There was an accessory sold a long time ago that consisted of a thing that looked like a tuning fork which mounted to the frame cross member that hung down behnd the axle with the two fork prongs facing down. To the axle was mounted a pin that rode in the fork.....it did the same thing as you describe.

As to the roller in the C channel, there must be some minor clearance since, if the roller were tight in the channel, one side of the roller would be rolling with the motion and the other rotating against the motion.

mike in tucson