So why do car brakes lose their effectiveness when they get hot?
I’m not sure if this is the answer you’re looking for, but if they get hot enough it can warp the rotors, causing a pulsing/pumping feeling when you have your food on it. It’s not like the pulsing of anti-lock breaks, it’s much smother, think sine wave. I’ve always wondered how much less effective warped rotors would be under normal breaking circumstances.
No, I mean the fact that they just don’t seem to grab any more and slow the car.
Could it be that they’re so hot that they can’t absorb any more heat, and therefore can’t convert any more kinetic energy?
It makes things worse when your peas roll off your plate and onto the aspalt:p
Higher temperatures encourage surface glazing and slip. Heat also promotes the shift of dimensions.
The easy answer is hot brakes work. Just maybe not as well.
Think of your brakes as energy dissipation devices. When you first stomp on the brakes the rotor can absorb the energy of braking by having its own temperature raised. As long as the braking is intermittent the surrounding airflow is sufficient to keep the rotor at ambient temperatures.
Braking effort is a result of normal forces on the braking surfaces. Normal force is a phyics concept for the force generated at a right angle to the related surface. This might be a brake rotor, or a brake drum, or the ground, if you’re the Bionic Woman. The normal force times the coefficient of friction gives the braking effort.
The bitch about hot brakes is that you’re changing that coefficient of friction. Hot, and I mean really hot, brake shoes have different properties and thus different coefficients of friction.
For fun you can watch a Formula One race on an overcast day. These guys use carbon shoes and rotors and when the hit the brakes both glow bright orange with the energy of braking.
FWIW
Excessive heat “thins” the brake fluid making it progressively less effective as a hydraulic fluid. In a worst case scenario the fluid can boil, effectively leaving you with no brakes. In this case the emergency (manual) brake and engine friction are your only means of stopping.
Decreased effectiveness of hot brake coverings and the risk of the brake fluid boiling are only part of the answer.
Basically, there are two kinds of brakes used in modern cars (there may be more, but these are most common):
- disk brakes, where two brake shoes press against the brake disc from both sides: For this kind of brake, heating the whole thing will deteriorate the effectivity of the brakes just as described above, but nothing more.
- drum brakes, where the brake shoes are pressed against the inner wall of a cylinder (“drum”): If this cylinder expands due to heat, there will be a point where the brake shoes will not touch it at all, leaving you with no brake. When this happens, not even the hand brake will work, because it uses the same drum brake (just triggered by a linkage instead of a hydraulic connection). This is possibly what you meant.
Most modern cars have at least two disc brakes (at the front wheels), but they are more expensive than drum brakes.
Be aware that while passenger car brakes, which have to work when cold, are susceptible to fading when hot, race car brakes are designed to work when hot, and don’t do well when cold.
This site answers the OP question pretty well: http://www.elephantracing.com/techtopic/brakefade.htm
This one also addresses it in the “High performance brake pads” section: http://www.precisionbrakescompany.com/why.html
All modern cars have had front disc brakes for decades.
Surely correct for middle to upper class cars, but there are some low-priced cars that have all drum brakes. For now I have only one cite: http://www.citroen.mb.ca/citroenet/html/number/2cv-1a.htm
Anyway, from my memory I’d say that there are even some newer cars to which that applies (2CV is from 1964 after all). I’ll keep searching.
MartinL, I just noticed you’re in Europe. There may well be some more recent vehicles there that have front drum brakes. I was thinking of the U.S., where the last cars with front drum brakes were from the mid-70’s. All passenger cars sold in the U.S. have had front disc brakes for well over 20 years.
There are 2 kinds of brake fade- pedal fade and pad fade.
Pedal fade is when the brake system temps get so high that the fluid boils- making the pedal have more give and a spongy feel. The boiling point of most common fluids (DOT 3) start around 400 , but drop significantly as moisture is absorbed(down to a min wet boiling point of 284) , so change your fluid fairly often!
Pad fade is when the CF of the pads goes down because of heat- the pedal stays firm, but you don’t slow down much. This is basically when the pad materials start melting and acting more as a lubricant instead of friction materials.
Some high performance brake pads actually have an minimum temperature range somewhere in the 200-300 range, meaning that they won’t work worth a damn unless they’re heated up to that temperature beforehand. However, once at that temperature, they’re good up to 1200 or so.