SSCX Only!

I'm doing one myself, with 42 front, a 16/18 freewheel on one side of a flip flop and a 20 or more on the other side of the flip flop. Got myself a couple of gears for roads and tracks. And flip it over to the 20+ for off road. Running 700x30.

Comments welcome.

If you're not using a tensioner. You'll have to have the wheel slammed forward for the 20t for this to work. My guess is you need 2/3 inch of rear wheel movement to get this to work well. So it is doable.

42:20 sounds like a nice offroad CX gear.