Yes, it is possible, but no, it wouldn't be better. Heres why...
Disc brakes are used primarily where extreme stopping power in adverse conditions is needed. A BMX bike would never encounter these types of conditions.
Current BMX brakes are made so that the speed of the bike can be modulated carefully to reduce speed on a gradual but even level. This is why many people that get bikes with U brakes complain that they don't stop... they really aren't supposed to. Racing BMX bikes usually use V brakes which WILL stop pretty quickly but these types of bikes usually don't require precise brake control.
Are there 20" wheeled bikes with discs? Yes
yill never no???????????
October 2008