Brake noise, like all sound, is the result of vibration. A “slow” vibration sounds like a groan, while a “fast” vibration will sound like a squeal, because the pitch is determined by the frequency of the vibration. A vibration in one part can induce a secondary vibration into another part sometimes making it quite difficult to track down the source of the vibration. In most O.E. brake designs, dampening is performed by guide clips to assure rigidity or by insulator shims behind the pads. There are two ways to solve noise, dampening and preventing vibration.
Using high-temperature silicon grease on caliper slides, and floating pins will prevent caliper binding and chatter.
Shim insulators create a permanent cushion to dampen vibration. Some pads use applied shims as insulators which can migrate or delaminate during use. Wagner ThermoQuiet ® brake pads include an Integrally Molded Insulator (IMI) which will not delaminate or migrate during use.
Bend any retaining tabs on the pads to make sure that the pads are secure in the calipers to prevent rattle.
Block sand rotor smooth and flat using 120 to 150 grit sandpaper, to get the full pad surface contact, while preventing grab and chatter.
Wash rotors with soap and hot water to remove metallic fragments and abrasive grit trapped in the rotor surface and to clean oils from the surface of the rotor. Scrubbing with soap and water will remove all contaminants that cannot be removed by spray cleaners which can become embedded in the new pad causing chatter and squeal noise.
Replace all anti-rattle clips, springs and pins, which can lose their spring tempering due to the higher brake heat. Make sure guide pins are not binding which would cause the brake caliper to not release properly and drag on the brake rotor. Clean any rust from the caliper surfaces where the anti-rattle clips mount to allow free movement of the pads.
Brake System Noise Summary
The primary cause of brake system noise is vibration and all brake systems vibrate to one degree or another creating an initial noise. For the most part this initial noise is beyond detection to the human ear. In part this is due to the dampening effect, in which the larger mass brake components absorb the vibrations. The key factor in dampening initial noise is the stability or rigidity of the brake system components. In most O.E. disc brake designs, dampening is assisted by guide plates or abutment clips and the caliper mounting brackets assure rigidity of those components and/or insulators called shims or noise suppression gaskets which reduce vibration between the disc rotor plate and the caliper brake pad contact surfaces.
When the initial noise in the brake system is amplified (instead of dampened) to a level which becomes disagreeable to the vehicle operator, this amplification is called a secondary vibration. The secondary vibration occurs when the initial vibration frequency reaches an audible level with a high enough frequency for the driver to hear and be concerned.
Friction materials are not normally the cause of a secondary vibration. A secondary vibration generally results from rust, distorted or loose components, and/or the wearing or weakening of an original part. Components will no longer fit as tightly as necessary to maintain proper rigidity as they weaken or become fatigued from the heat and stress generated in the brake system. Any vibration resulting from the pad backing plate to caliper contact points will result in a very audible, irritating noise such as squeal.
Many technicians believe that changing to disc pads with a “softer” (more forgiving) friction material will cure noise problems. In reality, changing to the “softer” material changes the balance of the brake system, and this results in a change in noise frequency to an inaudible noise level. The important tradeoff to consider; wear rate of softer pads is much higher, life expectancy is greatly reduced. The thermodynamics of the brake system may also be altered affecting brake safety and efficiency.
Understanding Vehicle Brake Balance
– 70%-85% or more of the braking is performed by the front disc brakes.
– Older rear wheel drive vehicles performed 55%-60% of braking up front.
– Front wheel drive disc operating temperatures are much higher, resulting in increased glazing and wear.
Repeated braking in heavy traffic or hilly driving conditions is very demanding on the brake system. Under these operating conditions, temperatures can reach very high temperatures. At these temperatures, brake squeal is a common occurrence due to a metallurgical structure change that occurs in the rotor or drum surface.
Any brake system is capable of generating noise while braking under these extreme conditions. Heat buildup causes glazing of the friction surfaces (a thin surface hardening or flaking is formed on the surface of pads/shoes) and all these conditions can produce noise.
Brake noise may be heard during light brake applications, and usually when the brakes are cold. Make sure caliper and pads move freely and are lubricated. Make sure the guides or slides are not making contact with the rotor.