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Unread 08-06-2013, 10:22 AM
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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.

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

Ron Sutton

Ron Sutton Race Technology
Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

Check out our 400 Page Car Building Catalog HERE

Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

Last edited by Ron Sutton : 12-07-2014 at 05:27 PM.
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Unread 08-06-2013, 10:41 AM
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Welcome,
I’ll outline some the different types of rear suspensions … pros & cons of each … key tuning goals … and how to tune them for optimum performance. We’re going to keep the conversation to typical Pro-Touring type cars … front engine, rear wheel drive … but for improving track performance.

If the conversation bleeds over into general handling … I may suggest we move the conversation to that forum thread. Same with front suspensions.

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15 of the 20 CRITICAL TUNING CONCEPTS:
These 15 are very pertinent to our rear suspension & geometry discussions on this thread. The full list is in the thread: Overall Handling & Tuning for Track Performance

4. After the car is built, tires are selected & the geometry is optimum … most chassis fine tuning is to control the degree of weight transfer to achieve the traction goal.

5. Force (weight & weight transfer) applied to a tire adds grip to that tire. With the exception of aerodynamics, weight 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. Stiffer bars reduce roll angle, keeping the car flatter, working the inside tires better.

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” … with the springs’ primary role of controlling dive & rise, anti-roll bars’ primary role of controlling roll & shocks primarily role controlling the rate of both. They all affect each other, but choose the right tool for the job & you create a harmonious team.

9. A production based car, 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.

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.

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 & under works the inside tires. Too little roll angle under works 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 utilized optimally.

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 car can be pushy because it has too little roll angle in the front & is not properly working the outside front tire. A 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 the same is not, 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.

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

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

Ron Sutton

Ron Sutton Race Technology
Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

Check out our 400 Page Car Building Catalog HERE

Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

Last edited by Ron Sutton : 08-06-2013 at 11:53 AM.
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Unread 08-06-2013, 10:42 AM
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Let’s talk rear suspensions …

Other than singular purpose built race cars … like Formula, Indy, GTP, Midgets & Sprint Cars … production type bodied cars’ typical limiting factor … is front tire traction during cornering. In PT cars we can’t go any faster through the corners than the front end has grip.

As we optimize front end grip & maximize cornering speed … tuning the rear suspension is the key to achieving a balanced, neutral handling track car. This is assuming you have a tunable rear suspension. The key areas of tunability are roll center, anti-squat, lift/push ratio, rear steer, spring & sway bar rates, shock valving & track width.

If you haven’t gone off the range with track width split or tire size split … it is relatively easy to balance the rear grip to the front for a neutral handling track car. But you need to have a tunable rear suspension … and the knowledge to do so.


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There are four common types of rear suspensions utilized in Pro-Touring cars today ... 3-link, Parallel 4-link, Triangulated 4-link & Torque Arm. 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.

The typical Torque Arm suspension is similar to a 3-link, using two lower 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.

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. 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 many 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. You want the push & pull forces going through the links to be parallel with the chassis … not angled within the chassis.

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.

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 AutoX racers.

3-links, Parallel 4-links & Triangulated 4-links can be made very adjustable if designed & installed with multiple or variable mounting points. But most "street kits" are sold with little or no adjustment to protect non-tuning novices from themselves. If you know set-ups or plan to learn, you may want to pick a system designed for adjustability. If not, pick a system designed for your application, install as directed & run it.

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. There are pros & cons both ways.

A suspension with a panhard bar or watts linkage ... "can" allow for easy roll center changes, if the mounting brackets allow for adjustment. (Many ProTouring focused kits offered do not have adjustment capabilities) Again, decide if you want to "set it & forget it" (sorry Ron Popiel) or if you want a suspension that is tunable for optimum performance & varying track conditions.

For a track car, I can’t fathom not having an adjustable rear suspension. But if your PT car doesn’t have a rear suspension, that doesn’t mean you shouldn’t do track days. 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.

For the best adjustable rear suspension for road racing, track car, or AutoX car, is the adjustable 3-link, as it has the best articulation. 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.

For the best non-adjustable rear suspension for road racing, track car, or AutoX car, is the Torque Arm suspension, as it offers good articulation. The non-adjustable 3-link & parallel 4-link “can work well” … providing the instant center location provides a decent anti-squat percentage for your application. 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.

For drag racing, the advantage goes to the Parallel 4-link, with the Torque Arm suspension 2nd (for handling launches BUT not very adjustable), Triangulated 4-link 3rd & the 3-link 4th (only due to strength concerns).

For a "cruiser/driver" that will only occasionally see the track, with little or no tuning ... any of them will work fine ... but the Torque Arm suspension is best here & the triangulated 4-link 2nd ... which is why you commonly see these two suspensions in factory production cars. They both work fine in many hot rod & street performance applications. They are not better than the others, just simpler & effective. Plus these two allow you to keep the rear seat if that is important to you.

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Offset 3-links

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. Very few people can tell you accurately how far to offset it, because it changes with gear ratio & friction within your rear end. I have my own proprietary formulas I use, based on my knowledge of where the force comes from & high tech testing of dynamic loads. 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 service as one of my 70+ tech services you can see HERE.

The formulas I’ve seen other people use involve rear steer, which makes no sense for handling cars. A rule of thumb is 8-12% of track width. 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.

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

Ron Sutton

Ron Sutton Race Technology
Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

Check out our 400 Page Car Building Catalog HERE

Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

Last edited by Ron Sutton : 12-06-2014 at 07:09 PM.
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Unread 08-06-2013, 10:43 AM
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Rear 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 … then how to calculate them … and finally how to tune with them.

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.

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.

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

Ron Sutton

Ron Sutton Race Technology
Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

Check out our 400 Page Car Building Catalog HERE

Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

Last edited by Ron Sutton : 08-06-2013 at 11:25 AM.
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Roll Centers Continued

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 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. Excessive chassis roll angle is your enemy, because it is over working the outside tires & under utilizing the inside tires.

Some people like to look at the car as a unit. I look at it as two halves. Here are some examples … using a typical 3500# Pro Touring Car with 53% front weight… to provide more clarity:

If the CG is 20” high … and the front roll center is 2” below ground … the car has 53% of the 3500# weight with 22” of leverage to roll the front of the car.
If the CG is 20” high … and the rear roll center is 11” above ground … the car has 47% of the 3500# weight with 9” of leverage to roll the rear of the car.
* Rolling the car that much more in the front overloads the outside front tire & under utilizes the inside front tire when cornering.

If you lowered the car 2” … the CG drops 2”. The front roll center probably moved too … but it’s not linear … it is based on A-arm angles. Let’s say it dropped 1” in the front to 3” below ground and the rear stayed the same at 11”.

Now …
If the CG is 18” high … and the front roll center is 3” below ground … the car has 53% of the 3500# weight with 21” of leverage to roll the front of the car.
If the CG is 18” high … and the rear roll center is 11” above ground … the car has 47% of the 3500# weight with 7” of leverage to roll the rear of the car.
* The front now rolls over less & the rear too, making the car run “flatter” … not flat, just less roll angle … working the inside tires better.

Any weight you can remove from high up … or relocate to lower in the car … moves the CG down … reducing the leverage it has over the roll center … allowing the car to have less roll angle during cornering … working all four tires more evenly … and the grip of four tires is faster than two.

We’ll discuss moving the roll center in the final section. Next, let’s cover how to figure out where your front roll centers is at.

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Calculating the Rear Roll Center is MUCH Easier than the front ...
Unless you have a double A-arm rear independent suspension … then it is exactly the same as the front.

For all other common rear suspension types, here are the quick methods.

Panhard Bar/Track Bar: The RC is located horizontally & vertically at the center of the two pivots. If the bar is level and both sides are 8” off the ground … the roll center is 8” above the ground. If the bar is at an angle with one side at 11” & the other at 12” … the RC is at 11-1/2”. (But this angle will make the car handle differently on LH & RH corners.)

The RC is located horizontally exactly in the center of the two panhard bar pivots … which is why it makes sense to have the bar centered in the chassis on street, road race & AutoX cars … so the RC is centered in the chassis. Some oval track cars use a J-bar, which is not centered horizontally, and therefore neither is the RC.

Watt’s Link: The RC is located horizontally & vertically at the center of the bell crank pivot that is attached to the rear end housing.

Leaf Springs: The RC is located horizontally in the center … halfway between the two sets of leaf springs. The RC is located vertically at the height equal to the mating line where the leaf spring connects to the housing spring pads. If lowering blocks are utilized, the RC height is in the center of the lowering blocks.

Triangulated 4-links: Draw a line connecting the IC of the lower set of trailing arms … to the upper set of trailing arms … and where that line crosses the axle CL … is the rear RC.

Diagonal Link: The RC is located horizontally & vertically at the center of the two pivots. If the diagonal link is centered, so will the RC be. If the diagonal link is at 6” on one side and 7” on the other … the RC height is at 6-1/2”.
* The diagonal link should be used for drag racing only.

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Optimizing the Rear Roll Centers:

First off, most PT guys aren’t tuning on their rear roll centers to balance the car’s handling … except the thinkers & tuners that are winning most the events.

Many find tuning with the rear roll center tedious and/or confusing
… and therefore they don’t do it. Most don’t even have adjustment built into the car to tune the rear roll center. I love those people as competitors because they’re easy to beat. Getting fast … faster than everyone else … takes work, testing, work, smarts, more work & more testing. And the rear suspension … which is simple … is the final key to having a well balanced, neutral handling car.

You don’t have to become a tuner to have fun with your Pro Touring car. You can buy & install many good suspension packages available on the market that have a “much better than factory” set-up for your car … because the aftermarket manufacturer worked out a good basic geometry package. The car will handle well, outperform most factory cars and be a lot of fun. Just don’t disillusion yourself into thinking you’re going to show up at serious competitions & beat the “thinkers & tuners” with a bolt on package.

If running “good” isn’t good enough at autocross & track day events, and you want to compete at a higher level and win events … you need to learn about suspension tuning … and do some testing & tuning. I figure I have over 2500 test days under my belt in my 35 years of racing. I’m not the smartest guy at the track … but when he goes home … I’m still there testing, tuning, learning & getting faster. To win … you gotta be willing to do the work. If you’re not, be clear on that and set your goals accordingly. We’re all here for fun. Some of us find the fun of winning worth the effort & sacrifices it takes to do so.

.
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Last edited by Ron Sutton : 12-06-2014 at 07:10 PM.
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Tuning with Rear Roll Centers:

If you have a double A-arm independent rear suspension … then it is exactly the same as the front. Change the term from “spindle” to “upright” and party on. Everything else is the same.

For all other common rear suspension types, here are the quick methods to tune.


Panhard Bar/Track Bar:
Each end has a rod end as a pivot point. In race cars, we use a variety of different styles of mounts on the chassis & housing that allow us to relocate these pivot points up or down on both sides. I see a lot of aftermarket suspension kits & frame clips that have no adjustability.

My first thought is “what the … @$&% !”

Then I realized they’re doing this so tuning rookies don’t get themselves in trouble. Frankly, I can not fathom not having the rear roll center adjustable. It is one of the most predictable tuning tools & with the right mounts, simple, quick & easy. It is so easy, that in racing circles, if a racer is “lazy” that’s the tuning tool they use first & most, which isn’t correct, just reality of human nature.

Watt’s Link: Very simple to move the bell crank pivot up or down, as long as there are holes available.

Leaf Springs: Do not have an adjustable RC unless you change the mounting points of the springs or use lowering blocks.

Triangulated 4-links: Do not have an adjustable RC unless you change the mounting points of the links. Great for street performance cars with owners not desiring to tune.

Diagonal Links: Are for drag racing … as they are not independently adjustable for RC in the typical mounting. They mount to the ends of the rod end bolts, so change only when the lower bars are being moved to different holes.

The two best methods of centering the rear end … and have tunability … are the Watt’s link & panhard bar. Many people favor a Watt’s link because it keeps the rear end perfectly centered during travel & roll. I like them, but they’re not my personal preferred method, because as a veteran tuner, I sometimes WANT to load the LR or RR tire more ... depending the number of left & right corners at a track. Most Watt's link designs have limited adjustability … sometimes only a few holes … farther apart than I like to make changes … and often mounted with the RC too high for modern low roll suspensions. This limited tunability of these designs requires the tuner to rely more on other tuning items such as springs, shocks & sway bars. It reminds me a little of torque arm suspensions. They work well, just not much tuning adjustment. I think they’re the ticket for car guys that want to get a good set-up & just drive it.

But if you want a watt's link with a wide range ... 6" of roll center adjustment ... and very fine tunable ... you may want to consider using one of THESE threaded adjuster.s like we use in the Track-Star kits

As a tuner, I like to have a full tuning “tool box” at my disposal, and a panhard bar (or track bar, depending on where you came from) is more tunable. With the right adjustable mounts, I can move the RC 1/16” if I wish. I can make it super low … super high … or anywhere in between.

If the car is working the rear tires different in RH corners versus LH corners … for any variety of reasons ... I can put a little “tilt” in the panhard bar … while keeping the same RC height … and even out how the car works the rear tires in RH & LH corners, making a more balanced, faster track car. Just lower the bar down on the side you want to work the tire more … and raise the bar up on the opposite side the same amount. If I’m at a road course where I need a “lil sumthin” extra in one corner, I can achieve that with a little panhard bar tilt.

As far as keeping the rear end centered … with the newer technology low roll suspensions … if the outside rear tire is compressing much more than an inch during cornering … I’ve got bigger problems than the rear end shifting a few thousandths off center.

Direction … regardless if you’re using a Watt’s link or panhard bar:
a. Raising the RC, places it closer to the CG, reducing the CG leverage, reducing roll angle … and working the rear tires less.
b. Lowering the RC, places it farther from the CG, increasing the CG leverage, increasing roll angle … and working the rear tires more.
c. With low roll suspensions utilizing mean stiff rear springs … or medium springs & significant rear sway bar … for faster corners found at big road courses I’ve found the happy window to be 8” to 12” … and 6” to 10” for tight AutoX events.
d. Softer sprung, higher roll rear suspensions run higher rear RC’s.

*KEY NOTE: For optimum cornering ability, you need to position the rear RC low enough to work the rear tires … allowing just enough rear roll angle to disengage the inside rear tire to a degree … but high enough to prevent the outside rear suspension from compressing so much that the car rolls diagonally … and unloads the inside front tire.

Old school, conventional, soft sprung rear suspensions achieve this with higher rear RC’s. Modern low roll suspensions achieve this with stiff rear springs (or or medium springs & significant rear sway bar) to control the roll angle, while utilizing a lower RC to work the outside tire more. In other words, modern suspensions don’t use the rear RC as the primary tool to control the car’s roll angle.

For the Warrior Cars I designed & offer as turn-key, complete track cars with either Watt's link or double adjustable panhard bar, we have four rear baseline RC locations from 7" for AutoX to 11" for high speed road courses. The Watt's link adjuster takes one adjustment, literally taking under a minute. Because both sides of the panhard bar adjust easy, it is about a 2 minute change.

Last Key Note:
Once we have tested & worked out an optimum front end set-up for a type of track … we “lock it in” and don’t change it at the track. So this is NOT something you’re constantly tuning on … just initially. At the track, as long as the front suspension is fully optimized … we’re tuning on the rear suspension to keep the car handling balanced for neutral handling as the track changes throughout the day.

This is why it is so important to have an adjustable rear suspension for serious track & AutoX cars. We need to be able to adjust the anti-squat, track width, rear steer & roll center.

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

We got into detail of rear roll centers (RC) ... but I think it's helpful for car guys & tuners to take several steps back & look at the big picture of handling ... to better understand the role of the rear RC with the rest of the car.

Total weight ... weight distribution front to rear ... and height of this weight (CG) act like a lever over the roll centers. As discussed earlier, lowering the CG shortens that lever, as does raising the RC ... but works the tires less. Raising the CG lengthens that lever, as does lowering the RC ... and works the tires more.

Your goal is to move them both ... to the degree possible ... where you find the optimum balance of working the tires & roll angle. BUT ... and this is KEY ... modern day tuners do not use the RC height as the primary means of controlling roll angle. They use the suspension tuning items as their primary tools & the RC height as a secondary tool.

So to achieve the optimum balance of roll angle & working the 4 tires optimally ... this all has to work with your suspension ... springs, anti-roll bars & shocks ... and track width ... to end up at the optimum roll angle for your car & track application.

Hopefully, this brief overview, helps clarify roll centers place in the tuning picture.

.
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Ron Sutton

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Last edited by Ron Sutton : 12-06-2014 at 07:29 PM.
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For Optimum Corner Handling ... You Need to Disengage the Inside Rear Tire ... To a Degree

Below is a good Youtube video, showing how the inside rear tire needs to "disengage ... to a degree" in race car cornering. The gentler the radius, the less it needs to disengage ... the tighter the radius, the more. You can learn a lot, just by watching the tires. Expand the video to full scene & watch the inside rear tires of the cars in front of the driver with the video. Freeze frame it if you need to see it better.

"Disengage ... to a degree" means we are reducing the load & even reducing the contact patch on that inside rear tire. This is critical to a good handling car on grippy tires & tight corners.

How much is right? The answer depends on many factors. The video of the Porsches shows the "degree" to be quite high. That's why I posted it ... so it was clear to see. The "degree" the Porsches are disengaging the inside rear tire is correct for these cars, but they have the engine in the rear, so they have to disengage it more than an evenly balanced, or front heavy car. The amount the Porsches are disengaging the inside rear tire in the video ... would be excessive for a typical P/T car with 50-58% front weight. But the concept is the same.

"To a degree" means we are not lifting the tire off the ground completely. We may not even be close. It simply means we are reducing the load & the contact patch on that inside rear tire.

"To a degree" also implies ... correctly ... that we want to control how much. This is key. If we do not disengage the inside rear tire enough, we struggle with a tight/pushy car. If we disengage the inside rear tire too much, we now have a loose car to deal with.

"Re-engage" means we want to "plant" the inside rear tire hard on corner exit ... to regain & achieve a full contact patch ... so we have 2 full tire's worth of contact patch for accelerating. This actually happens as the driver unwinds the steering wheel. "How much" is determined by suspension geometry, shocks, springs, etc.



There are almost ALWAYS exceptions to the rule. It is NOT critical to disengage the inside rear tire to a degree if …
a. You can get the rear end to "glide" (think slight, controlled slide) around a tight corner (easier to do on hard tires).
b. The car has a complete open diff, like Formula Fords.
c. You're oval track racing & running staggered tires (outside rear tire a much larger diameter than the inside rear tire.)
d. The corners are not tight, but instead they are big sweeping corners.
e. The corner has significant banking.

.
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Ron Sutton

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Last edited by Ron Sutton : 12-07-2014 at 04:37 PM.
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Unread 08-06-2013, 10:54 AM
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Tuning the Track Width with Wheel Spacers

In case new people drop in on our conversation, track width is center of tread to center of tread. Tread width is outside tread to outside tread (not the sidewall). Track width difference is a tuning tool … if the track width is adjustable … or a tuning challenge if it is too far biased on one end and NOT adjustable.

I typically don’t want to see the track width difference more than 1” on either end. Even that can be a challenge if it the difference is the wrong direction. There are exceptions to almost every rule ... this one too.

Within reasonable differences making the front track width wider than the rear … or the rear wider than the front can be a practical tuning tool. Making the front track width wider than the rear = tightens the car … too much makes it push. Wider rear track width than front track width = frees the car … too much makes it loose.

Put even more simply, regardless of what is going on at the other end …
a. Moving the front tires out tightens the car.*
b. Moving the front tires in frees the car.*
c. Moving the rear tires in tightens the car.
d. Moving the rear tires out frees the car.

*Note: Adding spacers to the front hubs, increases the scrub radius. Not good. So when I can use either adjustable LCA’s, or simply replace the LCA’s, for the track width I want … that is how I prefer to do it, versus adding wheel spacers to the front. I have used wheel spacers in the front many, many times. It’s just not my preferred method. Sometimes class rules dictate the decision.

A LOT of racers use small (1/8” to 1”) wheel spacers to tune … and in fact, many design & build their rear end & front hub combination with spacers as a part of it, so they can be removed to go “in” if needed … and of course, go “out” with more spacer as needed.

I prefer to use spacers on the rear end only, so as not to affect the scrub radius in the front.

For the Warrior Cars & Track Warrior Front Frame & Suspension Packages I designed the lower control arms use adjustable threaded ends to allow up to 3/4” width change per side … 1.5” total track width adjustment, without affecting the scrub radius. The 315 front tire set-up has a “standard track width” of 56”. We can adjust the front track width from 55-1/4” to 56-3/4" without spacers. The 335 front tire set-up has a “standard track width” of 57”. We can adjust the front track width from 56-1/4” to 57-3/4" without spacers.

The rear end in the Warrior cars, and almost all cars I build or design, I “start” with spacers that can be removed … and of course, spacers can be added. At a minimum, I like to build the rear end with 1/2" spacers on each side placing the rear tires at their widest point ... and allowing the track width to be narrowed by up to 1". This is an simple, easy, predictable tuning tool at the track to balance the front to rear handling. In our Track Warrior cars, we use 1" spacers on each side … for a 2” adjustable track width range in the rear.

Additional tips: For cars with “high travel/low roll” suspension strategies, the front to rear “track width split” favoring the front works best. For cars with “low travel/high roll” suspension strategies, the front to rear track width split favoring the rear works best.

If the car is already “designed & built”
… and the track width is a wider in the front, that will “contribute” to the car being tight/pushy a degree. If the track width is a narrower in the front, that will “contribute” to the car being free/loose to a degree. I use the word contribute, because there are a lot of other suspension & geometry factors affecting the grip at each end of the car. If the track width is not adjustable at either end, you will need to tune with roll center, springs & sway bars to balance the car.

.
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Ron Sutton

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Last edited by Ron Sutton : 12-06-2014 at 07:42 PM.
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Unread 08-06-2013, 10:56 AM
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Differentials

For the track, I run & recommend Detroit Lockers, because they will handle 600-900hp, hard launches & aggressive shifting ... live ... and completely disconnect the inside rear tire in the roll through zone of a corner ... then re-engage like a spool on corner exit. BUT most people find the loud clunking noise very distracting & annoying for everyday street driving.

For a more civilized approach on the street, especially with daily drivers, many people prefer a tamer Detroit Wavetrac, Truetrac, clutch or cone style positraction. None of these will hold up like a Detroit locker, but they behave more civilized. Any differential, that doesn’t fully & completely disconnect the inside wheel from power … is contributing to a tight handling condition in the middle of the corner known as the “roll through zone.”

"Most" of the small helical gear type differentials use a method of pre-loading the unit which affects how the power is distributed under throttle on corner exit. And unfortunately they wear ... and as they wear … and the power distribution changes ... making the car handle differently on corner exits. Not good.

For track performance, nothing beats a Detroit Locker ... with the right spring rate in it. In racing, we tune the lockers with different spring rates from 25# to 125#. (See locker springs HERE) The softer the spring the easier it releases when turning. The stiffer the spring, the more it takes to get it to release.

New stock, off the shelf Detroit Lockers come with too stiff of springs (78#) for what we do, so they need to be changed. A lot of guys are finding deals on used lockers, but the challenge with buying a used Drag Race or NASCAR locker is you don't know what springs are in it. They are easy enough to take apart & see ... or replace the springs. On big tracks like Daytona & Talladega the last thing you want is for the locker to release. So they run the 110# to 125# springs. If you bought a used NASCAR locker with that much spring rate, it would be a bear to get it to unlock. With street tires, I don't think the tires have enough grip to force those 110# to 125# springs to unlock consistently ... if ever ... and would on occasion/often simply slide the tires instead. Drag racers typically leave the stock 78# spring in, which is still too much spring rate for what we do here. For road racing & autoX, the hot ticket is softer springs. The key to picking locker springs is based on rear tire grip. The less you have the softer the spring needs to be.

From my experience, this is what I recommend …
• 40# ... Street, autoX & track cars with 10"+ TW200 street tires.
• 55# ... Track cars on 10”+ slicks & race cars.

As a rule of thumb, you can go softer 1 step, but don’t go stiffer. The key to making lockers work well is getting them to disengage in tight corners. Of course they need to re-engage too. So you wouldn’t want to run way too soft of a spring. Having said that ... there is no disadvantage to running a 40# spring with slicks on road courses. My Warrior cars come with 40# springs in the locker, so they perform just as well on TW200 tires as they do on slicks.

.
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Last edited by Ron Sutton : 12-06-2014 at 07:50 PM.
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Unread 08-06-2013, 10:59 AM
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For track cars, the most tunable & best suspensions for many applications are the 3-link & 4-link.

The 3-link has two lower links … often called lower trailing arms or lower control arms … and one upper link … often called the top link, 3rd link or upper control arm. The top link can be centered, offset or angled. The reason to offset or angle the top link is to counteract the torque transmitting through the rear end housing.

The 4-link has two upper & two lower links. For racing & track applications the triangulated 4-link is not optimum, as you don’t want any forces pushing or pulling in a direction other than parallel. So we’re only discussing parallel 4-links here.

The design, set-up & tuning concepts are the same for both 3 & 4 link rear suspensions. The 3-link allows for more rear end housing articulation within the chassis. Both 3 & 4 link rear suspensions will bind at some angle different than the chassis … but the 4-link will bind at a lower angle.

For drag racing … high power, high rpm, side step the clutch launches … the 4-link is better suited to deal with the launch forces. While the bottom links “push” on launch … the top link(s) “pull.” Pulling the rod ends is more susceptible to failure than pushing. Two links with four rod ends are obviously twice as capable of handling this “shock” that can be so powerful, the 4-link suspension is lifting the front of the car off the ground.

So, either 3 & 4 link rear suspensions can be used for corner handling & drag racing, but the 3-link has a slight edge for cornering & the 4-link a slight edge for drag racing.

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

If you can buy an adjustable 3 or 4 link suspension that fits your car & fits needs at a fair price, for Pete’s sake buy & install it. You’ll be miles ahead. If on the other hand you either can’t find an adjustable suspension that fits your car … or you can’t afford to buy the kit … then maybe buying the components & building it yourself makes sense. Maybe.

This is providing you and/or your friends have the metal fabrication equipment & skills, time to hunt for all the parts, time to lay it out & install it … the TLC to get it right … and the desire to do it yourself. If you’re not sure about this … hire a shop. If you know you want to build your own … here’s the “how to” ... in posts #11 & #12.

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

Ron Sutton

Ron Sutton Race Technology
Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

Check out our 400 Page Car Building Catalog HERE

Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

Last edited by Ron Sutton : 08-06-2013 at 11:42 AM.
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