Chain still keeps coming off. Need to mount a chain tensioner somewhere.

[Synthman]

My bike has always had this problem from new. Local bike shops have tried pulling the rear wheel back to take up the slack, and this has only worked for a short while. Last summer my freewheel failed and when the shop fitted a new one, the problem went away completely for the rest of the year. The chain never came off once the entire time.

Now it's starting to come off on nearly every journey. Most of the time it only comes off when I'm reversing the bike, or when pedalling hard on bumpy roads, but it is guaranteed to come off when I'm backing the bike up. It is coming off at the crank end.

The other day I saw an A2B Metro going past and noticed a tensioner mounted on the frame. This has given me an idea to fit one to my bike, but the one place it can't go is by the freewheel as there's no space. There are several parts of the frame I could attach it to though. I'd want something that doesn't require drilling and ideally would clamp to the frame, but I haven't seen anything like this for sale.

 

[flecc]

Here's one that adds easily to the rear spindle:

Shimano chain tensioner

If you have a hub gear or single gear this one is ok. If you enter chain tensioner into Google you'll see plenty with various fittings.

However, it sounds like your chainwheel and rear sprocket are out of line, your chainwheel is buckled or you have some chain links not freely rotating on the adjacent links.