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Thread: Cracked crank?

  1. #1
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    Default Cracked crank?

    My left crank made some worrisome creaking/ticking noises on the ride home last night, so I pulled it to inspect. Here's what I found. It's a Shimano Deore LX, c. 1990. I will try sanding and polishing to see if it's just a surface scratch, but I'm thinking crack. The red arrow is the part that looks worst, the orange arrow is pointing to what looks like an unpolished surface to me.

    Click image for larger version. 

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    Time to go shopping...

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    Speaking as someone who has broken left cranks before after hearing creaking noises, you're definitely going to need a new one.

    Proof that I know what I'm talking about
    Quote Originally Posted by bobco85 View Post
    Well, this just happened today:

    broken left crank
    Attachment 5947

    scratched right foot
    Attachment 5948

    While riding to Trader Joe's in Clarendon on Highland just a few blocks south of my destination, I was in a standing position to pedal off from a stop sign (I was a PAL) when the left crank on my hybrid suddenly broke. I luckily did not fall down, but I did scratch the back of my right foot. A cyclist coming toward me witnessed it and asked if I was okay. I replied that I was fine but tried to hide my disappointment at knowing that I'd have to get the crank (and hopefully JUST the crank) replaced. At this point, my bike (GT Zum City bike) is a year and a half old with about 3,460 miles on it.

    I did, however, continue on my grocery trip, getting a bandage from Trader Joe's (it was blue!), and using the downhills to coast with walking for the uphills to make it home. I got into a good rhythm of half-pedaling by using my left foot to push the broken half of the crank back, almost like rowing a boat, to stay above walking speed, but really, it was rather pathetic.

    Anybody know what kind of $$$ will be emptied from my wallet to fix/replace this with average parts?

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    TwoWheelsDC is offline I really need to log off the internet and go for a ride.
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    Sounds like the perfect opportunity to switch to an external bearing bottom bracket and a two piece crank!

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    Quote Originally Posted by TwoWheelsDC View Post
    Sounds like the perfect opportunity to switch to an external bearing bottom bracket and a two piece crank!
    An upcoming build will be external BB/2 piece for sure. For now, I think a $20 replacement arm will do.

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    Quote Originally Posted by bobco85 View Post
    Speaking as someone who has broken left cranks before after hearing creaking noises, you're definitely going to need a new one.

    Proof that I know what I'm talking about
    Yoiks. I'll take your word for it.

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    Quote Originally Posted by EasyRider View Post
    My left crank made some worrisome creaking/ticking noises on the ride home last night, so I pulled it to inspect. Here's what I found. It's a Shimano Deore LX, c. 1990. I will try sanding and polishing to see if it's just a surface scratch, but I'm thinking crack. The red arrow is the part that looks worst, the orange arrow is pointing to what looks like an unpolished surface to me.

    Click image for larger version. 

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ID:	14695
    I might not condem that crank arm only on the image. The line has no black oxide and does not extend to a corner of the BB socket. It might be a casting line. The crank has some other similar casting lines. Install it back on the BB at full proper torque and check if the crack line changes.

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    Quote Originally Posted by Vicegrip View Post
    I might not condem that crank arm only on the image. The line has no black oxide and does not extend to a corner of the BB socket. It might be a casting line. The crank has some other similar casting lines. Install it back on the BB at full proper torque and check if the crack line changes.
    Vicegrip, I'll try sanding and polishing it to see if the "crack" remains. The outside of the crank has a better finish than side pictured. Understandable for the side that no one would see on a mid-grade part. Here's another view:Click image for larger version. 

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  12. #9
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    Update: I was able to sand and polish out the flaw, so I'm going with casting line, not crack. The creaking appears to have been an issue with the drive side of the BB. After trying everything else, I backed it out a half turn, retorqued it, then left a sip of beer on the workbench for the gremlin that got stuck in there when I first installed the BB two weeks ago. All quiet on the morning commute.
    Click image for larger version. 

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