On The Tree
Join Date: May 2003
Location: Apache Junc, Az
| | Re: Brake question
Since I have been attending more and more driving events, one comment I have heard many times is that 'this car has huge brakes on it and wow, does it stop quick'. It seems that next to horsepower, large calipers are next in line for must-have modifications. The popular thinking is that the bigger the brakes, the faster it will stop.
I've always had my doubts about this but now thanks to an article by physicist Brian Beckman, titled 'The Physics of Racing,' I have some authoritative back-up. Excerpts from Brian's article state. . .
"Do brakes dissipate energy at a constant rate? My guess as a physicist is 'probably not.' The efficiency of the braking process, dissipation, will depend on details of the friction interaction between the brake pads and disks. That interaction is likely to vary with temperature. Most brake pads are formulated to grip harder when hot, but only up to a point. Brake fade occurs when the pads and rotors are overheated. If you continue braking, heating the system even more, the brake fluid will eventually boil and there will be no braking at all. Brake fluid has the function of transmitting the pressure of your foot on the pedal to the brake pads by hydrostatics. If the fluid boils, then the pressure of your foot on the pedal goes into crushing little bubbles of gaseous brake fluid in the brake lines rather them into crushing the pads against the disks. Hence, no brakes."
We now arrive at the second way of looking at this problem. Let us assume that we have good brakes, so that the braking process is limited not by the interaction between the pads and disks but by the interaction between the tires and the ground. . . .
"The assumption that the brakes are better than the tires, so long as they are not overheated, is probably right, and the assumption that brakes dissipate energy at a constant rate is probably wrong because it leads to the conclusion that braking takes more time than it actually does. You see, how fast you stop is determined by your tires, not your calipers. For one minute forget about threshold braking and think about braking with all four wheels locked (and with no directional control). Most any car can do this (that is, lock-up all four wheels). Once this happens you have shown that your brakes have a higher grabbing power than your tires. Your wheels have stopped moving and that is all your brakes can do. Now your stopping distance is a function of the friction between your tires and the road surface. A higher adhesion track or a stickier tire compound will shorten your stopping distance and nothing else."
Well, lightening your car or changing the weight balance will also affect it, but that's a different direction.
"Then why do the newest cars stop faster? Check out the tire and wheel combinations and you'll see why. ABS also plays a key roll in a non lock-up situation. Take a new 911 Turbo and disable the ABS and then put the same tires and rims on a 1969 911S, run them both up to the same speed and lock-up the brakes and the 1969 911S will stop quicker due to its lighter weight."
Big brakes be damned.
If what is said is true, you ask, then why the big brakes on the new cars and on all race cars? Good question. The answer lies in the first paragraph of Brian's article. What hasn't been taken into account is the heat generated by the brakes in the process of stopping the car. If we go back to the 911 Turbo vs. 911S example and keep repeating the test, the brakes on the 911S with their lower surface area and poorer heat dissipation qualities would start to fade and soon you could not lock-up the brakes and therefore couldn't stop as quick (soon you couldn't stop at all!).
One thing that fools a lot of people too is the feel of bigger brakes. Because bigger brakes can give a much bigger bite with less effort, they appear to brake better. Did you ever stall a car without power brakes? It seems like you'll never stop but the brakes are just as good, you just have to really PRESS. Many people just don't press hard enough on the pedal to approach threshold braking.
Another factor that can play a big part is the brake balance. In most rear wheel drive cars the balance is biased toward the front to prevent rear lockup. If, however, you take full advantage of the front/rear weight bias and increase the front/rear braking forces to better match the car's weight transfer, your braking will improve. If one caliper setup is naturally sized to better take advantage of this, the braking performance will appear to be better. By simply adding a brake balance adjuster, things will again be equalized.
So folks, we have discovered the true reason you may need bigger brakes. If your car is driven really hard, such as in track events, and you encounter brake fade, you need to get better heat dissipation for your system so your fluid doesn't overheat or your pads don't go off. To correct this you can use a better brake fluid, get effective brake coolers, get better pads, or (finally) get BIGGER BRAKES.
So, the next time someone with the same tires as you with the big red calipers brags about how fast he can stop, you can just smile to yourself and brake with him into any corner, but maybe just not as often.
One last comment. If you take the proper route to stopping quicker and go with better/wider rubber, just remember this will put an additional strain on your existing braking system and you may (make that will) experience fade sooner. So be really careful on those long hard runs until you know your system can handle it. Here's to better braking.