Subject: Brake Squeal
From: Jobst Brandt
Date: May 11, 2001
Most car, motorcycle, and bicycle brakes squeal at one time or another because they involve stick-slip friction whose frequency is supposed to be out of audible range. Squeal is not only annoying, it decreases brake efficiency, especially in the lower frequencies where the length of slip motion exceeds that of stick.
Brake noise requires elastic motion (vibration) at the sliding interface, with at least one element in rapid stop-start motion. Because bicycles use hand power and demand light weight, they use relatively flimsy mechanisms and demand pads with a high coefficient of friction. The brake material must be soft and pliable enough to achieve good contact on relatively rough rims. The brakes generally have a mechanical advantage between 4:1 and 6:1 from hand to rim, as described under "Brakes from Skid Pads to V-brakes." That's not much compared to motorcycles that have hydraulic disc brakes with practically no pad clearance. For a hand brake, free travel (pad clearance) and flexibility define the limit of mechanical advantage.
Soft brake pads and lightweight (flexible) calipers promote squeal and chatter, chatter being the mechanically more detrimental version of stick-slip behavior. Brake chatter is caused by gummy residue on the rim together with excessively flexible (skimpy dimensioned) brake mechanism. Rims can be cleaned but flexible brakes can only be fixed by using better brakes. If the rim becomes gummy again after cleaning, then either the rims are being contaminated by something like riding through tar weed or the pads are no good. My solution for pad quality is Kool-Stop salmon red pads.
Squealing brakes, the more common problem, involves mainly brake pads that generate caterpillar-like surface waves. The common advice is to bend the brake caliper to make the trailing edge of the pad (with respect to rim motion, the forward end of the front brake pad) contact first. This is not entirely without merit because toe-in is the natural state of a used, non squealing brake. Elasticity of the caliper, however small, allows the pad to follow the rim and rotate forward about the caliper arm, wearing the heel of the pad more than the toe, causing toe-in. Toe-in is preferred because a pad that makes full contact as it first touches the rim will rotate slightly from frictional drag, reducing contact...and drag, which allows it to snap back and repeat the action. This causes surface waves in the pad, especially when it is new and thick. For this reason, some pads are made with thin friction material to reduce elasticity.
If the pad contacts the rim trailing-end first, it develops full contact stably as pressure and frictional drag increase. However, the brake may squeal anyway. This can occur with new rims or one with wax or oil, or from other contaminants like riding across a moist lawn. New pads often have a glossy, sticky skin that should be removed by either sandpaper or use. Many types of rim contaminants that increase stiction (stick-slip) can be removed easily by abrasive scrubbing. This can be done by braking at moderate speed with a dusting of household cleanser on a moist rim, followed by a water bottle squirt rinse (also while braking). This process is more conveniently achieved by slowly riding through a long mud puddle while braking or by descending a mountain road in the rain where there is usually plenty of fine grit and where rain supplies the rinse.
Some rims have machined brake surfaces with fine grooves whose roughness reduces squeal tendencies so they don't have to be "broken in". Martano rims of old had somewhat larger grooves as part of the extrusion for this purpose.
Avoid bending brake calipers. This is "cold setting" in its worst form. Aluminum in such cross sections doesn't bend far without structural damage. Besides, this remedy could lead to more bending with each occurrence of squeal, and that is better abated by other means.
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