Well, my front brake gave a little metal-to-metal squeal yesterday. I
looked down when I got home, and sure enough, it's time for new brake
pads. And, for that matter, possibly a new brake rotor, because this guy
is looking a bit tired and worn -- the previous owner put sintered
metallic pads on the thing. We'll see what happens when I clean it up. The
good news is that the cycle accessories place down the street is a Galfer
dealer. The even better news is that his Galfer distributor is about 1
mile away from his shop, so that even though he'll have to order pads,
they shouldn't take long to arrive.
I looked at the "big rotor" kit. I'm not convinced. Braking efficiency is
detirmined by three things:
1) Temperature of the rotor and brake pads. This is reduced by the big
rotor, but modern brake pads aren't as sensitive to temperature. However,
the big rotor does allow more heat dispersal.
2) Area of brake pad in contact with the rotor. This is identical between
the two.
3) Amount of force exerted against the rotor by the brake pads. This,
along with the coefficient of friction, detirmines how much directional
energy will be transformed into heat energy. The calipers continue to
exert the same amount of force against the rotor, and steel continues to
have the same coefficient of friction that it's always had.
Now, there seem to be people absolutely convinced that the big rotors make
their brakes stop better. They point to the amount of suspension dive.
However, suspension dive is not only a matter of weight transfer, it is
also a matter of moment arm. The relocated brake pads, being further out
on the fork, will transfer additional torque to the fork. This is not
causing additional braking by any means -- amount of braking is detirmined
by how much forward energy is converted into heat energy. This has
absolutely nothing to do with fork dive.
The other thing they point to is how much easier it is to do a "stoppie".
But again, this is a matter of moment arm, not of braking efficiency.
Another confounding issue is the fact that most people also upgrade to the
Galfer stainless steel lines at the same time. The Galfer lines will cause
greater braking power with less lever motion, because they have a slightly
smaller inside diameter, meaning less buffering by the fluid column in the
brake line. More of the force instead goes against the calipers. So people
may be confusing the better feel from the Galfer lines with the effects of
the rotor. People also often update to the Galfer green brake pads at the
same time, which will increase the coefficient of friction between brake
pad and rotor, and thus stopping power.
It appears to me that the temperature issue is the only one where the
larger rotor will produce better results. If I were intending to go down a
4% slope using my brakes rather than my engine to slow me down, I might
think it's worth it. But at the moment, looking at the simple physics
involved, it seems to me that it'd make more sense to just put another
OEM-style rotor by EBC or etc. on there (for half the price or less of the
big rotor kit) and go with the Galfer Green pads and stainless steel brake
line.
From what I can tell, the only way that the bigger rotor would result in
more braking power would be if there were bigger pads and more powerful
calipers to go with it. Without that, it seems like a vanity project to
me, not to mention that it puts forces onto the fork that the fork wasn't
designed for. Am I overlooking something? Is physics lying to me? Did I
overlook a coefficient of friction somewhere that is improved by the
bigger rotor? Do the big rotor kits include something that isn't shown on
the web sites (which show a rotor, and a bracket for moving the calipers
upwards)? Scientific minds want to know!
-E