Your vehicle's brake pads will eventually wear out. There's no avoiding it, largely because that's what they're designed to do. Friction is what makes your vehicle stop – if a brake pad was made to slip gently over the surface of a brake disc, it wouldn't be terribly good at slowing you down. So, over time the friction material on the pads will wear thin, exposing the backing plate.
This is A Bad Thing, and it's vital that you replace your brake pads before they get to this point. That’s why it is important to regularly check the thickness of your brake pads.
What is the minimum width for brake pads?
Brake pads should be changed before they wear down to 1.5mm. A new brake pad will be around 10mm thick. Most manufacturers and mechanics will all agree that you should probably replace your brake pads once they wear down to 3mm.
Checking brake pad thickness and disc wear
1. The brake pad thickness and the condition of the disc can be assessed roughly with just the wheels removed, but for a comprehensive check the brake pads should be removed and cleaned. The operation of the calipers can then also be checked, and the condition of the brake disc itself can be fully examined on both sides.
2. If you're checking the front brakes, apply the handbrake, then jack up the front of the car and support it securely on axle stands. Remove the front wheels. If it's the rear brakes you're checking, chock the front wheels and jack up the rear of the car before using axle stands.
3. With the wheels removed, the pad thickness can be seen through the front of the caliper.
4. On completion, refit the wheels and lower the car to the ground.
How do brake pads work?
They may only be small, but the brake pads provide a vital role. The braking system consists of hydraulically actuated pistons housed within metal (usually cast iron) calipers.
The pistons force the brake pads (which are fitted in pairs) onto the brake discs and use friction to convert kinetic energy into thermal energy – which is why brakes get so hot in use.
The actual pad consists of a steel backing plate and a 'friction material', and as you brake minute amounts of this material are transferred onto the disc, and it is this that gives brakes their 'bite' (and the reason why new pads need to be 'bedded in' correctly).
The actual friction material is what dictates how well the pads perform, and different compounds can affect the service life of the pads and the ability to resist brake fade (when the brakes become less effective the hotter they get).
There is always a trade-off between performance and service life, and some pads are designed to recover from extremes of heat that would otherwise damage a 'normal' brake pad.
Pads are specified to work within an optimum temperature range for their fitment – hence you'll get different pads designed for a 1.0 hatchback, compared to those fitted to a supercar.
Not only is the friction material different, but the sizes can vary, with large or performance cars featuring substantially bigger pads than more humble vehicles.
As the pads wear down they need to be replaced well before the steel backing makes contact with the disc. Some pads require a visual inspection, some incorporate audible wear indicators (that squeal as the brakes wear low), and others contain sensors that trigger a warning light on the dashboard when they get to a prescribed limit.
Check out more FREE brake-related features here
What kind of brake pads do I need for my car?
There are lots of different types of friction material but the main groups are:
- Non-metallic/Organic – a combination of synthetic substances bonded together. Short service life, but reduced brake disc wear. These are the pads most commonly fitted to standard road cars.
- Semi-metallic – Synthetic substances, plus metal compounds. Harder wearing than non-metallic, but harder on brake discs, and can give poor pedal feel until warm/hot.
- Fully-metallic – Generally reserved for race applications. Can withstand huge temperatures, but need to be very hot to work effectively and are very hard wearing on discs.
- Ceramic-metallic – Composed of a dense ceramic material and copper strands. The most expensive of all the pad materials, consistent performance whatever the temperature, extremely quiet operation and brake dust that doesn't stick to your wheels!
How else will I know when my brake pads need to be changed?
Pedal feel
If you exhaust the friction material, you’re going to go metal on metal. This means the metal pad carrier will be pressing on the disc. You will feel this and hear it through the pedal. It will vibrate and rumble when you apply the brakes. Don't drive any farther - change the pads immediately.
Noises
If there is a deep whooshing or groaning sound when you brake hard, that’s a sign your pads are getting low. Think of it as a deep, bassy groan.
Mileage
There is no hard and fast rule regarding how long a set of pads should last. However, you can apply a bit of common sense. Generally speaking, pads can last from 40,000 to 100,000km. If you have three kids and you drive a big, heavy people carrier, you’re looking at the lower number. If you drive a Mini, then you’re going to be looking at the bigger number. But also, think, really think about your driving. Are you an early braker? Are you late on the brakes? Do you gently come to a stop or tend to stand on the brakes? All this can affect the life of the pads. Heavy driving means a short life.
Pulling to one side
Does your car pull to one side of the road or the other when you hit the brakes? If it does, this could be a sign the pads are wearing unevenly. This is where you need to go back to a visual check and both sides for even amounts of wear. Brake pads should always be changed in pairs. Never change just one side.