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Old 08-06-2013, 10:22 AM
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Ron Sutton Ron Sutton is offline
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Default Rear Suspension & Geometry for Track Performance

This Forum Thread is for discussing & learning about “Rear Suspension & Geometry for Track Performance.”

This thread has a narrow focus, just as the title says.
For the Parent Thread on: Overall Handling & Tuning for Track Performance … click HERE.
For a thread focused on: Front Suspension & Steering Geometry for Track Performance ... click HERE.
For a thread focused on: Designing Aerodynamics for Track Performance ... click HERE.
For a thread focused on: Safety for Pro-Touring Track Cars ... click HERE.

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. I’m writing some books and want car guys to understand them. When you’re really not clear on something I said … please bring it up & help me improve.

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 & I think it 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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Before we get started, let’s get on the same page with terms & critical tuning concepts.

Shorthand Acronyms
IF = Inside Front Tire
IR = Inside Rear Tire
OF = Outside Front Tire
OR = 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.

ARB = Anti-Roll Bar
FLLD = Front Lateral Load Distribution
RLLD = Rear Lateral Load Distribution
TRS = Total Roll Stiffness
WT = Weight Transfer

RA = Roll Angle
RC = Roll Center
CG = Center of Gravity
CL = Centerline
IC = Instant Center is the pivot point of a suspension assembly or “Swing Arm”


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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.

Anti-Squat percentage is a calculation of mechanical leverage of the rear suspension. It is calculated utilizing the location of the rear axle CL at ground level, the car’s center of gravity point & the instant center of the rear suspension. Each rear suspension model is different, but all have an instant center forming the swing arm arc the rear suspension pivots on (think side view).

Percentage of Rise is different term for Anti-Squat used in the drag racing world. It also is calculated utilizing the location of the rear axle CL at ground level, the car’s center of gravity point & the instant center of the rear suspension … but 100% of rise occurs as the ground line crossing through the IC intersects with the car’s CG. Percentage of Rise is always exactly half of Anti-Squat percentage or Anti-Squat percentage is always twice the Percentage of Rise … however you want to think of it.

Total Roll Stiffness 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 Angle: is the amount the car “rolls” on its roll axis (side-to-side) in cornering, usually expressed in degrees.
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.

Dive = is the front suspension compressing under braking & cornering 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
Roll = Side to side body rotation … aka body roll.
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.

Track width = is center to center of the tread.
Tread width = is outside to outside of the 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 weight transfer) on the spring. If that same 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.

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.

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.

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Feel free to chime in or ask technical questions. I am here to help where I can.

Ron Sutton

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Last edited by Ron Sutton; 12-07-2014 at 05:27 PM.
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