[Ron Sutton]

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. 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 9” above ground … the car has 47% of the 3500# weight with 11” of leverage to roll the front 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 9”.

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 9” above ground … the car has 47% of the 3500# weight with 9” of leverage to roll the front 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 centers in the final section. Next, let’s cover how to figure out where your front & rear roll centers are at.

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Front: 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.

Quick Acronyms:
UCA = Upper Control Arm
LCA = Lower Control Arm
BJC = Ball Joint Center
IC = Instant Center
RC = Roll Center
CG = Center of Gravity
CL = Centerline

Your UCA & LCA have pivot points on the chassis … and they pivot on the spindle at the BJC’s. Forget the shape of the control arms … the pivots are all that matter.

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