Why temperature and speed affects braking force?



Planes and cars have problems braking if brakes are too hot. F1 teams complain also if brakes are too cold. What’s the reason behind so much variance?

Also, I have noticed, that basically any vehicle have smaller deceleration when moving fast than when moving slow. From elementary school physics – when I apply some force onto surface, I should get some amount of friction, so I don’t know why it changes. It this also related to the temperature, or is there unrelated mechanism behind it?

In: Physics

Elementary physics is idealized. It typically assumes that the coefficient of friction is constant. In real life, the friction coefficient will change depending on temperature.

The brakes are designed to operate within a certain safe temperature window and typical driving. For cars (not sure about planes), the brakes are NOT designed for repeated high braking operations done frequently. Doing this overheats the brake pads (boils the brake fluid too) etc and causes the brakes to fail or degrade.

F1 cars, on the other hand, are designed to have repeated braking. The compounds used are most effective when the brakes are hot (much hotter than road cars) but conversely they operate poorly when below a certain temperature.

You are almost certainly incorrect. Braking will cause more deceleration from a high speed compared to low speed. This is because the air resistance/drag at high speeds is much greater and this contributes to the car slowing down. But most drivers are conditioned or trained NOT to stamp on the brakes at high speeds because it results in the car sliding. So while it probably “feels” as though brakes work less effectively at high speed – that is related to the driver and not the physics.

So for your first point, the reason brakes become weaker if they’re too hot is that as materials get hotter they generally become softer which makes them worse as brakes. F1 brakes can reach temperatures as high as 1000C, and if you were to heat a regular steel brakes to that temperature it becomes so soft to be effectively useless (F1 cars and other high performance cars use variants of carbon brakes which work at higher temperatures).

For your second point, brakes convert the kinetic (movement) energy of the car into thermal (heat) energy. Brakes convert the energy at a fairly constant rate assuming they’re not too hot/cold. However kinetic energy is to the square of its speed, meaning an object travelling twice as fast will have 4 times the kinetic energy. So the brakes will have to get rid of 4 times the energy, meaning it will take 4 times as long, even if it’s only going twice the speed.

Planes and cars normally brake from a cruise. The brakes are cold when they start working, and the material they use needs to have high friction from cold for the brakes to work. The downside is, that the material that can brake from cold starts to melt, degrade and deform when it gets too hot. The upside is that they don’t need to brake so often so they can work like that.

A race car needs to brake every few seconds, if you use normal car brakes they’ll overheat super quick. You can drill holes in the brakes, add air channels and direct air at them to try and dissipate that heat to make them last a bit longer in that environment, and it will help. But also, because you’re braking every few seconds the brakes will always be hot, you don’t need them to work from cold any more. So race car brakes can use different materials that only start to work when hot, but also survive much higher temperatures. Thus, if the car goes too slow and the brakes get cold the drivers will complain because they don’t work so well anymore.

Brakes on a car is basically a clamp on a spinning metal disc. The clamp is operated with hydraulic force using the brake fluid. Most street cars use “organic” material to create friction when squeezed against the disc. Organic material is not tolerant of high temperatures. In some cases these materials melt and leave a “glaze” on the surface of the disc and the brake pad. The glaze is low friction and can cause a shudder under braking if not distributed evenly on the disc. Additionally some brake materials will off-gas under high heat and the gas can create a small buffer layer between the pad and the disc.

Better performing brake materials are semi-metallic or metallic and then into ceramic and other exotic materials. In addition to higher heat tolerance they are also less prone to the glazing problem.

The additional issue that arises is within the hydraulic system. As temperatures rise the brake fluid will boil and generate air bubbles. When hydraulic force is applied to “clamp” the brake disc, force will first compress these air bubbles and then close the clamp. This means for the same force applied you would allow the disc less with all other factors being equal. This problem is overcome by using brake fluid with higher boiling points (dot 3,4,5 ratings) and brake cooling ducts.