How long should a crank last?

[fossyant]

I'm rolling on some 30 year old cranks.

 

[Danny]

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

I was hoping one was going to spot that. Replacing the chainset has been on my 'to do' list for a while.

 

[Danny]

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.

Thanks for the detailed explanation - I'll do some face on photos this evening.

If a crack happens is this something you should be able to spot?

 

[Danny]

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 ...

Why would over or under tightening the crank bolts impact on the other end of the crank? Or did you mean over/under tightening the pedals.

For those who were asking, I usually screw in the pedals until they are just starting to tighten - so I don't think that over-tightening was the cause of this particular problem.

 

[ColinJ]

Why would over or under tightening the crank bolts impact on the other end of the crank? Or did you mean over/under tightening the pedals.

For those who were asking, I usually screw in the pedals until they are just starting to tighten - so I don't think that over-tightening was the cause of this particular problem.

Sorry - I was multitasking and speed-reading ... My cranks failed at the other end of the cranks!

I have never seen your problem before.

 

[Danny]

Sorry - I was multitasking and speed-reading ... My cranks failed at the other end of the cranks!

I have never seen your problem before.

Thanks for clearing that one up

 

[Supersuperleeds]

It looks like Shimano? They give a two year warranty, so if it is not that old you could either take it into your lbs or contact Madison yourself.

My crank on my Tricross sheered off a couple of weeks ago, but at the other end.

 

[Yellow Saddle]

Why would over or under tightening the crank bolts impact on the other end of the crank? Or did you mean over/under tightening the pedals.

For those who were asking, I usually screw in the pedals until they are just starting to tighten - so I don't think that over-tightening was the cause of this particular problem.

Overtightening is better than undertightening in this case. When undertightened the pedal spindle will move in the crank cause more fretting and quicker crack propagation. If you take the pedal off and look at the pedal-end of the eye, you'll see that it gouged a groove in there.

 

[Danny]

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.

Here's a face on photo of the broken crank

 

[Danny]

Which appears to show some aged crack, the darker bits, with the majority being shiny new crack.

Does that mean I should have been able to spot the crack before the crank failed?

 

[LarryDuff]

Something similar happened to my son's bike a few months ago.

 

[mickle]

I'd warranty that.

And use pedal washers next time.

 

[slowmotion]

The dark patch could be a fault in the casting...I think. I would have thought that an old crack would have a sharper edge to it.

 

[Danny]

Possibly so. As Mickle says, I would want it sent back and replaced under warranty.

It was more than two years old so out of formal warranty.

 

[HLaB]

I've never touch wood broke a crank. The longest Ive had one (distance wise) is 40k miles. The crank had spds for about a 1/4 of its life and spd sl for the remainder ; managed to wear out a set of Spd sl's though and the large chainring, the crank is still going strong though.