Disclosed are exhaust systems for two-stroke engines mounted on bicycles. The exhaust systems include an expansion chamber having a divergent portion and convergent portion. The expansion chambers have different dimensions for the divergent portion and the convergent portion. The exhaust systems improve the power performance of the two-stroke engines operating at below maximal RPM by between about 15% and about 85% when compared to that of the two-stroke engines equipped with a stock exhaust pipe.