Usually, the first sign that you have issues with your car brakes is a squealing or squeaking noise.
New vehicles rarely experience brake problems, but as the cars accumulate mileage and the brake parts begin to wear, this can change and your car could make a noise when braking. Even after a brake replacement, problems can be experienced if non-original brake pads, discs or shoes, of unknown specification are fitted.
| Noisy Car Brakes
The sound of metal clashing and grinding indicates that your brake pads need replacing. Ignoring this sign and not acting can result in car brake failure and severe damage to your car that could serve a hefty price to get fixed, and more importantly poses as a danger on the road, putting your life and others in jeopardy.
There are different degrees of noise coming from the vehicle and these are defined as Noise Vibration and Harshness (NVH), commonly described as:
- Car brakes judder
- Car brake squealing
- Car brake squeaking
These are defined by the frequency range in which they occur, with judder, which can often be felt as a vibration, occurring below 300 Hz, squeal below 5,000 Hz and squeak higher than 5 kHz. At 12 KHz, the ultrasound is above the range for human hearing and therefore is of no consequence when a car makes a noise when braking.
| Check the Friction Surface of the Brakes
The first step in diagnosing any car brake problems is to examine the appearance of the friction surfaces, and these can be found on the brake pads and brake linings. You can install a Brake Wear Indicator which will produce a high pitched squeak if the brake pads have worn down to 2.5mm and it comes into contact with the brake disc, which will only be silenced when the brake pads are replaced. You can initially check the brake lining thickness and you can do this by looking into the inspection hole located on the brake cylinder (you need to take off the wheels and make sure the vehicle is supported).
Observation: Tapered or unevenly worn car brake pads
Appearance: Unevenly worn or tapered disc brake pads.
Result: Excessive disc brake pad wear, uneven braking pressure, noise.
Cause: Incorrectly located car brake caliper, sticking or seized caliper slides, excessive caliper sliding-pin clearance.
Action: Replace the disc pad set with approved parts, service and lubricate the caliper.
Observation: Inside and outside pads exhibit different wear-rates.
Appearance: One brake pad within the caliper set displays excessive wear (Note: It is normal for the ‘fixed’ pad in the caliper to wear slightly more than the piston side).
Result: The car pulls to one side under braking, uneven and excessive pad wear, brake squeal and judder.
Cause: Caliper guide pins or piston seized or sticking.
Action: Service all caliper sliding pins and pistons, replace disc pads.
Check discs for thickness and runout.
| Diagnosing and fixing noisy car brakes
While the sources of brake noise are varied there are a few basic checks and interventions that should be done first when trying to diagnose and cure a noisy or brake vibration problem:
Car Brake Squeak:
High frequency vibrations that occur at a frequency higher than 5 kHz.
- The most common cause of squeaky brakes on a car is the vibration setup within the friction material when the brake pads are forced into contact with the brake disc.
- Worn disc pads.
- The first thing to check is that the brake pads have more than 3mm of friction material remaining – the resins that are used during the bonding process tend to concentrate in this area, which can change the braking performance.
- It’s also important to check that the anti-rattle springs are (correctly) fitted and to ensure that the anti-squeal shims are located correctly.
- There’s a widespread view that copper grease, otherwise known as copper slip, applied to the back of the brake pad will reduce the noise. This is no longer the case: Advances in shim technology has made this practice obsolete if not undesirable.
- If the noise persists the specification of the replacement brake pads (With regards to the coefficient of friction over the operating range) may be incorrect which will require the brake pad set to be replaced.
Car Brakes Squealing:
This noise occurs when the pads vibrate at a frequency in the range of 300 to 5,000 Hz, resulting in a noise with a higher pitch than the squeak described earlier.
- The pads do not clear the disc properly because the caliper piston doesn’t retract correctly.
- The pads are fitted incorrectly or misaligned in the guides.
- The anti-squeal shims are damaged or located incorrectly.
- The surface of the car’s brake disc is ‘wavy.’ This is usually found on high mileage vehicles or when the incorrect pads (coefficient of friction) are fitted.
- The brake disc has a thin film of rust on the operating surfaces from standing for a period
- The disc has reached the minimum operating thickness.
- Clean and lubricate the car’s brake caliper and sliding surfaces.
- Make sure the mating surfaces of the hub disc are clean and free of rust.
- Ensure the disc’s runout, when fitted to the hub, remains within a tolerance of 0.1 mm.
- If the runout or waviness is out of specification it may be possible to skim the disc to rectify the condition. In this case it is important that the disc rotor thickness remains within the manufacturer’s specification.
- If the disc is worn and out of specification, then it needs to be replaced.
Car Brake Judder:
This is a low frequency vibration that occurs at a frequency below 300 Hz, which can often be felt through the steering column.
- Excessive disc runout due to poor alignment on the hub or excessive disc thickness variation.
- Brake disc wear.
- If the brake disc is still within specification it may be possible to skim the disc before reassembly.
- If the brake disc is outside the wear limit it will be necessary to replace the disc and clean and lubricate all surfaces of the brake assembly according to the vehicle manufacturer’s instructions.
Even though there is always a temptation to save money by fitting non Original Equipment this is not recommended for motor car brakes as they are safety critical systems.
While this guide covers the more common car brake problems it does not attempt to deal with Hybrid and Electric Vehicle braking systems that are designed with regenerative braking in mind.