Gatorfreak

10sp Di2. Shifts perfectly but if I pedal backward it often shifts to a smaller cog. When I start pedaling forward again it will go back where it was.
Any idea what causes this?

ljsense

Are you running it 2x10 or 1x10? Sometimes 1x10 setups have more of a tendency to do this, for various reasons, but mostly due to chain angle.

I would check the chain to be sure it's not worn and also that it's the right length (number of links, I mean here). Then I'd check the hanger to make sure it's perfectly straight.

See if it does the backward shift in all cogs. If it's just one, it could be a damaged tooth. Or who knows, maybe someone put a nine-speed or 11-speed spacer instead of a 10-speed spacer between 2 cogs; if you suspect someone did a DIY build, you never know what kind of surprises lurk deep within the heart of the bike.

If all those foundations are okay -- the chain is good, you're running 2x10, the cassette is right and the hanger is true -- then I think you might be stuck with the same solution as the guy who went to the doctor and said, "hey, doc, it hurts when I do this." Doc says: "Don't do that."

A few years ago, a Shimano rep stopped by to go over all the e-shifting stuff, and as I understood it, pretty much all the 10-speed stuff is no longer supported. So if you're hoping there will be an answer in the shift algorithm or firmware or something like that, my impression is that it will take some diligent hacking.

Gatorfreak

Thanks for all the tips! It's 2x10. I'll check your other suggestions.
And yeah, I realize it might just be one of those things you live with/work around. Mild annoyance.

alcjphil

Is there a particular gear combination where this happens? It is pretty common when running in one of the most severe cross chained situations on the large chainring for the chain to derail to a smaller cog when you pedal backwards especially if your bike has short chainstays. This is not a phenomenon unique to DI2

ljsense

One other basic thing to check: the derailleur pulleys. If they're worn or wobbly, that might do it.

dsaul

Totally normal. When pedaling forward, the derailleur pulleys align the chain with the selected cog. When pedaling backward, the top run of chain has nothing to keep it aligned with the selected cog, so it will try to align itself with the chainring. Bicycle drivetrains were not designed to be pedaled backward.

alcjphil

Derailleur pulleys do not come into the equation when pedalling backwards. When a chain derails while pedalling backwards, it comes off the cog long before it passes through derailleur pulleys

ljsense

That's a pretty good point, in theory, but shift your rear derailleur when pedaling backwards and see what happens. They can work like a front derailleur, in some cases, and make a back pedaled chain fall to a smaller cog.

It shouldn't be an issue unless the pulley wheels are totally shot.

alcjphil

Shifting in which direction? In any case totally irrelevant since the OP did not mention anything about shifting while pedalling backwards and in fact said that the chain went back to the larger cog as soon as they pedalled forward

ljsense

I just don't think it hurts to do a two-second check of the pulleys. They're part of the system. If they wobble, don't turn freely, etc., there's a chance they could be playing a role.

alcjphil

Fine to check them, but they will not play a role in this scenario. Pedalling backwards, the chain will derail at the top of the cassette, not where the derailleur pulleys guide the chain onto the cassette at the bottom while pedalling forward. Don't forget, as per the OP, the chain goes back onto the correct cog as soon as forward pedalling recommences. A worn pulley might not be able to get the chain back in place quickly as was described

alcjphil

+ 10
Derailleurs shift gears while pedalling forwards. Inadvertent gear shifts pedalling backwards do not involve the rear derailleur, mechanical or otherwise

Trakhak

True, but there are people who insist that derailleur-equipped bikes should never have a problem with being pedaled backwards.

To any of those people who happen to read this thread: yes, it's possible that there any given bike might be capable of being pedaled backwards in most gears without a problem, but if so, it's a fluke. No bike in the history of bike manufacturing, and no derailleur system, electronic or mechanical, was ever designed with pedaling backwards in mind (with the possible exception of some odd Italian or French system in the early days of derailleur design that relied on backpedaling to shift between the two or three sprockets of an early freewheel).

JohnDThompson 

Chain angle is a major factor in this. Pedaling backward when in one of the extreme gears (large-large or small-small) will exacerbate it, as do clusters with many cogs which require wider sweep across the cluster. Short chain stays can also figure in, and shift ramps can facilitate cog changes in either direction.

Bottom line is, if your bike has problems when pedaling backward, don't pedal backward.