Chrys,
As set up now the drive wheels have both the rubber covering you can
see in the photo plus the 2 pins Neil mentions.
As it happened the rubber fell off one drive sprocket after the last
battle. I glued it back again. Short story.
The 2 pins and rubber seem to work perfectly satisfactorily- there is
probably just about as much movement as in that photo (it's raining
outside and I can't be bothered to go to the shed to check) and it
seems pretty successful.
Are you having problems?
Phil