I just finished converting and old Giant MTB to a singlespeed using a cassette cog, PVC spacers, and the deralleur with a piece of brake cable holding it in place as a tensioner till I buy a real tensioner. I love riding it, and I've found a gearing (1.9:1) that seems to work pretty well, but a few of the trails I ride are a lot more technical and steep than the rest, and the 1.9:1 is a little steep for me (at least for now). I was messing with the barrel adjuster on the deralleur and realized there is enough play in it for another (easier) cog on the back. Then I could have two gears- one for fast trails and one for technical/steep ones. I know this wouldn't be true single-speed, but I would only switch it before and after rides (I promise, no cheating in the middle of a ride ), so it might as well be a flip-flop hub. Anyone else have experience with a set-up like this?