A car's brake pads are classic 'wear and tear' items – pieces of equipment that, like engine oil, coolant and drive belts, eventually wear out and need to be replaced.
Brakes are a concern for all drivers, and over time the friction material on the pads will wear thin, exposing the backing plate. 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 brake pad thickness?
In the UK, the legal minimum thickness for brake pads is 1.5mm. A new brake pad will be around 10mm thick. Most manufacturers and mechanics will all agree, though, that you should probably replace your brake pads once they wear down to 3mm.
Checking your front brake pad thickness and disc wear
1. Apply the handbrake, then jack up the front of the car and support it securely on axle stands. Remove the front wheels.
2. The brake pad thickness, and the condition of the disc, can be assessed roughly with just the wheels removed. For a comprehensive check, the brake pads should be removed and cleaned. The operation of the caliper can then also be checked, and the condition of the brake disc itself can be fully examined on both sides.
3. With the wheel 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 your 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 back 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.
What are the different types of brake pads?
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!
Other ways to tell when brake pads need to be changed
1 Have a look
You don't need to be a mechanic to identify if your pads need changing. Brake pads are made of two parts: the metal back plate and the friction pad. It’s the latter that presses on the disc, stopping the car. When new, this friction pad will be around 10mm thick. It may sound obvious but the less pad you have, the less effective the brakes will be. Less friction material means less heat dissipation, which in turn means your brakes could overheat. As such, you really want to be changing your pads when you have around 3-4mm of friction material left.
2 Pedal feel
If you exhaust the friction material, you’re going to go metal on metal. This means the metal back plate 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.
When you have to brake hard, if there is a deep whooshing or groaning sound, that’s a sign your pads are getting low. Think of it as a deep, bassy groan.
There is no hard and fast rule with how long a set of pads should last. However, you can apply a bit of common sense. Generally speaking, pads can last from 25,000 to 60,000 miles. 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.
5 Pull over
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.