# Why do softer brake pads stop a bike faster?

• B
Why do softer brake pads stop a bicycle faster, and wear faster, and harder brake pads stop a bicycle slower, and wear slower? Thanks Much in advanced!!

## Answers and Replies

Related Other Physics Topics News on Phys.org
scottdave
Science Advisor
Homework Helper
My initial answer is that the coefficient of friction between a softer pad and the rim is higher, which does work out, but only provides part of the answer.
There are some factors which are not known, like how hard the calipers are pressing in each situation.
But to simplify, think about this: if the pad is wearing, then small bits are being torn off. It takes force to tear them off. If you could zoom in on the pad as it brakes, you would probably see it shearing (angling) some as it brakes. This shearing force comes from the rim.

I hope this helps.

jerromyjon
My initial answer is that the coefficient of friction between a softer pad and the rim is higher, which does work out, but only provides part of the answer.
There are some factors which are not known, like how hard the calipers are pressing in each situation.
But to simplify, think about this: if the pad is wearing, then small bits are being torn off. It takes force to tear them off. If you could zoom in on the pad as it brakes, you would probably see it shearing (angling) some as it brakes. This shearing force comes from the rim.

I hope this helps.
I do mean the question assuming the same amount of brake force is being applied with each type of pad. I suppose I could have clarified better. I meant the question in terms of where does all the kinetic energy go exactly. I know some of it goes to thermal energy but does pad wear count as a type of energy loss? How does the pad wearing faster cause the bike to slow down faster? Is it that softer pads rely more on non classical friction than harder brake pads. For example I'm guessing it can deform to the surface of the rim better which creates a mechanical bond to some degree, and even then when the particles of break pad and rim break off how does that slow the bike down faster? I don't think it necessarily causes the rim or pads to heat up proportionally more. Is it that the particles carry thermal energy with it when it leaves the bike and they cool down when they hit the air or that the particles carry momentum with them and then those particles rely on a series of inelastic collisions to convert their kinetic energy back into thermal energy after they leave the bike? Thanks Much in advanced, and sorry if the question doesn't make sense or if I worded it improperly!

sophiecentaur
Science Advisor
Gold Member
Why do softer brake pads stop a bicycle faster, and wear faster, and harder brake pads stop a bicycle slower, and wear slower? Thanks Much in advanced!!
The contact of a softer pad is greater than that of a harder pad. That gives more opportunities for interaction between the two surfaces. You have to chuck out the simple concept of a linear coefficient of friction which we are taught. The coefficients of friction will be different for different materials, for a start. There are plenty of 'soft' substances which would be really poor as brake pad material. I think what I am trying to say is that the implied 'experiment' that the OP describes is not really a fair test. The only thing in favour of hard pads is that they will probably last a lot longer, which is what your everyday (low power) cyclist probably wants.

scottdave