About Brake Power
Ones up on a time (many years ago)..., I was selling brake pads, rotors and other brake parts at race tracks. When calculating dimensions for brake system components, I used pen, paper and calculator. To do this faster and more reliably, I created ABC (Automotive Brake Calculator) that you see on the home page of this site.
Soon I realized that ABC could be a useful tool for you as well, so I thought about 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 brakehorsepower; see below), I thought BrakePower.com would be a good name.
After selling brake pads and related parts for more then ten years, 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.
Enough about the past. Here is the (fun) 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 
= 
0.002667Wd_{max}S (or in SI units: P_{b} = m_{c}a_{max}V_{c}) 



0.002667 
= 
constant factor to convert the metric SI units into Standard units 
W 
= 
Weight of car in pounds 
d_{max} 
= 
maximum deceleration in g's 
S 
= 
Speed of the car in miles per hour 



P_{b} 
= 
Power of brakes in Watts (= J/s = Nm/s = kgm²/s³) 
m_{c} 
= 
mass of car in kg 
a_{max} 
= 
maximum deceleration in m/s² 
V_{c} 
= 
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) ; d_{max} = 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) ; d_{max} = 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.