Topic: Can somebody explain this? Drilling a rotor increases it's surface area.

[gnarlycharlie4u]

I'm surprised noone mentioned this but when you drill a rotor, yes it does cool faster but it heats up a lot faster too

[Syscrush]

(Pi x D) x L x N

Pi =3.14159....
D=drill dia
L= thickness of rotor
N= number of holes


Opps, what can say I'm old.

That gives the gain due to the sides of the drilled hole, but it doesn't account for the loss in swept area on the faces of the disc.  So the correct formula for the net increase would be:
((Pi x D x L) - (2Pi x (D/2)^2)) x N

Consider the case where D = 2L (hole twice the size of the rotor thickness):

(Pi x 2L x L - 2Pi x (2L/2) ^ 2) x N
= (2Pi x L^2 - 2Pi * L^2) x N
= 0

So, if the diameter of the hole is less than TWICE the rotor thickness, then you'll have a net gain.  If it's more, you'll have a net loss.