Left Crank Fell Off

[Venod]

Last week my lad said he was having difficulty changing onto the big ring on the crank set, he asked me if I would have a look, so I adjusted the gears and got it changing ok, I said it was probably a bit of cable stretch.
This morning he again was having difficulty, then his left crank fell off it must have been coming loose last week allowing the crank set to move out a little this was causing the bad shifting.
Its a Shimano 105 crank with the two bolt fixing and the plastic end plug that pulls the crank into position.
Has anybody else experienced this?
I can only think it hasn't been fitted correctly to start with.
Its a Giant TCR which he purchased from Tredz.

 

[Punkawallah]

Purchased when? Any pictures?

Don’t think the gear change problem is related.
I would think if the crank was improperly fitted it would not help matters, but until we can see the ‘results’, it’s hard to say.

 

[Cycleops]

Oh dear, that's very strange. it's a Hollowtech I guess. Maybe the two pinch bolts weren't correctly torqued.
Difficult to prove but if no damage has been done just refit. Have you managed to retain the thin plastic packing piece which slots into the gap?

 

[DCLane]

The Hollowtech II fixing bolt can crack if it's over-tightened as it's plastic. Otherwise it seems the pinch bolts, and more likely the small protector, weren't tightened enough.

One thing to check is whether the splines are now worn. If it's been working loose for some time the splines can wear, meaning it'll never fit properly again.

When was the bike purchased?

 

[Ming the Merciless]

Sounds like the left crank had moved to extreme left of splines. Allowing the right crank to move out right which would have put the big ring further out than derailleur was adjusted for. As above check the splines are not damaged.

It may not have been fitted properly or might not have been tightened properly. Having said that always worth periodically checking all bolts on the bike. I caught a bottle cage bolt about to set itself free that way.

 

[Venod]

Don’t think the gear change problem is related.

It's 100% related, the left crank coming off allowed the shaft and crankset to move to the right so that when he tried to change into the big ring, the derailleur couldn't move the chain onto the ring as it was further away than it was set to move.
I checked the splines and all seemed good, refitted the crank arm and tightened the bolts to the correct torque.
All seems well and gears working OK, just a mystery why it came loose and fell off.

 

[bikingdad90]

I am afraid it is a problem with 105 cranksets. My mam had one fail under warranty after about a month of riding her new bike in the same way. Shimano and the bike shop swapped it over no problem and put threadlock on the bolts to stop anything coming undone.

 

[Arrowfoot]

You get this when there are 2 bolts that have to be tightened equally. You slot both in and some distraction takes place and it never gets tightened. Guess the LBS guy got distracted. It clearly has shifted along the splines hence the misaligned gear change.

I twice forgotten to tighten the rear QR skewers after clean-up. Gravity held it in place until I lifted the bike to park it.

 

[Sittingduck]

Check there is no plastic spacer inboard of the nds crank arm. I left on one once after changing from a triple to a dbl and the left crank fell off. Unlikely as it was probably never a triple but worth a look?

 

[Cycleops]

I'm sure it'll be fine once refitted. It's when the right hand crank comes off you need to worry

 

[silva]

Having said that always worth periodically checking all bolts on the bike. I caught a bottle cage bolt about to set itself free that way.

Quite true, last year when doing such a bolts check I discovered the bolts of the frame slot (that I never touched) ment to allow usage of a belt instead of a chain, as loose, one almost out. The discovery probably prevented the bikes frame total loss, and me an ugly crash.

 

[Dolorous Edd]

Check your bolts regularly. And for the men out there, check your nuts regularly too (joke but serious)

 

[Punkawallah]

It's 100% related, the left crank coming off allowed the shaft and crankset to move to the right so that when he tried to change into the big ring, the derailleur couldn't move the chain onto the ring as it was further away than it was set to move.
I checked the splines and all seemed good, refitted the crank arm and tightened the bolts to the correct torque.
All seems well and gears working OK, just a mystery why it came loose and fell off.

More information is always good! We’re back to ‘was it fitted properly’ in the first place, then?

 

[Venod]

More information is always good! We’re back to ‘was it fitted properly’ in the first place, then?

I am not sure what you mean, I stated in the original post what had caused the gear change problem.

 

[figbat]

Hollowtech II effectively has three retaining devices: the plastic tab, the preload nut and the two pinch bolts. For all three to fail would, IMHO, mean incorrect fitment.