When brakes start to wear out they can begin to make strange noises, especially when braking at low speeds like when in traffic.
What causes noise when braking at low speeds? Noise from your brakes at low speed can be caused by worn out brake pads, worn or warped rotors, trapped dirt or rust on the brakes and sticky brake calipers.
During their lifetime, the brakes on your car are exposed to all sorts of dirt, debris and weather conditions along with extreme temperature fluctuations. Over time, this will cause wear and tear and will inevitably lead to a reduction in performance and an increase in noises and vibrations.
It's often the case that you'll be more aware of changes to the brakes when driving at low speeds or when stuck in traffic. Stop- start driving is when the brakes are under the most pressure and are often running hot.
There are a few common causes of noise from the brakes and these are often more pronounced when driving at low speeds. Here are a few described in more detail.
1. Trapped dirt or rust on brake pads and rotors
Probably the most common cause of noisy brakes, especially at low speeds is brake dust, dirt or rust on the brake pads and rotors. Brakes pads, rotors and calipers are constantly exposed to dirt, mud, water and debris from the road surface.
These contaminants can get trapped between the rotor and brake pad surfaces causing a squealing or grinding noise when braking. The build up of foreign material on the braking surfaces can also increase the dynamic friction coefficient of the brake pads.
The dynamic friction coefficient is a measure of how much force the pads apply to the rotors when braking. A normal range for this coefficient is between 0.35μ and 0.42μ. If this increases too much, this can result in sticky, noisy brakes, especially at low speeds when the temperature of the brake pads is often higher than normal.
A build up of rust on the surface of the brake rotors can also cause noises when braking. The rust can prevent the pad from gripping smoothly when the brakes are applied.
A small amount of rust on the surface of the rotors is common if the vehicle has been parked up for a few days and usually wears away fairly quickly when the brakes have been applied a few times at higher speeds. The rust may linger on the rotors longer if the brakes are only applied at low speeds.
2. Worn out brake pads, brake rotors or brake shims
The second most common cause of noisy brakes at low speed are worn out brake parts that no longer work smoothly or efficiently.
As brake pads and rotors wear down, they rarely wear evenly. It's very common for brake pads to wear down at a slight angle and this uneven wear will become more pronounced the more worn out they get.
Uneven pad wear will create brake judder and brake noise. At low speeds, the brake noises will be more pronounced because the rotors are moving slower and are less likely to overcome resistive forces caused by the uneven wear.
If the brake rotors or pads become badly worn, or worn out completely, they will no longer work smoothly and this will invariably cause noise when the brakes are applied. Old rotors can develop ridges, surface scratching and cracks. They can also start to rust, especially around the wheel hub and this can lead to strange noises when the brakes are applied.
If the brake pads are not changed when necessary, this can also cause noises at low speed. Brake pads most commonly consist of a steel backing plate attached to a semi-metallic braking material. The braking material will generally harden and degrade as it ages and wears away. An old, hardened brake pad will be noisier than a new one, especially if some of the steel backing plate is coming into contact with the rotor when the brakes are applied.
3. Warped Rotors
Another cause of noisy brakes at low speed are warped rotors. When new, a rotor should have a minimal sideways (lateral) runout of less than half of a millimetre as it rotates. This will ensure it wears evenly and performs smoothly.
As the rotor wears, there are several reasons why a rotor can become warped and to wear unevenly. Uneven wear leads to variations in thickness across the surface of the rotor that can produce vibrations and noise when the brakes are applied.
Warped brake rotors can be caused by a build up of dirt and rust on the surface. This build up causes an increase in friction and increased temperatures when the brakes are applied resulting in hot spots on the rotor surface and uneven wear of the rotor.
4. A sticky brake caliper
Brake calipers can stop working smoothly as they get older and this can be a cause of noise from the brakes.
A sticky brake caliper is one that does not squeeze and release the brake pad quickly and smoothly. The symptoms of a sticky brake caliper include noisy brakes, a vehicle that pulls to one side when braking, a burning smell from the rotors or brake pads and brake fluid leaks.
The brake calipers are responsible for creating the brake force that squeezes the brake pads against the rotors when you put your foot on the brake pedal.
They are simple devices, consisting of a metal body that houses a movable piston. The piston moves outwards when forced from behind by brake fluid. To stop the brake fluid from escaping, there is a rubber seal behind the piston and a boot at the front that stops dirt and debris from getting in.
Brake calipers can fail when the rubber seals harden and split. This allows the brake fluid to escape, causing a reduction in braking force. A worn outer seal will allow dirt to collect around the piston, causing it to corrode and get stuck in place.
If the piston does not move backwards fully when the brake pedal is released, this can cause a noise as the brake pad will be partially rubbing the rotor as the wheels turn.
This noise can become more pronounced at low speeds as the rotor is not moving fast enough to overcome the extra friction being applied by the brake pad.
5. using Ceramic brake pads fitted instead of semi-metallic pads
Ceramic brake pads have been known to create more noise than standard brake pads, especially at low speeds. This is because they are harder than standard pads and don't grip the rotors as well when they are cold.
Most new cars are still factory fitted with metallic brake pads. This is because they are cheaper and offer better all round braking performance for normal road driving.
Ceramic brake pads are often fitted as standard to premium car brands such as Porsche and Bentley.
Their surface brake material is made from a clay and porcelain compound with copper flakes added to enhance the thermal conductivity. Most cars won't have them fitted as standard, and because of their unique properties they may not be suitable for all braking systems.
Ceramic brake pads are very hard wearing, and should last much longer than standard brake pads while producing a finer, lighter dust as they wear.
On the downside, the ceramic compound is not as good a conductor of heat as metallic pads and take longer to reach optimum operating temperatures.
If there are noises coming from your brakes at low speed then there is more than likely something wrong with one of the parts of the brake system.
You should get your vehicle checked out as soon as possible to ensure that the problem is not serious or could cause an accident.
Usually, noisy brakes can be resolved easily by doing one of the following:
- Change rotors for new ones
- Change brake pads for new OEM quality
- Clean away any buildup of dirt or rust between the rotors and pads
- Check and replace sticky brake calipers
If you have been keeping your car serviced and well maintained, then most brake problems can be fixed easily and without too much expense.