Originally Posted by 46x17

No experiences with freewheels. I have a 17t shimano on the flop side of my flip flop but have never used it. Currently it is just keeping the threads clean. You might find the same thing happening to you.

I am quite sure you could fit the largest shimano freewheel (i think 18) toghether with your 20t cog without needing to change the chain lenght when flipping the wheel. Depends on your dropouts though.

Also 20t cog is pretty large. I run an 18t and it is definitely on the spinnie side. What chainwheel size are you using?