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I have a 2017 Giant Escape 3 bike where the chain occasionally falls off the smallest sprocket (when it's on gear 7) and goes between the frame and the freewheel. It mostly happens when there is an inadvertent backpedaling for half a crank, or when stopping relatively quickly. There has been a lot of adjustments done at the Trek store I've been going to for maintenance, but nothing solved it.

Some thing that may be of interest is that according to 99spokes and another question on this site, the original freewheel is TZ31, and the shop has replaced it with TZ510. Also, I have replaced the rear wheel with a new wheel with Formula FM-31-QR hub, as the original wheel's spokes broke and the rim is close to being worn out.

It used to be that I could pull the chain out without removing the rear wheel by backing it up, but now the gap between the sprocket and frame seem to have gotten larger after the wheel change, and I have to completely remove the rear wheel to get the chain out.

Looking for a quick solution, I see chain catchers being mentioned a lot for front derailleurs when I searched for the problem, but I couldn't find any information on such a thing for rear derailleurs.

Is there such a product that can prevent the chain from getting stuck between the frame and rear sprocket?

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    Are RD limit screws set correctly?
    – Art Gertner
    Commented Sep 30 at 7:22
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    The limit screws should completely solve this issue when they are set correctly. There is no need for any additional device. On the other side, one could use a dork disc, but it is not needed either.
    – Vladimir F Героям слава
    Commented Sep 30 at 8:48
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    @ArtGertner "On the other side"
    – Vladimir F Героям слава
    Commented Sep 30 at 10:22
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    @VladimirFГероямслава one could use a dork disc, but it is not needed either. Until a 100 kg cyclist has to shift under load up a 17% grade and frame flex makes an otherwise well-adjusted bike drop the chain between the spokes and the cassette, and the chain cuts into the DS spokes so badly the wheel needs to be rebuilt.
    – Andrew Henle
    Commented Oct 1 at 0:08
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    Note though that the chain may fall off even if you're an elite (even legendary) cyclist, you're pedalling only forward, and your bicycle is tuned by a world-class mechanic: cyclingweekly.com/products/…
    – Sam7919
    Commented Oct 2 at 21:16

2 Answers 2

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There are no chain catchers to keep the chain from going between the smallest cog and the frame.

Picture of rear hub and cluster with drop outs

If the rear derailleur is functioning correctly (nothing broken or bent) then correct derailleur adjustment will prevent the chain from going between the smallest cog and the frame.

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    Not if you pedal backwards quickly enough to dump a lot of loose chain on top of the cassette. "Don't pedal backwards" is the lesson here, because you can always spin in reverse hard enough to dump untensioned chain onto the cassette, and without any tension there's no way to control where it goes - and it doesn't matter how well adjusted the RD is.
    – Andrew Henle
    Commented Oct 1 at 0:10
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    @AndrewHenle back pedaling is quite useful in a number of situations though, and usually it should work without any problems. Perhaps we can say "don't pedal backwards whilst in a very small cog", and/or "don't pedal backwards too sudden and quick", but don't throw out the baby with the washing water.
    – leftaroundabout
    Commented Oct 1 at 19:41
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Is there such a product that can prevent the chain from getting stuck between the frame and rear sprocket?

(Sorry for a slightly joking answer.)

Yes, there is.

It's called a big chainring.

If your big chainring is big enough, there is no reason to utilize the smallest sprocket which is typically 11 teeth. However, if your bike has old-style freewheel, there is possibility that the smallest sprocket is bigger than 11 teeth (which is ridiculously small), and a conversion to a freehub/cassette system could be expensive.

And if you never utilize the smallest sprocket, your chain won't fall off it either.

I recommend at least 48, maybe 50 teeth big ring. I also recommend you to use the biggest chainring in most situations. The smaller rings should be reserved for steep hills.

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