[Ron Sutton]

Tuning with Roll Centers:

First off, most people find tuning with the front roll center difficult, tedious, confusing & laborious … and therefore they don’t do it much. 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 front suspension … which is the most complicated … is the most important key to cornering performance. Because I understand it so well, that has been to my advantage over the years.

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, and you want to compete at a higher level and win events … you need to learn about suspension geometry & tuning … and do lots of 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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Tuning with Front Roll Centers:

Let’s start with the understanding that to move the front RC … you are changing the angles of the UCA and/or LCA … to achieve a different IC. Some changes affect RC only dynamically in dive … while most changes affect the RC both statically at ride height & in dive. I’ll put an * next to the item that only changes RC dynamically & doesn’t show up statically.

What are (or can be) your tuning tools to change angles:
1. Spindle heights and/or distances from spindle pin to ball joint surfaces
2. Ball joint pin heights
3. Control arm length*
4. Adjustable control arm mounts on the chassis.
5. Also, obviously, any changes in ride height.

Direction:
a. Raising the RC, places it closer to the CG, reducing the CG leverage, reducing roll angle … and working the front tires less.
b. Lowering the RC, places it farther from the CG, increasing the CG leverage, increasing roll angle … and working the front tires more.
c. For faster corners found at big road courses I’ve found the happy window to be 1” to 2.5” … and -0.5 to 1.0” for tight AutoX events.
d. If the RC migrates to the inside of the corner under dive … it will work the front tires more … but roll more if not controlled by the suspension.
e. If the RC migrates to the outside of the corner under dive … it will roll less work the front tires & roll less.

*KEY NOTE: For optimum cornering ability, you need to WORK the tires … and low RC’s combined with big sway bars work the tires while keeping the roll angle low. In other words, don’t use the front RC as your primary tool to control the car’s roll angle.

For the hot rod I’m designing & building for myself, I have two front RC locations. For AutoX it is 0” in dive & 1.2” in dive for road racing & high speed events. All I have to do is change the slugs in two control arms & reset the toe & bump steer shim packet. Yes, I have big sway bars.

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Also know, when you are changing these control arm angles, you are changing the camber gain. Use this to your advantage.

A lot of stock production cars have the swing arms so far out … there is little to no camber gain … often camber loss. Plus, in many stock production cars the A-arm angles put the roll center so low it is below ground ... and the CG is high … giving it a ton of leverage to roll the car … which is part of why many stock production cars roll so much.

Typically, when you dial in your front geometry … you’re goal is to place your RC for optimum handling for the type of driving you do (or find the best compromise) … and end up with the desired camber gain.

Some quick tips:
Anytime you’re shortening the “swing arm” … you’re increasing camber gain … regardless of how you did it.
Anytime you’re shortening the swing arm length … & keep the IC at the same height … you’re raising the RC.
Conversely, lengthening the swing arm length … & keeping the IC at the same height … lowers the RC.
Anytime you’re raising the IC of the swing arms … and keeping the same swing arm length … you’re raising the RC.
Conversely, lowering the IC of the swing arms… and keeping the same swing arm length … lowers the RC.

There are several software programs out there to calculate roll centers. I own & use several. I suggest Performance Trends to car guys & gals often because it is the easiest to use.

*KEY NOTE: When you change the swing arm IC … length or height … you are changing the bump steer … because you are changing the arc the UCA/Spindle/LCA assembly travels in. Another reason why so many people don’t like tuning on it.

For the hot rod I’m designing & building for myself, I have two front RC locations & have worked out the bump steer for both setups … which includes a tie rod slug & shims. I simply keep a “set” for each of the 2 RC locations, making the change over quicker & easy … after the initial work is done.

Last Key Note about Front Roll Centers
once I 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, the optimum front end geometry is the optimum front end geometry … so as the track changes throughout the day … we’re tuning on other stuff to keep the car “balanced”.


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