Brake Pedal Ratio

[EngineerDoug]

Hey guys,

I am converting my stock manual braking system to a power system. I have installed a complete kit, but am puzzling over the pedal ratio. I understand the concept, but there are a couple of details I hope you can clear up for me:

1) There is a linkage that connects the rod that goes into the power booster to the actual brake pedal - the correct term might a bellcrank. Because of this mechanism, a movement of 0.1" of the pedal rod causes a movement of 0.2" of the booster rod....this would effectively cut the overall pedal ratio in half. My first question is, does the desired pedal ratio (4:1 for power brakes) take this into account? Or is the pedal ratio simply this:

Distance from pedal to pedal pivot/distance from pedal rod hole to pedal pivot

2) So let's say I need to reduce my pedal ratio, because I had manual brakes before. I can't move the hole in the pedal arm very far without altering the angles of the bellcrank. So how does Mopar differentiate between power and manual brake pedal arms? Are the holes in the same locations but just different lengths to the actual pedal?

Sorry if this came out convoluted, but I am wondering if the correct way is to just find a power brake arm with the correct ratio - I presume it would be shorter overall?

Thanks.

 

[Yatzee]

Hey guys,

I am converting my stock manual braking system to a power system. I have installed a complete kit, but am puzzling over the pedal ratio. I understand the concept, but there are a couple of details I hope you can clear up for me:

1) There is a linkage that connects the rod that goes into the power booster to the actual brake pedal - the correct term might a bellcrank. Because of this mechanism, a movement of 0.1" of the pedal rod causes a movement of 0.2" of the booster rod....this would effectively cut the overall pedal ratio in half. My first question is, does the desired pedal ratio (4:1 for power brakes) take this into account? Or is the pedal ratio simply this:

Distance from pedal to pedal pivot/distance from pedal rod hole to pedal pivot

2) So let's say I need to reduce my pedal ratio, because I had manual brakes before. I can't move the hole in the pedal arm very far without altering the angles of the bellcrank. So how does Mopar differentiate between power and manual brake pedal arms? Are the holes in the same locations but just different lengths to the actual pedal?

Sorry if this came out convoluted, but I am wondering if the correct way is to just find a power brake arm with the correct ratio - I presume it would be shorter overall?

Thanks.

Pedal ratio is calculated from the top pivot point center of the pedal arm to the center of the attachment point of pushrod further down the arm. This is a basic 2nd Class lever calculation. The second calculation of the ratio was the distance from rod attachment center to the actual foot pedal. But you must also keep in mind the distance the pushrod travels and it's swing. As you push the pedal, the pushrod rises somewhat and in extreme cases binds somewhere down the rods length or at the booster/master unless you use a swing bellcrank mounted to the firewall behind the booster. Ideally, the pushrod rises and drops in the same amount of degrees from center when the pedal is pushed. So let's say, when the pedals at rest the rod is 5 degrees higher off lateral center, and when the pedal is fully depressed, the rod drops 5 degrees off lateral center. This would be ideal. Measure the stroke of the pedal at the mounting hole for the pushrod starting at full up to where the peddle is about 1 inch off the floor above the carpet. You have the overall stroke. To get the stroke length and the mounting point for the pushrod exact you may have to weld a tab on the back of the pedal arm (or not). In all cases I prefer a swivel rod end at the attachment point for the pushrod to the pedal arm. You can buy a kit that contains adjustable threaded pushrods and a swivel end to make installment easier and more exact. Two cars I've done previously used the exact same hole in the pedal arm that was factory. One car was from manual to power brakes, the other power to hydro. I believe Chrysler used similar arms in some cases and just adjusted the mounting method and distance on the engine side of the firewall. Under no circumstances allow the pushrod to travel forward laterall a greater amount than the stroke of the master or master/booster. Bottoming out at the master is bad.

 

[wyrmrider]

just try it before you make any changes
My disc brake conversion works fine with manual drum master cylinder and linkage
changing the bore on the master cylinder is easier than changing mechanical linkage
if your rear tires slide on hard test stop put in smaller wheel cylinders if drums in rears
otherwise adjustable proportioning valve where you an reach it from under front of your seat

 

[5.7 hemi]

If your brake pedal has 2 holes, use the bottom for power and the top for manual. Manual is 6:1 and power is 4:1.