Back pedaling - chain comes moves to smaller sprocket
 

Hi - Just took my bike to the shop to have the bottom bracket bearings and the rear cassette replaced. Rear cassette is exactly the same as original. I have a cross country mtn bike with 2x10 gearing. Now when I back pedal, as little as a half a rotation, when chain is on the front largest ring and the rear is on the 1st or 2nd largest sprocket, the chain drops off the rear sprocket and goes the next one or two sprockets smaller in the rear.

Is that normal behavior or should the chain remain on the same sprockets during back pedaling in all gears? I don't think the bike was acting this this way before I took it in, but not 100% sure.
Thanks much for the feedback.

Ian

Was the chain also replaced?
You should check your indexing.

Well that makes sense. Back pedaling while cross chaining big-big is sure fire way to jump a chain to a smaller cog.

John

Chain rings, spider, and chain were replaced. Shifts well.

I didn't think the bike chain jumped before getting the work done. Thought somehow the front chain rings got moved to the right during BB bearing replacement, thus enhancing the crossing of the chain. Can't verify this though.

So it's normal for the chain to move to a smaller sprocket during back pedaling in big to big position? My road bike doesn't do that. Just try to determine "normal" behavior for a mtn bike before returning to the shop.

Thanks again -- Ian

Yep, don't do that, either direction.

I had the similar problem before. It was fixed after did re-indexing

Indexing has nothing to do with pedaling backwards. The problem is "cross chaining". There is nothing to guide the chain when pedalling backwards. The chain is stressed sideways, and is pulled towards a center cog. Combine that with gravity pulling the chain to a smaller cog, and jumping sprokets is the result.

FIX...Don't pedal backwards when cross-chained.

None of my road bikes do this but my mountain bike does. They all shift excellent.

I'm not going to change the chainline on it, it works too well to mess up.

The only way it's a problem is starting on a rough hill. Sometimes whatever made me stop caused me to backpedal a bit.

It is a pick your poison. If you ride big-big and you hardly, or never, ride small-small, you might be able to bring the crank closer by removing any spacers. As long as you have chainstay clearance, you can see how it works.

Mountain bike chainstays are usually longer and shading the crankset inward will help with the cross chaining. There can be quick terrain changes riding trails so set it up for how you ride.

John

The root cause is cross-chaining. You should work on shifting to a smaller front ring earlier on hills instead of staying in the big ring and going to larger cogs, to avoid getting into the situation.

Why did the chainrings got moved to the right? they should be at the same place if everything was done correctly.

I said mountain bike.

Anymore, unless we're talking about the vintage MTB groups, we should assume that mountain bikes are 1x unless explicitly stated otherwise.

And yes, 1x itself is likely the culprit here. The rider can't adjust the front chain position relative to the rear, get it close and functional, and just ride.

The OP does have 2x, and something the shop did is causing the problem.

Possibly a thicker body on the chainrings effectively moving them right. Possibly didn't get that right cup seated all the way in. Possibly didn't tighten the crank all the way.

A shorter chain can do this easier too, so when they replaced it, they may have properly measured it rather than just laying the new one next to the old one.

There was a recent thread on this. A member had the same problem than after a year posted a fix. Can't remember what it was, can't seem to find the post. Might have been a bent derailleur hanger

We shouldn't assume that anyone's assuming anything:

That was in response to someone who responded to me, to school me on how I ought to use the chainrings on my mountain bike.

The OP was clear about what he has and I gave suggestions based on that.

I still say, in 2020, if someone mentions a mountain bike drivetrain, it's 1x unless otherwise stated.

^^^^^ Likewise, in 2020, if someone mentions a "bicycle", it's electric unless otherwise stated, I suppose.

Too funny.

I just converted a 3x MTB to electric.

Now it is 1x.

Everybody wins.

1x are really dependent on the terrain and how well someone can climb. If you can push a 30t to a 40t/42t cog then a 1x is the way to go.

On more level terrain it is not an issue and you can use whatever.

John

Exactly! My fear that the chain rings did get moved to the right for some reason. A mistake I presume. That's the only explanation I have for why the chain is more crossed than it was before I had the work done.

I can back pedal on my three other bikes when on the two big rings. That's enough of an indicator to me that I need to go back to the bike shop and have them fix this issue. One would think that I would be able to use all the gearing on my most expensive bike.

Thanks all for your input!

You might be able to have them center the big ring on the cassette, if they have the room on the inside. This will kind of give you a 1x with a granny just for those times you need it... if that is how you ride. I’m assuming it is since you were in the big-big.

I will admit that if you lined up the big ring with the center of the cassette and took off the inner ring, there would be those that would say it’s 1x so you’re not cross chaining. But put that small ring back on and the big-big is now cross chaining.

John

I had this issue on a MTB and found that the B-screw was set too far in (jockey wheel too far from the cassette in the largest rear cog). Backing out the B-screw to the right spot, which took some trial and error, solved it for me. Not saying it's THE cause-solution to back-pedaling gear jumps, but it worked in this case.

Simply put if you cant backpedal with out the chain jumping sprockets, something is wrong, if you are not in an extreme cross chain situation.

If you want to know whether your chainring moved outward, why not simply measure and find out?

Never heard of this. Will file this in the back, well maybe way back, of my brain just in case.

John

Keep the chain in a straighter line.

Old worn chains can flex sideways easier then a new chain.

There's nothing to guide the chain when back pedalling


nothing to guide the chain + a chain that want's to stay straight = a chain that jumps sprockets to a straighter line when backpedaling



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