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The chain on my girlfriend's bike came off the front gear today, it looks like the back gear changing mechanism isn't handling back-pedaling properly, whenever she pedals backwards the chain slacks rather than feeding through.

The chain doesn't seem to be taut enough and has at one point gone round the front gear and caught in the front gear changing mechanism.

Any idea what might be wrong? I'm sorry if I've got my terminology completely wrong here.

It's a 21 speed mountain bike if that helps.

EDIT I've looked up the terminology now and it seems like there might be a problem with the rear derailer? It doesn't seem to be keeping the chain taut at the top and is just getting pulled almost to its limit towards the front of the bike.

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Chain tension is maintained by the rear derailler; make sure it's keeping the chain taut. Especially in mountain bikes, dirt and muck can cause this sort of problem (google 'chainsuck') –  Ian Howson Jul 11 '11 at 1:49
    
(Of course, generally there's no reason to back-pedal more than a revolution or so on a derailleur bike, but the chain and derailer should handle it, within reason.) –  Daniel R Hicks Jul 11 '11 at 11:49
    
One possibility I omitted in my post below is some frozen links in the chain. Shouldn't be a problem on a bike that is used with any frequency, but can occur if a bike sits in the weather for a few months. –  Daniel R Hicks Jul 11 '11 at 23:15

3 Answers 3

up vote 3 down vote accepted

Yeah, first clean the rear derailer well. Simplest way to do this is to lean the bike against a wall (outside, where you can make a mess), slide some newspapers up behind the derailer and drape them down the the wheel (to keep from dripping all over the tire), then spray the derailer well with WD-40, using an old toothbrush to clean it up a bit.

Afterwards, wipe the derailer dry, then oil it up well with a heavy oil (preferably bike chain oil).

If the bike has more than maybe 2000 miles on it then likely the chain is worn out, and possibly the rear sprockets have developed a "hook". A bike shop can check for this with a special gauge, and replace the chain if it's too badly worn. (If "hook" has developed a new chain will help a bit, but the rear sprockets may need to be replaced.)

Regardless, likely the rear derailer is out of adjustment. It's not very complicated to adjust, but it takes a touch of mechanical ability to understand which way to turn things. But mainly it's a matter of adjusting the cable tightness, which is usually done by turning a knob on the cable near the shift lever or elsewhere along the cable route.

The other odd possibilities for this problem are that spring has broken in the derailer (or sometimes the spring has just "jumped" off the tang where it's supposed to be hooked), or some jackleg replaced the chain and didn't get the length right.

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+1 Excellent answer. Complete, and to the point. 1 point: Only oil the derailleur pivots and pulley wheels. And a light oil, like TriFlow, would be ideal. (Heavy oil generally indicates 5w or higher, which is not appropriate anywhere on a bike.) I realize you said bike chain oil, just tring to clarify a point of possible confusion. –  zenbike Jul 11 '11 at 7:09
    
Wow thank you, really concise and complete answer. –  Omar Kooheji Jul 11 '11 at 20:11

If you have crashed or if your bike has fallen over, you may have bent the derailleur hanger, the part that attaches the derailleur to your frame. If this is the case, it cannot be fixed simply by adjustment of the derailleur. The remedy for a bent derailleur hanger is either to bend it back in place with a derailleur hanger alignment gauge or buy a new derailleur hanger.

Make sure the limits are set correctly for your front and rear derailleurs in addition to making sure the cable length is proper. This is done by tightening or loosening the screws on your derailleur that are marked L and H (for low and high, respectively). Sheldon Brown has a good article about adjusting derailleurs that explains this more in depth.

If the chain itself is the source of your problems, it may need to be cleaned and/or lubricated.

If the chain is gummed up, I recommend using a citrus degreaser that you can buy at your local hardware store instead of WD-40 as a solvent because WD-40 leaves a film when it dries and citrus degreaser does not.

Then lubricate your chain:

Depending on how much you ride, you should lubricate your chain every few rides to every few weeks so your chain doesn't seize or wear itself down. If you are riding in dusty, dry areas, consider using a dry lubricant such as teflon that will not pick up dust like an oil lubricant will. Alternatively, if you are riding in normal conditions, chain oil is your best bet.

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I just had this problem on my town bike. Its my old swiss bike and its been suffering alot of small problems recently due to cycling in -10 degree temperatures. the most recent of which was the chain constantly falling off every time i stopped pedalling or back pedalled. So to successfully stop i was having to pull the breaks while still gentily pedalling until i stopped, or just put the chain back on every time i free wheeled or stopped. the main cause of this it turns out is cycling here in the snow in zurich city. the snow itself is not great for the bike but apparently the generous amounts of salt they put on the roads was the real cause of the trouble. I managed to fix the problem by just turning the bike upside down getting an old tooth brush and light bike oil and just brushing the gears and oiling the mechanisms until it started running smoothly , making sure i was pushing the pedals in the reverse direction alot and starting and stopping quickly. it seems to be working fine now.

Thanks for the tips everyone.

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