Another car that has well known bump steer related issues is the AP1 S2000. The S2000 is somewhat unique compared to other modern Honda models in that (bar the NSX) it was designed from scratch as a sports car, with suspension geometry to suit that application. Contrary to what you might expect Honda has deliberately built in specific bump steer behaviour, rather than targeting zero toe change.
Peter asked us to help him with the adjustment of a J’s Racing rear bump steer kit that he had purchased. These parts are intended to reduce the amount of toe change in the rear of the AP1, which according to common Internet wisdom has large amounts of toe in with bump travel. This has the expected opposite effect when under heavy braking and turning into a corner. As the rear suspension enters droop travel the rear tyres toe out significantly, making the rear unstable and often resulting in surprising and spectacular oversteer. A typical S2000 alignment features significant static rear toe in, to help alleviate this issue.
Here’s the J’s parts and our measuring setup fitted, the basic concept is much like our own tie rod ends; the outer pivot point of the toe link is relocated downward to adjust the bump steer curve. 3 spacers are supplied per side, adding more spacers above the rod end moves it further downward and increases the effect on the toe curve.
First up we measured the car with the OEM toe link in place, to see what we were starting with. The measurement setup is the same basic tools we’ve used before, our budget gauge bolted on in place of the wheel, some magnetic stands, and dial gauges. Pete then installed the J’s toe link with all three spacers in place, giving the maximum bump steer correction possible.
Here’s a graph comparing the OEM toe link with the J’s Racing bump steer kit
Looking at the data a number of conclusions can be drawn. Firstly the stock Honda toe curve gives substantial toe out in droop as expected, however it is surprising just how much. As a rough estimate the static toe should be set to a minimum of 3-4mm of total toe in to avoid toe out when the rear suspension is unloaded.
The J’s Racing bump steer kit works as expected, with the downward movement of the outer toe link pivot reducing the toe change substantially, and even changing the direction of the toe change in the lower end of the suspension range.
Following these measurements we recommended using 2 of the 3 spacers, which will give a small amount of toe in with bump travel for corner exit stability when under power. The reduced toe change at the rear should allow Peter to run less static toe in, while also stabilising the rear of the car during braking and turn in.