Why Are Hot Brake Pads Harder to Manage?

[kurious]

Why is it harder to slow a car down when the brakes are hot?
Is there less friction between pads and disc?
Is it harder to remove bits of pad material which can carry away kinetic energy?

 

[Nenad]

when the brakes are hot, the pad expands because of the heat, making the little cravaces on your pad move away from each other leaving larger gaps between them and causing less friction when you slam on the brakes.

 

[Erienion]

If it was the case that the thermal expansion of the pad was significant at the temperatures it gets up to during use, wouldn't the area of the pad in contact with the brake dick also scale up with the size of the gaps?

Does having heated brakes always make it harder for stopping, i thought that racing cars pre-warm their brakes and tyres in the warm up before a race to improve the performance?

 

[BobG]

The primary reason brakes don't work as well when they're hot is because of they are heating the brake fluid. I'm not sure how much effect heat has on the brake pad material, but it's not the limiting factor (although, finding an alternative for asbestos in brake pads was an issue at one time, so break pad heating is at least something that must be dealt with). I couldn't tell you exactly what happens to the fluid when it becomes too hot.

Racing cars warm up their tires, not their brakes. Their brakes are much more heavy duty than the average family car, primarily to offset the fact that they need to provide extra surface area on the piston mechanism (the thing that squeezes the pads closed) to cool the brakes.

 

[Nenad]

Does having heated brakes always make it harder for stopping, i thought that racing cars pre-warm their brakes and tyres in the warm up before a race to improve the performance?

Recing cars wet and heat up their tires in order to get more friction with the ground in order to get a faster takeoff. Once the tire is heared, the rubber expands giving more surface area for the wheels.

 

[Cyclotron Boy]

Brake fade

When the brakes get too hot, the fluid indeed gets too warm. What happens? It boils! The brake fluid begins to boil, and gas bubbles form in the hydraulic lines. As anyone who has ever changed brake pads on a car can tell you, a hydraulic brake line with bubbles in the fluid can't make any pressure on the pads. Basically, the hydraulic pressure is wasted compressing the gas bubbles instead of clamping pads against the brake rotor. Check out the CarTalk website for more info.

-F

 

[Stingray]

There are a lot of misconceptions here. I invite anyone who thinks the pads don't care about temperature to put real racing brake pads on their regular car and go out for a drive. You might want to stock up on insurance first.

The car will not stop unless you stand on the pedal. Racing pads are designed to work at high temperature, and will not grip at all until they are warmed up to several hundred degrees.

Brakes can be operated over a very wide temperature range, and there are no materials which are sticky throughout that range. Chemical changes will occur within the pad to change the friction coefficient. This is why you should buy a pad for your car's intended usage. A street car will have pads that work at very low temperatures, but the tradeoff is that they will start to lose grip once the brakes become hot enough. Most forms of racing are unconcerned with cold grip, so their useful temperature band is shifted up.

Brakes can also fade due to fluid boiling, but this is usually less significant on a properly designed and maintained system. You can usually feel the difference also. Fluid fade makes the brake pedal mushy, whereas pad fade will just require a lot more pressure (but the pedal will still be hard).

 

[kurious]

Does an application of the brakes cause a tyre to exert a greater downward force on the road as though the car has got heavier?

 

[Stingray]

The front will get "heavier" and the rear will get lighter. This is why most of the braking force is distributed to the front wheels (except in cars with most of their mass in back).

 

[kurious]

Do any cars spray cold air over the brakes for long journeys downhill?

 

[Erienion]

When the front brake of a car is applied, there is a force acting on the tyre where is touches the road. This creates a torque on the cars center of mass which has the effect of lifting the back of the car a bit and increasing the amount of weight on the front tyres, which in turn means better friction there.

This is why the brakes for stopping a car are on the front tyres. If you pull the handbrake up while the car is driving, the torque that the car applies is much less and so the wheel just loses traction and you just do a big slide out

 

[Stingray]

Most high performance cars that have ducts specifically to channel air onto the brakes.

Additionally, most cars have rotors with vanes in the middle that are shaped in such a way that the disc's rotation forces air through it.

Even so, any car's brakes will eventually overheat if you abuse them enough.

 

[kurious]

Could a road surface with an intricate pattern like a car tyre pattern slow the car down better in dry conditions or is a flat road surface best?

 

[Stingray]

A lot of a tire's grip is derived from the rubber "moulding itself" into the little bumps in the road, so I think that some roughness helps. I don't know that any particular pattern would be beneficial though (at least in dry weather).

 

[kurious]

Is steel the best performance material for a building a tyre or is it just "good enough and cheap."

 

[Stingray]

You mean the steel belts? I think that it is a case of being "good enough," but I'm not sure.