How To – Fix Your Roll Center With Whiteline
By Richard Gantry, Photography by Joey Leh
Beyond possible suspension travel and alignment issues, some of the biggest concerns when it comes to lowering a car should be the effects on the roll center and on bumpsteer. Looking at a car head on, the roll center is quite literally the point around which the vehicle rolls when cornering.
It’s determined by drawing a straight line from the instant center to the middle of the opposite tire’s contact patch. Do that for both sides and the point at which the lines intersect is your roll center point. For front strut cars, like the E36 M3, WRX and Evo, the front instant center point is the intersection of a line parallel to the strut top and a line intersecting the lower control arm.
The problem with lowering your car excessively is that the roll center is always lower than the center of gravity and will actually fall more than the center of gravity with a decrease in ride height. As your front roll center approaches the ground, your car will actually begin to roll over more because of the increased torque leverage from the larger roll couple. This is bad for steering response, handling and basically ruins the entire point of buying a lower, stiffer suspension. This also means that you’ll need overly stiff swaybars or springs in order to compensate.
Excessive lowering also amplifies bumpsteer, which is where your toe settings will actually change based on suspension travel. Imagine you’re flying through a sweeper, cornering steady state, you come up on a slight undulation in the road and bam, your car darts even though you didn’t turn the wheel and you didn’t bottom out.
Thanks to excessive bumpsteer, your wheels just self-adjusted their toe setting. It’s caused by the increased difference in arc travel between the lower control arm and outer tie-rod end, and it only gets worse after changing ride height. It’s bad too, for obvious predictability and control reasons.
Curing roll center and bumpsteer problems has generally been a race-only proposal, but drop-in products do exist for garage mechanics to get their fix. Ideally, the best way to perfectly cure roll center problems is to create new control arms, alter the control arm mounting points or raise the front subframe.
Conversely, bumpsteer should be adjusted by using heim-jointed tie-rod ends and specially measured stacks of spacers to drop the connection of the steering arm to the spindle. These solutions, however, require careful race shop measurements and custom fabricated parts.
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