[Front Suspension & Steering Geometry Fundamentals]

Front Suspension & Steering 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 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.

[First, Let's Talk About What We're Measuring & Why]

First, Let's Talk About What We're Measuring & Why

We cannot go any faster through corners than the front end has grip & the front roll center is the #1 priority to front end grip.



Front Roll Centers:
I'll be very basic for any readers following along that are completely new to this & apologize in advance for boring the veterans with more knowledge of this. Cars have two roll centers ... one as part of the front suspension & one as part of the rear suspension. I'll first explain what role they play in the handling of a car ... and then how to calculate the front roll center.

Think of the front & rear roll centers 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. Because the front & rear roll centers are often at different heights, the car rolls on different pivot points front & rear ... "typically" higher in the rear & lower in the front.

(See image above) If you were to draw a line parallel down the middle of the car connecting the two roll centers ... this is called the roll axis ... that line would represent the pivot angle the car rolls on ... again "typically" higher in the rear & lower in the front. (See image below) If you had a bird's eye view from the top, the roll centers should be in the center of the race car.



The forces that act on the car to make it roll ... when a car is cornering ... ... act upon the car's Center of Gravity (CG). With the race cars we're focused on, 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 as a lever. The farther apart the CG & Roll Center are ... the more leverage the CG has over the Roll Center to load the tires & make the car roll. While more grip is the goal, excessive chassis Roll Angle is your enemy, because it over works the outside tires & under utilizes the inside tires.

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Locating Your Front Roll Center:
Measuring all the pivot points in the front suspension to calculate the Roll Center in the front suspension of a double A-arm suspension car can be tedious ... but the concept is quite simple.

Your UCA & LCA have pivot points on the chassis ... and they pivot on the spindle at the center of the ball joints. Forget the shape of the control arms ... the pivots are all that matter.
 


If you draw a line through the Centerline of the Upper Control Arm pivots & another line though the Centerline of the Lower Control Arm pivots ... they will intersect at some point (as long as they are not parallel). The point of intersection is called the instant Center (IC). Look at the drawing below. The red colored dot on the left represents the Instant Center for the red control arms on the right. Blue is the other side. These are accurate for the static car at ride height.

The UCA/Spindle/LCA assembly travels in an arc from that IC point. However far out that IC is ... measured in inches ... is called the Swing Arm length. Longer swing arm lengths produce less geometry change through travel. Shorter swing arm lengths produce more geometry change through travel.



Still using the image above, next you draw a line from the Centerline of the tire contact patch at ground level ... to the Instant Center of the control arms on the opposite side. Do this on both sides.

Look at the image above again & notice the black dot. Where the two "tire contact patch lines to Instant Center Lines" cross (intersect) ... where the black dot is ... is the static front Roll Center at ride height. The black dot represents the Static RC at ride height.

In the image below, we compressed the front suspension 1" & rolled the car 1°. This is a dynamic condition under braking & turning. The red & blue Swing Arm Instant Center dots moved. Unevenly ... due to body roll. The roll center moved down (due to dive) & to the right (due to body roll). We call this roll center migration. We care about this MORE than the static roll center location. But we have to measure the car statically ... obviously. Then we plug the dimensions into chassis software & work out real world dynamic situations.



Make sense?

[Let discuss HOW to map out your front suspension geometry points]

Let discuss HOW to map out your front suspension geometry points

This may sound funny, but I do not know what spring & sway bar rates a car needs without running calculations based on the car's geometry. Frankly, I don't know the optimum camber, caster, roll centers & so on ... until I know the car's entire geometry picture. I offer two "Tech Services" ... Tech #24 & Tech #40... to assist Clients dial in and/or Improve their race car's performance.

Tech Service #40 entails:
* I send the client a link to these measuring instructions & 3 measurement forms
* They measure their car's suspension & steering points accurately & return the completed forms
* I plug all the info into my software & see what each area of their geometry looks like
* I evaluate each area that affects performance & gather the information for a client discussion
* We discuss what's wrong with their race car's geometry & how that affects our possible strategies
* We discuss front end suspension travel & car roll angle strategies & the client decides on the best one for them & their racing goals
* I share with them what to do & how to do the changes to improve the geometry & track performance
* The client makes the physical changes on their car - Some involve minor fabrication & welding
* If it is a production car from the 1960's to 1980's we can plan on truing up the lower control arm pivots. More on this later.

I then work out a "set up" which includes:
* Front spring rates based on the travel strategy chosen
* Front sway bar rate based on the roll angle strategy chosen for different courses
* Rear spring rates based on the roll angle strategy chosen
* Rear sway bar rates to achieve handling balance with holes for fine tuning
* Rear roll center height (IRS, Panhard Bar or Watt's link) for handling balance & grip
* Static Camber & Caster Settings for optimum tire contact patch dynamically when the car is in dive & roll
* Toe & Ackerman settings for optimum turn in, tire slip angle & inside front tire grip

Tech Service #24: Is just for new setups. It requires I already have the geometry in my software, from either doing a Tech #40 before ... or if I designed the race car's geometry. 

You can see more details on both Tech #24 & tech #40 HERE

If you are doing your own geometry optimization, you'll need to purchase a software. I used to keep up on the brands & versions available, but not anymore. So, I don't have any recommendations these days, other than Performance Trends. I've used several brands & versions & I always go back to Performance Trends Suspension Analyzer. It's simply to use, accurate & affordable. What else are you looking for?

None of them will tell you the results are good or bad. Everything is just numbers. You'll have to know what numbers you're shooting for. I share some of that in the other forum threads, like what kind of anti-dive percentage I target, FLLD percentages, etc. Being perfectly frank & transparent, I'll share you with a lot of what you looking for, but not all. Intentionally. Hey! A girl has to keep some of her secrets.

Part 1 of 3 - Preview:
Whether you're measuring your race car for you or me to enter into a suspension geometry software, it is critical you take your time, double measure & gather accurate dimensions. 

You're going to measure all of the pivot points in your front suspension, steering & rear suspension, plus front & rear track widths, sway bars, shock & spring points. You need what engineers call the X,Y,Z dimensions. In other words, you need the dimension to each pivot point.
•    OUT from CHASSIS CENTERLINE
•    UP from the GROUND/FLOOR
•    FORE/AFT of the respective AXLE CENTERLINE
•    I'll just refer to these as getting all three dimensions.

Once you, or I, input the dimensions into the suspension software & we'll get clear on your CURRENT:
•    A-arm Instant Centers
•    Static Roll Center locations
•    Dynamic Roll Center location*
•    Scrub radius
•    Camber Gain
•    Dynamic Camber in dive & roll*
•    Spring Motion Ratios
•    Actual spring rate at the wheel aka Wheel Rate
•    Sway Bar Motion Ratios
•    True rate of your Sway Bars
•    Shock Motion Ratios
•    Shock Valving rate at the wheel**
•    Anti-Dive
•    Ackerman
•    Anti-Squat
•    And more.

From there, we'll be able to make informed decisions on A-arms, ball joints, spindles, etc ... to achieve optimum front geometry for your goals. So, we can dial in the whole list above to the optimum settings (within physical & financial possibilities). Plus adjustments to the rear suspension to optimize it as well, and balance the handling.

I will show you how the information can guide us on part & tuning decisions, after we lay it out, along with what all the info means. If there are Engineers following this post, you're going to dislike some of my terms. I apologize in advance. Sometimes I use the engineering term. Other times I use what I call as "car guy" terms.

In this post, I'll lay out a method to measure all of the pivot points in your upper & lower A-arms, steering, rear suspension links, plus track width, sway bar, shock & spring points. There are hundreds of ways to doing most things. I'm laying out one way I think most Racers can do in their garage. You will need some basic tools & creative body language when it comes to getting into difficult spots. This method will take a little longer, but we will have accurate numbers ... AND ... as you make changes, you will need to take only a few measurements & we'll still know where everything is.

These dimensions are important, so taking your time, measuring several times & even measuring different ways will ensure we have accurate numbers.

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Tools:
You will need a good tape measure. Use just one ... so if the end isn't dead zero perfectly accurate, at least all the measurements are the same. Every 1/16" matters.

Tool check list or shopping list:
•    Good, readable tape measure.
•    4-6" dial caliper or digital caliper.
•    Short laser level (6-10")
•    Framing squares in sizes that fit under your car
•    2 Bob weights (See image below)
•    Chalk line (blue will come off your floor eventually. Red is not coming out ... ever)
•    Roll of quality string
•    Removable masking tape (Blue 3M works best)
•    Sharpie fine point marker(s)
•    Brake Cleaner & paper towels
•    Note pad & pens
Home Depot or Lowes has everything in one stop if you need something.



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Pre-measure Ball Joints:
If you have some spare upper & lower joints ... great. If not, make a stop at your local, friendly auto parts store where you buy parts. You'll need to borrow an upper & lower ball joint to measure at the counter.
Guys, don't use a "generic" ball joint. Use the part numbers for your car.



(See image above). Take a ball joint & lean it all the way to the right & draw a Sharpie line on the housing ... in line with the BJ pin. Now lean it left & do it again. Now straight up & do it. There is your true pivot center. Take your dial/digital calipers & measure from the pivot center to the top of the housing & again from the pivot center to the end of the stud.

Do both upper & lower ball joints & write down all the numbers. When you're under the car, getting to the top off the housing or the end of the stud will be WAY easier to achieve. You just need to do the math ... along with measuring ... to arrive at the true pivot center locations on your car.

What was the brake cleaner & paper towel for? To clean off your Sharpie marks on the ball joint housings. Plus clean stuff under your race car as you go.

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Before you start:
a. Plan for this to take at least a full day or more.
b. You'll need a helper to hold the other end of the tape and to "spot" for you.
c. Work out how you're going to put the driver weight in the seat. Don't use a person. (We use lead)
d. Put your track tires on & set the tire pressures just like you compete with.
e. Get the car as close to "race ready" as possible: fuel level, stuff out of (or in) the car, etc.
f. CRITICAL: Document all numbers & details on paper, so we can refer back later.

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Platform:
The tires & suspension have to be "loaded" just like the car is on the ground. So, you can't use a chassis lift or jack stands. You can use a drive on lift & I strongly urge you to find one to use if you can, because it is the easiest. Drive on Lifts have two advantages. One is you can adjust the height of the car to work under. The second is you utilize strings with weights on each end (like the image below) to create your "ground line" to measure from. This is pretty accurate. You just slide the strings where you need them to measure each time. I've also seen guys with shop access use muffler & oil change pits, where they can walk under the car.



There are 2 other good ways. Just remember the car has to be on its tires, just like ride height, but you need to be able to get under it. On Stock Cars & NASCAR Modifieds we use 10" Joe's Racing stands & simply add 10" to all of our height numbers. You "may" be able to do this on the garage floor, but it is harder. Depending on ride height your fat ass may not be able to get under the car well, or at all.



If you're measuring from the floor, pick a spot in your shop as flat as possible with minimal dips. With a friend, pull strings taut over the area & look for low & high spots. Avoid areas with high spots. Low spots are OK, because you can account for them. How? Simply put painter's tape on the low spots with the dimension of how low the dip is, below the taut string (3rd person?). Then you can subtract that dimension from your measurements when in that area.

For example, if the dip is 3/16" and you measure from a pivot point to that dip & get 15-5/16" ... subtract the 3/16" ... and your true dimension is 15-1/8". Make sense?  Don't add the dip number. Subtract.

I've seen guys use under tire, car ramps & custom built stands ... shimmed so all 4 are the EXACT same height. So, give this some thought on where & how works best for you. If you rig something up to raise the car, all 4 stands need to be the EXACT same height. Any height will do, but they have to be the same. Whatever the number is, you will be subtracting it from your height numbers to get true numbers, as if the car was on the ground.

Another option if you're using stands, is to level them to each other & run strings or straight edges across the tops of the stands, similar to using a drive on ramp lift.







Do NOT use a jack or jack stand under the lower control arm or ball joint. It is not accurate, because it loads the suspension at a different point. Do not use jacks under the ball joints like shown in the pictures above. No matter how careful you are setting the spindle pin in the correct height, this method will end up causing inaccurate measurements. Put tires on the car and get your measurements with the front suspension under weight and the suspension relaxed, not in bind. Most of you will be able to do this with the tires & wheels on. Do it that way if you can.

If you have deep back spaced wheels (like our road race cars) & the ball joints are deep inside the wheel & make this too difficult to measure accurately ... and the wheels have to come off ... use adjustable wheel stands like the photo below. Adjust them until the center of the hub is EXACTLY the same height as is was with the tire & wheel on, all four corners. Frankly this is how we do all of ours, but there is an expense to buy them. We also use them on top of perfectly level scale plates in our race shops, so we don't have to account for uneven floor surfaces. But you gotta use what you have access to.



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Pull the car into your "space" straight. Make sure the front wheels are "dead true straight" as you roll it in & do NOT turn the steering wheel once you're in your spot. Back up & do it again if you need to.

A simple way to check & confirm the front wheels are "dead true straight" ... is to pull a long, taut string across rear tire & front tire on one side ... at the same height (preferably axle centerline) ... and pay attention to how it lays across the front tire. Do both sides.

If the car is toed-in, the gap between the string & front sidewall, of the front tires, should be the same on both sides. If the car is toed-out, the gap between the string & rear sidewall, of the front tires, should be the same on both sides. If a gap is bigger on one side, the steering is not straight. You don't want to have to turn the wheels ... in their spot ... and leave them, as there will be "tire bind" (unless you have grease plates under one or both tires so they freely slide). Get it true & back the car up 2' & roll it into place again if you need to.

[Part 2 of 3 to measure suspension points]

Part 2 of 3 to measure suspension points

3 types of measurements:
1. Height ... from the ground up to the pivot point
2. Outward ... Left or right ... always from the car centerline to the pivot point
3. Fore or Aft ... from the axle centerline to the pivot point
We sometime refer to these as X, Y & Z measurements

What you're measuring ... in order:
1.    Car ride height front & rear
2.    Car centerline
3.    Axle centerlines front & rear
4.    Track Widths front & rear

Front:
5.    All four ball joints ... at true center of their pivot
6.    All 8 control arm pivot centers (2 for each upper & lower control arm)
7.    Shock pivot centers (upper & lower)
8.    Spring end centers (upper & lower) if not coil-over
9.    Sway bar attachment points to A-arms

Steering:
10.    Inner & outer tie rod ends
11.    Steering Box, Pitman, Idler & Centerlink pivots if steering box

Rear: (IRS – Same as front)

Rear if straight axle:
12.    All rear linkage pivot centers (2 for each link)
13.    Panhard bar or Watts if utilized
14.    Shock pivot centers (upper & lower)
15.    Spring end centers (upper & lower) if not coil-over
16.    Sway bar attachment points to & axle or chassis

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Here is detail for each area of measurements:

1. Car ride height:
Pick a spot on the bottom of the frame rails that will be easy to measure now & in the future. The further apart you go, the more accurate. Most Racers usually pick a spot in front of the rear tires & a spot behind the front tires ... if the frame rails are wide at that point & out near the rocker panel. Either pick a spot with identifying holes or marks on the bottom side of the frame rails ... or make something to
permanently mark these spots. You'll use these for the life of the car, no matter what changes you make.

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2. Car centerline & front axle centerline:
Before we start, "mark down" means using either the laser or plumb bob & string ... along with painter's tape & a sharpie ... to put a mark on the floor exactly where a point is. Some people use the term "transfer" ... meaning to transfer a point on the chassis or suspension onto the ground. I don't like marking on my epoxy coated floors, so I put blue masking tape down first & write on it with the Sharpie marker.

This measuring process is difficult & somewhat subjective. Pick a section of the frame that looks symmetrical ... and either go inside or outside (but use the same on both sides of the car) & "mark down" ... putting a dot or "x" on the floor. Do this in 3 spots, using 3 different sections of the front frame. Then, do this in the rear, using 3 sections of the rear frame. Now find the center of those 6 frame sections, using tape & sharpie again.



Next, with a friend's help, pull your chalk line taut ... down the center of the car ... on the floor ... and the string "should" hit all 6 frame section centers. If it doesn't, either recheck your measurements and/or re-evaluate your choice of locations. Ultimately, you're going to put a chalk line on the floor using the marks you trust. That is the car centerline that everything is going to be measured from & to.

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3. Axle Centerlines:
Start in the front. A simple way to do this is to pull a taut string behind the front tires, on the floor, and mark the string location on both sides. Do the same in the front. Split the difference & that is the front axle centerline. Problems only occur with this method with different tires, different pressures, suspension or tire bind from not rolling it into place straight.

Put a chalk line on the floor representing the front axle centerline. Many things, but not all, will need to be measured from & to this point. For all things in front of the front axle centerline, express them in negative or minus numbers. Example: if something is 2 & 3/8" in front of the front spindle centerline, write it down as -2 & 3/8". Everything behind the front axle, express as positive numbers.

Do the same for the rear axle. The only difference is everything behind the rear axle, express as negative numbers.

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4. Track widths front & rear:
Just for clarification ... "tread width" is outside to outside of the tread. "Track width" is center to center of the tires. A lot of people get those confused & our conversations get sidelined.

We need to know the track width ... the true center to center of the front tire contact patches. Several options, but here is a quickie. Start in the front. On the front side of the tires ... measure from the outside of the tread width on one tire ... across to the inside tread width on the other side. Do the same on the back side of the front tires ... and average the two numbers ... to account for any toe in or out. If you get 55-1/2 at the front of the tires & 55-3/8" at the rear of the tires, average that to 55-7/16".

Do the same in the rear. BUT ... if you have any "toe" in the rear, and your rear suspension is not IRS, you may want to address this.

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5. Locate all four ball joints ... at true center of their pivot:
Since you have already pre-measured ball joints, this part is not as tough as it is if you're trying to find the pivot centers while measuring. Eyeballing these pivot centers ALWAYS leads to inaccurate data. Use your measurements from the top off the housing, or the end of the stud, to the axle centerline & work out the math. Double check by eyeballing to see if your math places the pivot center where it actually is.



Your Roll Center locations & Camber Gain numbers will only be as accurate as your measurements. This is a time consuming & somewhat tedious operation. I always suggest people take the time to do it right the first time, so you don't have to do it over (or suffer handling problems from incorrect tuning).

You need to measure the location of each ball joint in 3 directions ... all based on the true pivot point of that ball joint. You need the ...
1. Height ... from the ground up to the pivot point
2. Left or right ... always from the car centerline
3. Forward or backwards ... of the front axle spindle centerline.

Reminder: for all things in front of the front axle centerline, express them in negative or minus numbers. Example: if something is 2 & 3/8" in front of the front spindle centerline, write it down as -2 & 3/8". Everything behind the front axle, express as positive numbers.

I can't stress this enough. Make sure your tape is both straight & a true 90 degrees to what you're measuring. Having the tape measure angled any direction other than a true 90 degrees will produce incorrect numbers. This is hard to see when you're holding the tape under the car in a tight spot.
Use your friend as a "spotter" to insure your tape is straight up & down for height measurements on 2 planes ... and 90 degrees to the car centerline & level for right & left distance measurements.

There are times when it is more accurate to ...
• Use a tape and measure directly.
• Hang a plumb bob on a string off the point, or near it, and measure.
• Utilize the laser (and level) to point up or down & transfer the measurements.
• Use the level to extend a point out to where you can use the tape measure.
• You'll just have to use your best judgment or try it different ways.







I like to mark on tape on the floor ... so I don't forget (I'm old). I put "points" on the tape along with the measurement ... so I can find the point again easily to re-measure if I want or need to ... and so I can re-check my measurements. I check my numbers & recheck them several times. So when I capture them all ... I KNOW THEY'RE RIGHT ... and can trust the data & results.

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6. All 8 A-arm pivot centers (2 for each upper & lower control arm):
You need to measure the location of all 8 A-arm pivots in 3 directions ... all based on the true pivot point of the bushing. You need the ...
1. Height ... from the ground up to the pivot point
2. Left or right ... always from the car centerline
3. Forward or backwards ... of the front axle centerline.



(See image above) For the upper control arms ... you can simply measure off the ends of the pivot shaft. For the lower A-arms, I need you to measure each bushing bolt ... on both sides ... and average the numbers.

For example: If the center of the bolt at front of the driver side lower A-arm is 8-11/16" in the front of the bushing & 8-9/16" in the back of the bushing ... you will average that to 8-5/8". That is the height of that bushing.

When measuring from the car centerline, the differences are bigger, because the lower control arm may be angled (top view). So, from car centerline, if the center of the bolt in front of the bushing is 10-1/16" & the center of the bolt on in the back of the same bushing is 10-7/16" ... you will average that to 10-1/4". That is the distance from the car centerline to the centerline of that bushing.

If this same bushing is 1-1/2" ahead of the axle centerline on one side & 1-7/8" ahead of the axle centerline on the other side ... you will average that to 1-11/16" ... and because it is AHEAD of the front axle centerline, you need to express it as a negative, so write it as - 1-11/16"

We need all 3 dimensions on all 4 bushings.

7. Shock pivot centers:
The process should be getting simpler by now. You're measuring to find the front shock pivot point centers. We need all 3 dimensions on the top & bottom of the shock bushings or bearings.

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8. Coil spring end centers:
If you have coil-overs, skip this step, as the spring does whatever the shock does. If you have separate shocks & coil springs, carry on this step. A little tricky to measure ... but we need measurements from the center of the spring top ... to the front axle centerline & car centerline ... and the height. We need the same thing for the bottom of the spring ... for both front springs.

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9. Sway bar attachment points to front control arms:
We need the 3 accurate measurements from the center of the sway bar link ends, where they connect to the lower control arm.

If your sway bar has linkages that mount off the side of the sway bar arm ... you want to measure to the center of the connection points on the A-arm ... NOT the center of the sway bar arm. Sway bar arm length matters to calculate your actual sway bar rate. So now is a good time to measure & record it. Measure from center of main sway bar to center of pivot on sway bar arm ... 90 degrees from the main sway bar ... regardless of angle or shape of the arm.

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[Part 3 of 3 measuring front suspension points]

Part 3 of 3 measuring front suspension points

Time to measure your steering:

10.  Inner & outer tie rod ends:
We need the accurate 3 measurements of your inner & outer tie rod ends. This is pretty easy if these are rod ends. If you have OEM type tie rod ends, you need to take them off the car & find the center ... the same way you did the ball joints. OEM style tie rods are basically smaller ball joints with a threaded stud off the said.



If you have a rack & pinion ... to get an accurate measurement of the inner rack pivot ... you should remove the boots to measure & replace then afterwards.



11. Steering Box, Pitman, Idler & Centerlink pivots:
If you have a steering box instead of a rack & pinion, you'll also need to measure the Steering box, Pitman, Idler & Centerlink pivots. This is trickier than the earlier tasks, because we have to get dimensions that allow the software to calculate the steering pivot axis of the steering box & pitman arm, as well as the idler arm. Be patient & do your best to be accurate.

On the steering box, we need to gather the 3 dimensions on the stud sticking out of the top of the box (See red dotted circle in photo below on left) & the 3 dimensions on the splined shaft sticking out of the bottom of the steering box (See blue dotted circle in photo below on left) that the pitman arm attaches to. Ultimately, this is to calculate the steering axis as shown by the purple dotted line in the photo below on the right. To help find the center, there should be divots in each end.



We need to do the same for the idler arm. This is harder if the idler shaft is stepped, like many factory units (on left in photo below). If the shaft is straight, like this Howe Racing unit (on right in photo below), it's easy. With the OEM unit, I ignore the step. Since I plan to mount it level on the side of the frame, I make my "Out from Chassis Centerline to Idler Pivot Axis" the same at top & bottom. Because that is how it will work.



Now to get the correct steering axis, we still need to measure top & bottom from front axle centerline and from the ground, to arrive at the Idler Arm's rotational axis. (See Green dotted line in Photo below)



Lastly, with your steering linkage still assembled (See photo below) we want to measure the 3 dimensions of the centerlink pivot points, where they connect to the idler arm & pitman arm. Remember we need the pivot points. There are several designs of centerlinks. Some have the pivots in the actual centerlink. Some have the pivots in the ends of the idler & pitman where they bolt to the centerlink. Regardless of shape of style, you need the 3 measurements to each of the centerlink's 2 pivot points.

[Part 4 of 4 measuring front suspension points]

Part 4 of 4 measuring front suspension points

Time to Measure Your Rear Suspension:
If you have IRS, follow the same steps as the front. The only difference is you have toe-links instead of tie rods & they attach to the frame, not a rack & pinion.

If your Rear Suspension has a Straight Axle:

12. All rear linkage pivot centers (2 for each link)
If you have a 3-link, 4-link or triangulated 4-bar rear suspension, you need to measure the location of all rear suspension link pivots in the 3 directions ... all based on the true pivot point of the bushings or rod ends. If you have a Torque Arm, measure the two lower links the same. Then measure the front mounting pivot point of the Torque Arm itself.



If you have leaf springs or truck arms, shoot yourself now. Just kidding. Sorta. Are you keeping these?  If yes, leaf springs ... measure to the front leaf bushing & to the hole in the frame where the upper shackle bolts through (not the rear spring bushing). If yes truck arms, may we assume this a NASCAR rule regulated Stock Car or Truck? I hope so. Just measure to the front bushings in the chassis. 



13. Panhard bar or Watts if utilized:
If Panhard Bar, you need to measure the location of the panhard bar pivots in the 3 directions ... all based on the true pivot point of the bushings or rod ends. Now is a good time to note if your race car has an adjustable panhard bar on one or both ends ... and how much range you have up & down on each end. I like to know if we're dealing with slots, jack screw adjusters or holes. If holes, note how far apart the holes are.



If Watts Link, you need to measure the location of the both Watts Link Tube pivots in the 3 directions ... all based on the true pivot point of the bushings or rod ends. Some people call the bellcrank a football. WTF? These are race cars for real men, not little boy games.  Shhhheeesh.

You need to know if your bellcrank is attached to the rear axle itself, or framework attached to the chassis. This will make a handling difference.  If your bellcrank is attached to the rear axle itself, one end of each watts link tube will attach to the bellcrank & the other end to the chassis. If your bellcrank is attached to the chassis (yeah!), one end of each watts link tube will attach to the bellcrank & the other end to the rear axle tubes.

Now is a good time to note if your race car has an adjustable watts link ... and how much range you have up & dow. I like to know if we're dealing with slots, jack screw adjusters or holes. If holes, note how far apart the holes are.



14. Shock pivot centers (upper & lower):
You're measuring to find the rear shock pivot point centers. We need all 3 dimensions on the top & bottom of the shock bushings or bearings.

15. Spring end centers (upper & lower) if not coil-over:
If you have coil-overs, skip this step, as the spring does whatever the shock does. If you have separate shocks & coil springs, carry on this step. A little tricky to measure ... but we need measurements from the center of the spring top on the frame ... to the rear axle centerline & car centerline ... and the height. We need the same thing for the bottom of the spring on the axle or lower links ... for both rear springs.

16. Sway bar attachment points to & axle or chassis:
We need the 3 accurate measurements from the center of the sway bar link ends, where they connect.
Be prepared for your sway bar to mount to the rear axle & the links to the frame, or vice versa.

If your sway bar has linkages that mount off the side of the sway bar arm ... you want to measure to the center of the connection points to the chassis or rear axle ... NOT the center of the sway bar arm. Sway bar arm length matters to calculate your actual sway bar rate. So now is a good time to measure & record it. Measure from center of main sway bar to center of pivot on sway bar arm ... 90 degrees from the main sway bar ... regardless of angle or shape of the arm.

Tips:
• Remember to subtract the distance you raised the car up from ride height for all height measurements.
• Reminder: for all things in front of the front axle centerline, express them in negative or minus numbers. Example: if something is 2 & 3/8" in front of the front spindle centerline, write it down as - 2 & 3/8". Everything behind the front axle, express as positive numbers.

• Your Roll Center locations & Camber Gain numbers will only be as accurate as your measurements & small #'s make a big difference. I measure everything I can on my cars with a digital caliper to the thousandth. It is that important.
• Take your time & be precise. Don't just "eyeball it."
• Getting true measurements is difficult. Experiment & be creative with measuring methods, but be accurate.
• This a time consuming & somewhat tedious operation. I always suggest people take the time to do it right the first time, so you don't have to do it over (or suffer from incorrect tuning).
• Because you got frame heights on 4 corners of the car, you will be able to make many adjustments to the car, and know what changed &
where you are, by rechecking frame heights.
• For example, if you lower the whole car ½" ... all of your frame mounted pivots just went down ½" ... and your ball joints did not. That is easy to change in the Roll Center programs & know what your geometry is then.

If I am doing Tech Service #40 for you, then I'll provide you Excel forms to fill out each dimension.  If you're doing this yourself, I suggest you look at your software first & make a form with all the data blanks in the same location & orientation as whatever software you're using.