"Tosspot" wrote in message
...
Why do they have holes in them?

Car discs don't, motorbike discs don't, aircraft discs don't. Why do
bicycle discs have them?

Damn - I was just seeking crowd funding for a start up in motorcycle disc
locks.

Vaguely remember seeing a car disc with holes - but can't remember where.

its to a considerable extent, a trendy thing, and not many aircraft cruise
past the girlies on the high street. Aircraft probably use cast iron discs
because it works better and is lighter than stainless - and cast iron might
be too brittle to make loads of holes in it.