Single Speed Cranks Issue

[Bullpup]

Good afternoon All

My teenage son has an ongoing issue with his Single Speed Cranks: they keep on suddenly coming loose and falling off!

This has happened with both X type and square taper Cranks. It has occurred on both drive and non-drive sides. In both cases I would suggest they were installed properly with the exception of the X type where the non-drive end cap was not installed although I had thought this was largely cosmetic.

The problem mainly occurs on the hills e.g. the Lammernuirs. The rings are Front 34: Rear 17

He doesn’t have any Crank issues on any other bikes.

What is going wrong please and how do we solve it? And is technique a contributing issue?

 

[Imlach]

Can't see any reason why any decent crank arms should fall off. I had the issue of crank arms coming loose years ago, and that was due to rounding of the taper and not applying enough torque when installing. I always use torque wrenches on all my bikes now, but I'm also a heavy rider. What are these 'single speed cranks'?

 

[Bullpup]

Fair question: shorthand for ordinary Cranks used with a single ring. The X type are FSA V drive and the square tapers are Raceface.

 

[Imlach]

It's odd that he experienced it with two different cranksets. The only thing that comes to mind is, as said earlier, lack of torque when installing. Also you say he rides hilly climbs, and probably applies a lot of force when climbing, which might allow them to wiggle loose if they aren't firmly installed. I usually go for around 40Nm as a rule, no grease.

Should also add that if they have come loose while riding multiple times, probably a good chance that the taper is bust.

 

[pete_mcc]

I’m with Imlach on this. In most instances loosening is caused by bad tapers on either the crank or bottom bracket. With square tapers it can also be caused by using an ISO crank on a JIS BB or the other way round. Once a taper on a crank is gone then it’s impossible to restore. You can crank up the torque on the bolt but it won’t do a huge amount in the long run.
Biggest cause is not having the cranks tight in the first place, riding up hills will exacerbate this and this in turn will screw the tapers meaning no matter how tight you go they will loosen off again, mostly on hills when you are really pounding on the cranks.

 

[Bullpup]

Thank you.

I am concerned about the JIS /ISO issue with the square taper bottom bracket as I am using a Shimano bb with a Raceface Crank. Is this really likely to be a concern?

 

[greencat]

I'm sometimes of the view that a persistent problem with a particular combination is the universe's way of telling to try something different. I'd bin the cranks and the BB and get one of the modern external ones.

It could be user error in installing them (too tight or not tight enough initially) and as others have said once a crankset has loosened once, it's pretty much trash.

 

[Woz]

There was a thread a while ago about Raceface cranks requiring their own BB. Worth looking into.