How long should a crank last?

[Danny]

I was cycling down a hill yesterday when the left crank of my Hallowtech chainset sheered off just where the pedal screws in. I have cycled for years and never had a crank fail before, and now feel a bit worried that this could happen on my another bike when i am cycling in heavy traffic.

Even though I wasn't pushing down hard on the pedals, I flew forward off my seat when the crank sheered and only just stopped the bike crashing - though was left with a nasty gash on my knee. Fortunately I was on a quiet route at the time, but if I had been pedalling hard in heavy traffic the results could have been a lot worse.

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[slowmotion]

Crikey! That's a bit worrying. Had it been stressed by a mishap earlier?

 

[Citius]

A crank arm should never fail like that. Something else must have contributed to that...

 

[ianrauk]

First time i've ever seen something like that.Blimey.

 

[Smokin Joe]

You were just unlucky, a flaw of some sort in the material.

I can imagine how scary it must be at the time, but the chances of it happening again are pretty remote.

 

[TissoT]

Could be the pedal has been over tightened .. and the arm had a defect in before hand
Or the pedal has hit the curb at some point..
again that's unusual

 

[MichaelW2]

A crank arm should never fail like that. Something else must have contributed to that...

Crankarms fail like that in a small but regular number of cases. Deore are cold-forged and not ultralight, so you would expect them to be amongst the most durable. High-end cranks fail just as much as mid-range.
Can you get some decent, in-focus, close up pics of the face of the crack and inner side of the crank.

 

[alecstilleyedye]

a club mate had a crankarm lose the thread for the pedal, a campag crankarm that was only 20 years old...

 

[Danny]

Thanks for the responses.

As far as I know there has been no serious stress put on that crank, though I have ridden that bike a lot and it has seen a fair amount of normal 'wear and tear'.

 

[PhilDawson8270]

There looks to have been some kind of impact to cause damage to the crank arm.

Also a chance of it being over tightened, and looking at the teeth on the chain ring, it looks to be close to the end of it's life anyway

 

[Drago]

I've only ever seen that once before, and that was for a rider in the region of 24 stones.

 

[ColinJ]

I have broken one crank like that. Fortunately, I was just setting off so it caused a low-speed crash.

I spotted another failing crank before it happened ... LINK

I think square-taper invites overtightening, but undertightening could also lead to problems! Best to use sensible torque on the bolts and check the cranks regularly. Add it to the list of checks you do when cleaning the bike ...

 

[Keith Oates]

I've never seen that before but as others have said knock on the end of the pedal sometime in the past could have been the cause.!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[Yellow Saddle]

A small point perhaps but....it did not shear. It broke off by way of a stress crack that developed where the pedal spindle screws into the crank eye. The type of attachment there is an imperfect legacy design that is faulty and will develop in a small number of cases.
The spindle has a flange at the end of the threaded section to dictate the depth of engagement and tighten the spindle against the crank. That flat section eats into the softer aluminium - especially on powerful riders and causes sharp nicks that start stress cracks that then travel through the pedal eye. Once the crack has developed, the continuation thereof is rapid. It would be interesting to see a face-on photo of the crack. You will see an old corroded section and a shiny new section. The former took months to develop, the latter, days.

The design should be different. It should be like a car's wheelnuts with a tapered flange instead of a flat one.

This was not caused by a crash, a knock or even a manufacturing fault, but a face-on photo will confirm that.

 

[User16625]

I have bottomed out my pedals once or twice in the past, on one occasion im pretty sure the back wheel lifted off the ground scaring the crap outta me.