[mstaples]

Mountain Bike: What would cause a chain to shift on its own back and forth between on cog and another? Originally, I thought the chain was skipping. But with a little more detailed look, I can see the chain being pulled off of it's rear cog and onto the one next to it. Then a few crank turns later, it will jump back to the original. Any ideas?

 

[Bikinfoolferlife]

Possibly as simple as adjusting the tension on the cable. Perhaps a dirty cable/housing interface causing the cable not to move smoothly. Could even be a misaligned derailleur hanger. Hard to say without some more analysis on your part. You might try to adjust your derailleur, using this article to start http://www.parktool.com/repair/readhowto.asp?id=64

PS It could be the chain itself if there's a kink in it...just realized what the title of your post was...any recent damage or changes to the chain?

 

[mstaples]

Thanks for the input Bikin. Sounds like all the things you are suggesting hover around the idea that something is pulling or not pulling that derailleur correctly. You didn't mention the cassette at all. That's kind of my direction as well. I don't think its the hanger. But maybe a combination of adjusting the derailleur and giving it a really good cleaning. This is a downhill bike that comes home buried in mud all the time. It may be that the derailleur is gummed up and kind of flopping back and forth somehow. I tried replacing the chain, by the way. So that isn't it. I'll pull the derailleur tomarrow, clean it off, then re-align and see if that helps. Thanks again. I'll check out the parktool site.

 

[Bikinfoolferlife]

Thanks for the input Bikin. Sounds like all the things you are suggesting hover around the idea that something is pulling or not pulling that derailleur correctly. You didn't mention the cassette at all. That's kind of my direction as well. I don't think its the hanger. But maybe a combination of adjusting the derailleur and giving it a really good cleaning. This is a downhill bike that comes home buried in mud all the time. It may be that the derailleur is gummed up and kind of flopping back and forth somehow. I tried replacing the chain, by the way. So that isn't it. I'll pull the derailleur tomarrow, clean it off, then re-align and see if that helps. Thanks again. I'll check out the parktool site.

Could be damaged teeth on the cassette now that you mention it, but again you have to take a close look at what's going on to eliminate possibilities. Be sure to check the condition of your housing/cable (and routing), more likely than the derailleur itself, especially in the conditions you mention (especially if you're not using an interrupted rather than full run of housing). You have a derailleur hanger alignment tool?

 

[FBinNY]

Bikingfool covered the basics, but since you say it's a downhill bike, I assume it has rear suspension.

If so, it's possible that the movement of the swingarm is affecting the RD cable length. Possibly a housing is kinked, a ferrule dislodged, or even that the housing section around the flex link is a bit short so a hard bump tensions the cable causing a down shift which then hangs up a bit before returning to the original gear.

After doing the basics, fully compress the rear swing arm and check that the housing is long enough so that it doesn't pull cable. A quick diagnostic gimmick is to slide a cable donut (or a bit of rolled tape) up against the forward chainstay fulcrum and ride a few jumps without shifting gears. If it moves off the fulcrum that's your problem.

 

[zebrahum]

The simplest answer is usually the best. A bent derailleur hanger is the easiest to cause problem of all the ones stated and it only takes a second to check for, I'd say it's worth a look.

Put your bike in a middle gear front and back. Lean the bike up and stand behind it. Do the pulley wheels line up in a straight line with your gears? If they're not exactly parallel, then you might have a bent hanger. A shop will have a gauge to tell you if it's bent and be able to bend it back.

Your cables are a great answer as to what may be going on, so check those too, but if it's a new problem, it probably got caused by something, and bumping the hanger is a frequent occurrence.

 

[dan0]

if its changing back and forth I would say bent cassette cog or stiff link
if its just dropping occasionally I would say cable tension
if it wont stay in gear bent hanger or deraillier adjustment

next time it does it look and see what gear you're in. then slowly turn the wheel and watch that gear and the one above and below for bent cassette cog. I've had it happen more than once