Sheldon Brown:

Michael wrote:

I just noticed that Avid's mechanical disc brakes come with 160, 185
or 203 mm rotors. It's claimed that the 185mm rotor has 15% more
"power", the 203 26% more.

Does a larger rotor really provide more stopping power? Why? I could
understand less fade due to heat, since a larger rotor would dissipate
heat better. Does that translate into more "power"?

I assume that dimension is the rotor diameter. But wouldn't a change
in rotor diameter change where the caliper gets positioned, i.e. would
require a change to the caliper mounting eyelets?

Certainly.

Avid's use of the term "power" in this context is sloppy and inaccurate.
"Force" would be more correct, and the force is proportional to the
rotor's diameter (assuming the caliper is repositioned appropriately for
the rotor size.)

Qualifying the term "force" is also required, since there is the force
applied to the pads which is perpendicular to the plane of the rotor,
and there is the force which the pad exerts on the rotor through
friction, which is parallel to the rotor plane. Both are related, of
course.

Braking force is inversely proportional to the tire radius too, so
larger wheels would call for larger rotors to maintain the same amount
of braking force.

Braking torque needs to be examined here as well. The braking torque
required is the same for the same load on any wheel diameter, and since
the moment arm on the caliper mounts is increased with an increase in
rotor diameter, the stresses on the mounts increase as well _despite_
the braking force remaining the same (oe even decreasing).