Too many posters with comments to address so I'll just fling a bunch out there. AS to car rotors, they are radially vented. Drilling a radially vented rotor would likely be certain death. That's why its not done. To the extent they want holes they are made that way.
Drilling bike rotors does not reduce breaking force due to a reduction in swept area.
We had an extensive thread on this subject many moons ago. It was led by my memory of reading a technical piece on why drilling a rotor does not reduce breaking force, but i couldn't remember why.. MAny of you stepped forward with slide rules in hand and presented the cae with formulas and physics etc. In laymans terms the force is determined by the master cylinder. It pushes out X hard. This translates into PSI at the disc surface. If there is less disc surface to push against the PSI increases. However, the force is not lost, or reduced, because there is less surface area. Its the same amount of force regardles. The trade off is increased pad wear. This is nominal in the drill patterns most of you are using. If you've seen mine, well there is more not there than there. If i ever get the chance to ride mine enough to wear out the brake pads, well good for me.
Cast iron is the best surface for braking. Ever seen a hard core bike racer that's been sitting? The discs are rusted up. Since auto discs are seldom seen, or at least less so, cast iron is fine. For cosmetic purposes bikes use stainless and sacrifice some braking. My FJR discs are some sort of blend. Let them sit long enough they'll rust, but its easy to clean. Any amount of regular riding keeps them clean.
But I think drilling cst iron is the no-no. SS is OK.??
So I'm going for the look. That makes me a poser? Guess so. What's wrong with that? bikes and race cars are toys. To think otherwise is the ultimate posing.
I've heard of bike rotors cracking never seen it though.
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