Race Car Brake Bias - Bias Braking

On short-track race cars, many of us have used brake bias to adjust the handling. Brake bias is a condition in which there is more braking on some wheels than on others. It can favor front or rear brakes. This bias can also favor the braking effect from side to side. In many cases right-front braking is reduced while the left-front is not. This bias can work toward getting the car into a turn.

There are many ways to achieve the brake bias effect, some more effective than others, and a number of these methods are illegal at some tracks. When racetrack rules call for braking on all four wheels, this means you cannot shut off the brakes to one wheel. However, the braking effect on a wheel can often be adjusted within the rules. As always, know your rule book. It never hurts to know how and what your tech man looks for.

I'll walk through some of the ways to achieve brake bias. When allowed, one of the best ways to achieve front-to-rear bias is with the use of a balance bar. This requires two master cylinders. An adjustable balance bar moves the leverage point of the brake pedal from side to side, thus favoring one master cylinder over the other. With this arrangement, using the Wilwood pedal/master cylinder/balance bar unit, a 57 percent change can be achieved.

What makes this setup more effective is the use of different-size master cylinders. For our purposes a 7/8-inch bore and a 1-inch bore master cylinder were used. The area of the 7/8-inch bore is 0.601 square inches. The 1-inch bore area is 0.785 square inches. Thus the 1-inch bore master cylinder is 24 percent larger than the 7/8-inch bore.

In testing, I used a pressure gauge at the outlet end of each master cylinder. Wilwood Disc Brakes contends that the balance bar assembly can adjust bias by 57 percent, a claim which my tests support. This effect is multiplied by the difference in master cylinder sizes.

For the same brake pedal (pressure) input, a small bore master cylinder will transfer more pressure to a brake caliper than a large-diameter one. The small cylinder will need more stroke to move the same volume of fluid as the larger one. Thus to achieve good braking, the small cylinder may need more pedal travel.

For further explanation, say you are standing on a loose-dirt surface. There is a washtub and a walnut on the ground. If you step in the tub, it may sink in the ground a few thousandths of an inch. If you stand on the walnut, it will push right into the dirt. You applied the same pressure to both. This is what happens with a large and small cylinder.

Let's say you are running on a dirt track and you would like to have more rear and less front brakes. Use the 7/8-inch bore master cylinder for the rear and the 1-inch bore unit for the front if you have disc brakes on all four wheels. With the same pedal pressure, the smaller master cylinder will put more pressure to the rear brakes. Now the balance bar can be adjusted to achieve more or less rear brake bias as needed. Wilwood's unit comes with an enclosed cable, a mounting plate, and knob. When the knob is turned, there is a detent every 90 degrees to help you keep up with where it is set.