chart. The only unit that is more progressive than a good Poly Bump Stop, is an Air Spring Bump Stop. More on this later.

* The image above makes it appear like the lower links & top link are outside the frame rails. The top link is always inside the frame rails & offset to a specific amount to the passenger side. Factory frame rails are quite wide, so the lower links mount on the inside. Track-Warrior frame rails are made narrower, so the lower links can mount on the outside.



Poly Bump Stops compress at a rate & distance determined by the load rate & number of Poly Bump Stops. You need to understand how rate & distance change with the number of Bump Stops and/or the hardness of the Poly Bump Stop. The durometers and exact rates depend on the brand of Poly Bump Stop you utilize. I typically run RE Suspension, Allgaier & Right Foot. Regardless, they all have a range something like 93. 90, 85, 80, 75, 70, 60 & 50 durometer (with an "A" Shure tip).

For conversation, let's say we have 1 red 85 durometer 1.00" tall Poly Bump Stop on the decel side. For conversation, let's say we'll see 1200# of force on deceleration. See chart below. Look at the red line. On the left side, go up to 1200# of force, then across (right) to the red line, then down to the Bump Stop compression at the bottom. You see the red  1.00" tall Bump Stop compressed .30" very aggressively. If we wanted to do this less aggressively & hits ofter on braking, we could run 2 reds on the decel side (assuming space available). Look at the blue line. That is two red 85 durometer 1.00" tall Poly Bump Stops "stacked." Note at 1200# of load on decel, the two of them compressed a total of about .57" ... about double. Look at the green line. That is if we ran 3. That's compressing the 3 reds .85".



On acceleration, I like the Accel Side Poly Bump Stops to compress about 1". Not going to do that with a single 1" poly bump. Frankly 2 of them compressing 1/2" each is a formula for failure. The poly bumps will split & pop off the shaft, somewhere on the exit of turn 6 at Webefast speedway. Running 3 is good, 4 is better, 5 is best ... to compress 1". The more Poly Bump Stops you run, less compression on each & they all have a longer life.

Some racers mix colors (different durometers & load ratings). I do not. The reason is the softest Bump Stop will compress the most & fail first. In my experience with poly bumps, running them all the same rate works best. Just as a note, if you're running a race car with poly bumps, you will need some kind of maintenance schedule to replace them, just like spark plugs & brake pads. The exception is if you run Air Spring Bump Stops. You don't replace them, you simply new seals in once a year.

Under hard acceleration we may see anywhere from 600# to 1500# load, depending on engine power, gear ratios & what transmission gear you're in. (Remember: gears are torque multipliers). For conversation sake, let's say we're expecting 1000# of load & want 1.00" Bump Stop compression.

The reason I like to compress the Bump Stops around 1.00" on the Accel Side, is the longer the travel, the better the shock reduction on the rear tires. If we ran 1 red 85 durometer 1.00" tall Poly Bump Stop on the accel side, we'd only compress it .25". That would be very little shock reduction to the rear tires. Look at the red graph above. Not going to get there with 3 reds. We'd only have .70" travel.



Race companies make torque absorbers in sizes to run from 3 (1" tall) poly bumps to 6. These utilize 2 separate shafts for decel & accel. With shaft length changes you can achieve just about any number of Poly Bump Stops on either end. Let's assume we have room in our rear suspension area to package a 5-Poly Bump Stop torque Absorber. I'd run 2 85 durometer reds on the decel side & 3 (70 durometer) poly bumps on the accel side. See graph below. The green line is 3 (70 durometer) poly bumps stacked, and that gets us right about 1.00" of compression with 1000# load. That will take a LOT of shock out of the tires produced by our bad boy motor ... allowing us to roll the throttle on more aggressively ... running faster lap times.



---------------------------------------------------------------------------------------------------------------------------------------- 


 

Racing is a Compromise ... Usually

The reality for all rear suspension designs ... except two ... is the Anti-Squat settings are always a compromise for corner entry & exit handling. With a 3-Link, 4-Link, Torque Arm, Truck Arm, IRS, etc., etc., if you shorten up the swing arm length (increasing the anti-squat percentage) to provide better rear grip on corner exit ... at some point short of optimum corner exit rear grip ... the car starts getting loose on corner entry. (The rear tires are being unloaded & lose rear grip.)

You do not want a pavement car to be loose on corner entry. That's not the fast way around the track. So, you end up with a compromise that is the "best corner exit grip, without being loose on entry." I've seen Racers do it the other way around. They lengthen the swing arm length (decreasing the anti-squat percentage) to provide better rear grip on corner entry. The goal is to drive it in deeper. At some Anti-Squat percentage short of optimum corner entry rear grip ... the car starts getting bad loose on corner exit. The rear tires just loose traction too easy & spin, killing the lap times.

To summarize this, with all rear suspensions (except the Decoupled versions) the rear anti-squat percentage is the same on corner entry & exit ... and that is a compromise. Depending on your priorities ... corner entry or corner exit ... you are tuning the Anti-Squat percentage to find the best compromise, that produces the quickest laps (or in some cases the happiest Driver.)

To clarify how all rear suspensions work, in linkage style suspensions, the links are placed to act as a lever. In IRS the control arms are placed to act as a lever. All rear suspensions have this lever. It just looks different & can be installed with various degrees of Anti-Squat. Anti-Squat is simply a term for this leverage.

Under throttle we have the torque of the engine going through the rear axle & this lever. If your rear suspension is set for 40% Anti-Squat, approximately 40% of that torque (there are other factors) is used to lift the car's CG mass & apply a significant degree of the car's weight as load on the rear tires for traction under acceleration. If we have 60% Anti-Squat, approximately 60% of that torque (there are other factors) is used to lift the car's CG mass & apply some degree of the car's weight as load on the rear tires for traction under acceleration.

Now, as Paul Harvey used to say, "for the rest of the story." Under braking we have the torque of the brakes going the opposite direction through the rear axle & this lever. If your rear suspension is set for 40% Anti-Squat, approximately 40% of that torque is used to pull on the car's CG mass. This REMOVES a significant degree of the car's weight as load on the rear tires ... reducing the traction under deceleration.

Anti-Squat is NOT our friend under braking! Under braking, that same lever that loads the rear tires on acceleration, is UNLOADING the rear tires on deceleration. The more Anti-Squat you run to help the grip on corner exit, the looser the car is on corner entry under braking. It is always a compromise.

What Racers do with adjustable rear suspensions is to tune it until the car starts getting "free" on corner entry. Adjustable 3-Links & 4-Links are adjusting the angle of the top link down in front or up in the rear ... bit by bit ... until they find the best handling balance for their preference or priorities. Most Racers want to leave the lower links level, so they do not introduce any roll steer into their race cars. For conversation sake only ... most Road Race cars end up in the 40% to 60% range of Anti-Squat.

Torque Arm rear suspensions are not as tunable. Again, most Racers want to leave the lower links level, so they do not introduce any roll steer into their race cars. That leaves very little height tuning range, if any, for the Torque Arm. The biggest factor to define the Anti-Squat percentage is the length of the Torque Arm itself. Most are not length adjustable, although I have seen designs that are. So, when deciding on a Torque Arm, it's wise to work out a length that gets you into the 40% to 60% range of Anti-Squat.

---------------------------------------------------------------------------------------------------------------------------------------- 

Offset Decoupled 3-Link Race Suspensions

I love Decoupled 3-Link rear suspensions. They are the highest grip rear suspension possible, with the occasional exception on REALLY rough tracks, or serious curb jumping, where an independent rear suspension may be equal or slightly superior. On most race tracks, even with curb jumping, the shock controlled Decoupled 3-Link will outperform independent rear suspensions for grip.

But, we're going to start with my least favorite of my favorite rear suspensions ... the Poly Bump Stop Decoupled 3-Link. I'd better clarify, my race teams have won a boatload of races with this poly unit. For racing, the Poly Bump Stop Decoupled 3-Link is superior to any other straight axle rear suspension ... except the Air Spring Bump Stop and/or shock controlled Decoupled 3-Links.

There are a lot of racing series & classes (NASCAR, Trans Am, SCCA, etc.) that put in a rule to prevent Racers from running the shock controlled Decoupled 3-Link. The rule simply states the race car can only have four shocks. I have run race cars in many road race & oval track series with this rule. So, we ran the Poly Bump Stop Decoupled 3-Link and had an advantage that led to many, many race wins.

The only rear suspensions that do NOT have to be a compromise of corner exit grip & corner entry grip are Decoupled 3-Links & Decoupled Torque Arms. I prefer the Decoupled 3-Link because they are more tunable than the Decoupled Torque Arm. The top link defining the acceleration characteristics is not coupled or connected solidly to the other top link defining the deceleration characteristics. That is why the word "Decoupled" is used.

In my racing experience Decoupled 3-Link is hands down the best route. I LOVE Decoupled 3-Links, because we can fine tune the corner exit grip to be 100% dead nuts optimum ... and the corner entry grip to be 100% dead nuts optimum ... completely independent of each other. The Accel link Sets Anti-Squat for Acceleration & the Decel link Sets Pro-Squat for Braking. No other rear suspension offers that.



As you can see in the illustration above, the "Accel Link" is in the typical location a top link would be in any 3-Link. In this example, the Accel Link is spring loaded & the front chassis mount is adjustable with a fine tuning jack screw adjuster. We can adjust the front of the Accel Link either direction by 1/16" of an inch or 2" if we want.

The angle of the Accel Link intersects with the angle of the level lower links in front of the Rear Axle Centerline (RACL) to achieve a percentage of Anti-Squat anywhere from 25% to well over 100%. Remember, with all other rear suspensions, most Road Race cars end up in the 40% to 60% range of Anti-Squat, as a compromise, so as not to be too loose on entry or exit. On Decoupled 3-Link rear suspensions we find Anti-Squat in the 90% to 100% range makes the race car launch off the corners. And there are no side effects. The rear grip on corner entry under braking is NOT affected by the Accel Link ... BECAUSE it DECOUPLES under braking.

Drivers LOVE it. But they really love the difference on corner entry. Again, most Road Race cars end up in the 40% to 60% range of Anti-Squat, as a compromise, so as not to be too loose on entry or exit. Not with Decoupled 3-Link rear suspensions.

The Decel link runs the reverse angle ... downhill to the rear ... and intersects with the angle of the level lower links BEHIND the Rear Axle Centerline (RACL). Instead of having some percentage of Anti-Squat ... it has NO ANTI-SQUAT on deceleration ... it has PRO-SQUAT.

What is Pro-Squat? The opposite of Anti-Squat. Under braking, the rear brake torque is lifting on the CG of the race car's mass to LOAD the rear Tires. On Decoupled 3-Link rear suspensions we find 100% Pro-Squat makes the race car amazingly stable under hard, threshold braking on corner entry. And there are no side effects. The rear grip on corner exit under acceleration is NOT affected by the Decel link ... BECAUSE it DECOUPLES under throttle.

Frankly it works so well, we run larger piston rear calipers to increase the rear braking force. The more rear braking force, the more it loads the rear tires.

---------------------------------------------------------------------------------------------------------------------------------------- 

There are Different Methods of Controlling the Travel Each Direction ... Poly Bump Stops, metal springs, Air Spring Bump Stops & shocks. The Poly Bump Stop Decoupled 3-Link works great!  Not as good as the shock versions, but very well. These are the same Poly Bump Stops utilized in the Torque Absorbers mentioned above. The Poly Bump Stop works the same, so read that section if you did not.

One drawback is Poly Bump Stops wear out from being loaded & unloaded 10-12 times a lap. Just like Poly Bump Stops in the front shocks or Torque Absorbers. When they get abused too much, the Poly Bump Stop splits & pops out. Same front or rear. Not good if it happens on track during a race. We simply changed them on a scheduled basis to prevent that. And we learned using multiple Poly Bump Stops together (Shown Here) spreads the load over that number of bushings & lengthens the life of the bushings. Two bushings will live twice as long as one. Four bushings will live twice as long as two. But they still need to be replaced with scheduled maintenance.



Many Racers ask why we don't run metal bump springs instead. They last longer, although not forever either. The reason is bump springs are not ... can not be ... as progressive on rates as the Poly Bump Stops are. Yes, I'm aware of progressive springs. But the progression rate is small. Too small for our needs.

Recently, Earnhardt Technologies Group (ETG) developed Air Spring Bump Stops that we have utilized in place of the Poly Bump Stops. Like Poly Bump Stops, the ETG Air Spring Bump Stops have a similar, very progressive rate curve to them. (See chart) This is perfect for our needs, as we want the initial compression to be soft & quick ... then to have the rate increase to slow down the sudden stop.
The Earnhardt Technologies Air Spring Bump Stops have several advantages over the Poly Bump Stops. Number one. They don't split & pop out! Second, we can tune them without taking them off, by changing the air pressure in them. The chart to the right shows the rate changes with air (Nitrogen) PSI changes. The ETG Air Spring Bump Stops come in four travel versions from 1/2" to 2" to fit our needs.

They can be used on regular Torque Absorber & Poly Bump Stop Decoupled 3-Link linkages, but we utilize the ETG "Bump Stick." It bolts in place of the shock. Frankly, it makes it easy to go back & forth between shock controlled ... with an Air Spring Bump Stop on the shock ... or just Air Spring Bump Stop controlled (with no shock) depending on the rules where the race car is run.



We have a version that only uses ETG Air Bump Stops ... and no shock ... that is legal for SCCA GT classes & Trans Am. On the Decel side ... when the Driver lifts off the throttle & applies initial braking ... the top link mount comes back that same 1.00" to 1.25" plus another .25" to .50" to "help" soften the transition. I used the word "help" intentionally, as one of the few cons of the poly Bump Stops & the Air Spring Bump Stops, is there is no shock valving to slow these transitions.

The top of the rear axle pulling on the Accel side under hard acceleration is relatively smooth. But the system hits pretty hard on the Decel side, when the Driver gets off throttle & on the brakes. Not enough to upset the chassis. But enough to irritate the Driver if they're not used to it. The image above shows an open shaft. We place the ETG Air Bump Stops, spacers & shims to fill that gap.



For that Accel Link, we utilize ETG's 2" travel "Pull Bar". Same technology & progressive spring rate as the Air Spring Bump Stops. Both have travel indicators. We like to 1.00" to 1.25" pull on the Accel Link. That amount of pull allows us to control the engine power from shocking the tire very well. Any more than that and we risk the rear axle hitting the watts link or other stuff back there. Any less than that & we find the system is not as effective at reducing the shock to the tires when the Driver cracks open the throttle. Especially with short sidewall tires on 18" wheels.

This is where the shock controlled versions of the Decoupled 3-Link shine. The unique shock valving we run smooths out the travel each direction. The Driver can still feel the stops each way, but it is minor. Believe it or not, the top link of the rear axle moves so much faster than any wheel on the race car. So, the valving is VERY different. To manage this well, the shocks we use have to be ultra quick responding. No twin tube crap. Only monotube shocks, with decoupled pistons & separation gas canisters will perform optimally.

---------------------------------------------------------------------------------------------------------------------------------------- 

With a Decoupled 3-Link rear suspension, we have a shared top rear axle housing mount (see below) that the Accel & Decel links attach to with a special V-block we make. The chassis mounts tie into the roll cage via a "J-Bar" assembly. You can see the upper brackets for the Decel link weld on & the lower adjustable mount clamps on the J-Bar. It has 4" of adjustability range, but can be unclamped, moves & reclamped on the J-Bar. The J-Bar welds to the rear crossmember. You can see it in the photo below next to the half round laser cut out for the driveshaft loop. This setup runs the exhaust down the passenger side of the car.




 
In the photo above, you can see how the J-Bar attaches to both cross bars in the main hoop of the roll cage. The top mount is gusseted both ways. You can see here in this photo, the rear crossmember has a gap. That is for the exhaust to come down the tunnel. The J-Bar always mounts to the passenger side of the roll cage.

---------------------------------------------------------------------------------------------------------------------------------------- 


 
The ARS shock shown above & below is the fastest responding shock on the racing market. It works excellent as long as we don't overheat it. It is a monotube shock with a small diameter decoupled piston. This piston allows the shocks to respond to small inputs (track surfaces) separate from the faster & longer stroke of acceleration & deceleration.

You can see how the decoupled 3-Link Accel & Decel system work in this video here:

It is of an autocross car just testing in a parking lot, so take that into account. But this shock in the video is not the decoupled piston.

The decoupled piston keeps the tires planted over irregularities. Then under hard acceleration the rebound valving allows the shock to extend fast, but smoothly. On Decel, the valving slows the shock compression significantly to take the violence out of the transition to braking. We like this shock up to 1000HP on track cars that see 5-10 laps or 750Hp if they're going to run 30 laps.





Anytime we're going to see more laps, or we have a lot more power to control, we step up to either Penske 8300DA or 8760TA shocks, with larger OD decoupled pistons, provide more control. The additional oil volume of these shocks allows them to see more abuse without running hot & thinning out the shock fluid. You can see these & the ETG Air Spring Bump Stops in the images below in our Race-Warrior underslung rear frame clip.

[Double Wishbone Independent Rear Suspensions]

Double Wishbone Independent Rear Suspensions

I have a love/hate relationship with IRS. My aversion for Independent Rear Suspensions is based on they're heavy & not easily Anti-Squat or Rear Roll Center adjustable. We can't offset anything to eliminate Torque Steer, nor can we Decouple them in any way. So, we deal with a compromise Anti-Squat on corner entry & exit. I am a "Tuner."  I love to dial in the total suspension at the track to be perfectly optimum ... in other words ... faster than all our competitors. IRS puts me in a box with less tuning tools at the track. A few different ones, but nothing that allows me to dial in Anti-Squat or Rear Roll Center.   

My love for IRS is based on the infinite range of roll steer & dynamic toe adjustability. We can make each side steer in or out to our heart's content.  If you're not familiar, low to zero ride height rule race cars, struggle to turn well due to little or no pitch angle change. In other words, if the ride height starts at 1/4", we're obviously not high travel diving the front end.

That lack of pitch change, CG migration & load transfer make it harder for these cars to turn well in tight corners. The solution & savior is IRS, because we can dial in Rear Roll Steer. With that, the rear of the car is helping the front of the car turn better. In my racing experience, cars with 2" & higher static ride heights don't "need" IRS. They can benefit from IRS if the front suspension is not up to par. But an offset, decoupled 3-Link will provide greater grip on entry & exit than any IRS. 

More love for IRS is based on the fact that ... well ... each side is independent. We can obviously keep the tires better loaded & following the irregularities of the track than a straight axle. The smoother the track, the less advantage this is. The rougher the track, the more of an advantage this is. Lastly, if we are running a rear engine configuration, IRS is our only choice. In this thread, I dive deeper into IRS setups & I'll show you why I love the Multi-Link IRS a little bit more than conventional control arm IRS.

First, let's start with the typical double wishbone IRS that utilizes unequal length control arms. In racing IRS designs, we can build the control arms any shape & size that will fit. Some Racers like to (or need to) utilize OEM control arms or copies of. Most OEM IRS control arms are unequal length to achieve goals they have for the production cars driven on the street. We don't have those concerns, but for this first example, we are utilizing C6 Corvette upper & lower control arms.

C6 Z06 Lowered Independent Rear Suspensions

First, let's start with the typical double wishbone IRS that utilizes unequal length control arms. In racing IRS designs, we can build the control arms any shape & size that will fit. Some Racers like to (or need to) utilize OEM control arms or copies of. Most OEM IRS control arms are unequal length to achieve goals they have for the production cars driven on the street. We don't have those concerns, but for this first example, we are utilizing C6 Corvette upper & lower control arms.




 
If you look closely at the toe numbers, you can see the static rear toe-in (-.26° = 1/16" toe-in each side = 1/8" total toe-in) changes from a -.26 to positive .25 ... meaning the outside rear tire goes from 1/16" toe-in to 1/16" toe out. The inside rear tire goes from -1/16" toe-in to 3/16" toe-in. That means the rear tires went into positive rear steer with both tires steering towards the outside of the corner. You can also see we lose a lot of our negative camber. We started with -1.2° & lost about .7° of that with body roll. 

This is fine ... If you have enough rear wing to keep the rear tires gripped up. In fact, this is common. The rear steer helps the car turn better in tight corners (where air speed is low & the wing is less effective). Then at higher speeds, when the rear steer would make the car loose, the wing saves our ass.

Both of these issues are geometry problems that can be fixed. The camber change is a control arm geometry issue & the rear roll steer is a toe-link geometry issue. All of this is fixable if it benefits your race car goals.

Below is a side view of the stock, but lowered C6 Z06 suspension. It doesn't show this, but how the lines from the control arms intersect produces a 59" swing arm length & 45% Anti-Squat. Both great for road courses. If we ONLY autocrossed this car, it would benefit from 10-15% more Anti-Squat.



Top view below. Nothing out of the ordinary. Easy to see the "unequal length" control arms.



A quick word on counter roll steer. We practically never run counter roll steer. Frankly, we practically never run positive roll steer either. If the front suspension is really good, it doesn't need any help turning from the rear with positive steer. What about counter roll steer? If you have plenty rear grip (both mechanical & aero), you do not need more rear grip from counter rear steer. There was ONE TIME I designed a C5 rear IRS with counter roll steer.

Stock Lowered Viper Independent Rear Suspensions

For years I had heard the handling in Dodge Vipers were "evil." I love the style & thought about buying one for fun. I had a client that was track & competing in his Gen 3 Viper he brought to us to improve. We started by measuring all the suspension & steering geometry points. After I entered all the data & looked at the geometry, I just knew the crew had made some measurement errors. So, I had a different guy remeasure. Nope ... the original dimensions were correct. 





When you look at the top view (above), things look pretty normal. It's when I looked at the rear view (below) ... that I agreed it's true ... these Vipers ARE trying to kill the Drivers. Holy cow!  I'd never seen a car with the Rear Roll Center UNDER the ground level! 



When you look at the image above & below, the upper image is static & the lower image is dynamic with body roll in the corner. The RRC below ground makes the Roll Axis backwards ... makes the rear of the car roll over more than the front (crazy) ... and the icing on the cake ... is the roll steer is positive (meaning the rear wheels turn towards the outside of the corner) ... AND the camber loss was huge! Had to be scary to drive at the limit.



He asked us to fix it, which we did. It required buying new control arm buckets from Dodge & remounting 6 of them ... two to improve the front ... and 4 to fix the rear. After we relocated the IRS control arm mounting points, the rear view of the Viper looked like the images you see below.





When you look at the images above, the upper image is static & the lower image is dynamic with body roll in the corner. You can see, simply by relocating the control arm mounts, the same control arms & uprights produce a very drivable combination.

You can see the RRC is 6-3/4" ... to mate up with his 0" ground level FRC ... and create a normal Roll Axis (Pitch down in front). Dynamically you can see rear camber (-1.2°) & rear toe-in (.26° = 1/16" toe-in each side = 1/8" total toe-in) change VERY LITTLE with the 1.2° rear body roll.

Track-Warrior Independent Rear Suspension

For my Warrior line of Tube Chassis cars, I make two very different versions of IRS. For the Track-Warrior, we utilize an (almost) equal length control arm IRS. And for the Race-Warrior, we utilize a Multi-Link IRS. The Track-Warrior version is simpler, less cost & very effective. The Race-Warrior version is more complex, more expensive, but can be configured to achieve damn near any geometry & final result with tire camber, toe & steer we can dream up. Here is the Track-Warrior ...



You'll notice this one is configured for a 4.5" RRC. That works well for high powered, fast track cars as long as the front suspension is doing its job. This low of an RRC does require a wing, but a modest one with 600-800# of downforce at 150mph is plenty.

The image below is dynamic, with 0.6° roll angle. (Without a wing, the roll angle would be higher.) You can see the camber change is about 0.3° on each side. In my racing experience I have found around 1.2° dynamic camber works pretty well on most 30 series sidewall race tires on 18" wheels for track cars. We run higher negative camber on our Race-Warrior stuff, because the corner speeds & g-forces are both significantly higher. 



If you compare both images above, the static & dynamic, you'll notice there is no rear roll steer. The 1/16" of toe in on each side stays intact during body roll. That is a function of the toe-link geometry. I like to make my stuff with easily adjustable (and fine tunable) toe-links on the chassis end & at the upright. That way you can really dial in zero roll steer, positive or counter roll steer. (*Some Racers say "negative roll steer" in place of "counter roll steer".)

It doesn't show in the image below, but the swing arm is 56.8" & the Anti-Squat is 47.7%. 



If you looked at the rear view thoroughly, you could tell the control arms were almost equal length. Since this IRS is for a tube chassis Track-Warrior, it's easy to design a frame with mounts the same width top & bottom. You can see this easily looking from the top view (image below).

The actual UCA & LCA length difference is 2" due to KPI of this spindle & the designed in negative camber. But that difference is much smaller than most designs. It makes it easier to achieve minimal camber change during roll.

Speedtech Independent Rear Suspension

When I was hired to design the Speedtech Independent Suspension, the original goal was to make two front mounting holes for the lower control arm. One that produced a setup optimum for autocross & the other for road courses. The autocross geometry is shown below.





This setup above was optimized for autocross with a 6-3/4" rear roll center, 59" swing arm & 70%+ Anti-Squat.



The image above shows the top view of the road course setup. You'll notice this IRS has the same almost equal length control arms to minimize camber & geometry change during body roll. This setup was optimized for road courses with a 8.11" rear roll center, 76-3/4" swing arm & 50%+ Anti-Squat.



This IRS also features tunable toe links on both ends, to fine tune any desired roll steer. The baseline is zero roll steer, but you can dial in counter roll steer for road courses, if you don't have enough rear aero downforce. And of course, you can dial in positive roll steer to help the car turn on tight road courses & autocross tracks. You can see from the two images above, even with 1.1° body roll, the rear tires are still straight & toed in 1/16" on each side.