Still Just Me... writes:

On Tue, 23 Jun 2009 20:49:34 -0400, Radey Shouman
wrote:

Also, the contents of the holes provide a bulk flow of air past the
*pad*. Without holes, when not braking, there is only a narrow space
for air to flow past the pad, which makes convection cooling very
slow. When braking there is no space for air to flow and cooling can
occur only by conduction, to the disk or to the caliper. With holes
there is significant flow of air past the pad, whether braking or not.
The air inside the holes will be well-mixed, and quickly approach
equilibrium with the surface of the pad. Once past the pad, air in
the holes will be exchanged with the free stream of air flowing past
the disk, thus cooling the pad.

This might apply in a system with a center vented rotor. The holes
would not cool the pads in a single, non vented disk configuration
except by virtue of the fact that they might lower the rotor
temperature while it's not between the pads. Even with the vented
rotor, the flow to the pad is minimal. I suspect that if the holes
have any cooling effect in either configuration, it's when the rotor
is NOT between the pads.

Holes that communicate with central vents would certainly have a
centrifugal pumping feature, as would the radial slots described
somewhere up thread. I think that even holes in a solid disk would
significantly increase air flow past the pad, since without them
air flow is near zero.

The disk is normally rotating, so there are two phases, when between
the pads the air in the holes is heated by the pads, and when not
between them the holes exchange air with the cooler free stream
around the brake. In order to cool the pads both are necessary,
which explains why clutch disks would not benefit from similar holes.