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  1. #1
    Half way there gmt13's Avatar
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    What happens when your crank breaks?

    Well, first you meet the pavement real fast. And then you need to figure out how to get home with only one functional pedal (shoe on the stump doesn't work all that well and to bystanders you look darn weird.)
    Crank-Broke.jpg
    So, this crank was a Sugino Mighty Tour that I had set up with a single 40t ring on my IGH commuter. It is early 80's vintage, but probably had less than 4000 miles on it. It was never damaged and I doubt that my 60 year old legs have suddenly developed mega power. So why did it break? Other than the groove, I see no real stress points. Microscopically, the fracture shows 2 colors to the metal, so I think there was a micro-crack half way through it that finally gave up. Does anyone think this may have been a manufacturing defect? It's got me scratching my head.

    I took my dive when pedaling up hill (of course) and luckily there was no traffic around. It was remarkably fast - I had no time to think about what was going on.

  2. #2
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    That's a classic example of metal fatigue. An object failed under a load which previously didn't cause a failure.

    Basically, aluminium has a finite life. Any cyclic stress, however small, will eventually cause failure. It's not something I've ever seen a crank do before though, admittedly. I'd suspect something provided a stress concentration which exacerbated the situation, to cause failure after that kind of distance.

  3. #3
    we be rollin' hybridbkrdr's Avatar
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    Is there a possibility that Deore or Deore LX cranks would be stronger than Alivio cranks even if they're lighter?

    Oddly enough, I have a Sugino Impel 150X crankset that I haven't installed on a frame yet. I wonder sometimes if it would be more realistic to buy another one and sell that one on eBay. lol
    Feeling Good by David Burns

  4. #4
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    Quote Originally Posted by hybridbkrdr View Post
    Is there a possibility that Deore or Deore LX cranks would be stronger than Alivio cranks even if they're lighter?
    Maybe, it depends on the alloy they're made from, and the heat treatment it's undergone.

  5. #5
    coprolite fietsbob's Avatar
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    >.. you go shopping for another crankset .. to use a 40t ring, .. there are 110 BCD cranks 3 Pc BMX stuff is pretty cheap.

    130 BCD is also useful as they start 2 t smaller [38t ], and as 39t is a common inner on doubles they are really common..

    single rings .,. I like Surly's Stainless steel are great.. super, Long Wearing..

    that one is probably over 25 years old..

  6. #6
    Senior Member
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    that looks like a pretty old crank. maybe 30 years or so?

    anyway, i had one break on me (american classic) after about 14,000 miles. it happens. getting home was fun! i leaned WAY forward then i hooked my broken crank side leg over the rear rack. that gave me some leverage. surprisingly, it wasn't too bad...

  7. #7
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    IME the most common cause of crank failure is pedal impacts. Each impact isn't enough to break a crank immediately, but it can accelerate the fatigue process, or can overstress the crank setting up a future failure.

    Also many (most?) aluminum alloys are prone to notch failure, meaning that a stresses tend to concentrate at small defects, nicks, dimensional steps, etc. Possibly time, use and a minor defect combined to lead to the failure.
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  8. #8
    Insane Bicycle Mechanic Jeff Wills's Avatar
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    A friend of mine had a Mighty Tour crank that broke while he was out touring. He hitchhiked to a town and caught a bus back home. End of tour, end of touring.

    A local shop gave him a replacement, which he never installed. Many years later, he gave it to me. I'm going to use it on a bike where I need a very low "Q" dimension.

    I think replacing the Mighty Tour with a mid-'90's MTB crank (with a 110mm bolt pattern) would be just fine.
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  9. #9
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    Quote Originally Posted by Jeff Wills View Post
    A friend of mine had a Mighty Tour crank that broke while he was out touring. He hitchhiked to a town and caught a bus back home. End of tour, end of touring.

    A local shop gave him a replacement, which he never installed. Many years later, he gave it to me. I'm going to use it on a bike where I need a very low "Q" dimension.

    I think replacing the Mighty Tour with a mid-'90's MTB crank (with a 110mm bolt pattern) would be just fine.
    I guess some people have limited coping skills and/or a low threshold for frustration. I wonder how that person would handle a real problem.
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  10. #10
    S'toon trail rider! MisterK's Avatar
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    steel is real

  11. #11
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    Quote Originally Posted by MisterK View Post
    steel is real
    So they say, however early in my cycling career I broke no less than 4 steel cranks. the first cranks I didn't break were my Campy Records in 1967.

    Since then I've used only Campy, and Suntour Spb Pro, or XC Pro, and haven't broken a crank.

    Not saying there aren't top quality steel cranks, but the assumption that steel cranks are stronger simply by virtue of being steel is overly simplistic.
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  12. #12
    coprolite fietsbob's Avatar
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    Campag cranks broke occasionally the quest for stiffness ocasionally crosses over the line to brittle hard.

    [i really noticed that with their steel hub axles ]

  13. #13
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    I would like to see a photo from from the broken ends.

    The small tear on the top of the arm is interesting. It may have been the last thing to let go.

    Also, there seems to be an abrasion on the end of the arm. A odd hit, a curb swipe, a parking
    barrier can all begin a stress or crack in the metal or enhance one that is already there.

    The shadows in the image are deceiving, but two colors would indicate to me that there has
    been a crack( break or flaw?) in the arm for some time. The raw, exposed alloy would oxidize and
    blacken over time. Surface cracks have the added factor of allowing rain, dirt and perspiration into
    the opening furthering the deterioration.

  14. #14
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    Oh yes, I failed to mention a couple of the very ugly injuries I have seen from this type of thing.

  15. #15
    bike whisperer Kimmo's Avatar
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  16. #16
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    Quote Originally Posted by Airburst View Post
    Maybe, it depends on the alloy they're made from, and the heat treatment it's undergone.
    I depends on weather they are forged or not. All of shimano cranks are forged. Heat treatment would negate the effects of forging.

  17. #17
    Senior Member Looigi's Avatar
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    A friend of mine broke his crank a couple of years ago. He hitched a ride in an ambulance.

  18. #18
    we be rollin' hybridbkrdr's Avatar
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    Quote Originally Posted by 100bikes View Post
    Oh yes, I failed to mention a couple of the very ugly injuries I have seen from this type of thing.
    I feel like staying in a seated position while riding after reading that. lol
    Feeling Good by David Burns

  19. #19
    Andrew R Stewart Andrew R Stewart's Avatar
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    The crank arm failure looks to have been right at the point that the end mill would center on (the tool that cut the groove) as it was at the end of the groove. I have seen this type of failure before. The cutting of the groove leaves a stress risor at the end of the groove, or actually at the center of the radius of the end. It's hard to see completely but there seems to be a shadow that radiates across the arm's cross section and is centered on the groove. This shadow could be the crack collecting grime over the years. At some point the cross section that was not yet cracked couldn't handle the stress of normal pedalling.

    This is why the drillium/relief groove is so questionable if not forged into the part. A surface stress relief like media blasting prior to anodizing migh have reduced the chance of a crack starting. BITD we would lightly file the sharp webbing between the crank arm and the spider arm of the Campy NR cranks to reduce the stress concentrations. As a young guy i found it interesting that a very little bit less material could improve the lifespan of Aluminum parts. Andy.

  20. #20
    Senior Member kmv2's Avatar
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    Quote Originally Posted by FBinNY View Post
    So they say, however early in my cycling career I broke no less than 4 steel cranks. the first cranks I didn't break were my Campy Records in 1967.

    Since then I've used only Campy, and Suntour Spb Pro, or XC Pro, and haven't broken a crank.

    Not saying there aren't top quality steel cranks, but the assumption that steel cranks are stronger simply by virtue of being steel is overly simplistic.
    Who still makes steel cranks ?

    edit: And not "stronger", but steel doesn't have a finite fatigue limit so its not necessarily going to fail like OP's crank.
    All aluminium will fail after a finite amount of time, this was learned the hard way via materials properties learned from failures in fields such as aerospace and electrical wiring.
    Last edited by kmv2; 01-17-13 at 02:51 PM.

  21. #21
    bike whisperer Kimmo's Avatar
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    Material doesn't matter nearly as much as appropriate design. If aluminium was as dodgy some folks here like to make out, we wouldn't make planes out of the stuff.

  22. #22
    coprolite fietsbob's Avatar
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    But there is a service log for airframes and when they pass the hours in service limit ,
    they park them in the desert or cut them up and melt it all into an ingot
    and make something else out of it.

    sometimes they try to stretch the usable life and that is why Nostalgic old WW2 aircraft
    come apart at air shows , or got re cycled for water bombers for fighting Forest Fires
    come apart in the Sky at the worst possible time.
    Last edited by fietsbob; 01-17-13 at 04:37 PM.

  23. #23
    bike whisperer Kimmo's Avatar
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    My point is that if bikes were aircraft, you'd see a lot less failures due to poor design.

  24. #24
    coprolite fietsbob's Avatar
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    Still apples and oranges.

    Bikes, Components, You would have to get rid of them sooner. NO C&V. except stationary in Museums.


    or have to stop caring about how much less they weigh than they used to.

    Roger Duham had his Cranksets Arms made from a Cromoly Rectangular tube
    and Tig Welded together, Rt one Onto the Tube spindle .
    [his patent expired, so now we see the tube spindles attached to Right crank arms ,
    But the arms are an Aluminum Alloy ...,gram counting won over durability. ]

    just be like the Sponsored Pros. New gear every Spring., and a trailer full of spares to
    always have the equipment seem invulnerable.
    Last edited by fietsbob; 01-17-13 at 05:16 PM.

  25. #25
    Half way there gmt13's Avatar
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    Here's another pic showing the distinct color boundary. The flash makes it look more pronounced than it appears to the eye. From the comments above, it is convincing that there was a crack there that finally let loose. I added the paint in the groove probably less than 3 months ago and did not notice the crack (which may have been too slight to notice anyway). So other than periodic Magnaflux inspection, what's a person to do? Retire the crank every 5000 miles? I am a C&V aficionado, and actually put some serious miles on my older bikes. Now I have to say that I am a bit spooked.

    Crank-Broke1.jpg

    Thanks to all for your insight.

    -G

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