RE: grooved, etc. brake disks

[Robert Wilkinson <wilk@domain.elided>]

I have followed with interest the thread on grooved, ventilated, drilled,
etc. rotors.  Regarding the question of grooves increasing/decreasing the
surface area (and I still don't see how area is increased), I wanted to
point out an overlooked fact:  The braking force (which is frictional drag)
is, to a first approximation, given by the normal force between rotor and
pad times the effective coefficient of friction between the two.  The
normal force is determined by the pedal force, the leverage of the pedal in
operating the master cylinder, and the ratio of slave cylinder(s) to master
cylinder cross-sectional area.  The coefficient of friction depends on the
rotor and pad material.  The swept area doesn't enter into it at all.
Decreasing or increasing it has no first-order effect on braking force
(think of drum brakes).  Second order effects include rigidity,
temperature-dependence, etc--here the pad size and a myriad of other
factors determine whose design is better. One can't easily predict the
virtue of modifying the rotor, but slight swept area change per se is
unlikely to be the dominant factor.  [Note that the same applies to tires,
whose braking/acceleration/cornering force is to first approximation
independent of contact patch area.  Moreover, unlike brake pads, contact
patch area is itself independent of the tire size.  Thus tire width is a
THIRD-order factor in determining traction!]

Bob Wilkinson
72 Spider 

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