The new Superior axles are stronger than the stockers for several reasons. First, they feature a larger diameter of 1.42 inches and 33 splines compared to the 1.31-inch diameter of the 30-spline stock axles. Also, the Superior axles are made of a heavy-duty alloy and feature rolled splines. You can also see here where the stock axles (bottom) neck down while the Superior axles do not. All of these features add up to make for a much stronger axle.