Originally Posted by dutret
1. The size of the cog is what matters, smaller means more force to put the same torque on the wheel. With a 16-18 you are likely to put a bit more force on the wheel when climbing and skidding but probably not starts.
Quite. I run a 42/15 setup, where I might more normally be starting in 42/17 away from lights or somewhere where hard acceleration is neccesary. This means that I'm torqueing the chain more, which is what would slide the axle forward in the dropouts. If anything, I'd guess this is what happened to the OP
Originally Posted by dutret
2. there is no reversal of forces. There is a few degree change in the direction of them but not much.
In terms of pulling the wheel forward, you're right. I was thinking of the twisting effect on an axle which won't be as strong as a solid, but that's not relevant to the post here.