I don't fold it that much Huw, but the inner ring is the one that has the exact same chainline as the original bike did. I always make a habit after folding, of lifting the rear wheel and spinning the pedals a couple of times to check that all is well. Once or twice I have had the chain come off the rear cog after folding. It would almost always be on the outer ring for me, since I rarely use the inner one except for climbing monster hills. On typical five mile rides, I can usually push the 45 inch bottom gear of the standard set up on almost all the hills I meet.