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So I have a Trek Fuel SLR 90 and whenever I ride on flat dirt roads, the back gears randomly keep switching on me. It gets really annoying to ride Please help

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  • Welcome to bicycles.SX! To be able to help you, we might need some more information. Things like: how old is the bike? How many kilometers have you ridden it already? Was this random switching suddenly there, did it develop slowly over time, or was it always there since you have the bike? Does it happen at special occasions (e.g. when riding over some bump)? You may also want to have a look at our help pages and take the tour to learn more about how good Q&A should look like. – Benedikt Bauer Oct 14 '14 at 7:49
  • Also, how is it "randomly switching"? Is it always going up or down or what? The first thing to check in all shifting problems in the back is if the rear derailleur is bent, though (especially if you've crashed recently). – Batman Oct 14 '14 at 15:19
  • I bought the bike from some friends recently and it is at least 10 years old so I don't know how much it has been ridden. Also I recently got the bike checked out at a shape and everything is still in great condition. The random switching has always been there only on super flat dirt paths. Otherwise it never happens. The rear gear shifts up and down on its own without using the shifting triggers. If I'm on any other type of terrain, the shifting works exactly how it should – David E Oct 14 '14 at 15:51
  • I'll only add that a 10 year old rear mech is probably at (or past) end of life. – Grimm The Opiner Oct 15 '14 at 15:06
  • @DavidE, thanks for the comment with the additional info, but please edit the question to include this info. People will see it sooner and it is more durable than the comments. – BPugh Oct 15 '14 at 19:11
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If its just up and down one gear then you probably need to calibrate the derailleur cage a little using the barrel adjusters. Depending on the make of shifter and derailleur there will be a barrel adjuster on one or both ends of the cable outer which will allow you to make micro-adjustments to the cage position.

The jockey wheel centre should be roughly aligned with the center of the cassette cog. What you might find is that the jockey wheel is slightly left or right of the cog at a given shift position (the one that is jumping) so use the barrel adjuster to eyeball the wheel to line up with the cog. It shouldn't take more than a few turns to do this.

Also if the barrel adjuster is fully extended or screwed in, you may have to release the cable at the derailleur end and move it in/out to give you some adjustment room before reattaching the cable.

On the other hand if you are getting more than one shift up or down something is seriously wrong and you may have to replace parts.

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Could be a lot of things. I had an Ultegra in good shape that was jumping around. It was two problems. 1) The cable was a bit frawed at the shifter. 2) I did not have the cassette tight enough - torque to specification.

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