We got our 10 year old a Novara Tractor 24" MTB from REI. It has a 3x7 Shimano Tourney drivetrain. He really likes it, but it will drop the chain at least once per ride. In bad cases, the chain has wedged itself between the BB and the crank and has been really difficult to remove.

I've tried adjusting the FD limit screws (I'm a competent bike mechanic), but I think it's a combination of things:

1) He'll often go from the big ring (it's a triple) to the small with one fast twist of the grip. The rear D can't take up the chain slack fast enough and so the chain gets dumped right off. I've told him to slow down his shifting in front, but sometimes he forgets. If we're on rougher terrain, sometimes the slow shift is not enough.

2) There's enough chain to safely run the big-big combo which means there is considerable chain sag while on the little ring.

The bike has a low-mount bottom-pull front derailleur. Because of that, I can't fit a chain catcher underneath it.

Looking for some options. Some things that have come to mind:

1) Get a high-mount bottom-pull FD and install chain catcher. I'm not sure what my options are with a 7 speed chain though?
2) Build some sort of chain guard out of 1/8" aluminum plate. Mount to granny ring posts (with appropriate spacers). I don't really want to do this though.

Ideas appreciated.

Thanks,
Bob

I would re-check the chain length as a first step. Big-big combination WITHOUT going through the rear derailleur, then add one pair of links (one inner, one outer). That is the best way to make sure that the chain is just long enough, and may allow the rear derailleur to take up the slack fast enough for his rough handling of the front shifter.

The momentum of the chain coupled with the "wave" caused by the sudden drop and change in angle is too much to expect the front derailleur and chainwheels to handle, no matter how carefully you adjust. A chain catcher with diff derailleur may help, change in habits is the only cure.

I can't imagine terrain such that one is forced to go from large chainwheel immediately to small. If in rough terrain I would advise avoiding the large chainwheel - if he somehow spins out of the middle chainring just coast - not going to add much speed on a downhill anyway. He just needs to anticipate better.