jumping chain on my boy's BMX - Help!
any advice would be appreciated
I am fairly able when it comes to bike maintenance, but struggling with my boy's bike
It is a 20" mini-mountain bile with 7 speed rear cluster (S-ram) and single 36T chain ring. It has only done about 200km, and now the chain jumps off all the time in the highest 2 gears (ie: 2 smallest rear sprockets). And I mean, All The Time. eg: 5 metres down the road
the bike has done a lot of off-road miles, so I have cleaned the chain, but it still does it. I took a link out as well, but that aint helping (on reflection, the chain jumps off even under tension when pedalling, so no point in removing links)
It has a cheap plastic outer ring, not really a proper bump ring, but this doesn't serve to guide the chain, as the gap is wide enough for the chain to fall between chainring and the plastic outer ring.
The chainline on a 20" is probably too angled in smallest sprocket, as with the smaller wheel, the crank and axle are much closer to each other, magnifying the chain angle.
But I am puzzled as to why this has started - it wasn't a problem for the first 2 months of ownership.
I've measured the chain - it isn't worn/"stretched".
I hoped cleaning it would allow it to mesh more cleanly with the chainring, but hasn't helped so far
My options seem to be
1. take off plastic guide and crudely sand it back to make it fit more closely (not my idea of a good solution)
2. buy a chain guide, but these cost! The most basic one I can see on the net which should work is
DMR Chain Cage Chain Guide, at 24 UK pounds, still quite an expensive fix
3. Have it out with the chain store (no pun intended) that I bought it from . However this seems likely to be an uphill battle. (from Halfords, UK)
Any bright ideas?