Fact: square taper bottom bracket spindles are no longer up to the punishment most riders dish out.  The main difference between square taper spindles and splines is the amount of surface area in contact with the crank arm; splines offer more contact area, making for a solid join.  The spline shape also means the arms are less likely to move on the spindle.  Most steel cranks use splines, and Shimano, TruVative and Race Face use them on their top aluminium alloy cranks.

    As the spline revolution took over, manufactures came up with their own fitting system.  Recently, however, the ISIS (international splined interface standard) system has been developed with different manufactures agreeing on a single spline system that fits different cranks,  This allows you to choose which crank and BB set that is best for you.

    The ISIS spline system also has the advantage that the cranks tighten down onto fixed stop points, so chain line remains constant if you remove or refit a crank.

Square taper and some other spline systems don't ensure a constant crank position if you remove and re-fit a crank.

Splined

Square