Square hole crank fit


i have a Qu-ax 24" inch unicycle, one of the older ones (2001) which i have refitted recently with standard square holed cotterless cranks. These were very cheap but fit poorly. Since i commute and unicycle a lot the cranks start to get al life of their own and move a bit at every revolution. When this happens the fastener nut loosens. It has not fallen out but it does not inspire confidence cycling in traffic when a crank may fall of at any moment.

I have used an ever increasing degree of violence using a hammer but the cranks stubbornly refuse to stay in place.

So my questions is do i buy more expensive cranks or is there something i am missing?


Well I’m using three sets of the cheapest available chromed steel square-taper cranks on equally cheap hubs and I haven’t experienced that, so I think it’s possible for it to work. What torque value do you use when you tighten the nuts? I use 25 ft-lbs, which Google converts to 33.9 Nm.

I’d definitely check to see that the crank sits along the hub taper, not all the way against the shoulder. (Damaged hole or wrong taper size.) Also it’s possible to need a washer under a nut or bolt head, if it runs out of threads before it tightens against the crank. The Sun hub that I redrilled and retapped for new crank threads is like that.

Good luck with it. I’m sure that’s annoying.

thanks for the tip, and happy new year:). The crank is not all the way in on the hub. In the original set up they used a single nut, (with grooves) and no washer. I have the feeling that the crank can move after tightening but it feels more like rotation than a linear slide. Maybe the square shape is not square enough, more like a rectangle with two sides longer than the other. I have tried various orientations but that doesn’t solve the problem. I will take it apart again and look for abrasion marks.

OK, I hope you find it. I have heard of putting thin metal shim material inside the holes to improve the fit on the taper, but haven’t needed do that myself–not yet at least. So you might be able to bring it closer to square with two small shims if that’s the problem.

Once the flats of the crank taper become rounded, even just a little, it is next to impossible to ever secure the tapered interface again. Using thin aluminum shims can be a temporary fix if the crank was pressed too far onto the spindle resulting in an enlarged hole int he crank, but that is a different issue.

After replacing the arm with a good one you want to tighten it securely, and then after riding it for a bit tighten it again. After initial installation the crank works itself further onto the taper, and if you don’t secure the nut/bolt at this point it becomes loose which ultimately leads to ruining another crank arm. With proper installation the crank should last years, if not forever, barring any significant trauma.