So far, Project tC has shown there is performance hope for the Scion coupe. In fact, there's a lot of hope. The last round of simple suspension bolt-ons yielded near Evo-rivaling handling numbers. Then we heard that the Celicas were being dropped in favor of tCs at this year's Toyota Grand Prix of Long Beach (see page 48). We figured if Toyota is willing to throw a bunch of celebrities into race-prepped tCs for their annual Pro/Celebrity Race, we might be on to something too (although this may not be an endorsement of the car's performance potential, but rather a clever way of disposing of annoying D-list celebrities like Frankie Muniz).
But numbers aren't everything. Although we realized huge improvements from just basic bolt-on parts, the tC is nowhere near as rewarding as many of our other project cars. We have to work out how to add some feel and character into the chassis, which is something far less measurable. In this installment, we'll try to fix the part of the tC we hate most, the brakes.
Braking Torque And Brake BiasBetter braking is a matter of tire selection. Since tires are what ultimately stop a car, the first place to look for shorter stopping distances is with stickier rubber.
For some cars, however, there's a way to cheat and get a little more braking out of the same tires by making the rear tires do just a little more work. In general, stock brake systems are designed to be front biased, meaning the braking torque in the front is a lot more than in the rear. Consequently, the front tires bear most of the stopping load. When ABS kicks in, the front tires are at their friction limit, but the rears most likely still have grip. By adding more rear brake bias we can take advantage of additional rear grip not currently being used. To do this requires taking an engineering perspective, so put on your pocket protector and brace yourself.
Powerslot's Plus kit for the tC uses two-piece, 12.8-inch vented front and 12.5-inch rear rotors compared to the stock 10.76-inch front and 10.5-inch rears. Powerslot includes CNC aluminum caliper spacer brackets and all additional hardware.
To start, any engineer worth his slide rule needs an equation-we're helpless without them. In an ideal world, braking torque, or how much your caliper can resist the twisting motion of your wheels, is approximated with a fairly simple equation:
TW = PS x AP x x 2 x RE
Braking torque, TW, is the product of the hydraulic pressure in the brake lines, PS, multiplied by the total piston area, AP, (for a sliding type caliper or one-half the total area of a fixed caliper) multiplied by the friction coefficient of the pads and rotor surface, , multiplied by the effective radius of the clamping force, RE. Everything is multiplied by two since there's another half of the caliper pushing on the opposite side of the rotor.
Without substituting in numbers, this equation tells us some simple things. The available torque to resist turning is a matter of line pressure, total area of the pistons (not number of pistons), friction material (pads), and size of the rotors. By increasing any one of these variables at the rear we can increase the braking torque and thus bring more braking bias to the rear.