Wogster

Other day, notice my left crank arm was lose (square taper), stopped at a bike shop, borrowed the crank arm wrench, and cranked down on the bolt, until it was nice and tight. Rode home, it's lose again.....

So, I am wondering, is it the bolt or the crank arm? I was thinking of pulling the bolt, cleaning it, greasing it, and putting it back in, if that doesn't work, then is it the arm itself, and if it is the arm, can you replace just the one, or do you need to replace the whole crank?

I am assuming that if I replace the crank, I should replace the chain and cassette as well, but what about the cartridge bottom bracket?

I don't follow this forum much, but I don't want to spend a lot on a solution either......

PDXJeff

Could also be the square post of the BB spindle has lost its taper.

If it is the crank and you can find a match, or similar that you like, then you could just replace one. The chain and cassette really just need to be replaced if you are also replacing the front chain rings, IMO.

The bottom bracket only needs to be replaced if that's the source of the problem. If it's not broken, don't fix it.

Bill Kapaun

Usually, once they start to loosen, it's a lost cause.
The square on the crank gets "wallowed out" and will move under pressure.

dabac

With an aluminum crank arm and a hardened steel spindle that ain't gonna happen. Unless properly torqued down a BB steel spindle will happily chew its way through several cranks w/o suffering more than a slight discoloration.

dabac

+1

On a real bad crank one can see how the hole has changed from square to something almost star-shaped.

dabac

Well, that's a familiar enough story.
Probably neither, but rather poor assembly.
Depends on how far the crank has gone. Whether to lube or not is something that generates heated discussions on a regular basis, but here's what I'd do:
- remove bolt
- remove the loose crank
- check that bolt goes in smoothly in bb spindle, if not chase threads and try again
- wipe off tapered part of bb spindle, grease lightly
- run a rag through crank arm hole, grease lightly
- lube bolt, then reassemble with proper torque

Possible further actions of undetermined effect are:
- instead of lubing bolt thread lock may be tried
- some sort of press may be used to seat the crank instead of merely relying on the compressing action of the crank bolt.

That is the highest probability, yes. Once it starts to wobble it quickly destroys the fit of the square taper hole.

You can replace just the arm. This being such a regular occurence, it isn't at all unusual for a lbs to have a couple of left, noname cranks in store. If they haven't, they certainly can order one.
It might be that the nicer cranks are only sold in pairs though.

The left arm crank is entirely independent of the toothed parts of the drivtrain, so there's no need to tinker with that because of the wobbly left crank. The bb is also entirely uninfluenced by it.