Broken Dura Ace 7900 Crankset
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Broken Dura Ace 7900 Crankset
Postby winstonw » Tue May 10, 2011 7:15 pm
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Re: Broken Dura Ace 7900 Crankset
Postby jules21 » Tue May 10, 2011 7:59 pm
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Re: Broken Dura Ace 7900 Crankset
Postby zero » Tue May 10, 2011 8:13 pm
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.
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Re: Broken Dura Ace 7900 Crankset
Postby Robdog » Tue May 10, 2011 11:48 pm
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Re: Broken Dura Ace 7900 Crankset
Postby winstonw » Wed May 11, 2011 12:24 am
zero, agree seized bearings and/or metal fatigue are the likely culprits.
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Re: Broken Dura Ace 7900 Crankset
Postby dobby » Wed May 11, 2011 8:16 am
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Re: Broken Dura Ace 7900 Crankset
Postby MichaelB » Wed May 11, 2011 8:24 am
I'd agree that it could be, but there is no way of knowing without having a close look at the actual part.zero wrote: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 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 !!!
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Re: Broken Dura Ace 7900 Crankset
Postby waynohh » Wed May 11, 2011 8:52 am
You just convinced me not to buy pressed BB's for another 10 years.
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Re: Broken Dura Ace 7900 Crankset
Postby twizzle » Wed May 11, 2011 9:25 am
But... NOT GOOD.
...real cyclists don't have squeaky chains...
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Re: Broken Dura Ace 7900 Crankset
Postby ZepinAtor » Wed May 11, 2011 9:41 am
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Re: Broken Dura Ace 7900 Crankset
Postby RobM » Wed May 11, 2011 9:52 am
Hope you heal well and fast
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Re: Broken Dura Ace 7900 Crankset
Postby jules21 » Wed May 11, 2011 10:00 am
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?zero wrote: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.
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Re: Broken Dura Ace 7900 Crankset
Postby Nobody » Wed May 11, 2011 10:12 am
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.
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Re: Broken Dura Ace 7900 Crankset
Postby ghettro » Wed May 11, 2011 10:25 am
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Re: Broken Dura Ace 7900 Crankset
Postby rustychisel » Wed May 11, 2011 11:43 am
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.
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Re: Broken Dura Ace 7900 Crankset
Postby Nobody » Wed May 11, 2011 12:10 pm
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Re: Broken Dura Ace 7900 Crankset
Postby jules21 » Wed May 11, 2011 12:17 pm
thus dispelling the oft-stated myth that DA is more durable than Tiagra or other cheaper level components. it's lighter.Nobody wrote: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.
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Re: Broken Dura Ace 7900 Crankset
Postby rustychisel » Wed May 11, 2011 1:24 pm
jules21 wrote: 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.
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Re: Broken Dura Ace 7900 Crankset
Postby jules21 » Wed May 11, 2011 1:31 pm
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.jules21 wrote: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.
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".
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Re: Broken Dura Ace 7900 Crankset
Postby Nobody » Wed May 11, 2011 1:44 pm
Bontrager on R&D:
http://www.bikeradar.com/MTB/gear/artic ... ger--27619"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.â€
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.
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Re: Broken Dura Ace 7900 Crankset
Postby Robdog » Wed May 11, 2011 11:04 pm
Not his first time, unlikely to be the lastrustychisel wrote: It tells me you've jumped to a conclusion unsupported by any facts.
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Re: Broken Dura Ace 7900 Crankset
Postby winstonw » Thu May 12, 2011 8:00 am
The crank definitely failed first.ZepinAtor wrote: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.
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Re: Broken Dura Ace 7900 Crankset
Postby ZepinAtor » Thu May 12, 2011 8:41 am
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 ?
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Re: Broken Dura Ace 7900 Crankset
Postby othy » Thu May 12, 2011 3:11 pm
Shimano don't make a 7900 bb30 crankset.Nobody wrote: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.
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Re: Broken Dura Ace 7900 Crankset
Postby Nobody » Thu May 12, 2011 3:45 pm
Thanks. So what is it then? Is it the standard diameter of 24mm? A smaller diameter should have even more torsional stress on it.othy wrote:Nobody wrote:Shimano don't make a 7900 bb30 crankset.
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