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Recently I discovered that my bike's back gears are shifted two gears off constantly. So when it says I'm in 1st gear I'm actually in 3rd and when it says I'm in 2nd I'm actually in 4th.

The shifter doesn't look bent or anything and I don't remember hitting it against anything. Is there some easy fix or this something for a bike shop?

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What do you mean by "it says I'm in 1st gear?" What says? Do you have some sort of gear indicator? –  Carey Gregory Aug 18 '13 at 5:48
    
the gear indicator on the bike handel. –  codedude Aug 18 '13 at 20:28
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4 Answers 4

Actually this sounds to me as cable tension problem. My advice would be to unscrew the

enter image description here

about two revolutions, a quarter at a time, and see if you reduce the shifting inaccuracy to 1 gear only. If nothing happens, stop -> I was wrong.

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Yeah, a previously working setup will tend to get off like this as the cable stretches. You get a lot of cable stretch when the cable is new, and several adjustments may be needed over a period of a few weeks. After that, only occasional adjustments are needed until the cable reaches end of life and starts to break one strand at a time. –  Daniel R Hicks Aug 18 '13 at 12:08
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I think what's happened is that your gear cable has become very badly misaligned. You're going to have to either take it to the shop to get adjusted, or figure out how to adjust it yourself.

There are a million articles out there to teach you to adjust a rear derailleur. The basic steps I'd follow would be:

  • Shift to the lowest gear (smallest cog)
  • Release the anchor bolt shown below
  • Adjust the low gear limit stop so that the chain runs quietly and is aligned on the smallest cog
  • Tighten the adjusting barrel "most" of the way, so there's still some slack to work with
  • Pull the cable reasonably tight (make sure the housing isn't caught anywhere along the frame) and tighten the anchor bolt.

At this point you should have the indicator reading 1 when you are in the first gear... a good start!

Next try changing gears and adjusting the "Adjusting barrel" until you're getting good clean shifts.

Finally change up to the highest gear and then tighten up the high gear limit stop so that you can't accidentally get the derailleur tangled in the spokes by over shifting.

enter image description here

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Image credit to sheldonbrown.com? –  James Bradbury Aug 19 '13 at 7:51
    
@JamesBradbury Where else does one get information about bike maintenance? –  Mac Aug 19 '13 at 23:07
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On your rear derailleur there are two boundaries adjustment screws marked H and L. These are the high and low limit adjusters.

Most probably your L screw is way screwed in and doesn't allow the derailleur to move to 1st and 2nd gears. You need to unscrew this until it can move there.

Have a look at http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur for more detailed info.

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I don't think it'd be related to the high and low limit adjusters. It's more likely a badly adjusted cable. –  Mac Aug 19 '13 at 5:20
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This is more of a fun anecdote than a likely useful answer, but I once used a bike with a rear indexed shifter that was off by one gear. It turned out that the shift cable, which ran bare under the bottom bracket, was caught under part of the kickstand's mounting bracket, which changed the length of its run by just that much (but still let it slide). When we freed it the cable had a shiny spot but no broken strands, luckily.

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