MOTOR CLINIC

Why your brakes squeal

In Summary

• Most brake pads are made chamfered/ tapered towards the edges.

• This reduces the angle of attack to the disk from 90° to around 45°.

Some road conditions and terrains can cause debris from the road to be trapped in the brake callipers.
Some road conditions and terrains can cause debris from the road to be trapped in the brake callipers.
Image: BOOSTED AUTO

The primary objective of the brakes is to slow down the vehicle or to halt its movement without loudly announcing to the surrounding drivers intention to retard the motion of the vehicle.

 Apart from the high-performance brakes that are usually loud on braking especially on low temperatures, normal road use brakes should be without any squeak, squeals or whistles.

 However, several underlying conditions within the braking system could bring about these noises.

Worn out brake pads

At least every car manufacturer has a way of conveying a message to the driver when the brake pads are worn. Some utilise the use of electrical conveyance by including a sensor on the edge of the brake pad that triggers a light on the dashboard to alert the driver when the brakes are worn.

 A larger percentage of the rest of the manufacturers employ a specially treated stainless steel clip attached to the back of the brake pads. Whenever the brake pads are worn, the clip scribbles on the rotating disk and makes a low-frequency audible squeaking sound. The sound is on at all engine speeds as long as the car is moving regardless of whether the brakes are being applied or not.

Debris trapped in the brake system

Some road conditions and terrains can cause debris from the road to be trapped in the brake callipers or around the back plate. For instance, if the debris is a piece of hard rock or glass, should it come into contact with the rotor, the result will be a high pitched squeal that will be audible only at low vehicle speeds. In some cases, the noise will disappear if the driver applies brakes and comes back on when brakes are not applied. The possible reason for this being that, when brakes are applied, the calliper movement disorients the initial position of the debris moving it away from the rotating disk.

The noise may fade away with time either because the piston crushes the debris or the road oscillations naturally causes the pebble to fall off on the road.

 It is always recommended to dismantle the brakes and wash away the pebbles or the dust with clean water before the noise becomes a nuisance. If the damage is too extensive, a disk skimming  must be conducted and a fresh set of brake pads replaced.

 Fake brake pads

Genuine brake pad construction must be made of materials being able to produce enough friction to grip on the rotating disk yet soft enough in order not to scribble on the disk.

 On the other hand, when fake brake pads are being manufactured, none of these principles is put into consideration. In some scenarios, fake brake pads are even made harder than the disk. This means  the amount of steel in the brake pads is so high to the extent that the pad is extremely hard which causes  the disk to wear. In such cases, the contact between the brakes and the disk would produce a sharp whistling sound whenever the brakes are applied. This kind of noise is what we referred to in the beginning as announcing your arrival. The remedy is to revert to genuine brake pads. Most likely the brutal damage caused by the none genuine would require disk skimming before OEM pads are fitted.

Absence of brake shims and copper grease

The shims are placed on the back of the brake pads to hold them in position when the piston is not acting on them. Failure to fit the shims gives the pad a lot of free play on driving which may produce a squeak.

 Authentic brake pads also come with high-temperature copper grease whose main purpose is to lubricate the calliper channels used as brake pad route.  The pads must move forward to the rotor when the brakes are applied and move back when brakes are released. When these channels are dry squeaky noise is produced. Due to its uniqueness, high-temperature grease lubricates the channels but does not melt out into the rotating disk.

Brake pad  chamfer

Most brake pads are made chamfered/ tapered towards the edges.

This reduces the angle of attack to the disk from 90° to around 45°.

Scientifically when brake pads come into contact with the disk at 90° a squeaking sound is produced and can only be heard at low speeds. The sound may be amplified if the vehicle is moving between 2 walls due to the refraction effect.

 In the initial stages when new brake pads are fitted, the contact area is usually about 45° ( oblique angle)  but with time, brake wear reverts the angle to 90°(right angle). At this point, the squeaks will be heard. The remedy is to shave off a small section from the edges of the pads to about less than 90° and within no time the noises will be no more.

 Direction of rotation

For slotted disks, the formation of the slots is always in one direction. Interchanging the disk distorts the direction of rotation causing the brake pads to contact the slats at a reverse angle which causes them to squeak. The remedy is to check the slots for correct direction of rotation.