ThE FINK

Lettersto the eDiTor

august 2013 | words in action 76

dear Fink,

the textar technical team recently came across a badly written article explaining “why do brakes squeal” on ioL motoring. we would appreciate it if you could find some space to allow us to explain why disc brakes squeal? Yours faithfully

Fabio dinino - product specialist – Brake: control instruments-automotive

eD’s CoMMenT: Fabio, The Floor is yours:

90% of all brake squeal happens between the backing plate of the brake pad and where the backing plate touches the metal areas of the

caliper anchor and caliper fist. During the fitment of brake pads there are a number of requirements that need to be adhered to that most

people do not address. Proper assessment is needed. Most people fit pads regardless of the condition of the brake discs, calipers and

caliper guide pins.

pads dragging on the disc. This causes the friction material to heat up and degrade as mentioned. In addition, guide pins must be removed and cleaned and inspected. If damaged, they must be replaced.

The next area to check is the caliper piston dust boot. This boot must not be broken, torn or have gone brittle. If torn or broken it will allow brake dust to enter the piston chamber. The piston will start sticking and will not release the pad causing it to drag on the disc. If the dust boot is brittle, this will indicate that the caliper has already been exposed to extremely high temperatures and that the “o” ring around the caliper piston has also been exposed to the same high temperatures. This will take away the “roll back” effect that the “o” ring has on the caliper piston and will cause the inner pad to drag on the disc.

We now move onto the caliper anchor which needs to be properly cleaned. A wire brush needs to be used to remove all the caked up brake dust. The dust needs to be removed. The question here is why did the “OE” pads last that long? It is because the calipers were clean and free of brake dust when the vehicle left the show room floor. The dust build up occurred over the first 50 000km. If this dust is not cleaned off, how will the new pads that have been fitted operate properly? The anti-rattle clips, if fitted, must also be removed. Lubricate and replace the new anti-rattle clips if supplied. Otherwise clean the old ones. Remove them from the anchors, remove all the caked-up brake dust, lubricate them with a high melting point grease and refit them.

When fitting the new pads, a high melting point grease should be applied to the areas of the brake pad backing plate that comes into contact with the caliper anchor or caliper fist. This allows the pad to move freely without restriction. These high melting point greases do not liquefy - so they do not absorb brake dust thus keeping it lubricated as opposed to turning it into a grinding paste.

Anti-squeal shims are another point of contention. These shims are fitted by the agents to stop their own pads from squealing. These shims are serviceable, which means that they must be removed from the old brake pads, cleaned and

refitted to the new pads. If they are missing, then new ones should be bought and fitted, although some pads are not fitted with shims from new.

Another aspect of the braking system when changing brake pads are brake fluid and the rear brakes. Brake fluid boiling point is the reason why the brake fluid needs to be changed. Brake fluid is “hygroscopic”. This means that it absorbs condensation or water. The more water absorbed by brake fluid the lower the boiling point of the brake fluid. When brake fluid contaminated with water starts to boil, air bubbles develop in the braking system, mainly at the calipers because this is where most of the heat is generated. This results in the brake pedal becoming spongy, affecting the efficiency of the braking system. A dot 4 brake fluid boils at 260 degrees Celsius.

And finally we come to the rear brakes. Most people only replace the front brake pads and never check the rear brakes. They expect this to have been checked at the last service? Remember what happens at services? One has to understand that a vehicle has four wheels and four brakes and all have to work. Rear brakes do not self- adjust as required if no one has serviced them after 50 000km. And excessive brake dust in the rear drum stops the self-adjusters from working. Remember, brake dust is a friction material and wherever it deposits it will stop these components from working. Rear brake drum systems must be checked, cleaned out and re-adjusted at every service and when brake pads are being changed. In addition, one must not only adjust the handbrake cable. This will cause only one brake shoe to work, resulting in uneven wear of the rear brake shoes under light brake pedal pressure. This is a problem with the Citi Golf and VW Polo brake systems.

If the rear brakes do not work during braking, then all the brake force will be applied by the front brake calipers. This will cause the front brake pads to run hotter and wear faster. Hence the brake pads do not give the required mileage as did the original equipment brake pads. For optimum braking system efficiency, all four brakes must work to the levels required of them.

Fitting brake pads to discs that have a ridge will cause the areas of the pad that bed into the grooves and ridges to overheat. This in turn causes the resin holding the ingredients of the friction materiel together (in those areas) to degrade, allowing the particles to become dislodged. It will result in a vibration causing an audible noise that we call a squeal. This is however only a small percentage of squeal issues. The new materials that have replaced asbestos are not the cause of brake squeal. In fact the new materials are much better than asbestos, creating higher levels of friction and are safer for the environment.

When fitting brake pads it is advisable to replace the disc rotor. But due to financial constraints most people cannot afford to do this, and therefore choose not to. This can be delayed if the disc is thick enough to be skimmed. Take into consideration the minimum allowable thickness of the disc before it is skimmed. It is important to note that for every 1mm skimmed off the disc, the disc will run 100 degrees hotter, resulting in the brake pads wearing faster. If it is below the minimum thickness the disc MUST NOT BE USED because this will put the safety of the vehicle and its passengers at risk. The disc may crack, resulting in brake failure.

Regarding the fitment of the brake pads into the caliper, here most people do not clean the calipers properly and don’t carry out caliper servicing. It is important to note that if a vehicle does 50 000km on its first set of pads, the braking system has only been checked by the servicing dealer, yet the engine has been serviced at least three times. As a safety-critical item, the brake system has only been checked once. This is a concern. When the brake pads are changed it is the perfect opportunity to carry out a brake system service. A brake system service includes cleaning all components and servicing them. Sometimes a screw driver is used to push the caliper piston back to fit the brake pads. This practice is unacceptable. Screw drivers were designed to loosen screws. It is a lever and levers have leverage which causes the guide pins to bend. When the guide pins are bent the caliper cannot move freely, keeping one of the