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  1. #1
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    Problem with my mountain bike...can anyone lend some advice?

    Hi,

    I am a fairly green mountain biker, I am primarily a BMXer in my early days and progressed to a basic GT mountain bike to keep myself in shape now I'm older.

    I have a small problem with my GT mountain bike - when I cycle OUT of the saddle e.g.up hills or for speed on flats.....the pedal slams violently from 12 o'clock position to 6 o'clock position occasionally. I had this same problem on a hire-bike also once.

    I am unsure wether it is a crank or rear sprocket etc.

    I went to my 2 mountain bike shops to ask (without the bike) - one of them said 'ooooo, that could be the crank, you might need a new crank'. The other said it could be rear sprocket - but both gave that impression that they would rack up lots of hours of labour....I am concerned I'm going to be ripped off here as the fault is intermittent and not obvious unless you went for a 15-20 minute bike run pedalling out of the saddle.

    Can anyone lend some advice?

  2. #2
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
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    I'd put my money on the rear sprockets & chain. Maybe an hour job if the wrench dogs it. Parts can be reasonable to pricey depending on what you get.

  3. #3
    Senior Member rjacob's Avatar
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    Possibly the chain is temporarily slipping partly off the front or rear sprocket, just for a single stroke? Or something in the freewheel is broken? I don't think it would be hard to fix.
    I would start off by re-adjusting the indexed shifting (it might be very slightly off). If that doesn't fix it, maybe swap out the freewheel...

  4. #4
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by Stacey
    I'd put my money on the rear sprockets & chain.
    +1. My betting line would be:

    90% rear derailleur adjustment. Maybe a little higher.
    9% worn chain and/or cassette.
    <1% crank. Maybe a little lower.

  5. #5
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    Thank you very much everyone for fast replies.

    I am relieved to hear my crank is OK probably!

    It should be noted the bike has maybe 60-70 miles in total on it, not a great deal. So I am wondering if the derail-adjustment would be more likely as a worn sprocket or chain would warrant more miles?

    How would I adjust the derail thing?

  6. #6
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    Also in addition to derailleur adjustment, how do I adjust the index shifting?

    Thanks!

  7. #7
    Non Tribuo Anus Rodentum and off to the next adventure (RIP) Stacey's Avatar
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    www.parktools.com ~ All you need to know and then some.

  8. #8
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by yoshi123
    It should be noted the bike has maybe 60-70 miles in total on it,
    The betting line just changed.

    Rear derailleur adjustment is now at 99+%.

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