[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.

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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.

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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 also 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 starts by drawing a line through your upper & lower ball joints, to the ground, that is your car's steering axis. Your car's Scrub Radius is the distance from the steering axis to tread centerline at ground level. That dimension 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, ARB's, track width & Roll Centers. Stiffer springs, bigger ARBs, 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.

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 center to center of the tires' tread, measuring both front or rear tires.

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

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

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.

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

ARB Rate = Pounds of torsional force to twist the ARB 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 & it causes problems. 


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15 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 your serious about winning.

C.   Weight (mass) location 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 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.   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 seat.

H.   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.

I.   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.

J.   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.

K.   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 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.

L.   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.

M.    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 you design an optimal high travel suspension, for example 3"-4" of dive, you may utilize long control arms to slow the camber during dive. This way you can run optimal static camber & achieve the optimal camber gain. If you 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 you to run significantly more than optimal static camber, to arrive at the optimal camber. Conversely, you'll have the reverse problem if you 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, you may start with a low travel strategy, for example 1"-1.5" of dive, with a 2.5" ride height & later decide you 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 you 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.

N.   Stiction & friction choices are not often thought of 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.

O.   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 your spindle, hub & bearing with a safety factor built in.   


20 CRITICAL HANDLING CONCEPTS:
1.   What you 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.

2.   Geometry design, settings & changes should be to improve how the tires contact the road Dynamically & how they are loaded.

3.   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 you 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, you will have different handling issues turning left & right. 

4.   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.

5.   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.   Anti-Roll 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.

7.   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.

8.   Springs, shocks & anti-roll 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 & you create a harmonious team.

9.   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.

10.   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.

11.   Tuning to allow a suspension corner ... to compress quicker or farther ... provides more force & therefore more grip to that tire ... up to the limits of the tire. Tuning to allow a suspension corner ... to extend/rebound quicker or farther ... provides more force & therefore more grip to the opposite corner's tire ... up to the limits of the tire.

12.   Softer front springs allow more compression travel & therefore more load transfer onto the front tires ... under braking ... for more front grip. Stiffer front springs reduce compression travel & therefore lessen load transfer onto the front tires ... under braking ... for less front grip.

Softer rear springs allow more compression travel & therefore more load transfer onto the rear tires ... under acceleration ... for more rear grip. Stiffer rear springs reduce compression travel & therefore lessen load transfer onto the rear tires ... under acceleration ... for less rear grip. There are pros, cons & exceptions to these rules.

13.   Optimum 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.

14.   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.

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.   Tuning is NOT linear 2 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.

18.   The car's Center of Gravity acts as a lever on the Roll Center ... separately front & rear. Higher CG's and/or lower RC's increases Roll Angle. Lower CG's and/or higher RC's decrease Roll Angle. Getting the front & rear of the car to roll similar is desired. Getting them to roll exactly the same is not the goal, because ...

19.   Goal: To have optimum grip on all tires and disengage the inside rear tire (to a degree) to turn well ... then re-engage 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 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.

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

[OKAY ... Let's talk rear suspension & geometry!]

OKAY ... Let's talk rear suspension & geometry!

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.

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.

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Rear Suspension Types
There are seven 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, 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 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.

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.





A Triangulated 4-Link is simple, and fairly common as a factory style rear suspension in production cars. It could be argued it will handle more torque under hard launches than 3-links, but if you were going to drag race it with slicks, you would want a Parallel 4-link, not a triangulated 4-link. 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 over 800HP.




Truck Arm rear suspensions started on half ton pickups in the 60's.  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 hate them. Yes they are simple.  Simply hard to make the rear do what you want. There are many tricks to making them work, 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.




A Torque Arm suspension is a great non-adjustable 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 can be made "a little" adjustable, but typically offer less adjustability than the other designs, as far as controlling the front Instant Center, rise leverage & 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.

The Torque Arm is similar, but has key differences, to a 3-link.  Both utilize two lower links (aka "trailing arms" or "control arms").  ... but instead of the third link being on top (centered or offset) & pivoting ... it mounts solidly to the housing & extends quite far forward (closer to the center of the wheelbase) with its 3rd pivot point. These are important differences. The 3-link, if configured as such, can be infinitely tuned.  The 3-Link is capable of short or long instant centers (swing arm length), as well as very low to very high instant centers (defining the rear suspension Anti-Squat).

Where the 3-Link upper link pivots freely with rod ends or bushings, the Torque Arm mounts solid to the underside of the rear axle housing. The length of the Torque Arm is the single most important factor to define its geometry & therefore its handling characteristics.  Since the length of most Torque Arm designs can not be adjusted ... what you have is what you're stuck with. For note, it is possible to build a Torque Arm that has adjustable lengths and/or different length Torque Arms that can be interchanged, but we're going away from the Torque Arm's simplicity.  Might as well build a 3-Link at that point.




3-Links 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.

Tech Tip 5: Why do we use a Torque Arm for Autocross & 3-Links for most Road Course applications?  Tuning takes time.  Autocrossers typically have less time for tuning between rounds, where there is typically more time between sessions for track days & road racing. 
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.  On road courses there are big time gains to be had dialing in the rear anti-squat for the track & conditions. 

A pro of the Torque Arm suspensions is that they are simple & easy to set up. Torque Arms don't require any tuning, because other than length, they are basically non-adjustable. The key is choosing the right length of arm, because in most cases, the length can't be changed.

Shorter arms provide higher Anti-Squat, more loading of the rear tires on initial & early acceleration ... less on later acceleration ... and even less on corner entry under hard braking, unless it is decoupled. A longer Torque Arm simply reverses these characteristics.

A good road race & track car rule of thumb is for the 3-Link Instant Center or Torque Arm pick up point to be under the Car's CG ... 55 to 60" in most 108" WB cars.  But for Autocross, we want a shorter arm, due to the lower corner speeds. We have found a 45-47" 3-Link Instant Center or Torque Arm pick up point is the sweet spot.

Shorter lengths add more forward bite (more grip on initial acceleration) which is critical for coming off slower corners. For the higher speed corners of road courses, with higher corner exit speeds, longer arms work better. With 3-Links, we can tune the Instant Center to form this longer or shorter "arm." With Torque Arms you need to pick your length based on priority.

The primary advantage of a 3-Link is the tunability. With multiples holes in top link brackets, or a jack screw adjuster on the chassis, we can change the intersect point of the upper & lower links (Instant Center) and therefore the Anti-Squat percentage.  We can tune the car for the optimum balance (compromise) of rear grip on entry versus rear grip on exit, for different tracks, drivers and/or weather conditions.

A good road race rule of thumb, or baseline, is to start with the Instant Center under the car's CG & tune from there. If you have a car with 55% front weight & 108" wheelbase, the CG is (108 x .55 = 59.4) 59.4" ahead of the rear axle centerline.  Run the lower link level with the ground & adjust the top link angle to form the intersection point (Instant Center) 59.4" ahead of the rear axle centerline. 

When you need more rear grip on corner exit under acceleration, we would adjust the top link (where it attaches to the chassis) downward, increasing anti-squat and rear grip on exit. This will reduce rear grip on corner entry, so we can't get greedy, or the car will get loose on corner entry under braking & turn-in.  We do not want that.  We're always looking for the best compromise that produces the best lap times ... unless we have a decoupled 3-Link.   



Parallel 4-links & Triangulated 4-links can be made somewhat adjustable if designed & installed with
multiple or variable 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-links can handle drag racing up to a point, but it wouldn't be my choice if the car was planned for super high hp, high rpm, clutch dropping, slick running, wheelie pulling launches ... as there are only 2 rod ends "pulling" through the top link to lift the whole car. 4-links can handle more launch load (like drag racing), because the force going through the rear end & rear suspension that "pulls" the top links(s) is spread over 4 rod ends.

Parallel 4-links, 3-links & Torque Arm suspensions require a device to keep the rear end centered in the
chassis, like a panhard bar or watts link. A triangulated 4-link does not require this, as the 2 or 4 links running at an angle keep the rear end in the location you put it. 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 everything tuning change is.  For this reason alone, I would not recommend a triangulated 4-link for track or road race cars. Having that rear roll center tunability is very important.

Triangulated 4-Links are rarely adjustable, but still ranks at the bottom as the push pull forces aren't parallel with the car (which is desired) and the roll center is not separately adjustable, as it does not use a panhard bar or Watt's link. If you have a triangulated 4-Link rear suspension, that doesn't mean you shouldn't do track days or autocross. Rock on & have fun. Just know if the rear needs to be loosened or tightened up, to balance out the car for neutral handling, you have less tuning options.

The best rear suspension for road racing, track cars & autocross, is the adjustable 3-link, as it has the best articulation & tunability ... plus a few added benefits if you design it optimally. The adjustable parallel 4-link will work well as long as the car doesn't require a high degree of roll angle for the suspension to work, but it's not my weapon of choice. The 3-Link is more adjustable ... can be offset to eliminate torque steer ... and can be decoupled to optimize rear grip on entry & exit separately with no compromise.

P.S. Torque Arms can be decoupled as well to optimize rear grip on entry & exit separately with no compromise, but can not be offset to eliminate torque steer.

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Offset 3-Links:
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 this in. Sometimes in the real world, packaging challenges play a role & prevent this.

[Offset 3-Links with Torque Absorber:]

Offset 3-Links with Torque Absorber:
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. 



It's common to use 2 or more poly bump stops on the acceleration side of the torque absorber & only 1 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. More on this later.




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 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 d 1.00", 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.