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Basically the angle between the bottom and top wheels is too great causing the chain to skip to the outside of the bottom wheel under force. If I use the higher adjustment screw on the shimano derailleur to align them better the top wheel is then not aligned with the smallest sprocket.

I'm a beginner and had an attempt at working it out but probably missing something obvious. enter image description here

  • Do you mean the jockey wheels are not aligned vertically, as they should be, and one is more inboard than the other? Adding a photo (press edit)will show exactly what you mean, but I wonder if the derailleur hanger is not straight – Swifty Jun 3 at 11:14
  • Something is crooked. The bottom of the arm is out too far -- the arm should be parallel with the wheel & cogs. – Daniel R Hicks Jun 3 at 12:10
  • Thanks, as you can see the bottom one (nearest in the image) is more outboard. If I press on it towards the wheel as I turn the pedals the chain doesn't skip or try to move outside. So maybe the hanger (the wheel housing right?) is just bent out of shape? – Matt S Jun 3 at 12:10
  • Yes that's it, I bent it inwards a bit and the skipping disappeared. Thanks. – Matt S Jun 3 at 12:14
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From the the photo of the derailleur it's obvious that the derailleur cage is not parallel to the sprockets and wheel. The bottom run of the chain returning to the crank is leaving the cage at angle.

The derailleur hanger that attaches the derailleur to the frame is bent and causing the misalignment, and I think the cage may be deformed as well.

There are special tools for realigning the derailleur that a bike shop can use, but it's possible to improve it by hand. From the back of the bike look down the chain. The cage should look parallel to the chainrings. You can grab the derailleur and bend it back into alignment.

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    Be careful when doing this that you're not bending the derailleur cage. Your force should be applied so that it's acting at the derailleur mounting bolt and it's hangar. – Jeff Jun 3 at 15:56
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    A technique I've found quite useful, having a few spare parts at hand, is to remove the derailer from the hanger and insert instead the right sized axle (I'm thinking it's a front axle from a skewer hub), with a nut threaded onto it. Once the axle is inserted and the nut tightened against the hanger, the direction the axle points tells you how things are bent. And you can then use the axle as a lever to straighten the hanger, until the lever axle is parallel, from all angles, with the rear axle. – Daniel R Hicks Jun 3 at 17:14
  • Yeah - is less-common to be bent outboard, often things are pushed into the wheel, not pulled out. – Criggie Jun 3 at 20:33
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The first thing to be checked is the derailleur's hangar alignment. A bent or misaligned hangar can produce this effect on the derailleur jockey wheels. There is a special tool to check and correct this, though one can fashion a homemade device to do the trick.

Park Tool's advice on their derailleur hangar alignment tool.

You intimated that you're using the limit screws to attempt to align the jockey wheels. This is only helpful on the two extremes of the cassette. The H limit screw should be set with the chain on the large chainwheel and smallest, outermost sprocket. The position of the upper jockey wheel of the rear derailleur should be aligned with the outside, vertical plane of the small cog. Loosening the inner cable fixing bolt so there is no cable tension involved prior to setting the high side jockey wheel position is necessary for the proper setting. Park Tool's site has a video on setting up a rear derailleur that further explains the process.

When the limit screws are set, adjusting the line up of jockey wheel and cassette cogs is accomplished by adjusting the Barrell adjuster, where the cable enters the derailleur. Another Barrell adjuster is often found at the shifter, again where the cable enters. Typical use of this one is when you're on the move, fine tuning to get smoother, quicker shifts or eliminate chain noise.

Another thing that needs to be determined at first is if there is a problem with the derailleur structure such as a bent cage ( the frame the pulley wheels sit in) or extremely loose linkages of the derailleur body.

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