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Every time I have my bike serviced, the bike shop sets up the gears on a stand in their workshop. The gears change perfectly on the stand, but when the bike is on the road, the rear derailleur doesn't change smoothly, and some gears are jumped.

I've fiddled with this myself, but can't improve over the default setup. Is it possible, or does this mean the Shimano gearsets are worn/the springs worn out on the rear derailleur?

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have you ever replaced the chain or rear cassette or front chain rings? –  Dog Ears Aug 25 '10 at 22:26
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5 Answers

The first rule is make sure the rear derailleur is adjusted correctly before adjusting the front derailleur.

There are three adjustments that you can do on the front derailleur:

  1. Clamp Position
  2. Low limit stop
  3. High limit stop

Clamp Position

Here you can adjust the height of the derailleur, normally this is recommended as a 2mm clearance between the bottom of the outer cage and the teeth of the large chainring.

front derailleur height adjustment

Picture from bicycletutor.com site

You can also adjust the rotational angle of the derailleur.

front derailleur rotational angle adjustment

Picture from Parktool site

Limit stops

The Limit stops are for adjusting the inward and outward travel of the front derailleur. Limit stops are marked "L" for Low and "H" for High. The L-stop will stop the movement of the derailleur toward the smallest chainring. The H-stop will stop the movement of the derailleur toward the largest chainring.

There is a good guide on the Parktool site that gives a lot of detail on adjusting the front derailleur.

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Also, while derailleurs generally operate similarly, you should download the manual for your derailleurs and read through them before attempting adjustments. Manuals are usually available from your derailleur manufacturers website. –  peppajiggapuff Aug 29 '10 at 11:39
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There are a number of reasons that the shifting on your rear derailleur is not working well:

  • The derailleur hanger (the bit of the frame that the derailleur bolts onto) could be bent. To check this look at the angle that the derailleur cage is at. When viewed from behind, it should be vertical, when viewed from above, it should be parallel to the centreline of the bike. Your bike shop should be able to either re-bend the hanger to align it properly or supply a replacement hanger if you frame has a replaceable derailleur hanger.
  • The cassette and chain could be excessively worn. The standard way to check this is to look at the distance between chain links - you can get a tool that does this for you. If you haven't replaced your chain for a while, this can often be a problem. The fix is to replace the chain, and if required the cassette.
  • There may be excessive friction in your gear cables. You try an interim fix of pulling the cables out of the outers, washing them through degreaser and then re-fitting them. You may need to repeat a few times to get the gunk out of the outers. A longer term fix involves replacing the cables.
  • The rear derailleur spring could be worn out and weak. Not much you can do about this short of replacing the derailleur. Only really high end derailleurs are able to have their springs replaced.
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+1 for checking the cables. Also checking for damaged cables can be very important. I had a rear cable that was frayed and stretched with each use, thus changing the derailer adjustment. –  Jack M. Sep 15 '10 at 19:47
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The difference probably arises because there is more tension being exerted on the chain when you are cycling as compared to the bike being up on a stand.

The screws on the dérailleur (as you probably know) are for setting it to stop at the right place (ie not to come off either towards the wheel or off the small gear). Do you have shifters which move the dérailleur a fixed amount (indexed?) or do you just have levers? On mine if there are problems shifting between the gears then I have to fiddle with an adjuster at the shifter, some dérailleurs also have adjusters at the dérailleur itself.

If you have a Shimano rear dérailleur (or a front one for that matter) go to the Shimano Technical Documents site and see if your model is listed, check there for a guide to setting up your specific model and troubleshooting it once you have set it up.

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I had this problem for a long time, and eventually I realised I was changing down gears too late when approaching a hill. The solution was simply to anticipate the hill and change gears slightly earlier before too much tension built up. –  Kevin Aug 29 '10 at 9:02
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I'm surprised they do it on a stand. The flex of the frame while in use makes a significant difference in how it needs to be setup! I put my bikes on a trainer when doing the shifting calibration. That way I can jump on the bike and do test runs under load. Sometimes ends up being a bit clunky if you spin it by hand while not riding, but always works perfect out on the road!

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A friend of mine had a similar problem lately, and it turned out that her cable was damaged in the housing and was slowly lengthening as it degraded. It took her several shops to find the problem, but eventually they did.

Pulling the cable out of the sheathing is easy and should eliminate the cable as a possible culprit.

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