This heavy duty shaft has a longer spline than stock so that the upper pulley of a belt drive has more support. This helps prevent premature wear of the splines which can cause excess wobbling.  It also has a bolt on the end to keep the pulley tight against the bearing to also help eliminate wobbling of the upper pulley. This is the 2015 style shaft, so if you have a 2012-2014 model you need the newer style bearing.