2 wheeler

Just like the title says, why does my chain shift when I roll the bike backwards?

The bike is a mid 80s Stumpjumper, so it's got a triple in front and IIRC 8 speed in the back. I can't see any misalignment in the derailleur hanger, but suspect that might be an issue.

Any ideas?

urbanescapee

If you look at the top of the cassette where the chain rolls off and goes back to the chainrings on the crank, if the chain-line is not straight (i.e. you have the front in lowest gear and the rear in the highest gear), you'll see that the chain comes off the cassette at an angle...not parallel with the gears of the cassette. When pedaling forwards, the chain is guided onto the cassette from the bottom side. The rear derailleur ensures that the chain is lined up for engagement with the cassette, that's its job. But when the system is run backwards, the chain advances onto the cassette from the top side where there is no guidance to keep it in one particular gear...so it naturally tries to shift to the gear which offers the straightest chain-line to the front chainring. To illustrate this better, try rolling the bike backwards in the hi/lo combination mentioned above (be careful, you could potentially damage your drive train) and then use the middle/middle combination. with a well adjusted drive train, the middle/middle gear combination should not cause the chain to shift when rolled backwards. This doesn't mean that there's anything wrong with your drive train...it's just not designed to operate in reverse. That's why there's no "R" on the shifter indicator, lol. Hope this helps your understanding.

2 wheeler

I'm not often guilty of overlooking the obvious, but in this case, I am.

I've been riding derailleur equipped bikes since 1969 and I'm astounded that I haven't figured this out. An old dog can learn new things (and tricks).

Thanks for the great reply!

supremekizzle

Happened to me the other day. I was cross chained pretty bad and I went to peddle in reverse around a corner to get my pedal in position. Low and behold, my jockey wheel retracted all the way and I had no movement in the peddles. Didn't know what it was at the time, but I'm going to guess it was this. Thanks for the great explanation.

urbanescapee

Yes. So the derailleur wants to keep the chain on the gear selected by the shifter on your handlbars. But when you pedal backwards, the chain will follow the tendency I mentioned above and the derailleur won't move with it. This can cause a pretty bad jam, as you mentioned, supremekizzle. And that is the point at which the drive train could become damaged if enough force is applied. You're welcome.

rydabent

Actually rolling backwards or turning the crank backward should not make the chain switch gears. There are several things that might be wrong. Bent RD, bent RD hanger, bent chain stays, and a bad chain line. I have always used the check of spinning the cranks backward as a check that the RD is properly adjusted, and that there are not the other problems I have listed.

cny-bikeman

As Urban very thoroughly explained there are some combos where one can do what you describe and some where it will not work. It depends mostly on how cross-chained one is (outside rear to inside front primarily). Yes, if the derailleur is not lined up that will also cause problems, but it happens at the bottom of the freewheel, not the top in that case. The problem due to cross-chain tends to happen more when rolling backward vs. spinning the cranks because the chain is under tension in the former situation, so it more easily catches on the next larger cog. All it takes is simple observation to see the how/why of it.

cxwrench

Can you explain how a bent RD or hanger will cause a backwards moving chain to shift gears? Since the chain goes directly from the chainring to the cog, I'm thinking you're gonna have a helluva time telling us how this happens.

Homebrew01

Read post #2

fietsbob

..or just the RD pulley is not dead on in line with the cog above it .



Bene Sugg: lock the rear brake , pull your bike to stand on the rear wheel, turn the bike 180,

and set it back down, facing the other way..

bobotech

That is one of the mysteries of tuning that is always a pain to figure out. I did recently notice that my Giant TCX with a triple kept shifting when I tried to pedal backwards.

My shop manager at the co-op checked my derailleur hanger alignment and sure enough, it was way off. He straightened it and I can back pedal all day long now without the chain shifting.

itsmoot

If you're basing your opinion on recumbents, most have a great enough distance between crank and cogset that chain angle doesn't get acute enough to cause the problem.