… when I take a biggish hit / land off a biggish step, always off the chainring, but can be off either chainring (I have a 2×10). I’m fairly sure the gears are tuned ok. Do I need a tensioner to keep it in place or is there something else I’m missing?

Chain a little too long? has it always done it?

or Mech setup..

Mech should be mounted as low as possibe so as to “enclose the chain”, not using a triple mech are you?

Failing that a stinger or clutch equipped rear mech.

As above, is the chain too long?

Do you have a clutch mech? Is it adjusted and working properly?

If all of the above is right then you have too much Gnarr.

I have a Blackspire Dually up for sale of you are interested.

This was one of many reasons I went 1×10 on my bike a few years ago. I kept dropping the chain on rough terrain with my 2×10 set up.

A clutch mech will help if you haven’t got one.

A bit of fine adjustment of the front mech height-wise and with the limit screws will help make it more difficult for the chain to physically drop off the bottom ring or out over the top ring.

Chain devices for 2x set ups do exist. I cannot comment on how effective they are though.

I dropped my chain repeatedly on a ride a month or two ago and found it was because there was so much dried mud in the rear mech the tension spring wasn’t able to keep things tight. That’s with 1×10 and a narrow wide chain ring. With 2×10 you probably need an additional tensioner/guide/clutch mech as the shifting rings aren’t great at holding a chain on.

I’d have said that a chain device was pretty much essential on any 2x set-up in the rough-stuff.

E13 DRS or Blackspire Stinger were the two that worked best for me.

Blackspire Stinger were the two that worked best for me.

Are these relatively easy to set up?

Keep pedalling, it’ll stay on then!

Long flappy chain, no clutch mech and the odd bit of involuntary back pedaling? could all be factors… Hard to be certain really though.

OP.
I have a stinger you can have for postage if you want to try it.

OP.
I have a stinger you can have for postage if you want to try it.

That’d be grand. I’ll send an email to the address on your profile.

Check your chainstay hasn’t snapped off.