Should Brake Discs Get Hot?

Your car’s brakes work by converting kinetic energy (the motion of your wheels) into heat so that your vehicle can safely come to a stop. This means that it’s completely natural for brakes to get hot in order to stop your vehicle.

But how hot is too hot? And when do hot brakes become dangerous? Let's find out.

Before we can think about how hot our vehicle’s brake discs get, we need to understand how the brakes work. 

So, to start from the very beginning: 

1. To slow your vehicle down, you press down on the brake pedal. 
2. What’s happening when you do this is that brake fluid travels to your brake calipers. The harder you press the brake, the more fluid is applied to them – this is called hydraulic force multiplication. 
3. The brake calipers are essentially a squeezing mechanism. They’re positioned over the brake disc (also known as a “rotor”) and clamp down on it when the brake fluid’s applied to them. 
4. Now, so that the calipers don’t need replacing all the time (because they’re quite fiddly), brake pads are inserted between the calipers and the discs. These are made out of a high-friction material, which significantly slows down the wheel by making it harder for the disc to turn through the pads. 

Think about it like the difference between wearing plimsolls on a slippery floor and wearing sturdy rubber-sole work boots – you’re a lot less likely to slip in the work boots because there’s more friction between the boot and the ground. 

However, the main big catch here is that friction always produces heat, which is why your brake discs can become hot after braking for a prolonged period of time. 

It takes a lot of pressure to stop a one tonne vehicle. So it’s crucial to keep your brakes maintained and ensure that they're working properly, otherwise you could cause a serious accident. 

Now we’ve determined that it’s okay for your brake discs to get a little bit hot, what’s the maximum temperature they can reach while continuing to work safely? 

• For standard road cars, you should expect your brake discs to reach around 130°C. 
• The brake discs on track cars can reach around 500°C, but these vehicles are built so they can cope with such an extreme temperature (and the discs & pads are often replaced between races). 

The most common way brakes overheat in standard road cars is when driving on a long stretch of downhill road (especially when in the wrong gear). If you’re not taking advantage of engine braking, you’ll tend to have to ride the brakes quite a lot, and this doesn’t allow enough time for them to cool back down. 

If this goes on for too long, you could experience something known as brake fade. 

This is the most common cause of brake fade and happens when the heat from the friction sort of “glazes” the rough surface of the brake pads. This happens in much the same way that sandpaper stops becoming effective quite quickly when you’re sanding fine, glossy material; all the gaps in the sandpaper that cause it to be rough get filled up with particulate matter, and then it just glides over the surface. 

Moreover, as happens with any friction interaction, your brake pads tend to shave off tiny amounts of the brake disc any time they’re used – and this dust accumulates when the brakes are clamped down because there’s nowhere for it to go! 

If you’re already experiencing friction fade, you’ll need to allow the brakes to completely cool for at least an hour before you drive the car again.

Most car manufacturers advise that you change your brake fluid periodically (the period varies depending on the manufacturer). If you don’t change your brake fluid, it can degrade and begin to absorb water over time. This excess water can cause the brake fluid to boil, which means it’s not applying that pressure to the brakes. As such, they may begin to feel spongy and could even stop working. 

Ideally, you should have your brake fluid changed at least every couple of years – or every 45,000 miles if you’re a frequent driver

If your brake discs have been subjected to particularly high heat for a long time (or repeatedly), they may need replacing altogether. 

• Brake rotors can begin to warp if you don’t allow your brakes to cool down correctly. The rotor should have a flat surface so that the brake pads can keep in contact with it at all times. 
• If the rotor is warped, you may feel a movement in the brake pedal or the vehicle might jerk as it comes to a stop. 
• You may also be able to tell if a rotor has overheated by looking at it. The surface could be discoloured or it could be a blue or purple colour. This discolouration can cause the surface to harden and means the brake pads may not work properly to slow the rotor down. 
• This might make your vehicle come to a stop much slower than it should, which, of course, could be very dangerous in an emergency stop. 

Always be careful when looking at your brakes. If you’re checking your car’s brake discs, you should make sure that they’ve had ample time to cool down as touching hot things is rarely advisable! 

Hot brakes shouldn’t be something to worry about unless you experience any of the following:

• Vibration in the pedal when you apply the brake
• Squealing or screeching noise when you apply the brake
• A burning smell
• Smoke coming from the tyres
• Spongy/soft brakes

If you think your brakes are overheating, there are a few ways you can try to cool them down while you’re on the road.

First, you should try to avoid overusing the brakes. Always look ahead of you for hazards so that you can see them more quickly and bring your car to a stop without slamming on the pedals.

As previously mentioned, be mindful when you’re going downhill that you aren’t constantly applying pressure to the brake pedal. You should allow your brakes to cool down in between uses.

You could try engine braking. You shouldn’t do this all the time, but it can be a useful technique if you think your brakes are overheating. You should downshift the gears, moving down from one gear to another, allowing your revs to increase but keeping your foot off the accelerator so the car is continuing to slow down. 

This way of coming to a stop can be slower than applying the brakes, so make sure you have plenty of room in front of you to decelerate this way.