MOBI > Sweet Wings Cranks

Cranks go around and around, but the loads on them are offset and alternating. Thus, crank joints must be tight, else they will work back and forth under load and either unscrew their holding bolts or wear away the joint and get loose.

Sweet Wings cranks use a loose press-fit spline and an axial bolt. The two halves are clamped together around the inner race of the right bottom bracket bearing.

ic3076.025.jpg   ic3077.025.jpg   ic3079.025.jpg   ic3080.025.jpg  

A loose press-fit spline is appealing because it can be made with standard industrial tools and relatively low precision (by contemporary standards) and thus relatively low cost.

However, imagine standing on the pedals with your left foot forward: it will tend to "take up the slack" in the spline by letting the pedals "sag" slightly towards the ground. Now suppose you turn the cranks half a turn (180°) and again stand on the pedals. Once again, the spline will shift, to take up the slack the other way.

The problem is there is lash in the spline. The only thing that resists the back and forth motion is axial friction between the two halves. However, over time the tiny back-and-forth motions tend to either unscrew the bolt or wear away at the axial faces, thus making the joint slightly looser. The looser it gets, the more often it moves.

Users report after weeks or months the crank starts to creak, click, or make other noises. Assembly with generous use of a strong LocTite™ seems to solve the problem for some users and make it rare for others. If the joint were designed with a large enough spline, the LocTite might be strong enough to work for almost everybody. But as-is, many users report LocTite is not enough.

Approaches used in other cranks include a much tighter press-fit spline, a tapered spline, an axially-tapered spline or a split spline with a pinch clamp.