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# Help Measuring Brake Pedal Ratio

Discussion in 'The Hokey Ass Message Board' started by 3spd, Jun 2, 2012.

1. ### 3spdMember from Portland, Oregon

I want to put a reverse swing master cylinder under the dash of my 56 Buick. I have a 1" master cylinder and I am aiming for a 6:1 pedal ratio. I will also need 1.5" of push rod travel to bottom out my master cylinder. To do this and clear the firewall/floor I am going to have to make the pedal arm bent. How does one measure the pedal ratio on a bent pedal? I have attached a picture of a pedal I cut out of cardboard that will give the proper push rod travel, clear everything, and hopefully give me the correct pedal ratio.

I figure the pedal ratio is 5.7:1, am I correct? The top hole is for the push rod yoke and the middle hole is for the pivot. To get the ratio I just measured the straight line distance from pivot hole to the end of the pedal (10") (will be the middle of the brake pedal pad) and divided it by the distance from the top hole to the pivot hole (1.75") giving me 5.7. Is this the correct way of measuring?

Thanks,
Ryland

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2. ### EDPACECARMember from POTTSTOWN, PA.

Formula for Pedal Ratio:

Pedal ratio is the ratio of leverage you brake pedal applies to the master cylinder. To determine the pedal ratio you need to measure the height of the pedal to the pivot point then divided the measurement of the pivot point to the lower arm that controls your rod to the master cylinder.

A = height of pedal
B = center to center measurement of the lower arm
C = pedal ratio
A divided by B equals C
Or example 9" divided by 1.5" equal 6 to 1 ratio

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4. ### FendersMember from 2,800 miles from Bisbee

(1) I believe you are correct in calculating the ratio, and
(2) I my opinion 6:1 is fine (FWIW)

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5. ### 3spdMember from Portland, Oregon

Correct me if I am wrong but how I am thinking about this is that it all comes down to torque, and boiled down torque is force times radius. It doesn't matter what the pedal does (if the pedal does not flex) in the end the radius from the point of force to the pivot is what matters. This also means that it does not matter if the push rod side and the pedal side are in line with each other because the are separate systems tied together by the central pivot. As long as the pivot is solid it will transfer the force equally.

Thanks,
Ryland 