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.

If you have rear drum brakes and front disc brakes, you should realize that drum brakes take much less pressure to activate. The reason for this is that drum brakes are self-energizing. Because of the location of the pivot point of the shoes, one of them will have a tendency to pull into the drum, creating a braking effect with little pressure. With disc brakes, the hydraulic pressure of the caliper clamping on the disc is the only braking force. Therefore you might want the larger master cylinder on the rear drum brakes. With the balance bar, you will still have considerable adjustment.

If for some reason you don't use a balance bar setup, Wilwood makes two types of proportioning valves. One has a threaded adjustment, while the other has a handle with a number of detents. With the detents, it is easier to know where the setting is. With the threaded adjuster, one can get a finer adjustment. They both have the same amount of proportioning effect. In my testing, I found a pressure reduction of 25 percent with both valves. I mounted a pressure gauge before and after the proportioning valve to get these readings. A proportioning valve can also be used between the front brakes to achieve less braking force on the right-front.

If you are challenged by rules that call for no proportioning valves or balance bars, and you have front disc brakes and rear drum brakes, try this. It has worked for me on several dirt cars. Use the stock master cylinder with two lines coming out. Connect these lines with a crossover tube. You will have equal pressure to all brakes. This will result in about 30 percent more rear brakes. This is not adjustable, but it is close enough to work much of the time.

At times racers have used a shut-off valve on the right-front brake. This can be effective under most conditions. However, before the valve is turned off, the brake pads should be pried away from the disc, forcing fluid back to the master cylinder. This assures that little or no heat is built up in that brake. Heat will expand brake fluid. If the valve is shut and the pads rub the disc, then the heat generated can expand the fluid and apply braking force to that wheel. If a shut-off valve is used, it can be opened while racing, but should not be closed on the track.

For quite some time, I have felt that cutting down the area of the brake pads on the right-front to reduce braking effect was a myth. When you have 1,000 pounds of pressure on a pad that is 10 square inches, then there is 100 pounds of pressure per square inch. If the pad area were cut down to 5 square inches, then there would be 200 pounds per square inch. However, there would still be 1,000 pounds of pressure on the pad. So there should be almost the same braking effect. I have spoken with some brake engineers who agreed with me, and I've talked to racers who didn't.

Well, I decided to test the theory. In the pictures, you will see the cobbled-up machine I put together. First I obtained a spindle, along with a caliper, a brake disc, and a good set of pads. I welded the spindle to enough angle iron to make it stand up. An electric motor was mounted to this. A V-belt drives the brake disc. A Wilwood master cylinder/pedal assembly was bolted on. Next a Wilwood brake pressure gauge was installed in the line going to the caliper. I used an Amprobe to check the amperage going to the motor.

Here is what I found: The motor with no brake load pulled 9 amps. With full brake pads, I applied pressure to the brake pedal. At 450 pounds of pressure, the Amprobe read 12.1 amps, indicating an increase in load. The difference in the amp readings is an indication of the amount of load being applied to the brake. Or, in other words, this is the power that is being absorbed by the brake (braking effect).

My next step was to cut down the pad area. If some's good and more's better, then too much should be just right. I reduced the pad area by two thirds. Of course, the pads will wear faster when cut down.

Back on the machine, I found that 450 pounds on the pedal only loaded the brake to 10.2 amps on the motor. I pressed on, pun intended, until the Amprobe read the same 12.1 amps as in the first test. At this time, the brake line pressure was standing at 600 pounds. It now took 150 more pounds of force to get the same braking effect of the full-size pad. Or, to put it another way, the same brake line pressure produced 17 percent less braking force with the cut-down pads.

Understand that my test may be crude. I didn't have a way to put a lot of heat in the brake, nor did I run tests on a number of different pads. Still, the tests showed a reduction in braking force by reducing the pad area. Specifically, a two-thirds reduction in pad area reduced braking effect by 17 percent. I think less reduction in pad area would result in reduction in braking effect.

This was more reduction than I thought I would find. Yet 17 percent can have a substantial effect on handling.

To make sure there is some objectivity to all this, a couple of brake companies were contacted about cutting down pads. They believe there are better ways bias braking can be achieved, fearing the cut-down brake pad could fail and come apart due to heat. They also expressed concern that the pad backing plate could flex and cause similar problems. One of their suggestions was to use a combination of hard and soft pads to achieve the needed bias.

There are a number of ways to achieve brake bias in a race car. Rules and wallets usually determine how this is accomplished. Brake bias is just one more way of adjusting the handling.

This is the pad tester in operation. An Amprobe reads the electrical load on the motor while a pressure gauge records the brake line pressure. Less load on the motor for the same pressure means the brake is absorbing less power. With the cut-down pads, the braking force was reduced by 17 percent.

SOURCE
Wilwood Disc Brakes
4700 Calle Bolero
Camarillo
CA  93012
8-05/-388-1188
www.wilwood.com
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