Originally Posted by
PepeM
How thick are bicycle discs?
If we assume that the holes are circular, for the surface area of a drilled disc to be larger than that of a solid disc, the following would need to be true:
surface area of exposed material due to hole > surface area of the material that would be there if there was no hole
2*pi*r*th > 2*pi*r*r
th > r
Is that usually the case? What am I doing wrong?
Your formula is correct (yay, math!). However, the heat transfer coefficient (call it "h") is affected, too. Here's an approximate equation:
Heat transfer rate = h * Area * (Rotor Temperature - Air Temperature).
The accurate set of equations are way too much for a forum post. But this equation can be used to understand the issue. The heat transfer coefficient h is affected by the flow of air over the rotor, and I suspect that the holes increase turbulence and hence increase h more than they decrease surface area.