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I got a folding bike 2 months ago and got my first hiccup. It has a 3-speed hub gear and while changing gears today, it seems like the chain slipped off the smaller chainring near the back wheel into the gap with the wheel. As the chain is quite tight, my basic knowledge or Youtube info on how put chains back on in dérailleur bikes is useless.

How do you actually slide it back on? Is there a simple technique? I tried simply pedalling but it doesn't seem to make a difference.

If this matters, it's an S-RAM iMotion hub gear.

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    You may not have had enough chain tension. Solution is probably to loosen hub nuts, slide wheel forward to get the chain back on the sprocket, and re-tighten hub nuts with appropriate chain tension.
    – Jason Brown
    Commented Mar 20, 2014 at 17:38
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    You should first put the chain on the rear sprocket, then pedal it on to the front one. This way it's easier.
    – Alexander
    Commented Mar 20, 2014 at 23:10
  • The chain is actually quite tight now, I can barely move it and I don't see immediately how to take it off the front sprocket. Any possibility to do this without tools? Thanks for your input by the way.
    – Kurt
    Commented Mar 21, 2014 at 15:11
  • Get it of the front sprocket by pulling it to the right while turning the sprocket forwarth. It should come of. Now put the chain on the back sprocket. Get the chain on the front sprocket again by pushing it back on a very small part of the sprocket and turning the sprocket so that it will 'pick up' the chain. If your chain came of, it will might have been to lose as Jason Brown said.
    – Foxhill
    Commented Mar 26, 2014 at 16:17
  • Slowing down or stopping pedaling completely to avoid a chain drop works OK IF YOU SEE THE BUMP IN TIME. That hasn't always been the case with me on urban streets where bumps and dips come thick and fast. A suspension would probably help but for some reason, hub gears and suspensions don't go together.
    – Randolph Selig
    Commented Sep 23, 2018 at 20:49

2 Answers 2

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I also have a folding bike with a 3-speed hub (Shimano Nexus 3). My bike has a chain protection cover in the front chainring, so I find it harder to follow those other suggestions. I usually prefer to put the chain over the front sprocket in the first place.

The chain is also as tight as I can, but sometimes it happens that it falls over. I have noticed this happens when pedaling on a more irregular road or over bumps, so I just stop pedaling for a moment or go slower.

This trick is pretty easy and fast:

  • Grab the chain and put it over the rear sprocket (assuming it is already over the top of the front sprocket).
  • Because the chain is tight, probably, it will only be possible to move it over one or two teeth -- no problem.
  • Let the bike go backwards a little bit. The rear sprocket will spin together with the wheel and it will pull the chain over the sprocket.
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  • Might be that your chain is too loose or too tight. Also watch out if you've pedalled and got the chain wedged down beside the spokes.... I've managed to munch off some spoke heads doing this.
    – Criggie ♦
    Commented Nov 9, 2015 at 20:58
  • Thank you for your comment. When I touch the chain it looks properly stretched, anyway, it is really hard to make it even more tight, but it should be possible. I don't have that problem with the spokes, this hub has around 2 cm of clearance between the sprocket and the spokes.
    – iled
    Commented Nov 10, 2015 at 12:08
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There are two parts to this: Emergency recovery, and fixing of the cause.

Emergency recovery

This is already described this in iled's answer. I'll just repeat for clarity:

  • Get off the bike and stand on the side of the chain.
  • Grab the upper part of the chain right in front of sprocket and pull it away from the rear wheel if it's between sprocket and wheel. If the chain has fallen to the other side, you need to push it towards the wheel.
  • Move your bike back to let the sprocket grab the chain. Make sure that the chain makes the transition smoothly enough, otherwise you may damage chain and/or sprocket due to over-tension.
  • Once your sprocket has firmly caught the chain, continue moving backwards so that the chains sits correctly all around the sprocket. That means, you must turn the sprocket more than a full half turn. If you forget this step, the chain will be back off the sprocket immediately.

Fixing the cause

When a single speed chain falls off the sprocket, that's usually down to one of two reasons:

  1. Your sprocket has lost teeth.

  2. Otherwise, a jumped chain is a sure sign of the chain having too much slack.

The first case is obvious to diagnose. Just look for missing teeth, and replace the sprocket if some are missing. If you find none, you need to readjust the rear wheel for correct chain tension:

  • Loosen the nuts that hold your hub's axle in the dropouts of your frame. You don't need to unscrew them, you just need to loosen them enough so that you can easily move your back wheel in the dropouts.
  • Pull the rear wheel back until the chain has the correct tension. That does not mean that the chain should have no slack, but that the slack should be relatively limited. Try to get as little slack as possible while being able to turn the cranks without any significant resistance. If the tension is too high, you get unnecessary wear and loose pedaling power, if the tension is too little, you get jumping chains.
  • Retighten your hub's axle nuts. Take care to fasten your rear wheel in such a way that it is in-line with the frame. Also take care to apply enough torque: When you pedal with your full weight, your chain is pulling with roughly twice your body weight on the rear wheel, and the axle nuts have to withstand that force.
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