2008 Subaru WRX Project - Tech

The Whiteline bumpsteer adjusters are just outer tie-rod ends with a taller ball joint and bent endlink casting. Bumpsteer is basically unwanted toe change that occurs as the suspension compresses and extends. This is because of the difference in angles and lengths of the tie rod and the control arms. Simple geometry means that as the control arms swing and the total horizontal distance from the upright to the pivot point of the arm changes, but because the steering rack and tie rods are mounted on a different horizontal plane than the control arms, the change in total horizontal distance as the tie-rod ends swing is different from the change in the control arm. And because the tie rod is mounted offset to the upright vertical centerline, so that there is torque moment to steer the upright, the upright will ultimately change toe as horizontal lengths of the tie rod and control arms differ as the suspension travels. This is all designed to work by the OEM for a car at stock height, but by changing ride height we've compromised this. The rough rule of thumb is to keep the tie rods level with the ground. By essentially pushing the outer tie rod downward with a shimmed ball joint, the Whiteline bumpsteer adjusters compensate for lowered cars by bringing the tie rods back down to level.

Similarly, the Whiteline roll center adjusters (RCA) are shimmed ball joints designed to push the lower control arms to a more static downward angle to increase camber gain in the MacPhereson front suspension. Since MacPhereson suspensions have a motion ratio of nearly 1:1, the amount that the ball joint has increased in height is a ballpark distance that a car should be lowered by to maximize the RCA's function.

With our streetcar now truly tuned, we headed back out to the figure eight for a retest. While we didn't pick up any additional time, the feel is utterly different. The geometry corrections on the front suspension, along with the added camber, have eliminated the washed-out sensation at corner exit. Project WRX still understeers but at least now it can be forced to rotate. Although we have solutions for even better handling, we opted to keep the antiroll bars stock along with the TEIN spring rates. In the end, it's still a streetcar.

We lost some time in the corners because the car does roll more now with its slightly higher CG and heat-cycled Advan AD07 Neovas. Although the car averaged the same lateral grip in both directions, the spread was wider than before on account of the tires' wear. What we lost in the corners, we picked back up in acceleration and exit speed. It's not faster on the lap, but far better overall and that's what matters on a streetcar.

Previous installments
January 2008
Part 1: Brakes, Wheels, and Suspension

March 2008
Part 2: Back in Black

July 2008
Part 3: Measuring the Par

By Jay Chen

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