About Brake Power
Ones up on a time (in the 90's) I was making a living selling brake pads, rotors and other brake parts at race tracks. When calculating dimensions for custom brake system components, I first used pen and paper. To do this faster and more reliably, I created this ABC (Automotive Brake Calculator) that you see on my home page.
Realizing it could be a useful tool for you as well, I had to come up with a name for the web site to make it available. Since the power of brakes is hugely underestimated (the brakes of an average Ford F150 for example, can generate over 1000 brake-horsepower), I thought of BrakePower.com.
After selling brake pads and related parts for almost a decade, I quit traveling from race track to race track in exchange for a steady mechanic job. Do I miss "the good old days"? Sometimes, but a regular paycheck throughout the year is nice too. UPDATE: I retired from my job on 2/5/2017, but working on brake systems is still one of my hobbies.
Enough about the past. Here is the theory behind brake power:
If a brake (rotor clamped by a brake caliper with brake pads) is driven by an engine via a dynamometer, this dynamometer shows that power into the brake equals power output of the engine.
The speed (kinetic energy) of a moving car represents energy that originally came from the engine. This kinetic energy, when converted by the brake system into thermal energy (heat), can be expressed in horsepower with the following formula:
| Horsepower |
= |
0.002667WdmaxS (or in SI units: Pb = mcamaxVc) |
| |
|
|
| 0.002667 |
= |
constant factor to convert the metric SI units into Standard units |
| W |
= |
Weight of car in pounds |
| dmax |
= |
maximum deceleration in g's |
| S |
= |
Speed of the car in miles per hour |
| |
|
|
| Pb |
= |
Power of brakes in Watts (= J/s = Nm/s = kgm²/s³) |
| mc |
= |
mass of car in kg |
| amax |
= |
maximum deceleration in m/s² |
| Vc |
= |
Velocity of car in m/s |
Let's take the most sold truck and car for example and assume some numbers:
Late model F150: W = 6500lbs (GVWR) ; dmax = 0.75g ; S = 100mph (maximum speed)
»»» maximum possible brake power = 0.002667 x 6500 x 0.75 x 100 = 1300 brake horsepower
Late model Corolla: W = 3800lbs (GVWR) ; dmax = 0.8g ; S = 120mph (maximum speed)
»»» maximum possible brake power = 0.002667 x 3800 x 0.8 x 120 = 973 brake horsepower
Of course these values will become quickly less impressive after the first hard stop, due to fading of brake lining and sooner or later boiling brake fluid. To see how to take care of these problems, go to "Deceleration Optimization".
Another way to illustrate the power of vehicle brakes is comparing the distance (or time) it takes a vehicle to reach a certain speed, versus the distance (or time) it takes to come to a full stop from that speed. For this example we run the quarter mile (1320 ft) and reach 75 mph. Stopping the vehicle from this speed took 220 ft. (even less if the tires had more grip!). So if we divide 1320 by 220 we find that it took 6 times more distance to reach 75 mph than to get rid of this speed. Of course air drag, rolling resistance and friction forces in the engine/drive line and other parts contribute to the fact that it is harder to get up to speed than to get rid of it, but still, this example illustrates that the brakes are (usually) much more powerful than the engine.
