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Pedal and Pedal Pressure

Q:   How do I measure the pedal ratio and what pedal ratio is best suited for my application?

A:  Pedal assembly ratio, or mechanical leverage, is the ratio calculated as the length from the pivot point of the pedal to the center of the foot pedal (A), divided by the length from the pivot point to the master cylinder pushrod (B). Refer to the figures below. Mechanical leverage is simply a means of increasing the brake force without increasing your leg effort. As “A” gets longer and “B” gets shorter, the mechanical leverage increases brake force without pushing harder on the pedal. The disadvantage is that the pedal stroke also increases, requiring you to push the pedal further. With a 1” master cylinder stroke, a 100-pound push on the pedal, and the pedal having a 4:1 ratio, the force is 4 x 100 = 400 pounds, and the stroke is 4 x 1 = 4 inches. With a 100 pound push on the pedal, with the pedal having a 6:1 ratio, the force is 6 x 100 = 600 pounds, and the stroke is 6 x 1 = 6 inches.

If uncertain about which pedal ratio is right for your application, a 6:1 ratio is an excellent starting point.

A = Distance from pivot point to middle of push / pull point

B = Distance from pivot to point of push on master cylinder

P = Pivot point

F = Force or push

For further information, review Wilwood's Tech Tip Guide. You can also contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.

Q:   I have a hard pedal, but the car is very difficult to stop. What is the problem?

A:  Common contributors to "hard pedal, won't stop" issues are an oversized master cylinder bore and/or inadequate pedal lever ratio. Another contributing factor is the “aggressiveness” of the pad. Disc brakes require approximately 900-1200 PSI at the caliper for effective functioning. We recommend that you use Wilwood Quick Check Pressure Gauges to measure your pressure at the caliper. If you are not generating the required pressure, we recommend increasing your pedal ratio, and/or going to a smaller bore master cylinder. See our Troubleshooting Guide for more complete information, and make sure you have Wilwood Quick Check Pressure Gauges available to assist you in evaluating the problem. You can also contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.

Q:   Why does my pedal "fade" or "go away" after I've warmed up my brakes?

A:  Old brake fluid is the main cause of this problem. Brake fluid deterioration occurs from heat cycling and absorption of moisture. As brake temperatures increase, the old fluid boils, causing the pedal to fade. See our "lose your pedal" section of the Troubleshooting Guide, and make sure you are using fresh Wilwood brake fluid. You can also contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.

Q:   Do I need to use a pressure valve?

A:   These in-line pressure valves retain a minimum brake line pressure to help eliminate excessive pedal travel in both disc and drum brake systems. The two pound valve is used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers and fluid drain back occurs from gravity and vibration, thereby causing excessive caliper piston retraction and a longer brake pedal stroke. The minimal two-pound residual pressure prevents fluid from flowing back without causing the brakes to drag. With drum brakes, a ten-pound valve is used to compensate for return spring tension in the drums.

Q:   What is a proportioning valve and do I need one?

A:   A proportioning valve is a pressure reduction device. It is typically installed in the rear brake line to reduce braking efficiency and compensate for premature rear-wheel lockup; a result of incorrect front to rear brake bias. An adjustable proportioning valve permits incremental adjustments to fine tune brake bias. This ability to adjust front-rear brake bias is particularly important in race applications, as changing track conditions and vehicle dynamics usually require the brake bias be adjusted throughout the race.

Normally, you do not need to purchase a proportioning valve with a Wilwood four-wheel disc brake kit. Because Wilwood manufactures calipers with the correct piston area for each application, our kits will work with your dual-chamber stock master cylinder and stock pressure limiting valve. There is no need to modify or remove the existing pressure-limiting valve, and no additional proportioning valve is needed. A Wilwood kit will also work with your ABS control systems.

However, if you significantly change your vehicles weight and/or chassis dynamics, such as is common with muscle cars, hot rods, street machines and customs; you will likely need to remove the factory proportioning valve and install an adjustable proportioning valve when installing Wilwood brake kits. The factory valve was designed for a specific weight car, on a specific tire, with a specific suspension system, and a specific amount of brake torque at each wheel. If any of these specifications have been altered, the factory valve will not allow optimum performance of the braking system by either limiting too much pressure, or not limiting the pressure adequately. A Wilwood adjustable proportioning valve will provide easy adjustment to obtain the optimum pressure for your modified vehicle. For further clarification, contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.

Wilwood Engineering, Inc
4700 Calle Bolero
Camarillo, CA 93012
Phone: (805) 388-1188
Fax: (805) 388-4938

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