## Braking Systems…in Plain English

### Article Index

by James Walker, Jr. of scR motorsports
...as published in Grassroots Motorsports, Dec 2000

Back in February, an article describing brake upgrades made to the GRM project Ro-Spit introduced a number of basic braking concepts and reviewed several brake component upgrade possibilities that are available to racers and enthusiasts. However, before any of us go running off to the aftermarket for our own NASCAR 6-piston calipers, F1 carbon-fiber rotors, and 50 feet of stainless steel braided brake lines, it would be wise to take a deeper look into braking systems. We just might find that once we gain a fundamental understanding of what each of these components really does (and more importantly, what each does not do), we will be better prepared to make the right decisions when modifying (or choosing not to modify) our own rides.

What Do Braking Systems Really Do?

Although Tim touched on this critical fact briefly in his story, this is one of those statements that every racer should be forced to write on the blackboard 1,000 times:

Your brakes do not stop your car. (999 to go - start writing…)

Of course, the next question is: "What DO your brakes do?" In plain English, your brakes convert ‘the energy of motion’ into ‘heat’ An engineer would say the brakes are responsible for turning the kinetic energy of your speeding car into thermal energy (see Sidebar 1 – Speed vs. Heat for more details). But in either case, your brakes are not stopping your car. Surprised?

Sidebar 1 – Speed vs. Heat

Just for fun, let’s have a closer look at the equations for the conversion of kinetic energy (KE) into heat (Q):

KE = ( ½ ) x (vehicle weight) x (speed of the vehicle)2

Q = (rotor weight) x (rotor material constant) x (temperature rise)

Since Mr. Isaac Newton stated that energy can be neither created nor destroyed, all of the KE from the car must be completely turned into Q. What does this tell us? KEbefore = Qafter!

Now, if we assume that…

1. the weight of the car stays constant with us
2. the weight of the rotor remains constant with use
3. Newton was right

…then the ‘speed of the vehicle squared’ is proportional (directly related) to the ‘temperature rise in the brakes’. If you pull out your calculator, you can prove that if the speed at which the brakes are used increases by 40% (60 mph vs. 95 mph, for example), the temperature rise in the brakes resulting from that stop would increase by nearly 100% {1.4 x 1.4}!

This factor is often overlooked by racers who add horsepower to their cars and can’t figure out why their brakes don’t seem to work as well when they ‘only gained 10 mph’ on the straights. Lesson learned: small changes in speed can have a huge impact on brake temperatures!

So what DOES stop the car? Good question. There are many "things" that can stop your car– and several of them have nothing at all to do with your braking system. We all have experienced this first hand as we let off the accelerator pedal and feel the vehicle begin to slow - even without stepping on the ‘pedal in the middle’.

In theory, any "thing" that can generate a force which opposes the motion of the car (like the wind pushing on the front of the car or gravity as the car climbs the hill) will eventually stop the car; however, there are often times that we need to slow at a greater rate than what headwinds and gravity can deliver. In these cases, we depend upon the brakes to ‘assist’ in the stopping process.

The next logical question then would be, "how do the brakes ‘assist’ in stopping the car?" To answer, we need to look at each of the pieces of the braking system puzzle.

The Brake Pedal
Most GRM readers are probably somewhat familiar with the brake pedal. But while most of us probably think of the brake pedal only as the flat part that makes contact with the foot, remember that an equally important part of the brake pedal assembly, the output rod, continues out of sight. Together, these parts compose the brake pedal assembly.

The sole function of the brake pedal assembly is to harness and multiply the force exerted by the driver’s foot. It does this thanks to a concept known as "leverage." We all learned the concept of leverage on a teeter-totter – the farther you sit from the middle (the pivot), the more weight you can lift on the other end. In the case of the brake pedal assembly, the pivot is at the top of the brake pedal arm, the pad (where we step) is on the opposite end, and the output rod is somewhere in between. In the example illustration (figure 1), a driver input force of 90 pounds is multiplied by the 4:1 ratio into 360 pounds {90 lb. x 4} of output force.

Does the output rod directly stop the car? No. So a) would we want to make any changes to the brake pedal, and if we did b) how would this impact the brake system performance?

There are several answers, each with their own set of pros and cons:

1. Increasing the ratio (8:1, for example) would further amplify driver input force, but would make the pedal travel through a longer distance to achieve the same output. In the given example, the 90 pound input would generate 720 pounds output, but with twice the pedal travel.
2. Decreasing the ratio (3:1, for example) would reduce the overall size and weight of the brake pedal assembly, but would decrease the amount of amplification – the 360 pounds in the example would fall to just 270 pounds. To generate the same 360 pound output, the driver would need to press the pedal with 120 pounds of effort!

So, will changing the brake pedal make the car stop any faster? Not by itself. But one can tune the pedal output force and pedal travel characteristics by making changes to the pedal ratio.