Hi CE,

1.) On my Jet XP the chain guard is only on the outside of the chainring.

2.) It is because of the shorter chainstay that chain jump is common on bikes with 20 inch wheels. The chainline angle is more obtuse in the highest and lowest gears, and I find that chain jump is the most likely when I skip gears up to the highest gear, when the chain is moving to the smallest sprocket on the cassette and the chain tensioning spring on the derailleur is under the least tension. It definitely helps if I don't skip gears above 6th, and shift up slowly and deliberately, one gear at a time.

Aside from the roller, here are a couple of other ideas:

Get another chainguard from Dahon and sandwich the chain on the chainring.
Convert to hub gears, which has other benefits aside from solving the chain jump issue.