Originally Posted by
David Broon
There is, but I've never seen them enforced except at nationals, and at a few huuuuuuuuge cadet (15-16) races.
It's enforced every time in the Mid Atlantic; you better test it ahead of time to be sure.
It's probably a good idea not to be too close to the limit, as you never know if you'll get an official who flunks it because of a slightly short marking on the road or such.
My juniors use 46 chainrings so they can use 13x25 or 13x28 10s cassettes.
Sheldon Brown (RIP) has a gear calculator here->
http://sheldonbrown.com/gears/