Technique and elaborations Mon 03 July 2017

When the brakes whistle

 If the brake pads are consumed they need to be replaced, if they're superficially crystalized (vitrified) they need to be sanded or replaced. The brake system must be perfectly organized (no matter if the brakes whistle or not; it must be perfect since it's the most important active safety’s system on the motorcycle) and, nonetheless, clean. So, starting from a correct maintenance situation, if the noise (which only represents an annoyance) persists, go on reading.
The whistle we perceive is only a vibration fomented and amplified by a resonance which in specific circumstances establishes between the pad and the disk. The problem appears more often when the brakes are overheated, for example after the constant use during a long slope. Also rain (or better, water) often makes the phenomenon worse. Anyway, our goal is to avoid this resonance, and the consequent establishment of the loud vibration which we perceive as an annoying whistle.
There are two methods that we would like to suggest you, and they could also be used together, since the one doesn't exclude the other. The first is the application of a particular grease or an anti-seizing paste which resists high temperatures. Copper grease is perfect for this application. It's not a grease easy to find, but if you can't find it at the local hardware store, you can easily find it in the great distribution (amazon, eBay,…), sometimes like “anti-whistle paste for brakes”. You have to applicate a light coat of it on the back of the brake pads, only where they touch the pistons. It's important to apply only a veil of it, a bigger quantity is not necessary since the excess would be expelled anyway, causing problems in the case it meets the disk or the rubber seals (making them swell, grow), because this type of grease is incompatible with rubber. So the supports of the pliers doesn't need to be greased with copper grease, but with siliconic grease or other products compatible with rubber.
In this picture PICTURE 1 we can see a little tube of the product we use, and on a consumed brake pad the signs left by the piston of the back brake pliers. The image is useful to understand where to spread the copper grease, that is only where the piston touches the back of the pad.
In the following picture PICTURE 2 we observe the same brake pad after the application of enough quantity of the product.

The second method consists in rounding to 45 grades the angles (or corners) of the brake pads using abrasive paper (grain 120), because it's the external perimeter of the pad that mainly activates the vibration.
In this first image PICTURE 3 we can see how it is the original brake pad.
In the following picture PICTURE 4 we see the same brake pad after rounding the angles

 

Who’s not used to mechanics probably will be surprised, but  the disk inexorably consumes too  because of the effect of friction, even though more slowly than the pads.
It is of fundamental importance, for your safety and for the performances of the braking system, to check regularly the state of use of disks and pads. The first have a limit thickness normally stamped on the disk itself (after the initials MIN. TH. Which means in fact “minimum thickness”), which when it's reached by the disk, the disk needs to be quickly substituted. If you use a disk not thick enough, besides of obliging (specially using used pads) the pistons of the pliers going out a lot, you risk another problem probably less obvious but still important particularly in the racing or exasperated use: the disk also absorbs part of the heat generated by the friction with the pads and disperses it in the ambient. If the mass of the disk decreases (as in the case of a consumed disk, of not enough thickness) its thermal capacity will be compromised too, not only its mechanical resistance. There is also the risk, specially on off-road motorcycles, that stones or bumps damage the disk or other components of the system, so it is necessary to check them regularly. Little by little the disk and/or the pads consume, you will notice that the oil level in the tank of the brake pump will decrease. This happens because the piston is obliged to go out more to compensate for the minor thickness of disk and/or pads, and this new space will have to be occupied by the oil, which level will decrease. It's not about (in this case) a consume of oil, but a low level is an important index of wear of the disk and/or of the brake pads.
The check of the disk is the verification of its being planar (making the wheel rotate, the disk shouldn't laterally swing), it’s the verification of the absence of scars or important scuffing, and it also consists in verifying that the thickness is greater than the minimum indicated by the constructor (usually, for specialized enduro, 2,5 mm for the front disk and 3,5 mm for the rear one), measure that has to be taken using a calipers or a micrometer. It also needs to be checked, specially on fixed disks, that the flange and the connections to the hub don't have cracks or mechanical distortions.
To rebore a crooked or damaged by deep scars disk is usually not suggested, because it also unavoidably implicates a reduction of the thickness, and so, despite the reparation would have a useful life, it would be brief.
The pads are usually considered consumed when they reach the thickness of 1mm, but also in this case it is good to consult the manual of your own motorcycle.
Always remember that the brakes are the most important part of the motorcycle in matter of active safety, and that efficient and well maintained brakes (which also includes the periodic substitution of oil) could save your life.

Curiosity

This phenomenon was quite unknown until the 80s. In fact, back to those times, in the composition of the brake pads was present asbestos too, which thanks to its characteristics avoided the appearance of this annoying effect. Since when the carcinogenicity of asbestos and it has finally been banished, it was necessary to use other materials, which present more this inconvenience.In the long slopes, instead of constantly keeping a light pressure on the brake, it is better to brake intensely to reduce the speed, then let the motorcycle go (using the engine brake when possible) to let the pads cool down and clean themselves, so brake intensely again for a limited period of time, which means using the brakes discontinuously. This behavior will avoid the crystallization (vitrification) of the brake pads.

by Fabio Bizzetti