| MadSci Network: Engineering |
Brakes work by friction. Simple bicycle brakes apply a pad against the rim of the wheel to slow down the rotation. The pads can be made of all sorts of hi-tech materials or simple rubber. You're right that the squeal is in part a mechanical oscillation. But as annoying as the squeal may be, it is almost a necessary evil of the braking mechanism. The heat/energy of braking must go somewhere. That is why over time, brake pads wear down and must be replaced. The squeal can come two sources: mechanal oscillation of the brake and wheel or from the heating of air between the brake and the wheel rim when the brake does not hit the rim perpendicular.
Let me propose one possible mechanism that could produce a squealling situation and oscillation.
Sometimes brakes experience a very rapid "slip-stick" interaction. "Slip-stick" means that the brake grabs for a bit of time then slips for some time. In a sense then, the brakes are working in spurts like antilock brakes only with less control over the duration of on and off. This "slip-stick" can very easily create a feedback loop.
First the brake is applied and sticks. The wheel produces a torque on the brake lever arm (the caliper) so the brake leg moves in the direction the wheel is turning. When the brake lever arm moves the brake shoe will probably turn and some of the brake shoe will not be in contact with the wheel rim and the brake-wheel contact will slip as the brake heats up becomes less sticky. Once the brake becomes less sticky the torque on the brake will be lessened and the brake lever arm will move back to its original position. Now the process will repeat itself.
Quick Summary of Brake "Slip-Stick"
The harder the brake is applied the more squealling that occurs. So one way to reduce the squeal is to reduce the coefficient of friction of the brakes. In otherwords, make the brakes more slippery to reduce the squeal at the expense of longer stopping distance.
Another effort might be to shorten the brake lever. A short lever arm would mean less torque on the brake. This will probably change the pitch of the squeal to a higher pitch which could be even more annoying and painful or it could reduce the effect away to the point that the squealing is much less intense.
Yet, another thing is to adjust the spot at which the brake pad hits the wheel rim when applied in effort to ensure the braking force is directed perpendicular to the wheel rim. This should help braking efforts and could reduce the squeal because less air is right near the brake-wheel interface. This will make the brake more efficient because more brake pad area will apply the stopping force to the wheel rim and the rider will not need to hit the brakes as hard.
Another option is to get fancier (and more expensive) brakes in which a combination of the above has been considered and either a new brake mechanism has been developed or the caliper system has been optimized.
I found a sight that has hydraulic brakes for bikes.
www.hayesbrake.com Hydraulic brakes tend not to squeal as much because the unwanted oscillation that occurs during braking is dampened by the fluid. The brake can sort of "swish" around on the fluid that applies the force to the brake foot.
Sincerely,
Tom "Old Bike Mechanic" Cull
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