The front suspension's lower control arm had a huge rubber bushing at the forward pivot. This bushing would allow for over a quarter of an inch of deflection when loaded via a prybar. Although this might be acceptable for grandpa's Buick, it was a source of much steering slop. Although we liked our tC's refined cruising ride, all this monkey motion made for a lack of turn-in crispness and some rear-end twitchiness when pushed to its limits on the track. The key was to keep the rear suspension from moving around.
We found our solution through Energy Suspension (ES), who provided us with one of its recently released bushing kits for the Scion tC. Originally prototyped for track use, ES bushings replace all rear subframe-to-chassis bushings with super-stiff, 95A-durometer, solid polyurethane pieces. Large 88A-durometer rear trailing arm bushings are also available.
According to ES engineers, subframe bushings actually contribute less to NVH than control arm bushings, so our race bushings should minimize subframe movement without rattling our teeth out. ES will initially release the subframe bushings to the public in the softer 88A-durometer form. Since the tC's rear suspension requires some bushing compliance to prevent bind, ES only replaces select bushings, mainly those that have the largest affect on wheel tracking.
Up front, the two inboard lower control arm bushings were replaced with 88A-durometer pieces. The front item is a conventional rotation bushing while the rear vertical ball bushing is a work of art. ES created a multi-piece urethane ball-and-socket affair to eradicate most of the gush, while having less bind than the factory rubber part. Revisiting the suspension with our semi-scientific prybar test revealed that we had eliminated about 80 percent of the suspension's deflection.
In previous installments, our horsepower additions to the tC had created severe engine torquing issues. The compliance of the hollow rubber engine mounts added so much lash to the drivetrain that the engine felt like it was attached to the throttle with a yo-yo string. This lurching contributed to a pounding case of wheel hop. We reduced the engine motion with the use of an engine torque damper from Ingalls Engineering. But it wasn't enough. The piece transferred all engine vibration to the strut tower and sheetmetal, essentially making the car a rattle can between idle and 3000rpm.
Desperate to address the engine's wind-up and subsequent wheel hop, we replaced two of the engine's four mounts with ES's polyurethane parts. While we knew stiffer engine mounts typically add more vibration (especially to the steering column at idle), our hope was to sacrifice some comfort at idle in order to eliminate the perpetual rattle from the engine damper. ES makes a complete four-piece kit, but to preserve some smoothness, we opted to change the forward and transmission mounts only. Unfortunately, this operation upset the harmony of our car's inner being and our once-smooth Scion turned into a vibrating beast.
This turned out to be far shakier than any other car we've ever experienced with polyurethane mounts. Even road-race tCs with four solid urethane mounts and no harmonic mass dampers had much less vibration. Our theory is that the cantilevered-forward weight of the TRD supercharger-and perhaps the exhaust attachment-had something to do with this. While the mounts and the bushings effectively eliminate all wheel hop, it was unbearable on the street. So we reinstalled our stock mounts. Vibration gone, although we're now back to the same sloppy wheel hop and engine lash problem.