Understanding your brakes will not only help make your car safer; it can also help improve your lap times. Correct brake bias will help you maximize the braking force available from your car, allowing you to brake later, harder, and more confidently.
Safety FirstProper brakes are one of the most important safety features on your race car. There are plenty of ways to reduce the cost of racing; your brakes are not one of them. No car should ever race without separate brake systems for the front and rear-if one system fails, you still have a second to provide some stopping power.
Separate systems are easily achieved using either dual master cylinders or a production-style, tandem master cylinder. Tandem master cylinders use one cylinder bore with two pressure ports and pistons. They are designed so that if pressure is lost in either port, the other one maintains its pressure. Dual master cylinder setups completely isolate the two hydraulic systems. However you choose to do it, dual brake systems are designed to provide braking, even if one system completely loses pressure.
Bias DefinedWhat exactly is brake bias, and how will it help you? Brake bias is just a fancy way to describe how the total braking force is distributed between the front and rear tires.
Many factors affect the amount of braking force a tire can generate. Most important is the force (weight, downforce, and so forth) pushing the tire against the ground (see sidebar on friction). As your car decelerates, weight is transferred from the rear to the front tires. This weight transfer reduces the amount of braking force the rear tires can produce. Apply too much braking to the rear wheels, and they will lock up, causing the rear end to lose traction and possibly swing around violently. For most of us, that's one of the last things we want to have happen. However, some people actually use this principle to their advantage. Rally drivers quickly apply the parking brake to turn tighter corners. How many of us haven't used the parking brake to spin "doughnuts" in a parking lot freshly covered with snow? For now though, we will concentrate on getting the maximum braking force from all four tires without losing control.
Losing traction on the front tires is not as bad as the rear. You usually just plow forward until the driver lightens up enough on the pedal to regain control. As a general rule, 60 percent of your braking capacity should be on the front tires. Whatever the percentage is for your particular car, the front tires should lock up slightly before the rear tires under hard braking.
The Braking SystemThe driver sees the wall approaching a little too fast and applies pressure to the brake pedal with his foot. That pressure is multiplied by lever action on the pedal or a power brake system. The multiplied force is transmitted to the master cylinder(s) either directly or through a balance bar. The master cylinder(s) uses a piston to transmit the applied force to the calipers by increasing the pressure of the brake fluid in the brake lines. The increased fluid pressure causes the calipers to squeeze the pads against the brake rotors.
Torque, pressure, and friction are discussed in the sidebar.
* Brake PedalThe brake pedal multiplies and transfers your force to the master cylinder(s), either directly or through a balance bar. A typical brake pedal will increase your force three or four times through lever action. If you run power brakes, the power booster is located between the pedal and the master cylinder(s).
* Balance BarA balance bar (also called a bias bar) on dual master cylinder systems divides the force from the brake pedal to the two master cylinders. It is called a balance bar because that is exactly how it acts: The torque on one side of the bar must balance the torque on the other. Remember that torque is the product of a force applied over a distance. Therefore, the master cylinder closer to the pivot point on the bar has a shorter lever arm and will receive a higher braking force.
Balancing bars take force from one side and give it to the other.
* Brake Proportioning ValveBrake proportioning valves on tandem-style master cylinders act much like a balancing bar on dual master cylinder systems. The proportioning valve is usually used in the rear brake line. Normal proportioning valves can reduce the pressure by 0 to 50 percent. Proportioning valves only reduce the pressure in one system, unlike balance bars, which equalize two pressures.
* Master CylindersThe size of the master cylinder's piston has a direct result on brake fluid pressure. Higher line pressure will build up on a smaller master cylinder piston to react to the force applied by the pedal. A smaller master cylinder will create more brake fluid pressure but will also require longer brake pedal travel. The smaller-diameter cylinder requires a longer stroke to move the volume of fluid necessary to move the caliper pistons during braking.
Larger master cylinders will create the same pressure with less brake pedal travel.
* CalipersGreater piston area on the calipers, whether by using larger pistons or more of them, will result in greater squeezing force on the rotor. More caliper-piston area also increases master cylinder piston movement. Different brake pad compounds will affect the friction developed between the pads and the rotor. Larger brake pads will not significantly increase braking capacity, but they can improve wear and lower temperatures.
* Brake RotorsA larger rotor is the equivalent of a longer lever arm and increases brake torque on a wheel. Smaller-diameter rotors reduce brake torque. Ventilated rotors do not significantly increase brake torque but do improve cooling.
* TiresBrake torque on a wheel assembly is determined by the rotor diameter and the amount of squeeze from the caliper. The torque caused by the friction forces developed between the tire and the ground must balance the torque from the brakes, or slipping will occur. The result is that a larger-diameter tire (bigger lever arm) will actually reduce the braking force created by the caliper squeezing the rotor. Larger-diameter tires require bigger brakes. It's all about torque.
TestingAs always, there is no substitute for proper testing.
Throw on some old tires (the same type you will be racing), and ask a friend to help. Find a place where you can safely test your brakes. Have your friend stand where he or she can safely observe the car. Drive at a decent speed, but not too fast, and slam on the brakes to lock them up. Have your friend observe whether the front or rear wheels lock up first. You can tell which tires are locked by the smoke billowing up. Adjust your brakes until the front tires lock up just before the rear. Record this setup as your starting point. The bias may need adjustment from time to time as factors change.
Always use extreme caution when making brake bias adjustments. Cars have been destroyed due to overly aggressive bias adjustments.
TorqueTorque is a twisting force caused by the multiplication of a force by a lever arm. Torque is usually expressed as inch-pounds (in-lb) or foot-pounds (ft-lb). The inches or feet refer to the lever arm length, and pounds are a measure of force. For a given torque, as the length of the lever increases, less force is required.Torque = Force x Length
We have all experienced a great example of torque. It seems there is always at least one bolt you just can't twist loose. But put a pipe over your good ratchet, and you can easily twist the bolt (or break your only socket). Why did the bolt turn more easily? You boosted your leverage by using a longer lever arm. The torque increased by raising the distance you moved your arm, while the force supplied stayed about the same. Remember, torque is force multiplied by distance.
PressurePressure is force spread over a given area. It is typically expressed in pounds per square inch (psi). Force is measured in pounds, and area is calculated in square inches. As a force is spread over a larger area, the pressure decreases. If a force is concentrated on a smaller area, the pressure increases.Pressure = Force/Area
FrictionFriction is the force resisting an object when it slides across another. Friction is measured in two parts: a coefficient of friction and the force clamping the two objects together. A coefficient of friction is a number that tells how much of the clamping force between two objects will be converted to friction to resist sliding. Different mate-rials have more potential friction between them (higher coefficients of friction) than others. Think sandpaper versus Teflon. For our purposes, the exact number is not as important as understanding the concept.
Friction develops when microscopic ridges on the two objects interfere with each other.
To make matters worse, there are two types of friction: static and dynamic. Static friction is when two objects are not sliding relative to each other-friction has effectively overcome the force of movement. Dynamic friction occurs when the parts move relative to each other (sliding). Static friction is stronger than dynamic friction. Once the parts start sliding, frictional forces go down considerably. This explains why once your tires lose traction, it is so hard to regain it.