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Help with rear shifting problem

dmousedmouse Posts: 18
edited July 2009 in MTB workshop & tech
Hello

My spec:

Kona Kula 2008 in standard form expect for upgraded stem and bar.
Parts in question:

Shimano HG53 chain
Shimano XT Shadow rear mech
Shimano Deore shifters

Okay so the problem I have relates (I think) to the tension of the rear mech or the length of the chain...but I could be wrong. This is not related to wear as the problem has exisited from new (the LBS I bought it from have done their best but setting it up on the stand and a quick test ride aren't working out and the bike works fine unless you actually ride it (and even then its a bit intermittent).

The initial problem was that the bike tended to randomly stick in gear, in that I would move the shifter and the chain did not move up/down the sprockets. This was not a simple cable tension or upper/lower stopper problem as it would tend to stick moving either up or down the gears. I solved this problem by adjusting the mech pretension screw (the one that moves the mech body back from the hanger. In order to get relaible shifts this screw needs to wound all the way out.

This solves the problem but I now find that the tesnion arm hits its stop in the lower gears at the back when on the small chainring leaving the chain slack.

I believed the correct fitting of the mech is to put the bike in both smaller gears (I know it is not recommended to ride in the gear) and adjust the tensioner screw until the slack is just taken up followed by steeing the upper and lower stop screws and then cable tension. However on my bike this results in poor shifting.

What I am wondering is whether the chain is in fact too long and I should remove a link or two so that the mech can be positioned in the "reliable" position without running out of the tensioner movement in the smaller gears?

Or is the issue related to chain tension and I need to run it as is and the chain slack is normal?

Your help is appreciated. I have searched lots of how to videos on the web etc. and even the actual Shimano one on YouTube does not mention the tensioner screw and starts straight into the stoppers and onto the cable tension.

Posts

  • Matt 24kMatt 24k Posts: 186
    Maybe the mech hanger is bent?
  • dmousedmouse Posts: 18
    The hanger looks fine, straight and in line with the rear stay.
  • supersonicsupersonic Posts: 82,708 Lives Here
    You must set the stops first. Then the tension. Park Tools has the best guide on how to do it. And for chain length.

    Sounds like the cables maybe routed badly and are rough.
  • dmousedmouse Posts: 18
    Thanks. I read the Park tools Guide - much better than the others I read butT it says that you should adjust the B screw in the top sprocket until the top wheel of the mech touches and then wind it back a little to get clearance. On mine the plastic wheel never gets near touching even with the B screw all the way out?

    This probably means that the mech is fine with the B screw all the way out....

    Cable routing and moving is fine, nice and smooth.

    Reading the chain length guide I will check the length tonight, I'm guessing that either the bike violates the deralleur capacity by design or that the chain is too long. I will measure it for top to top and see if any links can be removed to reduce slack when on the smallest rings.

    Any further advice appreciated.

    Si

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  • dmousedmouse Posts: 18
    Just been reading this:
    http://www.singletrackworld.com/forum/t ... y-problems

    Seems like the XT Shadow needs to run close as possible which mine is, nothing about chain slack in the smalls though....
  • supersonicsupersonic Posts: 82,708 Lives Here
    The chain wants to be set up so that if you hit the big/big gear, the chain will be long enough. Otherwise you will cause catastrophic damage! So loop around trhe two largest rings, bypassing the mechs and add two links.

    This may make it run slack on the small small. Is of little consequence.
  • dmousedmouse Posts: 18
    Thanks, I'll do that.
  • dmousedmouse Posts: 18
    Update:

    I did the two link measure and the bike came out 1 link two long. Took the link out and checked it on both big sprockets - shifted there okay and still plenty S still in the chain as it goes round the little wheels.

    Runs sweet now, changes well and no chain rattling when using the lower gears.

    Thanks for all your help.
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