How To Measure Brake Pedal Ratio
(Editor’s note: This article on how to measure brake pedal ratio is meant for skilled readers who are familiar with modifying or customizing their own vehicles.)
When the brake pedal (or clutch pedal) is too hard to push, driving your car becomes not only difficult, but downright dangerous. Disc brake conversions are great, but even converting to a power-assist braking system can alter the feel of the brake pedal, and not all vehicles are capable of using power brakes, so ensuring that the pedal ratio is correct is paramount to practical and safe operation of the vehicle. This is a common theme for customized cars and race cars, one that is too often ignored.
A common misunderstanding with master cylinders is selecting the right one. The main issue with manual brake master cylinders is that the larger the cylinder bore, the harder it is to press. This is due to the fact that there is more fluid in front it. The rule of thumb for manual brakes is no larger than 1”, with 7/8” being optimum for factory-type master cylinders. Power-assist master cylinders have a lot of help to move that piston, so they use a larger cylinder bore. Do not use a power master cylinder in a manual application.
Even with the correct master cylinder, the brake pedal ratio is the biggest factor in pedal effort. Pedal ratio difference in length between the pivot (fulcrum) of the pedal to the pushrod hole (Y) and the fulcrum to the center of the brake pedal (X). A power system should have a ratio between 4 and 5:1, where a manual system should be between 5 and 7:1. Consider this- a master cylinder with a 1-inch bore and a brake pedal ratio of 6:1 with 100 pounds of pedal pressure yields 600 pounds of pressure at the master cylinder. Cut that brake pedal ratio to 4:1, and the pressure at the master drops to just 400 pounds with the same effort, that is a significant difference.
This issue became apparent on a 1963 Buick Le Sabre project. The car had a single-bowl master cylinder with a power brake booster, but when it was converted to front disc brakes, it needed a dual-reservoir master cylinder, and the power brake booster was not useable, so it was swapped to a 1” bore manual brake master cylinder. The brake pedal was rock hard, to the point of not being drivable. The pedal was measured in the car, and the “Y” measurement (fulcrum to pushrod) is 5.5 inches; while “X” (fulcrum to center of the pedal) is 14 inches. That is just a 2.54:1 brake pedal ratio, which is ridiculously low for a power system, much less a manual master cylinder. The biggest problem that we face on this car is the master cylinder placement, which is too low on the firewall. The pedal itself is just an inch off the floor of the car at its lowest point, so we can’t just make it longer. Major surgery is required, but we need to make sure it will work before we cut.
In order to increase the brake pedal ratio, a new hole must be drilled in the pedal arm. To avoid a hard angle on the master cylinder pushrod, the master cylinder placement must change. In this case, the master cylinder needs to move up 3 inches on the firewall, any higher and it would hit interior structures that can’t be moved, so we drilled new holes through the firewall and support brackets. We also fabricated a support plate to relocate the master cylinder.
The pedal was removed from the vehicle for modification. Our target ratio is 7:1, however the limits of the pedal length and master cylinder placement would not allow for it. To maximize the pedal length, the pad was cut off, moved down 1-inch, to the top of the pedal and welded in place, along with a gusset for strength. This increased the overall X length to 15. The pushrod hole was placed 2.5 inches from the center of the fulcrum, which yields a brake pedal ration of 6:1.
The new placement of the master cylinder required making new hard brake lines for the brakes. With that completed and the brakes bled, we test drove the car. Unlike the previously impossibly hard brake pedal, the wagon now has a much softer pedal that actually stops the car without needed to use both feet and all nearly break the seat. If your custom car has a hard brake pedal and you just can seem to figure out the solution, take a few measurements, the brake pedal ratio may be the culprit.
Check out all the brake system parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to measure brake pedal ratio, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
Photo courtesy of Wikimedia Commons.