Dear all,

Could you please help me identify the problems with my 8 speed shimano deore bike shifting system?
The chain drops down to the smaller cog suddenly , it is very often, another problem is when I pedale back, the rear derailleur automatically downshifts the chain to bigger cogs, and if I pedale normal forward, the chain will be forced back to the the position set by shifter on handlebar again, but as I like back pedaling, it is really a big problem, because in this situation, the chain will be popped off, or get stuck in the pulley.

I checked the alighment of pulley, looks normal, I checked the housing of shift cable,very smooth, so I guess something is wrong with derailleur.

Just to be clear, are you referring to the chain moving to a smaller sprocket when you backpedal, or does it also do so during normal riding?

There's a big difference because the first is relatively normal, and the second is probably the result of a poorly trimmed derailleur.

If you look at your bike, you'll note that in one direction the guidance provided by either derailleur is BEFORE the chain engages, but AFTER when running backward.

So, when you back pedal the chain has nothing to prevent moving to the smaller sprocket if anything gets it started. It doesn't take much to start the shift because the sprockets are intentionally cut to make shifting easy and the system is dependent of the RD to feed the chain to the right sprocket (which it apparently does the moment you pedal forward again).

Odds are the system is less likely to shift when you backpedal when the chain angle is favorable, ie. outer chainring with outer sprockets, or inner with inner. So, my best advice is to simply stop backpedaling in those combinations where it's a problem.

actually when I back/pedaling, the chain was brought upward to the larger sprocket, having read your answer, I got a question, whe the chain is on proper combination of gears , I mean that the chain is in parellel with the frame plane, there are still the both problems I described above, I checked through the derailleur, it does not look worn, the pivot being smooth and rebounds forcefully at knock, so what is the most possible reason for that the chain is shifted to larger when I back pedal please?

I think the only thing wrong is that you like backpedalling. Nothing's wrong with the derailleur. They weren't meant to be used for backpedalling.

The derailleur isn't implicated in anything that happens when backpedaling foe the simple reason that it isn't there.

The chain is coming unguided to the rear sprocket, so anything can cause it to hop in either direction. If you lean the bike up and backpedal in a combination where this happens, odd are you'll note that the chain brushes the next sprocket very slightly, and it wouldn't take much for one of the teeth to snag and lift it, especially because they're designed do precisely that.

This is lie the old joke where someone tells the doctor, "my arm hurts whenever I lift it like this..," and your answer is the same as the punch line.

BTW - you didn't say, but your problem is more common with newer chains and sprockets. With some wear the edges dull and they are less prone to snagging and shifting.

quit cross chaining so much. if you want to backpedal without the chain wandering, it had better be parallel with your frame. quit leaving it on the small/small combo

also, don't backpedal, derailers don't work backwards

It's always been a mystery to me why some people get that "derailers don't work backwards" and others don't.

To the original poster: almost all single-speed bikes can be pedaled backwards. Some bikes with multiple sprockets can be pedaled backwards in a few gear combinations, but the bikes weren't designed with that in mind. What would be the point?

Bent hanger can cause issues as described. From the paint missing from the hex head area of where it mounts the derailleur to the hanger, I'd investigate why such paint is missing..

how dirty is the chain? if packed with dirt/grease/mung, the chain may want to false shift when backpedaling.

If you like backpedaling I suggest you get a one-speed bike with a freewheel. In spite of what you believe, the chain will only go to a larger cog in the back if it's at angled inward toward the front. If it's really parallel to the bike it will not shift out of the rear cog when backpedaling. The only possible other reason for a problem when backpedaling is a sticky cassette/freewheel, but that does not necessarily mean a shift to a larger cog.

Also, the chain cannot go to a smaller go unless the shifter has been moved to the position for that cog. If it suddenly jumps to a smaller cog it's because you shifted earlier but the cable is sticky, so the shift was delayed. That's a cable/housing maintenance issue, has nothing to do with the derailleur or rear cogs.