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My wife has a bicycle like this. It is only a few months old and it gets light use on weekends on (smooth) some dirt roads.

Recently it sometimes fails to downshift at the rear (i.e. shift to the smaller gears). When she stops I can see that the cable is loose. The spring in the derailleur doesn't seem to pull it tight. Moving the shifter or gently moving the derailleur fixes it temporarily, but then it may happen again an hour later.

What could have triggered the problem?

What should I try to fix it? Simply lubricate parts?

  • Sounds like initial cable stretch, which is normal. If you know how, let the cable loose, pull it tight and re-tighten the nut that holds the cable. Sounds like it has too much play in it now. If you got it from a bike shop they should adjust it for you free of charge. If that's what it is it is easy to fix yourself. – Nate W Apr 21 '16 at 18:15
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Once possibility is a rusty cable/housing. When the cable or cable housing gets rusty it can be the case that the spring of the derailleur can't provide enough force to overcome the friction of the cable moving through the housing. You can test this by disconnecting the cable from the derailleur and seeing if it is difficult it is to pull the cable manually (after you've switched the gear down). If this is the problem, the solution for this is to replace the gear cable.

  • If the bike is only a few months old the seller should fix this problem. – Carel Apr 21 '16 at 17:54
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It is not uncommon for new bike or older bikes with new cable to require a retune after a few rides. The cause is cable stretch. With use the cable stretches slightly. If you purchased your bike from a local shop it likely included a free adjustment and fastener check after 30 days of use. You can also search for tutorials on line dealing with derailleur adjustments. Park Tools' and Sheldon Browns' sites are the favorite of many cyclists.

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