Dealing With Body Roll

Body Roll Factors

Center of Gravity & Roll Center There are several basic terms that need to be explained in order to understand what affects body roll. They are center of gravity and roll center.
Center of Gravity: The center of gravity (CG) is that point that all the masses in the vehicle can be considered to be concentrated.

Roll Center: The roll center (RC) is a point where the car pivots. There is one in front and in one in the back and the car pivots about a line (roll axis) between the two roll centers. When the car is in a corner, it will lean (roll) about the roll axis.

The most dramatic impact to body roll is center of gravity height. This is affected by the CG distance from the roll center height. The greater the distance between them the more body roll. Consider the example below. This simplified vehicle has a high center of gravity and is offset considerably from the roll centers and roll axis. When a cornering force is applied the body will lean. You can see that if the CG was closer to the roll axis, in the same corner the body lean would be reduced.

roll-center

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Why is This Important?

From this discussion you would think that just lowering the vehicle by using shorter springs would result in an improvement to handling. While it is true that you will lower the CG by lowering the car, you can also adversely impact the roll center, steering geometry or both. Even if the ride height is not changed the roll center can be a problem that needs to be understood

We are only going to address the front roll center because it is the easiest to change and will provide the most amount of improvement. The RC of a symmetrical, independent suspension is determined by utilizing a front view of the suspension pivot points and drawing imaginary lines to connect the ball joint, point B and inner pivot, point A of each control arm and extending them to the point where they intersect, point C. This point is called the instant center and is the point at which the wheel (if attached with a single bar) will move about in an arc.

instant-center

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Drawing another line from the centerline of the bottom of the tire, point E to the instant center, point C and still another set for the other side of the suspension will give you the roll center. It is the point where these two lines meet. The height of the roll center is measured from ground level.

roll-center1

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While we can’t completely eliminate body roll without causing other problems, if the RC is changed to reduce these effects we can still improve things significantly.

Example: Raising the roll center from 1.28” to 4.21” above the ground will reduce the roll couple by 17% (assuming a theoretical CG of 18”).

You don’t want to raise it too far because that gets into other issues, such as jacking, that can have a negative effect on performance.

What’s Next?

If you are a Corvette owner we have our Shark Bite TM aluminum control arms. These a-arms raise the roll center on the C2-C3 Vette’s from 0.29” below ground level to 3.31” above ground level.

Stock Roll Couple Moment Arm = 16.79”
Shark Bite TM arms installed = 13.19”

This translates into a 21% reduction in roll couple moment arm thereby vastly improving the tendency to roll!

Other Advantages

They are 8.5 lbs lighter that the stock steel a-arms, and they improve the caster and camber over OEM. Overall, a good contribution to improved handling that is truly a bolt on modification.