The only restriction is that you cannot go less than 20inch tires. There needs to be a minimum distance from ground to center of the chain axis of 24 cms. I just checked the Downtube and its fine and it uses 406 wheels, so 451 would be even more clearance. Now the chances of anyone actually checking this is pretty minimal- I have never seen any official with a tape measure at any race! Its really more for the pros