This is a bit worrying

[EckyH]

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.
View attachment 705830

Roughly six years ago the right Truvativ Rouleur crank on my commuter bicycle broke at the pedal eye with a similar fracture surface: dark and relatively smooth like here on the right side and rough and light at the left side. The beginning was a hairline crack, very hard to notice on a silver crank and even harder to see on a black one. The only hint in my case was a faint creaking noise from the pedal area - where the pedal has had it's own creaking crack. Therefore I wouldn't blame the owner of this bicycle.

The crank broke suddenly and I hit the ground fairly hard. The result was a light concussion - and a broken helmet.
I hope the rider of this bike had at least as much luck as I had.

E.

 

[Ming the Merciless]

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.

View attachment 705955

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...

If a 3 speed brommie the low cadence thing becomes necessary at steep enough a grade.

 

[wafter]

If a 3 speed brommie the low cadence thing becomes necessary at steep enough a grade.

Aye; even with six speeds I do tend to find myself out of the saddle grinding up the hills..

 

[berlinonaut]

personally I think the original one piece seems a bit more robust, I’ve never heard of one failing

Now you have:

 

[berlinonaut]

Brompton recommends an exchange interval of 5000 miles for the Aluminium parts though most people don't seem to care. Snapped cranks are not very common but if you look you'll find a few cases. I.e. @Kell had his crank crack at ~8300 miles:
https://www.cyclechat.net/threads/snapped-crank.253599/
Her you have one that cracked after more than 6000 miles that included regular heavy mashing if I read it correctly:
https://www.cyclechat.net/threads/brompton-crank-snapped.190319/

 

[tinywheels]

I have the crankset I took off mine before fitting the hope one if anyone wants to make an offer

 

[Illaveago]

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.

View attachment 705955

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...

So freewheeling extends the life of your cranks .
I thought there must be a reason why I do a lot of it !

 

[dicko]

The only crank problem I ever experienced was a loose cotter pin leaving work one Saturday morning. A large engineers hammer got me home and a new spare cotter pin was inserted jobs a good un.

 

[fossyant]

Cheap cranks fitted on an expensive bike !

 

[tom73]

That RAAC get everywhere

 

[berlinonaut]

Cheap cranks fitted on an expensive bike !

To consider a Brompton expensive is a question of personal perspective. Many would agree, many wouldn't. I would however be interested on the foundation of your statement that Brompton uses "cheap cranks". Do you have any?

 

[fossyant]

To consider a Brompton expensive is a question of personal perspective. Many would agree, many wouldn't. I would however be interested on the foundation of your statement that Brompton uses "cheap cranks". Do you have any?

Erm, this is cheap.

 

[Drago]

Have they gone as upmarket as hollowtech level cranksets yet?

 

[berlinonaut]

Have they gone as upmarket as hollowtech level cranksets yet?

on the t-line, yes.

 

[T4tomo]

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

If a 3 speed brommie the low cadence thing becomes necessary at steep enough a grade.

I have, mine I had a pretty steep hill between station and home so necessitated a bit of low cadence high force pedalling and I managed to to pedal the crank arm bit away from the chainring join.

I replaced the 50T with a 46T to easy the hill climbing issues.