Crank arm had it ?

rmissin

I've only had my bike a couple of weeks, after about a week the left crank arm started to come loose, it kept having to tightening it up after every few miles, but it constantly came loose and creaked like Paris Hilton's bedsprings.
It's a Shimano FC M311 with 8mm allen bolt to tighten.

I'm sure it's had it and needs replacing, when I took it off this is what it looks like..





My main concern is does the main crank spindle need replacing as well ? bog standard tapered square, it looks in reasonably good nick, although a little battered.



Is it possible to buy just a new left crank arm or do I need to get a full set ? Any temporary fix in the meantime ?

tia

jimothy78

The crank arm is completely shagged, and although you can indeed buy left-hand arms independantly, it might not be worthwhile doing so in this case.

If the crankset is the FC-m311, then the chainrings are riveted (not bolted) on. This means that once the rings are worn, you're going to be faced with replacing the whole crankset anyway, so it probably doesn't make sense buying a left hand arm now, and then ending up replacing both a while down the line.

As for the bottom bracket (the spindle and cartridge bearings that carry it) I'd be tempted to replace this too - Firstly, while it doesn't look as bad as the arm, any damage to the interface area would lead to the new crank arm failing to sit properly, and likely lead to the same problem again. Secondly, when you do replace the crankset, you'll need to get a BB with the right spindle length for it (required spindle length is determined by the crankset, not the frame). A new square taper BB will only set you back about a fiver, anyway.

rmissin

Ouch....in that case I hope I'm covered with the 6 week back to base thingummy-jig that Halfords offer when you buy a bike.
Cheers for the info jimothy, the crank and gears on this thing are pretty basic, (Mongoose Tyax 29'er) probably the first port of call for an upgrade.
I have managed to get it back on pretty tight this time, I cleaned all the oil and gunk of the crank and spindle, sprayed on a bit of lube and tightened down, it "feels" tighter than it has before, but yeah it's shot to hell and won't last long.

jimothy78

rmissin wrote:

Ouch....in that case I hope I'm covered with the 6 week back to base thingummy-jig that Halfords offer when you buy a bike.

Oh, if the bike's that new, then take it back to halfords and ask for a replacement. The damage has happened because the thing's been ridden when the retaining bolt had come loose, but that suggests that it wasn't properly assembled (or that something was defective) from the outset.

rmissin

Herein lies the problem...I tool it home boxed, I was too impatient to wait another day for them to assemble it.

The crank arm was already on the bike, so "something was defective from the outset" is my only hope.

jimothy78

rmissin wrote:

Herein lies the problem...I tool it home boxed, I was too impatient to wait another day for them to assemble it.

The crank arm was already on the bike, so "something was defective from the outset" is my only hope.

Hmm, if the crank arm was already fitted, then you should still be able to argue that any assembly fault was at the manufacturer/supplier end, not yours (although they could try to counter that with the "check all bolts are fully tightened before riding" caveat that would have been in there somewhere).

Whatever the rights and wrongs, I reckon if you stick to your guns, ask to see the manager, etc (and talk quite loudly about your "unsafe" bike when the store's full of parents looking for bikes for their kids) that they'll be happy to sort you out.

Alternatively, you could chalk it up to experience - you can get a fairly decent square taper crankset for less than £30, and a good Bb to match for another £5.

http://www.rosebikes.co.uk/article/shim ... 1-crankset
http://www.chainreactioncycles.com/shim ... -prod34652

rmissin

Yeah, not too much of a sting, plus the cost of the tools to do it of course, I'll see what happens at Halfords. they seem a good bunch and I'm not one for kicking off in the store.

With hindsight I should have sorted it sooner, I wasn't "at one" with my bike so much then.

The Rookie

I doubt the hardened steel BB axle has been damaged by soft alloy arm....

Dirtydog11

Don't forget to mention that you checked the bolts before you set out and they where all tight.

rmissin

Dirtydog11 wrote:

Don't forget to mention that you checked the bolts before you set out and they where all tight.

Oh yes...tightened with my mates torque wrench to the specified 41n....twice