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Brake Noise In Detail

Brake noise is one of the major reasons for a driver to visit a mechanic. It can be a complex issue and is often difficult to solve.

WHAT IS BRAKE NOISE?

Brake noise typically originates from vibration of a part within the wheel end section of the vehicle – between the tire and the connection to the chassis. Noise is often blamed on the brake pad itself, but can come from any part, from the wheel bearing to the ball joint. Within the brake system, it’s normal to have some vibration, but it is not usually intense enough to be audible.



A. TYPES OF NOISE EXPERIENCED BY DRIVERS

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LOW FREQUENCY VIBRATIONS – JUDDER

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DESCRIPTION: 

A deep noise with frequency below 300hz.

CAUSE
  • Poor tolerance (poor fitment on the hub or excessive disc thickness variation)
  • Rotor damage
SOLUTION

Replace the rotor and clean and lubricate all surfaces of the brake assembly according to the vehicle manufacturer's instructions.

MEDIUM FREQUENCY VIBRATIONS – SQUEAL

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DESCRIPTION

Noise with frequency in the range 300 - 5,000 Hz.

CAUSES
  • Sticky movement on the caliper piston
  • Rough finish of the working surface of the rotor
  • Pad fitting errors
  • Hardware and shims are not installed properly
  • Disc thickness too low
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SOLUTION
  • Clean and lubricate components of the caliper
  • Ensure rotor surface finish (once fitted on to the hub) will stay within a tolerance of 0.1 mm
  • Clean the hub surface, machine rotor surface
  • Replace rotor and make sure brake pads, shims and accessories are fitted properly
  • Consider damping noise intensity with shims or brake pads with noise-reduction features

HIGH FREQUENCY VIBRATIONS – SQUEAK

DESCRIPTION

Noise with frequency higher than 5 kHz.

CAUSES

The most common reason for a squeak is a molecular vibration within the friction material during application to the brake rotor.

SOLUTION

Replace the brake pad set. Also check that the accessories (e.g. the caliper clips) are correct and properly fitted.

VERY HIGH FREQUENCY VIBRATIONS – ULTRASOUND

DESCRIPTION

Noise with frequency higher than 12 KHz, greater than the upper limit of human hearing.

B. FRICTION SURFACES GIVE VITAL CLUES

The appearance of the friction surfaces can give a good indication of the causes of noise problems. Jack up the vehicle, remove the pads and examine the friction surfaces to analyze potential problems. Below are five symptoms that relate to noise.

SYMPTOMS AND REMEDIES

TAPERED PADS

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APPEARANCE: Uneven wear, tapered pads

CAUSE: Distorted caliper, caliper slides sticking, excessive caliper clearance

EFFECT: Premature pad wear, uneven braking pressure, noise

REMEDY: Replace pad set, maintain & service caliper

UNEVEN WEAR

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APPEARANCE: Uneven wear on pad surface

CAUSE: Irregularly worn rotor (a wear lip will be visible on the disc)

EFFECT: Squeal & judder, premature pad wear

REMEDY: Replace rotors and pads

UNEVEN WEAR WITHIN AXLE

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APPEARANCE: One or more brake pad within the axle set will be excessively worn

CAUSE: Caliper guide pins or piston sticking

EFFECT: The vehicle pulls to one side while braking, uneven & rapid pad wear, squeal & judder

REMEDY: Maintain all caliper slides & pistons, replace pads. Check rotors

DAMAGED BACK PLATE

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APPEARANCE: Damaged back plate

CAUSE: Incorrect assembly, excessive force used during fitting

EFFECT: Braking inefficiency, irregular wear, noise & judder

REMEDY: Replace full brake pad set

PISTON DAMAGE

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APPEARANCE: Anti-noise features (rubber-coat / shim) damaged by piston

CAUSE: Piston not retracting fully, heavy brake use

EFFECT: Overheating, noise

REMEDY: Replace brake pad set, maintain caliper

C. AVOIDING NOISE PROBLEMS

The easiest way to avoid problems is to ensure that the brake pads and caliper are correctly assembled.

Tips for correct assembly:

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  • Strip and clean all the caliper slides and pins 
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  • Emery paper can only be used on the caliper and pad abutments in the case of heavy rust or pollutants 
    Note: If the caliper is not sliding easily, this can result in tapered pads or uneven wear within the axle, which creates squeal and judder.
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  • The pins must have the correct lubrication to ensure free movement so that the pin does not seize into the caliper body. Also, check the condition of the slide pin rubber boots to stop water ingress 
    Note: If the pins are not lubricated they may seize, causing wear taper and noise issues. 
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  • Check for free piston movement, and retract the piston fully – this is essential to avoid damage to the shim or the rubber coat 
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  • Check that the brake pads fit freely and easily into the caliper brackets 
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  • Clear burrs off the brake pad edges if required 
    Note: If the pad is not moving freely inside the brackets, there will be ongoing contact with the rotors, causing squealing as well as tapered pads or uneven wear within the axle. 
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  • In certain cases (e.g. an old rusted caliper), it is a good idea to lightly lubricate the contact points between metal back plate and slides with copper grease, according to the vehicle manufacturer’s instructions 
    Note: Take care that NO grease touches the friction material. 
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  • Insert wear indicators, when appropriate, in or on the brake pads 
  • Screw in the caliper fixing-bolts 
  • Replace the thread-lock bolts with new ones 
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  • Follow the correct tightening torque and sequence recommendations
  • With the brake caliper reassembled, pump the brake pedal until the stroke is around one third of the full stroke potential
  • Check that the brake operates correctly, including pad retraction 
  • Refit road wheel. If there is free rotation, the vehicle is ready to go back on the road 
    Note: Do a road test before returning the vehicle to make sure that the brake system is in full working order. When you return the car to the driver, advise them on the correct bedding-in procedure to ensure proper brake efficiency. 
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