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I tried to set up the perfect shifter + derailleur. For the last part it is better (for me) to have low-normal version, because reducing gears is essential for me (I bike mostly within city).

So I bought low-normal Shimano derailleur (Shimano-RD M770 SGS XT) and M770 shifter. To my surprise it is asymmetric shifter -- you are able to release gears by 4, and increase by 1 (I tuned M770 already, so I am able to increase gears by 2, just like with XTR).

So with current configuration I can more easily increase gears (by 4) than reduce them -- quite contrary what I wanted.

I can send back derailleur, and order a new one, with top-normal spring, but I would LOVE to improve M770 shifter even further by reversing the direction of cable, i.e. so the thumb level would pull the cable, and index level would release cable.

Such tweaking would result in dream machine (well, part of it).

The question is -- is such modification is possible at all (at home)? I would appreciate firm "no" or "yes" (+how to do it) answers. Thank you very much in advance.

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In general (without looking at that specific derailer) I think you'd need a machine shop. If you have one at home then you should be all set. –  Daniel R Hicks Aug 19 '11 at 15:42
    
@Daniel R Hicks, pity, I don't have one. I hoped that if tuning was so easy, that maybe reversing the mechanism would be at least possible. Btw. the question is about shifter not derailleur! Please post proper answer, so I will accept it. –  greenoldman Aug 19 '11 at 16:39
    
You have less chance of reversing the shifter than you do the derailer. The ratchets on the shifter are made to resist motion in one direction, and you'd be reversing the direction. –  Daniel R Hicks Aug 19 '11 at 18:37
    
You could do a kludge with a spring and double cable, though, where you have the derailer cable pulled by a spring, and your shifter pulls the spring back. Would greatly increase the friction and reduce the "liveness" of the shifter, though. –  Daniel R Hicks Aug 19 '11 at 18:39
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2 Answers

up vote 3 down vote accepted

You should switch your derailleur out for one with a reverse pull, but there is absolutely no chance that you'll get the shifter to reverse the operation of the cable.

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How to kludge up a "fix"

  • Obtain two stainless-steel "spiral" hose clamps, a little larger than your frame tubes.
  • Obtain some shift cable housing material and a new shift cable (or two), as long as possible (your existing will likely be too short).
  • Obtain an extension (pull) spring you judge to be about twice as stiff as the spring on the derailer (measured by pulling the cable).
  • Obtain some sort of cable clamp that can be used to clamp the end of 4 shift cables together (two cables doubled over). (Or maybe two separate clamps will work out better.)
  • If your shift cable comes up the down tube, you'll work on the top tube, and vice-versa.
  • Devise a hook of some sort that allows the spring to be anchored to the tube via one of the clamps.
  • Run a piece of cable housing from the shifter to maybe 6" short of the loose end of the spring, after it's mounted.
  • Run a piece of cable housing from wherever the old housing began in an arc (from down tube to top tube or vice-versa) and ending at the same point as the first piece of housing. (Note that this piece will be running "backwards", towards the back of the bike.)
  • Clamp both pieces of housing to the tube with the second hose clamp. (This may be a less than ideal anchorage, but remember -- this is a kludge.)
  • Thread cables into both housings appropriately.
  • Thread cable ends through the loose end of the spring and back on themselves.
  • Tighten the cables enough to pull the derailer all the way into low gear, then fasten the cables with the cable clamp.
  • Trim or secure the wild ends of the cables.
  • Adjust and ride.
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