Being a true car dork takes studying. Encyclopedic knowledge of the test numbers and driving impressions of hundreds of cars you'll never see is only the beginning. A true car dork also needs to understand how cars work. Supreme dorkdom means studying mechanical systems, electronics, hydraulics, material technology, acoustics I could go on, but I don't want to be a dork.
Brakes are an important part of a true car dork's repertoire, but because brakes are without the cachet of the noisier parts of the car, the study of brakes gets neglected. I've encountered professional car dorks who can't get the basic concepts of hydraulic brakes right. In fact, speaking with the biggest brake dork I've ever met, Stoptech's Steve Ruiz, I found my own basic brake knowledge to be lacking.
Time for a little studying.
Braking systems are really quite simple. Pushing the brake pedal squishes a bunch of fluid through a bunch of hoses to the calipers where it pushes out on the pads and makes them rub against the rotors. What could be simpler? It's in the details that things get interesting.
When you press the brake pedal, a rod gets pushed into the back of the master cylinder where it pushes on a piston. There will be some lever ratio between your foot and that rod, and usually there's a brake booster between the rod and the piston, but let's ignore both and just say the rod is pushing on the back of the piston with 100 lbs of force. The pressure generated is simply the force divided by the surface area of the piston.
Master cylinders are almost always measured in fractional inches. I have no idea why. I just bought a new 1 1/16-inch master cylinder for my Silvia, for example. 1 1/16 inch? What is this, woodshop?
Anyway, the car started life as an '89 240SX with a 7/8-inch master cylinder. The piston had a surface area of 0.60 square inches. Pushing on that piston with 100 lbs would generate 100/0.60, or 166.67 lbs per square inch of pressure. In my case, that's too much pressure and the pedal effort is too light, hence the 1 1/16 master cylinder (0.089 square inches), which, under the same circumstances, will generate 100/0.89, or only 112.36 psi.
First thing to remember: A bigger master cylinder means less pressure, and therefore higher pedal effort.
The other effect of master cylinder size is that a bigger piston pushes more fluid down the lines, so the pedal won't have to move as far to move the calipers the same amount.
Second thing to remember: A bigger master cylinder means less pedal movement.
At the other end of the brake lines we have calipers, and again, they have pistons in them. Now, for some reason, the pistons in the brake calipers are usually measured in millimeters. Great. We already have pressure calculated in pounds per square inch, so now we have to convert the piston sizes to inches. Thanks. There are 25.4mm in an inch, if you forgot.
The pistons can be arranged in several different ways, depending on the caliper design. Fixed calipers will have pistons on both sides of the rotor. A two-piston fixed caliper will have one on each side, while a four-piston caliper will have two on each side. You get the idea.
A sliding caliper only has pistons on the inboard side. The piston pushes on the inboard pad and the cylinder itself gets pushed away from the pad. That's the sliding part. When the cylinder slides backward, it pulls on the outboard pad, since it conveniently happens to be cast into a big fork that reaches over the rotor.
Let's say you have 40mm pistons in your caliper. To calculate the total working piston area, you might expect to calculate the surface area and multiply it by the number of pistons. That only works if you have fixed calipers. If you have sliding calipers, they push on the brake pads with both the piston and the back wall of the cylinder, so your calculation has to account for twice the actual piston area. A single-piston sliding caliper will clamp as hard as a two-piston fixed caliper with the same piston sizes.
Third thing to remember: Sliding calipers act like they have twice their actual piston area.
Now, let's say we have a two-piston sliding caliper like a WRX or Integra Type R. With two 40mm pistons (1.95 square inches each) and the back of two 40mm bores all working, you effectively have four working pistons, for a total surface area of 7.8 square inches. 112.36 psi x 7.8 square inches makes 876.4 lbs of clamping force on the rotor. Bigger pistons here would mean even more force, and at the same time, more fluid would be needed to move the pistons. That gives us two more things to remember.
Fourth thing to remember: Bigger pistons in the calipers mean more force and, therefore, less pedal effort.
Fifth thing to remember: Bigger pistons in the calipers mean more pedal movement.
We're almost to the part where we get to stop the car. Time to see how much torque this clamping force can generate. An approximate brake torque reaction calculation is quite easy. For comparing one brake setup to another, all you really have to know is the rotor size and total piston area. The effect of the brake pads clamping on the rotor can be assumed to be concentrated on one ring, called the effective radius, which is located almost at the center of the brake pad, but not quite.
The effective radius is like a balance point. In one rotation of the rotor, the same amount of rotor surface will sweep by on the inside of the effective radius as on the outside. Since more surface area sweeps by on the outside of the rotor than the inside, the effective radius is slightly outboard of the actual center of the pad.
If you know the overall radius of the rotor and the radius of the inner edge of the working surface, which is the shiny part the pad actually touches, you can find the effective radius with this formula:
For comparing one brake to another on the same car, you can simply multiply the total caliper piston area times the effective radius. If you want the actual torque generated, you need to multiply the piston area times effective radius times the coefficient of friction of the pad on the rotor times the fluid pressure. Most likely, you don't know the coefficient of friction, do you? Stick to the simpler calculation.
At this point, any dork can tell that a bigger rotor will mean more brake torque, and since you aren't changing anything hydraulic, it won't really affect pedal movement.
Sixth thing to remember: Bigger rotors generate more brake torque, which makes the pedal effort feel lighter without affecting pedal travel.
There will be a quiz next month.