Better brake pads:
Why Friction is Good

What is the job of the brake pad? Put simply the brake pad is supposed to create as much resistance to movement of the rotor over its surface as it can. The result of that is mechanical energy is converted to thermal energy – heat.

But it isn’t all that simple. The coefficient of friction C_f is a measure the resistance to motion of two objects sliding past each other. In this case it is the rotor and the pad. By multiplying the C_f by the psi of the pad pressing against the rotor you get the force acting against the motion of the rotor. F = C_f x P where P is pressure. That force figured into the moment arm of the wheel acting on the road will give you the force trying to slow your forward motion; assuming of course the tire hasn’t started to slip. This is oversimplified but it gives you the idea.

However, the heat mentioned in the first paragraph effects the C_f of the pad. C_f typically goes down after a certain temperature but it can also be needed to get the pad to even start really resisting motion in the first place. C_f can often be low when cold increasing with heat till a certain point after which is drops off again; like a mountain. Or it can be very high to start and then drop after it heats up; like a plateau. Hence why you will here people say that pads have a heat range rather than just a simple max temp.

On top of that the heat generated travels into the calipers, heats up the fluid and can cause the fluid to boil; fluid fade. Other things that happen with pads include wearing away the rotor surface, creating dust, and noise.

All of these things are taken into account when designing and choosing a pad. Do they need to be heated up to work (bad for the street), do they fade easily when hot (bad for the track)? Do they never really fade but also never grab all that well to start? Will they chew up rotors? Will they chew them up when they are cold? Do they dust a lot? Do they make noise when cold? All the time?

The problem here is that many of these specifications are work against each other. You can have a pad the grips like no other but it might dust a lot, or it might fade at a temperature that though is pretty high it is a cliff rather than a gentle slope. Or those pads might make a lot of noise when cold and even chew up rotors when cold. Conversely you could have a pad that works great right at the first stop when cold but has a low fade temp. Or maybe its fade temperature is relatively high but the overall grip of the pad is low (which limits the pads ability to get really hot to start with).

Then there are other things like some pads are "insulated" with a ceramic backing whose intent is to reduce the heat transfer from the pad backing into the caliper pistons and thus prevent fluid fade (boiling).

Ok so that is a basic run down about brake pads. By far not fully comprehensive as there is a good amount of technical theory and practice which goes into designing pads and the actual events that occur at the pad/rotor interface: many pads rely on a chemical reaction occurring at the interface to aide in increasing the co-efficient of friction, reduce rotor wear, and even noise. So here is a list of the most common pads used and a brief synopsis of their pro’s/con’s. I personally have tried the AXXIS and both Carbotech pads so I can comment on those; I am relying on digest posts and conversations with other racers for the other pads.
 
 
 

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