Snapped crank.

[Yellow Saddle]

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.

 

[Kell]

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.

 

[Kell]

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.

 

[Kell]

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

 

[silva]

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.

 

[Yellow Saddle]

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.

 

[silva]

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.

 

[wafter]

An interesting, if somewhat concerning thread..

I think this image tells the story best:

As already mentioned the crank is subject to a number of forces; including bending on two planes and torsion about its principal axis. In all of these situations stress is always highest at the exterior surface / edges of the beam / the points furthest from its centroid; depending on the direction of loading.

The main bending force acting in line with the bike / perpendicular to the axle will stress the front and rear edges the most; the front (top of this image) in compression, the rear (bottom of image) in tension.

The bending force acting across the bike will create highest stresses at the inside (tension) and outside (compression) of the crank.

The torsional forces will (simplified a bit) will give highest stresses furthest from the centroid, so in this case probably the outside corners of the inside face.

Materials tend to be more prone to failure in tension than compression and the cumulative stress of the three loading conditions above is most likely to have been highest at the rear, inside edge of the crank (bottom LH corner in the pic above) which fits with the visible failure characteristics. The crack will have begun here and propagated into / across the crank; the subsequent loss of strength and stiffness allowing a second crack to form at the front inside corner, the crank eventually becoming weak enough for the remaining, shrinking intact section to fail catastrophically.


The takeaway from this is to inspect your cranks regularly; paying particular attention to the areas of highest tensile stress - i.e. the rear / trailing, inside edge / corner. As has been mentioned as cracks propagate they often emit a black dust as the ally frets and oxidises; which should be a relatively easy tell on the silver cranks; less-so on the black ones.

You might also hear creaking during use as the sheared faces rub together.

I suspect it might be possible to do some sort of budget dye penetration testing; maybe using something like paraffin and talc..

 

[Kell]

You might also hear creaking during use as the sheared faces rub together.

I think this is the thing that confused me the most.

From memory there was a lot of creaking, but then there often is with a Brompton.

The last time I heard anything similar, it was because one of the rear hinge bolts was slightly loose and the whole rear end was swaying side-to-side.

Now, if I hear it again, I'll know to look out for it.

 

[tinywheels]

how old is the snapped crank?

 

[berlinonaut]

how old is the snapped crank?

Surprisingly, actually reading the thread you are posting to delivers the answer:

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

 

[Dogtrousers]

Surprisingly, actually reading the thread you are posting to delivers the answer:

And you can add four years to that due to the age of the thread

 

[tinywheels]

strava and actual mileage are not necessarily the same.
I only record some of the time However if we take the strava numbers as an accurate reflection it's somewhat worrying.
On my old cannondale mtb,non suspension I rode for twenty odd years. I covered many happy miles all over Europe and the UK. Nothing failed! Are modern bikes built to last or are some people just unlucky.
Cannondale guaranteed the frame for life if I remember correctly.

 

[berlinonaut]

On my old cannondale mtb,non suspension I rode for twenty odd years. I covered many happy miles all over Europe and the UK. Nothing failed!

Good for you. And what exactly do you think does that prove? Does this change the laws of physics? Singular evidence is random and is not usable as general prove for anything. There are 100.000s of Bromptons out there on the road for decades where the cranks did not snap over the last 35 years since their invention. And a handful where they did.

Cannondale guaranteed the frame for life if I remember correctly.

It was not a frame that snapped here but a crank, so I do not see what you want to express.

 

[berlinonaut]

An interesting, if somewhat concerning thread..

I think this image tells the story best:

View attachment 706074

On the positive side: This picture shows, that roughly 2/3 of the crank had disintegrated for quite a while already before the crank finally snapped. So there seems to be quite a bit of a safety buffer in the cranks, especially when having in mind that @Kell is probably not the most powerless rider you can imagine but seems to push hard on the cranks quite often.