Broken Dura Ace 7900 Crankset

[winstonw]

Anyone heard of a DA crank axle failing as in the pic? The guy was going 45kph at the time at Nundah Criterium circuit (Brisbane) and crashed heavily.

[jules21]

ouch. isn't that the bottom bracket that's failed?

[zero]

if the bearing seizes it can groove the axle. it looks like a fatigue failure of the axle starting at a groove at the edge of the bearing.

I'd look at that the half of that axle break that is not jagged and see how old/non shiney the cracking appears to be, and I'd pull that bearing off it and see if it isn't seized, and/or that the axle isn't visibly grooved under it.

[Robdog]

Wow. Hope he's alright. Was there any damage to the BB shell that you know of?

[winstonw]

3 rib fractures with a pneumothorax, shoulder rotator cuff injury, multiple abrasions. night in hospital.

zero, agree seized bearings and/or metal fatigue are the likely culprits.

[dobby]

I have read about the "quality" of Dura Ace being very high - maybe time to look at SRAM Red or Campagnolo Super Record for the next bike?

[MichaelB]

if the bearing seizes it can groove the axle. it looks like a fatigue failure of the axle starting at a groove at the edge of the bearing.

I'd look at that the half of that axle break that is not jagged and see how old/non shiney the cracking appears to be, and I'd pull that bearing off it and see if it isn't seized, and/or that the axle isn't visibly grooved under it.

I'd agree that it could be, but there is no way of knowing without having a close look at the actual part.

I'd be on to shimano straight away.

Nasty indeed !!! Mind you, shows how thing the axle wall is too. Saving grams aint always the important thing !!!

[waynohh]

Jeezus christ!

You just convinced me not to buy pressed BB's for another 10 years.

[twizzle]

Degrease the surfaces, look for the 'beach marks' that indicate where the crack originated. If the whole surface has a crystallised appearance then it's an overload failure. The former is more likely than the latter, I expect you will find a scratch/gouge in the shaft which acted as the initial stress raiser.

But... NOT GOOD.

[ZepinAtor]

Just to clarify the order of things here (chicken or the egg) did he crash at 45km/hr & the crank broke as a result of the impact or did the crank break causing him to crash. I'm assuming if the crank failed while applying reasonable pressure to the pedals the left crank arm would have let go resulting in the foot going down hard causing an imbalance & thus falling to the side.

[RobM]

Can we also have a photo of the mans quads/calves? Maybe he is just pure awesome and capable of sprint power the world has never seen. Cranks everywhere will be shaking in their dustcaps...


Hope you heal well and fast

[jules21]

if the bearing seizes it can groove the axle. it looks like a fatigue failure of the axle starting at a groove at the edge of the bearing.

obviously they should just check the bearing to confirm, but wouldn't it be strange for a competitive cyclist to ride long enough on a seized BB bearing without noticing it?

[Nobody]

I hate weak points in hidden places like steerers and crank spindles. Things hidden should have extra strength as you can't easily check for cracks.

So is that a BB30 crankset? If it is, I'm glad I don't own one. Not likely to by the look of that. I'm already a bit concerned about the standard two piece cranks I already own.

Is that crank spindle aluminium-alloy or steel? It looks to be a steel thickness. Surely they wouldn't make aluminium alloy that thin, would they? I just went and checked my standard two piece 4500 Tiagra cranks to find the spindle is aluminium-alloy by the look of it and magnet test. Square taper cranks are starting to look good again. No weight savings are worth serious injury or death.

[ghettro]

AFAIK, all hollowtech BB spindles, ultratorque, powertorque etc are all hollow alloy axles. And yes, I agree that is damn thin for alloy!

[rustychisel]

'Don't jump to sensational conclusions on the evidence of one.

Yes, that looks bad, very bad, the consequences could have been horrific. Yet, this is the only failure of its kind I have seen, and I still don't know what caused the failure though I broadly agree with the conjecture so far. More info is required, obviously. But it doesn't add up to an expected failure rate, a design flaw or anything else. I'm glad the rider didn't lose his nuts on the top tube.

[Nobody]

For a comparison of the standard BB spindle, here is a Tiagra 4550 crank. It appears to be an alloy spindle and has a minimum thickness of just under 4.8mm (can't measure the thread depth) between the trough of the spline and the top of the thread at the end. In the middle it measures 6mm thick.

[jules21]

For a comparison of the standard BB spindle, here is a Tiagra 4550 crank. It appears to be an alloy spindle and has a minimum thickness of just under 4.8mm (can't measure the thread depth) between the trough of the spline and the top of the thread at the end. In the middle it measures 6mm thick.

thus dispelling the oft-stated myth that DA is more durable than Tiagra or other cheaper level components. it's lighter.

[rustychisel]

thus dispelling the oft-stated myth that DA is more durable than Tiagra or other cheaper level components. it's lighter.

It tells you nothing of the sort!!

It tells me you've jumped to a conclusion unsupported by any facts.

It would be my contention, were I so inclined to go in to bat for Shimano, that the grades of alloy used are entirely different, that the yield strength etc etc are carefully calculated according to the material used, and so on. Jeez Louis, it ain't rocket surgery. What I can contend is that both with work well within their design parameters - in accordance with user intentions and the absence of other contributing factors like, eg a stress riser being created through other input. Furthermore, anecdotally it is expected that Tiagra is a relatively entry level groupset for amateur riders and might be expected to hit 10,000km when used by an enthusiast. DuraAce, being top of the line, is used by professional and amateur riders day in, day out, on their training bikes as well as race bikes, and might be expected to last until 25,000 ~ 30,00km in a solid season of use. I feel confident in saying you won't get that level of performance from Tiagra.

You're right on one small point. It's lighter.

[jules21]

perhaps what i should have said was:

thus dispelling the oft-stated myth that there is more of a design emphasis on DA components for durability than Tiagra or other cheaper level components. rather, a priority is on minimising weight.

it's self-evident from the above info that shimano have designed that axle to minimise its weight - by definition, this involves calculating how much material they can shave off without unduly compromising strength and durability. i'm not suggesting that was the primary cause of failure - it could have been something completely separate, like a manufacturing defect.

of course, if that was a Tiagra crankset and BB, there would almost certainly have been a chorus of "that's what happens when you skimp on componentry".

[Nobody]

I may be a simple man, but I know thin metal when I see it. Maybe it is just the photo, but I would have thought that is the correct thickness for 4130. Yes, there are some strong aluminium-alloys today, but have they got the fatigue resistance and durability of steel in that thickness? Time is the ultimate test of this and we'll see if it becomes a trend. Hopefully not. In the end, the R&D is racing toward the lightest and I'm not a believer that the lightest is also the most durable.

Bontrager on R&D:

"You’d have to be dumb to think everything can be as good as high-end stuff. Frankly, the engineering for the £5,000 bike is more fun, you get to play with space-age materials, but the engineering for the £500 bike is more demanding because if you make a mistake there will be a lot of them coming back.”

http://www.bikeradar.com/MTB/gear/artic ... ger--27619

I also have 5600 105 and M770 XT cranks if there is a need to compare measurements. From what I've seen they all seem to look the same though. I could be wrong.

[Robdog]

It tells me you've jumped to a conclusion unsupported by any facts.

Not his first time, unlikely to be the last

[winstonw]

Just to clarify the order of things here (chicken or the egg) did he crash at 45km/hr & the crank broke as a result of the impact or did the crank break causing him to crash.

The crank definitely failed first.

[ZepinAtor]

Not good & probably a similar result from a snapped chain.

Not fault digging here, but has the bike owner been the sole rider of this said bicycle & if so has it been crashed previously or to his knowledge ?

[othy]

I hate weak points in hidden places like steerers and crank spindles. Things hidden should have extra strength as you can't easily check for cracks.

So is that a BB30 crankset? If it is, I'm glad I don't own one. Not likely to by the look of that. I'm already a bit concerned about the standard two piece cranks I already own.

Is that crank spindle aluminium-alloy or steel? It looks to be a steel thickness. Surely they wouldn't make aluminium alloy that thin, would they? I just went and checked my standard two piece 4500 Tiagra cranks to find the spindle is aluminium-alloy by the look of it and magnet test. Square taper cranks are starting to look good again. No weight savings are worth serious injury or death.

Shimano don't make a 7900 bb30 crankset.

[Nobody]

Shimano don't make a 7900 bb30 crankset.

Thanks. So what is it then? Is it the standard diameter of 24mm? A smaller diameter should have even more torsional stress on it.