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Old 02-27-08, 06:00 PM   #1
Lamplight
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Ever had this happen? (crankarm failure)

Riding to work this morning, I came to a short but steep hill on the MUP I regularly travel. I could see a patch of ice crossing the path, so I got off the bike to walk around it because I didn't dare try to crank up a hill on ice. I got around it and mounted the bike, but realized I had left it in a fairly high gear. So I stood up and just let my body weight push the left crank downward and then *SNAP*. I heard metal hit the ground as my foot landed on the pavement. I assumed my pedal had broken, but I looked down and saw this:



And then this:



They were 1994 Shimano STX cranks with probably about 4-5000 miles tops. And for the record I weigh between 185 and 190, probably close to 200 considering how many layers I was wearing. Luckily I avoided injury; I just had to walk home and get another bike. So, have you ever experienced this? It was a terrible sound!
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Old 02-27-08, 06:08 PM   #2
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Old 02-27-08, 06:11 PM   #3
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Just a couple of weeks ago, my right crank arm broke about an inch below the pedal. It was a Galli that I had been using for about 25 years, so I gues it is not too shocking. When I looked hard, I found a crack in the left arm, too.
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Old 02-27-08, 06:24 PM   #4
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I've snapped cheap one piece cranks and there's at least one website showing pictures of, mostly older, failed cranks. Just visually check them when you're doing your weekly clean ( what clean?) and you'll be fine.
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Old 02-27-08, 06:26 PM   #5
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I broke a crank on a spin cycle at the YMCA. Actually, it might have been the bottom bracket that snapped where it joins the cranks. Tthe part probably rusted out from all the sweat in spin classes, but I like to think it was from the tremendous amount of torque that I generate.
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Old 02-27-08, 06:58 PM   #6
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Impressive.

No, never happened to me. Probably never will, given that I'm 120 pounds and a spinner.
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Old 02-27-08, 06:58 PM   #7
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After removing the arm (and replacing both!) it looks like it fractured about half-way through, and then bent what was left until it snapped completely. Looking at the cross-section of the arm, the metal on one half is much darker than the other and has a strange circular "grain" to it, where as the other half is nearly white and doesn't appear to have any "grain" at all. Of course now I'm nervous about all of my bikes.
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Old 02-27-08, 07:05 PM   #8
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Quote:
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...the metal on one half is much darker than the other and has a strange circular "grain" to it, where as the other half is nearly white and doesn't appear to have any "grain" at all...
darker section is the fatigue which is usually a gradual process

lighter section is your strength >> remaining metal

http://www.google.com/search?hl=en&q=metal+fatigue
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Old 02-27-08, 07:07 PM   #9
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Very interesting, thanks. I had some brand new Sugino cranks on the bike, but I had temporarily put them on a Miyata touring bike and replaced them with what I had lying around, which happened to be these. The Suginos are now back in place, needless to say.
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Old 02-27-08, 07:30 PM   #10
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Wish I were strong enough to break crank arms. Wish I could fly like Superman.
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Old 02-27-08, 07:40 PM   #11
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Doesn't the darker area indicate it was cracked for a while, and oxidized in the cracked section? Maybe there was a hairline crack there. I can't really tell, but it looks like there's a small nick in the crank right where the crack started.

It's lucky you didn't crash!

Cracked cranks pictures -- FAIL.

Last edited by rm -rf; 02-27-08 at 07:53 PM.
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Old 02-27-08, 08:18 PM   #12
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The closest I came to it was bending a Campy C record crank when I used to race. (It was a long time ago)

Bought a Dura Ace set after that, and did not bend that one.

Glad you are ok!
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Old 02-27-08, 09:33 PM   #13
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Glad you weren't hurt!

My 105 had a very big fracture. So big the LBS wouldn't let me out of the shop until it was switched. Good chance to upgrade to Ultegra!
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Old 02-27-08, 09:35 PM   #14
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I broke an Ofmega right crank about 22 years ago. It cracked from the pedal axle threads and the whole pedal broke off and I went down onto the pavement. I was pedaling up a mild incline at the time, luckily not far from home.
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Old 02-27-08, 09:58 PM   #15
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The cirdular "grain" is typical of a fatigue crack that spreads from the stress raiser. Then the white crystaline grain is the tensile fracture that happened under your weight.
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Old 02-27-08, 10:03 PM   #16
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I've never had this happen, though Sheldon sez standing up puts an unnecessary strain on the machine.

Guess I'll see, eventually.
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Old 02-27-08, 10:33 PM   #17
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Quadzilla!

I went through multiple bottom brackets and countless chainrings on my previous bike, but I had the same crankarms for 17 years. I've never busted a crankarm, but I have seen it on other bikes than yours.
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Old 02-27-08, 10:53 PM   #18
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Old 02-27-08, 11:32 PM   #19
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Broke a seat tube once but not a crank (yet).
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Old 02-27-08, 11:38 PM   #20
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I'm 165 pounds and a spinner and had a crank crack from the dust cap threads to the outside. That was enough to get the crank bolt to work loose and have the crank come off during a ride. But nothing like the OP!
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Old 02-28-08, 08:52 AM   #21
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Uh, don't wait for the crank to fail. Both the OP and myself lucked out. Usually there's some tell-tale hair-line fractures.
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I've never had this happen, though Sheldon sez standing up puts an unnecessary strain on the machine.

Guess I'll see, eventually.
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Old 02-28-08, 09:13 AM   #22
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The circular grain that you see is a textbook example of a cyclic stress fracture, which is exactly what you would expect to see in this kind of equipment.
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Old 02-28-08, 09:20 AM   #23
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I had a cheap Taiwanese one-piece crank fail, at the hole where the pedal was threaded.
The pedal just fell off when it rubbed against a curb [kerb].
I replaced it with a one-piece Schwinn crank that I had lying around.
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Old 02-28-08, 09:24 AM   #24
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I had a professor in a failure analysis class who did a bunch of consulting work. He had scores of bike cranks that were in some way involved in court cases, all of which failed like the one above. Glad you're ok!
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Old 02-28-08, 09:28 AM   #25
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Quote:
Originally Posted by Lamplight View Post
After removing the arm (and replacing both!) it looks like it fractured about half-way through, and then bent what was left until it snapped completely. Looking at the cross-section of the arm, the metal on one half is much darker than the other and has a strange circular "grain" to it, where as the other half is nearly white and doesn't appear to have any "grain" at all. Of course now I'm nervous about all of my bikes.
I'll bet it creaked before it broke. The one I broke did. Creaked like crazy no matter what I did to adjust it...I was just to dumb to figure out what it was
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