Brake Bias 101

By Dan Wagner (As seen in Circle Track magazine)

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 harder, deeper and more confidently.

Proper brakes are one of the most important safety features on you 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, period.

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 port maintains its pressure. Dual master cylinder set-ups completely isolate the two hydraulic systems. However you choose to do it, dual brake systems will still provide braking from one system even if the other one completely loses pressure.

Bias Defined

What 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. The most important one is the force (weight, downforce, etc) 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, losing traction on the rear end is one of the last things we want to have happen. However, some people actually use the knowledge of this principle to their advantage. Rally drivers use quick applications of the parking brakes to turn tighter corners. How many of us haven't used the parking brakes to spin "doughnuts" in a freshly snow covered parking lot? For now though, we will concentrate on getting the maximum braking force from all four of our tires without losing control.

Losing traction on the front tires is not as bad as on the rear. You usually plow forward in in the original direction until the driver lightens up enough on the pedal to regain control. As a general rule, 60% 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.

Braking force is applied to the brake pedal. The force is multiplied by the pedal output ratio (typically three to four) and the power brakes if present. The output force from the pedal is transferred to the master cylinder(s), either directly or through a balance bar. As the master cylinders' piston moves forward, the fluid pressure rises until the force applied to the face of the piston equals the force from the balance bar assembly. The fluid pressure is then applied to the face of the calipers' pistons causing the calipers to squeeze the pads on the brake rotors.

We will be talking a lot about torque and pressure. So, let's make sure we are all talking about the same thing.

Torque is a twisting force caused by the multiplication of a force by a lever arm. Torque is usually expressed as inch-pounds (in-Ibs) or foot-pounds (ft-Ibs). The inch or feet refer to the lever arm length and pounds are a measure of force. For a given torque, the shorter the lever the larger the force and the longer the lever the smaller the force.

Remember when you were a kid trying to use your dads' hammer? If you tried to hold the hammer at the end of the handle it kept twisting out of control. So, you ended up holding the hammer close to the head. What you really did was shorten the lever arm thereby reducing the torque. Now that you're hands are stronger, you hold the hammer the correct way to maximize the force on the nail due to the increased torque.

There is another example of torque that we all have experienced one time or another. It seems there is always at least one bolt that you just can't twist loose. You put a pipe over your good ratchet and easily twist the bolt (or your only socket). Why did the bolt turn more easily? You increased your "leverage" by using a longer lever arm. The torque increased while the force you supplied stayed about the same. Remember that torque is force times a distance.

Pressure is force spread over a given area. It is typically expressed in pounds per square inch (psi). Pounds for force and square inches for the area. As a force is spread over a larger area, the pressure goes down. If a force is concentrated on a smaller area, the pressure goes up.

Airplanes provide some great examples of pressure. Airlines want sturdy strong floors that don't weigh too much. Interestingly, the biggest problem they have is usually not heavy people but ladies with stiletto heels. Heavier people tend to wear shoes that spread their weight over their feet reducing any localized pressure. Even with a lighter woman wearing stiletto heels, the smaller force concentrated over a such a small area can cause enough pressure to puncture the floorboard.

Pressure applied over an area results in a force. Just a few psi pressure difference applied to the wing of a 747 can lift it into the air. Remember, pressure is force over an area and pressure applied to an area results in force.

The 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. If you run power brakes, the power booster is between the pedal and master cylinder(s). The booster provides another multiplication of your force on the pedal.

A 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 what it does. The torque on one side of the bar must balance the torque on the other side of the bar. Remember that a force applied over a distance causes torque. 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 valves on tandem master cylinder systems act much like a balancing bar on dual master cylinder systems. The proportioning valve is usually used in the rear brake line. It can reduce the pressure by 0 to 50% (typically). Proportioning valves only reduce the pressure in one system unlike balance bars that take from one side and give to the other

The size of the master cylinders' piston has a direct result on brake fluid pressure. However, it may not work like you would think... higher line pressure will build up on a smaller master cylinder piston to react the force applied by the pedal. A smaller master cylinder will create more brake fluid pressure but will also increase 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 less pressure but will require less travel.

Greater piston area on the calipers, whether by using larger pistons or more of them, will cause greater squeezing force on the rotor. More caliper piston area also increases master cylinder 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 can improve wear, lower temperatures etc.

A larger rotor has a bigger lever arm increasing brake torque on a wheel. Smaller diameter rotors will reduce brake torque. Ventilated rotors will not significantly increase brake torque but do improve cooling.

The rotor diameter and the amount of squeeze from the caliper determine brake torque on a wheel assembly. 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 have less braking force where the tire meets the pavement.

I'm sure that last statement will ruffle a few feathers. Please bear with me on this one. Many things (including contact patch size, tire compound, temperature, etc) will affect the amount of traction available from a tire. More traction means more torque will be required from the brakes to lock up the tire. Bigger (diameter) tires will require bigger brakes. It is all about torque.

Friction is the force resisting two objects from sliding. Friction is made up of two parts: a coefficient of friction and the force clamping the two objects together. Coefficient is just a fancy word for a number that multiplies another number. A coefficient of friction is a number that tells how much of a the clamping force between two objects will be converted to friction to resist sliding. Different materials have more potential friction between them (higher coefficients of friction) than others. For our purposes, the exact number is not as important as understanding the concept.

Friction is caused by miniature imperfections along the contact surface between two objects. Imagine looking at a piece of metal under a strong magnifying glass. The surface that once appeared so smooth now looks like a series of mountain ranges. Friction develops when those microscopic ridges on the two objects interfere with each other. If the objects are clamped together tight enough, you would have to break or deform those ridges before sliding can occur. Without adequate clamping, one of the objects can hop across or ride the ridges of the other part allowing sliding to occur with less effort.

As a very crude example to illustrate the point, think about our friend the Phillips headed screw. A Phillips head screwdriver tends to pop out of the screw if you don't push it in tight enough. On stubborn screws, you end up pushing on the screwdriver almost as hard as you are turning it. Don't push hard enough and the screwdriver pops out ruining the screw.

To make matters even worse, there are two types of friction: static and dynamic. Static friction is when two objects are not sliding relative to each other. Dynamic friction occurs when the parts move relative to each other (sliding). Static friction is stronger than dynamic friction. Once the parts start "riding the ridges" frictional forces go down considerably. This explains why once your tires lose traction it is so hard to regain it.

FRICTION = COEFFICIENT OF FRICTION x CLAMPING FORCE

It is true that other things can affect your tires’ traction. Rubber doesn't follow the rules like most materials. Because it deforms under load and shears off in friction (you know, the "marbles" you are constantly avoiding), other factors such as tire pressure, camber, temperature tire width, etc can affect tire traction. These other factors however, are secondary to clamping pressure. No matter how sticky or wide your tires are, you can't get any traction if they aren't touching the ground!