Here’s an idea that’s so obvious it’s amazing no one’s done it before…
It looks like a single ring chain guide but it’s actually a stiff polycarbonate guide securely bolted to either side of an XT front mech (included in the price). It’s designed for use on bikes that come with two chainrings and a bashring and is precisely shaped to extend the front mech cage and stop the chain from flapping around on rough ground.
It shifts the chain between rings far more securely than a front mech alone and it seems to be impossible to get the chain to fall off. It comes with a bottom bracket mounted 6061 aluminium plate with a roller that keeps the lower chain run to the rear mech in tension.
In rough conditions, where you’d normally be hesitant to shift gear, the glorified front mech cage works better than a standard mech. It does exactly what it’s supposed to do and does it well.
Check your tyre and swingarm clearances before you buy – a Dualrailleur needs a bit more room than a standard mech. It comes in top and bottom swing and e-type versions, and there’ll be a high impact plastic bash ring soon for another £15.