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Remember HOW a rear derailleur works. The upper chain guide shifts left and right forcing the chain left or right and thus over whatever gear you want to be in. This guide is controlled by your shifting level on your handlebar. To work the cable must be tight. The cable has a plastic reinforced coating that permit the metal cable to be bent from the Shit levels on the handlebars to the Derailleur. The cable extends from both end of the plastic Cable. The end of the Plastic Cable fit inside stays from the Front Shifter and from the derailleur. It is rare for the Plastic cover to be bad (it can be, but rare) most of the problem is from the stays to the Derailleur with the shift levels being a distant second.
Given the above (and most shift levels have their stays inside the shift levels so no adjustment is possible around the shift able) there can be three things wrong:
1. The cable is OFF its stays, i.e. the cable is lose. The stays holds the plastic cable in place and guides the metal cable to the Rear Derailleur. Thus if the plastic is off the stay, put it back on and the cable will be tight.
2. The cable has stretched so that you need to loses up the clamp holding the cable to the Rear Derailleur pull the cable tighter and then clamp down the clamp, making the cable tight.
3. The cable is damaged and needs to be replaced. If this is the case, it generally is caused when a bike hits something (or something hits the bike0 Smashing in the cable housing so that the cable can no longer move within the Plastic Housing.
If the problem is the cable housing I would have someone else replace the cable. Cost about $20-25 dollars. The main reason is that to work on the cables you need to work on the bike on a bike stand which runs $200-300 for a good bike stand. You can do it without a bike stand, but then you are working upside down (i.e. the bike is upside down) but it is easier to work with the bike held by a bike stand as you adjust the new cables to fit the bike.
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