After our stock brake baseline on day two, we noticed signs of caliper flex on the stock caliper, since the pad material transfer layer on the rotors were not the width of the actual pad, which it was at the end of day one.
Break Out The Pocket Calculators
So the problem really lies in brake balance. Acura did such a good job on the balance of the Type-S system that little to no room was left for improved braking distances, given the same set of tires. It already has the ideal brake balance to virtually maximize each tire's grip, while at the same time keeping the car stable during on-the-limit braking. To understand this, we're going to have to dust off the calculators and slide rules and run through some numbers.
In order to maximize stopping performance, each tire should be just near its limits of grip in deceleration. Most cars off the showroom floor have a front-biased brake system, which is much safer for the average driver in a panicked braking situation-more rear bias at the limits of grip will cause the car's tail to get loose, even in a straight line. Not a good thing when braking and turning at the same time, unless you're doing it deliberately on a track.
The cost of the greater front bias is more braking grip from the rear tires. In the case of the RSX, Acura's engineers found a way to make a safe street car that still had excellent brake balance and optimized tire grip on all four corners, making it that much harder for big brake kit manufacturers.
To maximize the work that both the front and rear tires are doing, it's logical to say that you want to generate the same amount of braking torque (created by the calipers on the rotor).
Braking torque is a function of several factors, like the frictional coefficient of the pads against the rotors, the effective radius of the rotors (since larger rotors mean longer force moments), hydraulic fluid pressure pushing against the brake pistons, and the pistons' total surface area. Multiply everything together for a simple equation that shows how much braking torque can be exerted on each corner.
It's pretty intuitive if you separate the equation into sections and have a simple understanding of physics. Friction between any two objects is a factor of frictional constant- (mu)-between the two surfaces (determined by its physical characteristics) and the force that's pushing down on it. So in the case of brakes, friction is just the total amount of force pushing the pads against the rotors times the friction coefficient of the pad material on the rotor.
Contrary to what many might think, total pad area doesn't play into the equation because (which is determined experimentally) is dimensionless, or unit-less, and already accounts for the area of the pad. The advantage of pad area is mostly in wear and increased thermal mass.
The red StopTech caliper designed for the RSX Type-S kit uses a pair of larger trailing pistons (bottom left) compared to the black RSX non-Type-S caliper. The 2mm difference in one set of pistons contribute to a six percent change in braking torque at the front wheels. The smaller pistons on the black caliper reduce force output in front, shifting bias towards the rear and allowing the rear tires to do more work.
The force pushing down on the pad is the area of the pistons multiplied by hydraulic pressure of the brake fluid pushing at the back of each piston. Much like a wing, pressure pushing on one side (let's say it's 100psi) times the surface area (let's just say there are two square inches of piston area on one side of the caliper) will yield a total of 200 pounds of force pushing the pad against the rotor. If the caliper is a fixed version with identical pistons on both sides, then add a factor of two to the equation.
The amount of friction force has now been determined. That force is stopping the wheel from spinning by exerting torque on the brake rotor. Torque is a force exerted on an object at a given radius causing it to rotate.
Since a rotor is already turning in one direction and torque generated by friction is, by definition, in the opposite direction, torque created by a brake pad acts to slow the rotor down. In geek: braking torque is expressed by friction force multiplied by the effective radius of the rotor.
The effective radius of the rotor is basically the radius at which the average amount of braking work is done. Different brake manufacturers use different methods to approximate the effective radius, but for the most part, it is larger than the centerline radius of the brake pads, since the outside of the pad travels a longer distance and does more work than the part of the pad closer to the center. Multiply all this together, and you have braking torque created at the rotor.
In terms of balance, the same equation can be applied to the front and rear brakes. Even without plugging in any numbers, we know rear brakes will yield less braking torque than fronts because they have a smaller radius, less piston area and probably less hydraulic pressure from the factory proportioning valve.
There are two reasons why rear-wheel braking torque is less than the front, even though we want to get the most work out of the rear tires. In addition to safety and stability under braking, the other factor we've ignored so far is weight transfer. Under braking, weight shifts forward, so the front tires do more work. With the same braking torque at both ends, rear tires lock up as weight is shifted off and available grip is diminished. Weight transfer can also be approximated by a simple equation, though the variables are hard to determine other than through experimentation.
Knowing this, StopTech's engineering team suggested a retest using their off-the-shelf RSX non-Type-S brake system, something few manufacturers offer for cars of different trim levels but the same chassis. The only difference between the two systems is a 2mm reduction of the trailing piston diameter and a conventional cast caliper instead of the squeezed-forged process used on the Type-S caliper. Everything else, including pads, remained the same.
Going back and crunching numbers in the brake torque equation, this means a six percent reduction in torque output at the front, which further shifts brake bias tothe rear.