Even though a braking system is fairly simple, there is a lot of confusion as to how it works and how can it be upgraded. Let us go over exactly as to what it does and how should one upgrade it.
A car in motion has some kinetic energy due to that motion. In fact, the amount of kinetic energy is proportional to the square of the speed of the car. In other words, a car going 60 mph has four times the amount of kinetic energy than a car going 30. You all remember the law of conservation of energy. When you slow down a car, that kinetic energy cannot disappear, but has to be changed to some other form of energy.
The job of the braking system is to change that kinetic energy to heat energy and dissipate this heat into the air. Once again notice that the amount of heat produced by braking from 60 to 0 is four times the amount of heat produced by braking from 30 to 0. This is why your braking distance will be significantly longer in the former case.
Let's cover the three most important elements of the braking system: tires, pads, and rotors.
Tires can exert a specific maximum amount of force on a car, which is directly related to the car's lateral g. This force is independent of speed, which is why it doesn't matter how fast you go on a skid pad when you're measuring the g. The stickier the tire, the greater the force that it can exert on the car. Since the force is independent of the speed, the rate of energy that the tire can deliver quadruples with each doubling of the speed.
Pads change the kinetic energy of the car into heat by rubbing on the rotors. Pads should not conduct heat, so that all heat produced is absorbed by the rotor instead of the braking fluid. The larger the friction coefficient of the pad, the faster the rate of energy that the pad can deliver to the rotor.
Rotors act as a funnel of energy between the surface of the pads and the surrounding air. Rotors should be an excellent conductor of heat and ideally the rate at which the energy leaves the rotor and goes into the air should be greater than or equal to the rate of energy arriving at the rotor through the pads.
Tires' lockup - we know that rotors were designed to absorb and release the heat energy produced by braking, tires were not. During skidding, all the heat energy is produced not by the pad on the rotor, but by the tire on the road. Since neither the road, nor the tire can release the energy as quickly as the rotor can, the braking distance will be significantly longer when the tires lock up.
Braking performance: At lower speeds, the rate of energy delivered to the rotor is much less than the rate that energy can leave the rotor into the air. The brakes are overwhelming the tires. The only way to improve braking is to exert more force on the car, which means to install better tires. At higher speeds, the rate of energy delivered to the rotor is significantly larger. There is a speed where that rate is larger than the rate at which the rotor can dump heat into the air and at that point the tires start overwhelming the brakes. To improve braking at higher speeds one must install larger brakes and/or have air ducts to the brakes, etc. Basically anything to help the rotors dump the heat into the air faster.
How can you tell that magic number? Easily - the point at which the brakes start overwhelming the tires, is the point where the wheels start locking up (ABS kicks in). Where it is safe to do so, accelerate to about 80 - 90 mph and slam on the brakes. Notice the speed at which ABS kicks in. On a stock car, this speed is usually around 55mph or so.
Street upgrades: If you want to reduce your braking distance on a street driven car, then you should first upgrades your tires rather than your brakes. Assuming a threshold of 55mph, better tires will reduce your 55-0 distance and better brakes will reduce your 60-55 distance.
Track upgrades: Basically the more you can do to increase the rate of heat exchange the better. There is no limit to track upgrades, as the constant heat exchange is enormous.
To increase the rate at which energy arrives at the rotor you can:
increase the size of the rotor
increase the size of the pad
increase the friction of the pad by going to a different one.
increase the friction of the pad by distributing the force more evenly (two or more piston calipers.)
upgrade your tires
To increase the rate at which the energy leaves the rotor you can:
increase the mass of the rotor
improve the shape of the rotor (directional vanes, larger surface, etc.)
improve the heat transfer properties of the rotor (ceramic brakes etc.)
install air ducts.
king bias: During braking there is a significant weight transfer from the back to the front of the car. A car that has a 50-50 F/R weight distribution at constant speed, can have 85-15 F/R weight distribution during braking. This means that the front brakes have to do a lot more work than rear. This is why the front brakes are usually a lot larger than rear brakes and why the braking system sends more force to the front than to the rear when you press the brake pedal. A correct bias exists when all four tires start locking up at the same time as you apply the brake. This makes sure that all four tires are delivering maximum braking force to the car.
EBD: All new Audis come with Electronic Brake Distribution. This is just a fancy name for an automatic bias system. This is why most front only upgrades work without a problem on Audis. The system automatically compensates and adjusts the bias accordingly. The way the system works is by removing some of the braking force from the wheels that lock up (ABS kicks in) and applying that force to the wheels that are still rolling. On a non-EBD car, you can figure this out yourself. Apply the brakes while listening which set of wheels front or rear locks up first. (Where does ABS kick in first.) That set of wheels has to much brake force. To adjust it, you can change the bias manually if you can, or upgrade the other (the non-locking) set of brakes.
The following information is courtesy of RangeR BoB:
You might point out that the braking in modern Audis is cross-linked to where the Left Front and Right Rear are on one circuit and the Right Front and Left Rear are on the other circuit. The biasing is basically the ABS system letting go of which wheel is locking. This is actually a problem with 4 piston calipers on S4's at the track. Both the front and rear wheels get the same line pressure and as a consequence the rears will lock up considerably ahead of the fronts, leading to ABS activations (which may cause you to back off the brakes) when the fronts are no where near out of grip.
Front Only upgrades: The assertion is that bigger front brakes will somehow stop better and that your rears won't do any work. With the 4-pot brakes replacing the stock one (A4) or two (S4) pot brakes, for a given line pressure the clamping force DECREASES at the fronts, so that the rears work harder. The upgrades work better because you get more friction on the rotor surface, even though the clamping pressure is less. Bigger swept areas make up for the reduced line pressures. When you brake really hard, line pressure goes up more. As a result the rears get even more pressure, so they work more than they used to, and brake bias shifts to the rear.
Don't believe me? Pull your ABS relay on a stocker and go out on a big parking lot or deserted road. Get going a steady speed, and when you cross a fixed mark nail the brakes and lock them up. Have an observer outside watch which set locks up first. Put a paint mark on the pavement and measure if you have a tool to see what your distances are. Repeat several times for consistency. Use a G-tech if you have one. Now go home, put on your monster brake upgrade, and repeat the test. Tell me how much EARLIER your rears lock up, then also tell me how much shorter your stopping distances are.
Upgrading the rear brakes on the A4/S4 cars is a complete waste unless you replumb and rewire the car to have front/rear hydraulic and ABS circuits, rather than the current diagonal circuits our cars have.
