This is a bit worrying

[Gunk]

I just saw this pop on Faceboast, don’t know the whole back story, ie rider weight/bike luggage weight etc, but pretty sure it shouldn’t do this.

hope his nadgers are OK

 

[EltonFrog]

Could’ve been worse, could’ve been my bike.

 

[wafter]

To be fair, it's well known that aluminium is highly susceptibly to fatigue / cyclic loading and I believe Brompton are one of the few who acknowledge this and recommend that you replace certain ally parts after a set mileage / time.

All that said of course I'm tighter than a duck's a*se so will insist on riding mine until it drops to bits and kills me 👍

 

[fossyant]

Eee they don't make em like they used to. Running 37 year old Shimano 600's and 33 year on Dura Ace. TBH they were both about the stiffest chainsets of the time.

 

[404 Not Found Anywhere]

What I can see of the bike looks a bit neglected in general…

 

[Gunk]

What I can see of the bike looks a bit neglected in general…

I don’t think that’s caused the problem, personally I think the original one piece seems a bit more robust, I’ve never heard of one failing

 

[dicko]

I’ve never seen that before in all my sixty three years cycling.

 

[Cycleops]

Eee they don't make em like they used to. Running 37 year old Shimano 600's and 33 year on Dura Ace. TBH they were both about the stiffest chainsets of the time.

As opposed to the rider of this folder who might be the stiffest cyclist due to that crank arm.

 

[Drago]

Holy Mary Mother of Starmer!

 

[craigwend]

I'm choosing not to worry about it
...

 

[Gillstay]

I have heard of it on a very cheap bike. Still surprised me that it was possible.

 

[tinywheels]

the solution

 

[Ming the Merciless]

Perhaps ask him on Farcebook?

 

[Cycleops]

I don’t think now I’ll be buying a Chinese classic double chainset my local bike shop sells for a fiver.

 

[wafter]

What I can see of the bike looks a bit neglected in general…

True; however I can't think of / see any consequences of neglect that should have caused the crank to have failed in such a way. That's either been massively over-loaded, or more likely seen a high degree of cyclic loading - as one might expect given its application.

From P31 here Brompton recommend that ally components are replaced every 5k miles. If one assumes a mean cadence of 90 rev/min and a mean speed of 12mph over this distance, this suggests a total number of crank rotations of 2.25 million; which appears to be the right ballpark for fatigue to be an issue; loading dependent of course.

This raises an interesting question; since no. cycles and load both play a part in defining fatigue loading, I wonder if, for a given power output there's an optimum speed / load condition for best component life..?

Looking at the graph below for the fatigue characteristics for 2014-T6 ally (sauce) suggests a curvilinear, exponential relationship between stress and no. cycles. It appears that the material will endure around 10^4 cycles at a stress of around 330MPa, or at the other end of the graph 10^7 cycles at a stress of around 160MPa.

So, by halving the stress / loading the cycling failure point is raised by a factor of 1000.. or, if we're looking purely at power transmission pedalling twice as fast at half the load should increase component life in terms of miles travelled / absolute service time by a factor of around 500.

It seems that load is the dominant factor here, so it seems highly likely that this failed crank arm is the result of a very heavy / powerful rider with a panchent for low cadence. Makes me feel not-so-good about grinding up the hill on the way to work...