# Bicycle Mechanics - Braking Theory Question

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View Full Version : Braking Theory Question

Stacey
02-09-07, 05:22 AM
Which brakes better, larger or smaller pads?

Same bike, same levers, cables & calipers. Same pad compound. The only variable is the contact size of the brake pad. Which brakes better & why?

demoncyclist
02-09-07, 06:09 AM
In theory, a larger pad will stop faster due to a larger contact patch. In practice, it probably won't feel much different unless you are trying to panic stop at 60 mph.

yairi
02-09-07, 08:05 AM
hmm, in theory actually, the equation is F=μN, where the force is found by the product of the normal force (how hard you squeeze) and the coefficient of friction (how grippy your pads are). This will hold if your rim is perfectly smooth. So the size of the pad should make little to no difference. It isn't like tire friction on pavement where you have to consider mechanical locking as the rubber conforms to surface irregularities.

TallRider
02-09-07, 08:17 AM
hmm, in theory actually, the equation is F=μN, where the force is found by the product of the normal force (how hard you squeeze) and the coefficient of friction (how grippy your pads are). This will hold if your rim is perfectly smooth. So the size of the pad should make little to no difference. It isn't like tire friction on pavement where you have to consider mechanical locking as the rubber conforms to surface irregularities.
+15 (more than +1, since his understanding of the physics/mechanics is correct)
So the upshot: practically, it doesn't matter too much if you're using larger or smaller pads, at least not for concerns over contact area.

There may well be other reasons for using longer brake pads (such as those typically found on v-brakes), such as toe-in mechanics and such. But I'm not familiar with those issues.

Rincewind8
02-09-07, 09:06 AM
Things to consider:
a) With water on the rim (e.g. riding in rain), the smaller pad might have a small advantage, since the pressure (P=Fn / A) is higher than with a larger pad.

b) In general, smaller pads are made from a harder/stiffer compound than larger pads. This is due to the different pressure on the contact surface. Since you specify that the pads are from the same compound, either the compound of the small pad is 'right', which would make the compound of the large pad to stiff (as in the pressure would be to low to engage the pad into all the grooves in the rim (Assuming you are using aluminum rims)). Or the compound of the large pad is 'right' and the small pad is to soft (which means it wouldn't last).

c) If you are using aluminum rims, the larger pad might have an advantage, since it "catches" more irregularities/grooves.

d) For prolonged breaking the large pads might have an advantage, since they have a larger area to dissipate the heat.

e) Brake pads work best at a certain temperature (the coefficient of friction between two given materials changes due to many variables, one of them being temperature (that's part of why Formula1 tires are preheated)). Before you reach optimal temperature the coefficient of friction is less and when the pad is to hot the CoF is lower again. (as a side note, melting/burning rubber has a really low CoF)