Harley Davidson began building Sportsters in 1957 and they are still being built today.These motorcycles started out  as the k, kk, kh and khk  models, built from 1952-1956 which had a flathead engine.Okay so what is the difference between a big-twin and a sportster engine. A big-twin has an independent transmission and a sportster has the  motor and transmission in the same case.In 1972  the displacment of these engines was changed from 900cc to 1000cc. Both of these models were know as “ironheads”. Which were built until 1986 when they were replaced by the Evolution design.Which introduced the  883 cc and 1100 cc models with alloy heads the previous being cast iron.1991 had another improvement on the design of the transmission went from a 4 speed to a 5 speed.In 1998 the XL1200S was introduced with dual spark heads and a performance cam. 2004 Brought about another change as the engines were now rubber mounted to cut down on vibrations from the engine.In 2007  was the end of carburetion from this point on it was fuel injection only. Sportsters are often referred to as girl bikes  due to there lighter weight.But on the other side of that is the fact that being lighter also means it is easier to build them up to be faster than a big-twin. I have seen some quick sportsters and as long as your riding who cares.