# Snapped crank.



## Kell (25 Sep 2019)

By the looks of it, this has had a crack in it for a bit.

Luckily, it went when I was sat down going uphill rather than when I was stood on the pedals.

Using it as an opportunity to switch to a black crankset.


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## fossyant (25 Sep 2019)

Get a lottery ticket ASAP.


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## presta (25 Sep 2019)

Got the T-Shirt:




Had the bruises too.


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## SkipdiverJohn (26 Sep 2019)

@Yellow Saddle will appear shortly to give a technical analysis on the cause of the failure.....


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## Kell (26 Sep 2019)

Probably because I don’t clean my wheels often enough.


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## Yellow Saddle (26 Sep 2019)

Kell said:


> Probably because I don’t clean my wheels often enough.



Yup.


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## Yellow Saddle (26 Sep 2019)

SkipdiverJohn said:


> @Yellow Saddle will appear shortly to give a technical analysis on the cause of the failure.....


Here's my best guess.

1) It is a right crank. Not right as in wrong, but right as in not left.
2) A chain "incident" or FD that moved over too far, scored the crank on the inside at two spots equidistant from the crank spindle.
3) A crack developed from a stress riser at that point and traveled inwards towards the junction of the two legs of the "U" A different photo may reveal this initial gouge. 
4) The failure has three distinguishable areas. The large rouged section on the left of the cross-section photo. The silver section in the centre and the smaller rouged section on the right. The two dull, rouged areas are the initial crack and the silver section represents the last straw that broke the camel's back. That held on until the last minute and broke suddenly and completely. The two darker areas indicate an area of the crack that has been there for months or more, gradually increasing in size and all the time, fretting, creaking and oozing black discharge. All of this would have been noticed had the bike been cleaned and inspected regularly.
5) A crank works in three ways: in torsion, flexion and extension. Torsion because of the pedal that's offset, not central like a piston on a connecting rod. Flexion because of the in-plane component of pedal torque and, extension (stretch) when you stand on the pedals when the crank is at the bottom of its stroke.
6) It was mostly torsion that killed the crank. That can be seen from the shape of the crack.

Before cranks became hollow, through some very clever engineering by Shimano, the U-shape or channel was one preferred way of making cranks lighter. However, a channel like that is a very poor structure for resisting torsion. You just have to handle a plastic rain roof gutter to demonstrate how weak it is in torsion. Nowadays Shimano cranks are hollow, essentially tubes, and they are stiffer under twisting.

As for Presta's photo of the pedal eye on a crank that broke off: it was probably not his fault. The stupid way pedals attach to cranks causes gouging at the eye which leaves stress risers which quickly crack and break off. One way to alleviate it is to install steel pedal washers and most cranks today come with pedal washers which users promptly lose and ignore.


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## Kell (26 Sep 2019)

I don’t recall the chain ever coming off and getting stuck, but I may just not remember an incident.

What’s FD?

As for the cleaning regime, i’ve given up cleaning anything that isn’t a mating surface. Doesn’t matter how much I attempt to clean the tubing of the bike, the grime it picks up will not come off.

Once a month the chain, chain ring, chain tensioner, rear cogs and chain pusher all come off to be properly cleaned down.

Every weekend, I run the chain through one of those park tools chain cleaners, but it really doesn’t do enough, hence the full overhaul once a month. I also do the wheels on a regular basis.

I’d normally give the cranks nothing more than a cursory wipe down but certainly not inspected them.

I will keep an eye on them in the future however.


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## Yellow Saddle (26 Sep 2019)

Kell said:


> What’s FD?



Front Derailer.


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## raleighnut (26 Sep 2019)

Yellow Saddle said:


> Here's my best guess.
> 
> 1) It is a right crank. Not right as in wrong, but right as in not left.
> 2) A chain "incident" or FD that moved over too far, scored the crank on the inside at two spots equidistant from the crank spindle.
> ...



I take it you mean 'roughed' as opposed to wearing makeup,





These pair are rouged.


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## presta (26 Sep 2019)

Yellow Saddle said:


> As for Presta's photo of the pedal eye on a crank that broke off: it was probably not his fault. The stupid way pedals attach to cranks causes gouging at the eye which leaves stress risers which quickly crack and break off. One way to alleviate it is to install steel pedal washers and most cranks today come with pedal washers which users promptly lose and ignore.



It was my fault, I'd grounded the pedal on a corner. The pedal spindle had already gone a few months earlier, but the penny didn't drop until the crank went the same way.


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## Kell (26 Sep 2019)

Yellow Saddle said:


> Front Derailer.



I figured that’s what you might be referring to, but this is a Brompton.


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## Grant Fondo (26 Sep 2019)

Yellow Saddle said:


> Here's my best guess.
> 
> 1) It is a right crank. Not right as in wrong, but right as in not left.
> 2) A chain "incident" or FD that moved over too far, scored the crank on the inside at two spots equidistant from the crank spindle.
> ...


You ought to do a Sheldon Brown type blog....epic.


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## Yellow Saddle (26 Sep 2019)

raleighnut said:


> I take it you mean 'roughed' as opposed to wearing makeup,
> 
> 
> View attachment 486673
> ...



No I don't. Rouged is an engineering term you won't find in everyday dictionaries.

But look here, for instance.

http://www.wwwrope.com/pdf/EL_TB_03.pdf

Fretting causes rouging.


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## raleighnut (26 Sep 2019)

Yellow Saddle said:


> No I don't. Rouged is an engineering term you won't find in everyday dictionaries.
> 
> But look here, for instance.
> 
> ...


Nice try but that refers to Iron oxidation in a wire rope and is related to the red colouration achieved by the oxide (pretty similar to the red in make-up)


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## Yellow Saddle (26 Sep 2019)

raleighnut said:


> Nice try but that refers to Iron oxidation in a wire rope and is related to the red colouration achieved by the oxide (pretty similar to the red in make-up)



No, we use it for all evidence of fretting. In aluminium it happens to be black, but it is still rouged.
It is also evident in stainless steel, where the rouge has not turned brown, yet it is still referred to as rouge.

It certainly isn't only used in wire rope either, that's just the first example I found on the web.

Also look up jeweller's rouge. I use it to finish off sharpened knives and blades on a leather strop. Here the outcome is black because the iron had not yet rusted.


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## KneesUp (26 Sep 2019)

Kell said:


> As for the cleaning regime, i’ve given up cleaning anything that isn’t a mating surface..



One for you here @Fnaar


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## 12boy (26 Sep 2019)

I've probably ridden at least 70 k miles in the last 60 years and never have broken a crank like that. I use pretty much Sugino and Shimano cranks, some of which are at least 25 years old. Having read your previous posts I gather road grime and grit and hard climb rides are every day for you but I still think there was a flaw in the crank to begin with. Salt is used here in the winter and that will trash components fairly quickly, so I do remove that after riding, I do protect the inside of my frames, and I do wax chains since the wax doesnt pick up grit like oil and grease do. Other than that, not a lot of maintenance other than rebuilding bearings when needed.


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## Kell (26 Sep 2019)

12boy said:


> I've probably ridden at least 70 k miles in the last 60 years and never have broken a crank like that.



Well, according to Strava I’ve done 8,278 miles on the Brompton in 4 years. 

Like other things that have happened with this bike, it’s proving to have a lot of firsts. 

It might be that everything is just that little bit lower to the ground which is causing more problems with ingrained dirt etc. But my previous commuter bikes never had these problems. 

Other problem, yes.


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## cyberknight (26 Sep 2019)

found mine


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## berlinonaut (26 Sep 2019)

Kell said:


> Well, according to Strava I’ve done 8,278 miles on the Brompton in 4 years.



From the Brompton manual, page 11:


Aluminium components: As on other lightweight machines, aluminium alloy is used in the construction of the Brompton, and this material has a finite design life before failure. In normal use, the risk of aluminium fatigue failure is remote, even after many thousands of miles. However, the risk of failure increases with use, especially with hard riding or other severe loading.As such a failure could cause injury, the hinge clamp plates, handlebar, *chain set*, seat pillar and pedals *should be replaced every 5,000 miles (more frequently if they are subjected to hard use), and we recommend that these items are checked regularly*.


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## Kell (26 Sep 2019)

berlinonaut said:


> From the Brompton manual, page 11:
> 
> 
> Aluminium components: As on other lightweight machines, aluminium alloy is used in the construction of the Brompton, and this material has a finite design life before failure. In normal use, the risk of aluminium fatigue failure is remote, even after many thousands of miles. However, the risk of failure increases with use, especially with hard riding or other severe loading.As such a failure could cause injury, the hinge clamp plates, handlebar, *chain set*, seat pillar and pedals *should be replaced every 5,000 miles (more frequently if they are subjected to hard use), and we recommend that these items are checked regularly*.




Pretty sure that it would be cheaper to buy a new bike at that rate.


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## Kell (26 Sep 2019)

However. 

On that advice, I’m going to order a new set of clamps.


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## Kell (26 Sep 2019)

Also, also...

Lightweight machines?


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## rogerzilla (28 Sep 2019)

Pedal washers don't help much. They're the same size as the pedal axle and they chew against the crank in just the same way. My TA cranks came with them, I use them but the crank's little recess for the washer still deepens with time.

If these were the original Brompton cranks (made by Stronglight) with the permanently-fixed chainring, they're just not up to the job. I replaced mine with Sugino cranks. The usual failure mode is that the ring shears from the crank when you're giving it some.


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## berlinonaut (28 Sep 2019)

rogerzilla said:


> If these were the original Brompton cranks (made by Stronglight) with the permanently-fixed chainring, they're just not up to the job. I replaced mine with Sugino cranks. The usual failure mode is that the ring shears from the crank when you're giving it some.


As one can see from the picture these were the newer style cranks with the spider, not the ones with the fixed chainwheel. Also clear with the bike being four years old - this makes ist date from 2015 while the newer style cranks were invented in 2013. With the old ones the ring shearing off the crank did happen but in my eyes by far not often enough that I would call them "not fit for the job" or the incident "the usual failure".
I second the analysis of @Yellow Saddle in https://www.cyclechat.net/threads/snapped-crank.253599/post-5749025 - this thing has been developing over a longer period of time and probably has a root cause that we do not know. There's a good chance that it would have been recognized early with the part being inspected regularly as Brompton even recommend in their manual. The cranks were well beyond their exchange interval, which, I assume, not too many Brompton riders stick to - still this kind of incident seem not to happen very often, given the amount of reports compared to the amount of Bromptons made. So basically, though not nice, for the most part a user created incident.


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## Banjo (28 Sep 2019)

In the marine industry a red die is painted on critical parts which shows up cracks invisible to the naked eye.

Get it on paintwork and it's there for good so maybe not ideal for bikes but could be carefully used on cranks.


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## rogerzilla (28 Sep 2019)

Does anyone know if the new cranks are forged or cast? A forged crank can still break, but it's a lot stronger than a cast one and less likely to have flaws in the metal. Brompton don't fit particularly high quality parts, so I'd be suspicious.


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## Kell (28 Sep 2019)

All points taken on board. 

I’ve never opened the manual, so didn’t know about the 5,000 mile limit. 

That would have been about a year and a half ago, so if I’d stuck to that then this wouldn’t have happened. 

I think even I could tell that this wasn’t a sudden failure, the darker patches clearly indicate ingrained dirt. Though now i’ve Taken the old crank off, I think you’d have been hard pressed to see any evidence of the crack developing from behind. 

It would have been totally hidden. 

Once it moved around the sides, however, a little more vigilance may have spotted it. 

But then I probably wouldn’t as I wasn’t looking for it. 

Who knows?

All I know now, is that it was a lucky escape. And i’ll Make a note of installation mileage of the new part.


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## Yellow Saddle (28 Sep 2019)

Banjo said:


> In the marine industry a red die is painted on critical parts which shows up cracks invisible to the naked eye.
> 
> Get it on paintwork and it's there for good so maybe not ideal for bikes but could be carefully used on cranks.


All industries use that die. Yes, it is lethal stuff and can even ruin white paint. However, on bicycles it isn't necessary. The aluminium rouges black and cracks are very evident.


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## Yellow Saddle (28 Sep 2019)

rogerzilla said:


> Does anyone know if the new cranks are forged or cast? A forged crank can still break, but it's a lot stronger than a cast one and less likely to have flaws in the metal. Brompton don't fit particularly high quality parts, so I'd be suspicious.


A photo of the unpolished bit inside the channel will clarify that. However, it is almost certainly forged. That particular shape is easy and relatively cheap to forge, unlike Shimano's hollow, forged cranks. Those are sheer wizardry.

Note that all forged cranks are also cast. They cast them first and them hammer the hell out of it with one big thud.


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## Kell (28 Sep 2019)

New stuff arrived this morning. 

Fitted the new cranks and new frame hinge clamps. 

Went for black edition versions and went back to a 50T chainring.


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## Kell (28 Sep 2019)

Some more pics of the other half of the crankset. 

I think I can see some evidence of the oozing you were talking about. 

But I don’t think I would have noticed it if I wasn’t looking for it. 

Think every time I replace the chain, I’ll make a mental note to check the cranks.


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## Kell (28 Sep 2019)

(Just for the record, on the picture of the fitted crankset, those are chips in the powder coat on the rear triangle, not dirt.)


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## silva (29 Sep 2019)

A mechanical question regarding a basic: why the choice for a tapered connection in general (in this case bicycle cranks-spindle/axle) instead of just straight (all 90 degrees corners) ones?
I rather see drawback(s) here, alike crank mount, undergoing a 90° angle force, that gets a sideway (away from mount-frame center) force component due to the sloping.


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## Yellow Saddle (29 Sep 2019)

silva said:


> A mechanical question regarding a basic: why the choice for a tapered connection in general (in this case bicycle cranks-spindle/axle) instead of just straight (all 90 degrees corners) ones?
> I rather see drawback(s) here, alike crank mount, undergoing a 90° angle force, that gets a sideway (away from mount-frame center) force component due to the sloping.


Because a taper is the most elegant way to get rid of lash. It is cheap, and elegant. Without it you need an expensive arrangement with splines and pinch mechanisms as in Hollowtech.


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## silva (29 Sep 2019)

I don't get that well Yellow Saddle, how does a taper eliminate lash? Lash (assume you mean play/room) is due to different dimensions/shapes - the force/power transmission has like "dead gaps", ie, the force pushes or pulls, but the engaged/driven mechanical transmission part only reacts/moves after that "dead gap" has been bridged, also causing a kind of hammering effect which worsens this over time. How would a tapered instead of 90° angled be an improvement let alone getting rid of it?
Imagine wear occurs. Particles are worn off. Dead gaps in the reaction to force direction changes grow. Also in the case of tapered...
... and now I start to get it heh. In the case of 90° angles, there is no other section dimension, the problem can't be compensated for without replacement. With tapered one can "shift up" the mechanical contact to a bigger, original, unworn surface.
So, if play is experienced, just tension further the cranks bolt until the play is gone, and a new wear cycle/life starts. At least until the end of the taper is reached.

At this same time, I now understand what I experienced the past days since left crank broke off friday.
I was able to remove the cranks remainder from the spindle (don't ask me how it wasn't exactly an "elegant" method) and mount an old / other brand left crank in its place, but the bolt kept on losing, yesterday I had to stop every 2 streets (not funny) to retension the bolt, in order to get at a dealer for a crankset replacement, to stand for a closed door due to vacation. Then later in the afternoon I reached a second dealer the same way, also closed, note on door just that day. A whole day wasted.
But I then already experienced that I had to retension less frequently. And today, 15 km, I stopped a dozen times with only at 2 of these being able to tension it abit. And in the return, didn't make a single stop, and when back home, no retension needed at all. 
At first I thought that I applied too much oil (I then just had spent hours getting the crank off without dedicated tool) when mounting causing easier slipping on the thread and that the retensions gradually pressed away oil. But with aboves explanation, the tapered slope on the spindle must have deformed so that the play (possibly due to different brands) disappeared.
In which case my intended crankset replacement may be not needed.
Still, I wanna play sure, walking 30 km home with a lot luggage in rain is a day-ruiner.
In the end, my "crankset" became now a combination of two brands (left crank Sugino XD, rest Stronglight Track 2000), which may even in case now "adapted" to eachother, cause problem on the longer term. There's a difference between those left cranks too: the Sugino's body at its mount is rounded-cilindrical, while the Stronglight is rounded-conical, the latters diameter becomes bigger towards the outside, whatever mechanical properties/strength improvement that may bring, I'd rather see benefit near its spindles contact area rather than away from it.

So it may be wiser to replace the crankset anyway, since the thing didn't suffer any major impact (as rider I would have noticed it I think), the next failure may well be on the drivetrain side, which would mean even no single leg propulsion (heh) anymore and a need to unmount various things in order to even just walk further without scratching the ground.
Also, I noticed serious thread damage on the broken crank, apparently the force required to remove the crank with a puller (by the dealer after he found out a longer spindle was needed to achieve a same chainline) damaged the thread that the puller used as backbone to get the crank off the spindle, which shouldn't.

And since the Stronglight set proved a big centering tolerance (a whole new drivetrain - ring+chain+cog) and already a 2 cm up and down chain tension variation while turning the cranks indicates not exactly precise manufacturing (remember the other thread months ago where you helped to eliminate other causes), there is even more reason for a whole crankset replacement.
I still need to find a precise centered set, though.
My previous bike has "Shimano 105" on the left crank and there (and also the other singlespeed>fixed bike) I never had a chain tension variation. Apparently more precise manufactured than Sugino XD and Stronglight Track 2000. Unless there is still some overlooked tension variation cause. I have had a KMC S1RB chain and a Surly chainring and it also had that tension variation from the beginning, only that it grew over time (likely due to the no wear spreading of the 48/16 gear) while now with 47/16 it just remains what it was.

So this crank break looks like an cost-opportunity to get rid of this problem too.


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