Driver Knowledge Tests

Brake fade: what is it and how do you prevent it?

Brake fade

Brakes are one of a number of mechanisms and forces in a car that slow it down. The others include air resistance, rolling resistance, engine compression and gravity (if travelling uphill).

Relying solely on the brakes for long periods of time will cause the brakes to overheat, and this causes brake fade – a partial or complete loss of braking power. This is because brakes work by using friction to slow the car down, and that friction causes heat which has to be dissipated using the components of the braking system.

What happens when you get brake fade?

Firstly, there are three types of brake fade and they don’t all feel the same. We’re going to deal only with disc brakes here, although there are other types of brakes, such as drum brakes, which also suffer brake fade.

Overheating the brake pads

The pads are built to work up to a certain temperature. They tend to increase in effectiveness as they get warm, but above a certain temperature they begin to break down. Components in them can melt and this lubricates the brake disc. Gases can be released as the brake pad material breaks down and that forms a microscopic layer against the pad that reduces friction.

If this has happened with your pads, as they cool down they will form a shiny surface on them – they become glazed. Glazing can also happen to the brake rotors (the discs themselves). The pedal feel then often becomes very wooden, and however hard you press on the brake pedal, you might find that you won’t be able to apply enough force to lock the wheels (or activate the anti-lock braking system).

In mild cases the pads can be sanded to bring them back to life; in worse cases you will have to replace the pads.

Not bedding in the brake pads

Brand new pads are ‘green’ (this isn’t the colour, it means they haven’t been bedded in, or ‘burnished’). While each manufacturer has its own specific guidelines for bedding in the pads, this usually consists of accelerating to a predefined speed then braking down to zero or almost zero and repeating that a few times. This video explains how to bed in the pads

Boiling the brake fluid

Pushing the brake pedal causes the brake callipers to push the pads against the brake disc using hydraulic fluid. Because the fluid is part of the calliper, as the calliper heats up, so does the fluid. If the fluid gets too hot, small air bubbles start to form as it begins to boil. Because air is more compressible than fluid, the brake pedal starts to feel soft and you can’t get as much braking power. If the fluid continues to boil, eventually the brake pedal will travel all the way to the floor without providing much force on the brake pads – you will lose all braking power.

Brake fluid is hygroscopic, which means it attracts and absorbs water over time. Water has a lower boiling point than brake fluid, so the older the fluid, the more likely it is to boil.

If you’ve boiled your brake fluid, you should replace it.

How do the brakes work?

This video explains how disc brakes work.

Front discs are almost always bigger than the rear discs because they shoulder more of the braking burden as the weight of the car is shifted forwards under braking. Too much brake bias to the rear wheels can cause the rear wheels to lock early and that’ll send you off the road in a spin.

You can see in this video the brakes glowing and how the front rotors are much bigger than the rear rotors. Skip to 45 seconds to see this.

Preventing brake fade

Obviously a race track is an extreme scenario where you will be using maximum braking power frequently. On the road, though, there are situations where you need to apply the brakes for long periods of time, or have the ability to slow down your vehicle over a long stretch.

For example, if you are towing a caravan or have a heavily laden vehicle and you are driving on a long descent, such as down the side of a large mountain range, you will put a lot of force on the brakes. To reduce this, change down one or more gears so that the engine is revving higher and let the engine compression do the braking for you.

Make sure you slow down early because a vehicle going slower has much less energy to dissipate during braking.

Ensure your brake fluid isn’t too old, and that your pads are in good condition.

What do to if you experience brake fade

As soon as you start to feel the brake pedal going spongy, you may have as little as ten seconds before the fluid boils completely and you have no braking power. Immediately change down through the gears so that your vehicle still maintains its revs under the redline but you will be getting maximum braking power from the engine. Pump the brakes if the pedal starts going almost all the way to the floor.

If this doesn’t work you may need to look for exits off the road. Side roads that turn back uphill will put gravity on your side, as long as you are not going too fast to turn into them. You want to try to avoid hitting another vehicle as this could injure its occupants or push it off the road and down a steep drop.

Sometimes natural features can be used to help stop the car, particularly hedges and bushes. They will cause damage, but possibly only superficial as opposed to catastrophic.

Cornering will reduce speed as some of the energy is used in changing direction. This also means that (theoretically) weaving can slow you down, but this is extremely dangerous as you can lose control easily, particularly if you are towing a trailer.

In a worst case scenario, if there is a risk that you might run head on into another motorist, or go off the side of the mountain you will need to find a more solid object to stop your progress. Smaller trees and saplings may break, while larger trees will be solid enough to withstand the impact of your vehicle. If it is only you in the vehicle, aim the passenger side to hit the object to reduce the risk to you.

If you succeed in stopping your vehicle with no further damage, bear in mind the brake components will be red hot – 600 degrees Celcius. These temperatures can melt hub caps and warp and crack brake rotors. To avoid this kind of damage try to keep moving, but ensure you will not lose control – travelling uphill is safer.

What are the chances of brake fade?

With modern disc brakes it’s extremely unlikely you will get brake fade. Older drum brakes were much more susceptible to them. However, you now have the knowledge almost eliminate the chance of this happening, and to deal with this emergency should it arise.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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Posted in Advice, Car, Heavy Vehicle, Motorbike
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