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Crank Failure

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Crank Failure

Old 03-21-15, 03:52 PM
  #1  
colnago62
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Crank Failure

Ok, I can't get the picture to load, but basically, left crankarm broke off at the spindle. Pulled the bearings out of the frame. It is a Shimano Dura Ace crank, so it isn't Cheap . I have a crank coming 2nd day air, but I am still bummed because I suffered through the rain this morning and right after I broke the crank, the sun comes out.
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Old 03-21-15, 03:56 PM
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Are you sure the bearing in the bottom bracket didn't seize, causing the crank to wear?
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Old 03-21-15, 03:56 PM
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A lot of watts
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Old 03-21-15, 03:57 PM
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Originally Posted by rms13 View Post
A lot of watts
Prolly over 400
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Old 03-21-15, 04:17 PM
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Use Imgur to load and post your picture
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Old 03-21-15, 04:48 PM
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Originally Posted by Pirkaus View Post
Are you sure the bearing in the bottom bracket didn't seize, causing the crank to wear?
That was the funny part, the bearings were still spinning smooth as can be.
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Old 03-21-15, 04:52 PM
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The bearing may only have an issue under load.
I wouldn't trust it, but I have been called a pessimist once or twice.
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Old 03-21-15, 04:55 PM
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https://imgur.com/OC8ljuI
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Old 03-21-15, 04:56 PM
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There was a guy a few months ago that claimed he went through a couple of cranksets per year or something like that. He was just too strong
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Old 03-21-15, 05:02 PM
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I don't touch my bike. I have a mechanic do all the work. He never said anything about premature wear; any wear for that matter. I am sure one they put it All back together, they will give their recommendations.
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Old 03-21-15, 05:04 PM
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How old are the bottom bracket and crankset? I'd want to see a picture of the ends of the spindle in order to look at what type of failure you had? Not going to be an internet metallurgist here, this might have had some defective metal in the spindle (a guess.) Sorry that you had the bad luck, it does happen to most of us, and usually at the worst possible time, too. Hopefully the new crank will be a good one.

Don't discount that this is a failure under warranty, if it isn't too old. Without knowing the year model/age, its up to you if you want to look in to that possibility.

rms: if someone is going through a couple of cranksets a year, they have something other than strong guads, or generate a hell of a lot more that 400 watts, they are intentionally doing something to trash a spindle or crank arm, or as full of as much crap as a Christmas turkey.

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Old 03-21-15, 05:20 PM
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The cranks have about 20,000 miles on them. They are 6 mos. out of warranty.

What is weird is that the failure was on the inboard side of the bearings. There isn't as much load on the spindle at that particular point.

Last edited by colnago62; 03-21-15 at 05:23 PM.
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Old 03-21-15, 06:00 PM
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if it's like my 7900, the drive side crank arm and spider are aluminum alloy and the spindle is steel. not sure how they are connected though. if this is correct, it wouldn't be too surprising if it failed at the connection between the two.
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Old 03-21-15, 06:04 PM
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i broke two nds crankarms two seasons ago. hope you didn't hurt yourself when the crank came out.
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Old 03-21-15, 06:34 PM
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Originally Posted by hueyhoolihan View Post
if it's like my 7900, the drive side crank arm and spider are aluminum alloy and the spindle is steel. not sure how they are connected though. if this is correct, it wouldn't be too surprising if it failed at the connection between the two.
The spindle itself failed. I have broke a square taper spindle back in the day, but always thought these were more durable.
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Old 03-21-15, 06:44 PM
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Originally Posted by colnago62 View Post
The spindle itself failed. I have broke a square taper spindle back in the day, but always thought these were more durable.
As noted, the spindle broke. IMO the jury is sill out about spindle strength. I suspect that they might have pushed too close to the edge in an effort to shave grams.
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Old 03-21-15, 08:11 PM
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Hey, it happens:

https://www.pardo.net/bike/pic/fail-001/000.html
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Old 03-22-15, 01:10 AM
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That's a defective crank. The spindle failed in shear from pedaling torque (you can tell because of the spiral break). This is not likely a fatigue failure and the bearings had nothing to do with it. This is very likely a manufacturing defect in the spindle. Contact a Shimano rep about this.

I can't see how this failure could have been caused by anything other than a manufacturing defect. The pinch bolts in the crank will strip long before spindle is damaged by over-tightening. The bearings are large and slow moving; there are very few instances that a frozen bearing will stay frozen long enough to wear all the way down to metal and notch the spindle. If the crank were installed wrong, I would expect the aluminum of the crankarm to fail long before the spindle. Under impact loading, I would expect the CF of the frame to fail before affecting the spindle.

I'm left with manufacturing defect.
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Old 03-22-15, 02:15 AM
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A NDS crank arm failure, and one might still be able to ride the bike home, although I could imagine it would be tough.

Shear the spindle like that, and I could imagine it would make a long walk home. Or, perhaps a good reminder why the Dandy Horse bike went out of style nearly two centuries ago.

Hopefully you were either close to home, or had alternative transportation.

If I was you, I'd push Shimano for warranty replacement, although it may be a little late now. Maybe they would cover the cost.

If it is a rare failure, I'd think they would want to get the crank back to inspect it. If it is a common failure, I'd think they might want it back just the same, unfortunately communication to the engineers in a big company is often not that great.
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Old 03-22-15, 03:40 AM
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Spent some time on the net and that this happens a fair amount. I found lots of pictures of XTR cranks broken in the same place as mine. I wonder if that went too thin 7900?
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Old 03-22-15, 05:36 AM
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Originally Posted by qcpmsame View Post
How old are the bottom bracket and crankset? I'd want to see a picture of the ends of the spindle in order to look at what type of failure you had? Not going to be an internet metallurgist here, this might have had some defective metal in the spindle (a guess.) Sorry that you had the bad luck, it does happen to most of us, and usually at the worst possible time, too. Hopefully the new crank will be a good one.

Don't discount that this is a failure under warranty, if it isn't too old. Without knowing the year model/age, its up to you if you want to look in to that possibility.

rms: if someone is going through a couple of cranksets a year, they have something other than strong guads, or generate a hell of a lot more that 400 watts, they are intentionally doing something to trash a spindle or crank arm, or as full of as much crap as a Christmas turkey.

Bill
Can somebody post the picture of the broken spindle?

Last edited by Campag4life; 03-22-15 at 05:47 AM.
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Old 03-22-15, 07:10 AM
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is it due to bad design flaw, or you're just too strong?
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Old 03-22-15, 04:32 PM
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I'm with the previous poster that the spiral nature of the fracture shows a shear failure from the torsion on the shaft. Could be a manufacturing defect and/or bad bearing causing the break. Any chance of seeing the actual break zone? It would be interesting to see if there is any indication of a long term problem. That would show as a smoother area within the break.
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Old 03-22-15, 04:43 PM
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Originally Posted by Point View Post
I'm with the previous poster that the spiral nature of the fracture shows a shear failure from the torsion on the shaft. .....
It's logical and predictable for a spindle to break torsionally. They always have, though it can be harder to see on an older 5/8" spindle.

Consider, the greatest stress on the spindle will be when the pedal is forward and loaded, as in hill climbing. At that point there are two forces on the spindle. The simple bending moment reflecting the twisting that happens within the crank arm, and the torsional load the arm normally exerts. So you'd expect the spindle to break with a forward twist and bend (think airplane rolling into a spiral dive).

As to why it broke, that's impossible to say. Underbuilt for the application and failing from a peak load, flaw or stress riser leading to premature fatigue failure, local impact damage from a pedal strike leading to premature failure later, etc.

As mentioned above, cranks have always broken in small numbers, and unless there's evidence of a higher failure rate than the historic average, we can chalk this up to a case of "manure happens"
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Old 03-22-15, 05:00 PM
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Tone down your guads.
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