Brake Ratio

Figuring out pedal ratio and master cylinder bore size                                                                                    
Wayne Scraba /

One area hot rodders, racers and custom car (and truck) builders tend to ignore is the brake master cylinder and, in particular, the actual brake pedal ratio. After all, it doesn’t make the car one bit quicker or faster, and if the thing eventually stops, why worry? Perhaps you should.


Pedal Ratio
The critical component in the braking equation is the pedal ratio. In operation, the brake pedal acts as a lever to increase the force the driver applies to the master cylinder. In turn, the master cylinder forces fluid to the disc brake caliper pistons or drum brake wheel cylinders. If you examine a brake pedal, you’ll see the pivot point (where the pedal swivels) and the mounting point for the master cylinder pushrod are usually different. By varying the length of the pedal, and/or the distance between the pushrod mount and the pivot, you can change how much force (from your leg) is required to energize the master cylinder. This is the “mechanical advantage” or pedal ratio. This formula will help you figure it out: Input Force x Pedal Ratio ÷ Brake Piston Area = PSI.


Mathematical babble? The arithmetic simply equates to the amount of force exerted by your leg times the pedal ratio divided by the area of the brake piston(s). FYI, the typical adult male can exert roughly 300 pounds of force (maximum) with one leg-and that’s a bunch. Something in the order of 1/3 or 1/2 that figure is obviously more comfortable, even in a hardcore racecar.





The average manual (non-power boosted) master cylinder requires somewhere between 600-1,000 PSI to be totally effective. Somehow, 100-150 pounds of leg force has to be translated into 600-1,200 PSI. The way it’s accomplished is by way of pedal ratio. While changing the overall length of the pedal is possible, it’s often easier and far more practical to shorten the distance between the pivot point and the master cylinder pushrod mount location. That’s precisely how many racecar chassis shops modify brake pedals.


Brake Line Pressure

Brake line pressure is a different thing than the force you apply to the pedal. Force acts in one direction and is addressed in pounds. Pressure acts in all directions against surrounding surfaces and is addressed in pounds per square inch or PSI. “Levers” (brake pedals) can be used to change the force. Inside the hydraulic system, the surface area of the piston is what is affected by pressure. Decreasing the bore size of the master cylinder increases the pressure it can build. Pistons in master cylinders are specified by bore size. But there’s a hitch: The area of a circle (or bore) is Pi-R-Squared. The area of the piston surface increases or decreases as the square of the bore size or diameter. For example, the area of a common 1-1/8-inch master cylinder is approximately 0.994-inch. The area of an equally common 1.00-inch bore master cylinder is approximately 0.785-inch. Switching from the larger master cylinder to the smaller version will increase the line pressure approximately 26.5% assuming that pedal ratio hasn’t changed.

As the pedal force or the pedal ratio (or both) is increased, the stroke of the master cylinder is shortened (brake line pressure is unaffected). When the size of the master cylinder piston increases, the output pressure of the master cylinder decreases. A smaller master cylinder piston will exert more line pressure with the same amount of force (pedal ratio) than a master cylinder piston with a larger piston area. There’s another catch: Since the brake line fluid pressure is working against the surface of the wheel cylinder (or disc brake piston), increasing the area of the cylinder will increase brake torque.


Improved Braking

The bottom line is, if the stopping power of a car needs improvement, or if there’s a need to reduce the pedal effort, several options are available: (1) Decrease the master cylinder bore size; (2) Increase the pedal ratio; (3) Increase the wheel cylinder bore size. If the pedal ratio is increased, there will be more travel at the master cylinder piston. If the master cylinder bore size is decreased, the piston has to travel further to move the same amount of fluid. Typically, a master cylinder has approximately 1-1/2-inch to 1-3/4-inch of stroke (travel). The idea here is coordinate the pedal ratio with the bore size to arrive at approximately half of the stroke (roughly 1-inch) in order to make the brakes feel comfortable, and of course, to bring the car to a grinding halt.


Resources: Mark Williams Enterprises, 765 South Pierce Avenue, Louisville, CO 80027, (303) 665-6901 Copyright 2000-2012



Real Life Mach 5!


Red Stripe Photography

Anyone growing up in the 80’s knows … Saturday morning cartoon were like the best thing ever!  I would wake up at 6am on Saturday morning, walk to the kitchen and grab a bowl, pour my milk and then I get to decide which cereal would taste best in my belly that morning.  It was almost always Coco Pebbles my favorite, because when you’re done eating cereal you had a bowl of chocolate milk.  How awesome is that!   Once I got my breakfast I’d sit in my spot on the sofa, flip on the telly and Saturday morning cartoons were on…instant heaven for a kid.
Some of my all-time favorites were Voltron, He-man, Thunder Cats, GI Joe and Speed Racer. If you grew up in the 80’s you definitely know the cartoons I’ve just listed and now you’re probably thinking of your own favorite shows, am I right?  
Speed Racer…

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Woodward Dream Cruise: Mercury Custom Street Rod

Check out this awsome piece of art

Mind Over Motor

It takes a special sort of car to pull off the color purple, and this does it as well as any of them. This beautiful Mercury custom was sitting at the Burger King on Woodward Ave during the Dream Cruise this summer. I am a huge fan of this type of car, so I am happy to add this to the features we have done on Sh’Boom and others. I wasn’t able to talk to the owner of this car so I really don’t know anything about it. I sure enjoyed seeing it though. Enjoy.

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SEMA 2012 Coverage…Part 7 of 7…Finale…

the rat rod look with the age engines under the hood

The Chronicles© - No Equal Since 2008 |

Alas we are at the final phase of our 2012 SEMA coverage…

SEMA is a blast every single year and it’s one of those events that I always make sure to mark down on my calender. What is great about the SEMA show is that it helps open my eyes to a lot of different styles of building as well as vehicles of all makes and models. While I’ll always be an import guy that is into Hondas and all other Japanese makes, it’s very interesting to be able to be in the presence of some really amazing domestic and European builds. It never hurts to expand your horizons and the diversity is what makes SEMA so enjoyable. My hope is that you’ve gotten that same type of feel from going through all 6 (now 7) parts of my coverage and you’ve learned a little something along the way. I…

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