Project Scion TC - Tech

The front lower control arm consists of a conventional rotational bushing in front and a ball unit in the rear. The outer cups of the bushing serve as dust covers.

The rear subframe bushings were replaced with prototype 95A-durometer solid units to reduce any unwanted deflection of the entire rear suspension. The stock bushings are semi-hollow pieces cast into a metal sleeve to keep the bushing from sliding and popping out, while the urethane pieces are solid.

The solid rear control arm bushing is critical in keeping the rear wheel from changing toe under cornering load. This will give a more consistent handling characteristic and increase cornering limits.

Progress disassembled our original CS-1 shocks to be revalved to Pro/Celebrity specs. The twin-tube shock is made up of an inner tube, where the shaft and piston ride, surrounded by a fluid-filled outer tube. The shock piston shown here is attached to the main shaft of the shock. As it moves up and down, the shock's fluid is forced through the holes to create damping. The size of the holes and the slots help determine the shock's damping force.

Progress' suspension for the tC works really well for sporty street driving. The fact that there is also an off-the-shelf version for track use is not common knowledge. Not being absolutely rock hard makes the Progress Pro/Celebrity track suspension reasonably streetable as well. A big plus with the Progress suspension is that it can be completely rebuilt and revalved to suit different spring rates.

The Progress shocks use a deflected disc valving system. These discs are stacked on each side of the shock piston and block the fluid flow through the holes in the piston. The fluid must bend the discs out of the way to flow through. The size, number and thickness of the discs control their flex characteristics and the force the fluid needs to generate to move them out of the way. The damping force curve can be adjusted precisely by changing the disc stack around. This contributes to most of the shock's tune-ability.

Here are the pistons and valve discs bolted to the shock shaft. The discs on the shaft side of the piston control compression damping, the discs on the opposite side control rebound.

The original ride height on Project tC was set way too low, with little travel. With a motion ratio of about 0.69 and a soft rear rate, the car should have sat higher. With double the spring rate and the rear suspension raised up, we're targeting a total shock stroke of two to 2.5 inches in compression and one to two inches in droop.

This is the foot valve. It sits at the bottom of the inner tube and controls the fluid flow between the inner and outer tubes. It mostly controls high-speed compression damping. This is the valve that reacts when a large bump or pothole is hit. It blows open and allows fluid to pass easily, so the wheel can respond smoothly to sudden bumps.

Our final addition was an Ingalls Engineering rear strut tower brace. It sits hidden beneath the trunk lining, since the rear strut towers only extend up to the floorpan sheetmetal. The brace uses all stock mounting points, with drilling required only to enlarge the two rear bolt-holes.

Ed Flores of Progress assembles our shocks after revalving them for the track suspension's stiffer springs. First, the inner tube with foot valve slides into the outer tube.

Next, the piston bearing is wrapped around the shock piston. This is a DU bearing, made of a bronze impregnated Teflon-type material. The bronze prevents the Teflon from squishing under side load and developing play.

The shaft oil seal slides down on to the shaft and the gland nut over it is tightened by hand.