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  1. #1
    Senior Member d2create's Avatar
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    Crank/Chain Ring Flex?

    What's a normal amount of flex under load?
    I have a Sugino XD double crank and my big ring is a 50t.
    When under load (pedaling from a stop, butt off the seat) I can see a good 1/4 inch of movement (side to side) of the chain in the derailler. It's enough that the chain scrapes the inside of the derailler. Is this normal? I didn't notice this before on my Sugino XD triple with a 48t big ring. Could be that I don't have my derailler set up correctly though since I did have to remove/replace it. But I still think I would have noticed that much chain sway.
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  2. #2
    Senior Member DannoXYZ's Avatar
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    Loose bottom bracket bearings? Also frame-flex plays a major part of the movement you're seeing as well.

    But most likely you have a bent chainring. Take the chain off and spin the crank by hand. Use the FD as a gauge and see how much the chainring moves laterally as you spin the crank by hand. Use a crescent-wrench to true it.

  3. #3
    Senior Member d2create's Avatar
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    Quote Originally Posted by DannoXYZ View Post
    But most likely you have a bent chainring.
    That was my first thought, but it's definitely more pronounced under load. During light pedaling it seems to be well within what I would think is spec for a relatively inexpensive ring. Then again, maybe it isn't.

    I also tried two different phil wood bottom brackets... one of them is brand new.
    Same frame as before so it shouldn't have more flex than it did before i switched cranks.
    2008 Rivendell A. Homer Hilsen
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    2010 Specialized Rockhopper 29er

  4. #4
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    Likely frame flex. If you are a heavier and powerful rider, the amount of flex as evidenced at the gate of the front derailleur can be noticeable.

  5. #5
    rebmeM roineS JanMM's Avatar
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    Trying moving away from a stop in a lower gear.

  6. #6
    Klaatu barada nikto cascade168's Avatar
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    Whatever the case, never underestimate the importance of a really stiff crank ;-)
    "Work is the curse of the drinking class."
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  7. #7
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    are you cranking the bike hard side to side? can you up the wheel and cinch it closer to the
    seat tube? this is a quality ride according to the pics and the bracket is bombproof--- what kind of tires are you running--- are you using spd and are they coming loose at any point?

  8. #8
    Senior Member vredstein's Avatar
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    Sugino XD cranks have a 110mm bsd. That's a pretty small diameter for a 50t ring. The distance from where the ring is secured and where the chain exerts force on the ring creates room for leverage results in flex. It's physics. Same reason a longer crank arm will flex more than a shorter crank arm.
    I'm pretty sure if the same make/model/length crank was 130mm bsd, using the the same bottom bracket, and using a 50t ring, you have less flex.
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