Simple question really- single ring on front of a road bike (with 8 or 9 gears on back)- will it work? Without a guide/mech to hold chain on, will the chain bounce off?

Ta, Duane.

Same as with mtbs – it can work fine but may also come off at inopportune moments (like in the middle of the TdF 🙂 ). I wouldn't do it myself.

never used to lose the chain when I was a kid on my 5 speed and 5 sprockets were widely spaced in the 70s

Chains were less flexible though and you were less gnar.

OK cheers, are there simple devices to keep the chain on? Rather than a bodged mech (not a MTB chainguide)?

MRP x.1 would help as long as you never back pedal,

Why though, whats wrong with a double setup?

I didn't get chain dropping on my 1×9 commuter and neither does Mrs tj. MTB frames with 1×9 set ups – the key thing is to use an unramped chainring

Simplicity. I would go singlespeed but Edinburgh has hills.

Mine has an old front mech bolted on, tight and low, it only ever dropped the chain off kerbs or very occasionally on big potholes etc, obviously I could ride round them instead of hopping them or slow down for kerbs but… Naaaaah. The mech does the job very nicely, though obviously it's ugly as all hell, but then it'd been in the garage since the early 90s so was free.