Crank failure: Town bike

[Falco Frank]

As the coolest place for cycling info that I know, I thought I'd post this here.

I'm not going to name people or brands but a friend has sustained moderate injuries after the CRANK LEG of his utility bike COMPLETLY failed, as in snapped clean (chain ring side) with injuries coming from impacting the bike and the (thankfully soft) ground, during a hill climb.
Sadly, even an inspection of the crank would not have revealed an impending failure unless the whole assembly had been removed.
The manufacturer who is no longer trading has been contacted; and have replied but I don't know details yet.

 

[Ajax Bay]

Welcome back after nearly a year away, Frank.
If the manufacturer is no longer trading, would it not be useful to name the make? Otherwise, what is the reason for sharing here?
The images are playing tricks (for me) as the two 'failure' sections don't seem to match.
Can I ask how you / your friend reasons that "an inspection of the crank would not have revealed an impending failure unless the whole assembly had been removed". The point of failure on that crank looks as if it would be in clear view of a visual inspection. You'd expect some kind of stress riser to be present.

 

[fossyant]

Not a big fan of cheap chanks, but even the fancy ones fail. Unless in warrenty or known faulty like Shimano hollowfalltobits, then not much you can do.

 

[Drago]

Holy Trump!

That  would have been visible prior to complete failure. You can see the mode of failure definiely extended to part of the sides and outsides. A good advert for keeping a bike clean and pre-ride checks.

I do hope they werent hurt badly?

 

[Cycleops]

If the manufacturer has gone then the only thing for your friend can do is grit his teeth (if still present) and buy a replacement.
Naming the make would be useful.

 

[Ajax Bay]

Not a big fan of cheap cranks, but even the fancy ones fail. Unless in warranty or known faulty like Shimano hollowfalltobits, then not much you can do.

Are you suggesting these are 'cheap cranks'? The revealed cross section suggests good solid aluminium cranks, till they weren't.
The widespread glue failure of the expensive Shimano ones is a separate issue.

 

[FrankCrank]

Nasty breakage that. My experience with aly is you get little if any warning of failure, it just goes in an instant. Not had a crank failure, but a few years ago had my aly folding bike go snap, and luckily I escaped unhurt. The point of failure was right next to a weld, and no visible warning signs that anything was amiss. My preference now is steel for folders, but am happy to ride aly on full size regular bikes.

 

[richardfm]

The manufacturer who is no longer trading has been contacted; and have replied

This doesn't make sense to me.

 

[MichaelW2]

What is the reverse side of the main crank like. I bet there is some reinforcement ip until the break.

 

[si_c]

Can I ask how you / your friend reasons that "an inspection of the crank would not have revealed an impending failure unless the whole assembly had been removed". The point of failure on that crank looks as if it would be in clear view of a visual inspection. You'd expect some kind of stress riser to be present.

Looks as though a crack developed on the inside of the crank arm at either side and then failed finally from the front. I'd have expected a regular check of the crank arm to have found that. You can tell there has been some oxidation of the aluminium at either side of the arm with the discolouration. Not had it happen to me, so can't be sure, but I'd imagine there would be some feedback that the crank wasn't entirely solid when riding.

I'm not sure what sort of damage could have caused it to fail from both sides, so I'd suspect a manufacturing defect, although I'm not an engineer or materials specialist so happy to be corrected.

Fortunately it appears nobody was injured and it's an easy fix to replace just the crankarms. I'd want to know the manufacturer of the cranks.

 

[rogerzilla]

Funny place to break. RH cranks (Hollowtech excepted, they're just weird) usually go at the pedal eye. Probably cast from something not much better than monkey metal.

 

[silva]

alu is chosen,to save weight, achieves same strength by 3D structural design, then they save again weight by decreasing the cross section area of the structures parts, upto the point that stress causes movement of material, for which alu is much less tolerant than steel. Fatigue cracks; growing to the point that the remainder of the 3D section isn't enough anymore for the load, and it just breaks. Look at that too narrow U-shaped cross section. The end result of saving cents per piece.

 

[wafter]

Looks as though a crack developed on the inside of the crank arm at either side and then failed finally from the front. I'd have expected a regular check of the crank arm to have found that. You can tell there has been some oxidation of the aluminium at either side of the arm with the discolouration.

I thought the same (and this would stand to reason); however the apparent oxidation on the remaining arm doen't appear to be present on the stub remaining on the crank..

I'm not sure what sort of damage could have caused it to fail from both sides

A beam in bending (as this is, as well as torsion to a smaller extent) experiences the highest stresses near its outside faces, so this failure mode is to be expected.

 

[Falco Frank]

I can round this post out now that my friend has received and (is still keen) fitted the new free chainset from the provider: Isla Bikes.
The broken crank has been returned for examination which, I hope is a good sign.
The crankset was their own design but, I suspect made in china.
The company changed hands and moved onto selling childrens bikes only I believe but, have fallen on hard times again and now only supply spares and accessories.
My intention I hope, was to inform not scaremonger and while I used to only give a cursory look over my bike at the start of the year
(or season 😉 ), I will be a bit more rigorous in the future. Stay safe.

 

[silva]

If you can be bothered, weight the old (2 parts now) and the new complete, and compare.

... oh missed already fitted, too late then.