Project Scion TC Part 4: Suspension and Brakes
By Mike Kojima
With our cage and safety equipment done, its time to do stuff to increase performance! The suspension on a road race car is critical for it to be competitive and we need to so some serious stuff on our car. Since the car is being built in its first stage for NASA’s PTD class, we have some interesting factors that we have to consider. To classify cars for a PT class a car is given a base number of points, then each performance part added to the car is assigned a point value. As the car is being built, the points for all of its parts are added up and the total amount of points determines which class the car ends up in.
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| In order to stay within NASA’s PTC class, we could not accumulate too many parts points, thus we opted to run with this non adjustable suspension from The Progress Group. The Progress suspension has been race proven in the Long Beach Grand Prix ProCelebrity races and is a high quality system. Progress custom valved the suspension for our stiffer spring rates. |
Since our car is going to run in PTD we have to be careful not to exceed the allotted number of points. Thus we wanted to get a basic suspension that could be valved for race spring rate but be non adjustable as a non adjustable shock is fewer points on the NASA PT scale. For help in this difficult choice we turned to the Progress Group. Progress makes some excellent components, we have used there sway bars on a lot of our race cars. Progress also makes a non adjustable coilover race damper. They also happen to make the suspension for the Long Beach Grand Prix Toyota ProCelebrity race cars.
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| The front Progress strut in place. It has provisions for the brake line and sway bar mounts. The sway bar mounts are reinforced so they won’t deflect or tear out. |
Progress supplies Toyota with the suspension for the TC’s driven in the Toyota ProCelebrity race. This is a place holder event run at Champ Car weekends where some celebrities and pro race car drivers bash it up for a few laps and wow the crowd for various charities. With inexperienced drivers at the helm, Progress could not dial up an edgy, G-Machine suspension calibration, in fact the spring and shock rates used in the celebrity suspension looked reasonable for a weekend warrior street car.
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| The rear strut in place. The inward location of the rear strut means that it works at a higher motion ratio than typical, thus a higher than expected spring rate is needed as the wheel has a lot of leverage on the strut. |
The ProCelebrity race calibration is a mild calibration and we needed something a little more extreme since this is a serious race car. We upped the spring rates over the LBGP calibration to run a stiffer 900 in/lb rear and 600 in/lb front springs. The stiffer rear springs are used largely because of the multilink rear suspension’s higher motion ratio that puts more leverage on the spring than your typical McPherson strut suspension.
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| The Progress rear swaybar is 3-way adjustable and has adjustable heim joint equipped end links to eliminate sway bar preload when corner balancing. |
To handle the extra rebound energy of the stiffer springs, Progress revalved our dampers to a custom spec. The front dampers had significantly more high speed rebound damping and slightly more low speed rebound damping. Nearly 50% more compression damping was added across the velocity range. In the rear much more high speed rebound was added as well as around 30% more low speed damping. Rear compression damping is nearly the same between the two rear suspension calibrations.
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| Wild Pony KMAC camber plates greatly help traction by allowing us to run more negative camber and caster. We normally frown upon camber bolts but the Scion TC has huge bolts stock so they can be tightened enough not to slip. |
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| In order to get more caster/camber adjustment, we ground the plates as shown at left to get more movement. There was enough room inside the strut tower to get away with this. |
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| We had to cut the inner plate of the camber plate as well to allow more room for adjustment. |
To reduce understeer by increasing rear roll stiffness and to aid with suspension tuning at the track, a bigger adjustable Progress rear swaybar was installed. The Progress bar is 3-way adjustable and features adjustable heim jointed end links and flex reducing urethane bushings. The heim joints help eliminate bar preload when the car is corner weighted.
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| The KMAC-Wild Pony camber/caster plate in place. |
As an essential part for any race car to help keep the tread flat on the ground for more grip and to improve stability by reducing flex, adding negative camber and adding positive caster, a set of Wild Pony Motorsports, K-Mac bearing equipped front caster/camber plates were added. Progress supplied front strut bolts with eccentrics built into them to add even more negative camber at the front of the car. We normally frown on camber bolts because they are notorious for slippage; the smaller bolt used usually stretches and allows the spindle to move. The TC has huge strut to spindle bolts so the smaller than stock diameter camber bolts are still bigger than most bolts.
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| Technosquare’s Richy Wantanabe fabricated this trick triangulated aluminum brace. |
