The Superbike racing motorcycles must have four-stroke engines of between 850cc and 1200cc for twins, and between 750cc and 1000cc for four cylinder machines. The Superbike racing motorcycles are derived from standard production models, so for a bike to be eligible, the manufacturer must first homologate the model and manufacture the required number of roadgoing machines. While rules vary from series to series, in general the motorcycles must maintain the same profile as their roadgoing counterparts, with the same overall appearance as seen from the front, rear and sides.In addition, the frame cannot be modified. Teams may modify some elements of the bike, including the suspensions, brakes, swingarm, and the diameter and size of the wheels.The restriction to production models distinguishes Superbike racing from MotoGP racing, which uses prototype machines that bear little resemblance to production machines. This is somewhat similar to the distinction in car racing between touring cars and Formula One cars, though the performance gap between Superbike and MotoGP racing is much smaller.