Wow, down to 39T? Now we are talking!

I just ran the numbers and this would give me roughly the same development as the middle-ring/big-sprocket combination on my big bike, which is as low as I go normally, without going into the granny ring.

Thanks for the idea; somehow I hadn't thought of looking for smaller chainrings than what Brompton offers.

The video shows that the chain wasn't shortened, but the tensioner handled it just fine... how does it feel that way?