you are exactly right and the rollers on our model are slightly tapered towards the inside end to allow then to track well along the wall. we took this engineering problem into account. and i know you will want to know how primative people could have done this. there are two answers. 1. if they could erect the trilithons surely they could taper a wooden roller. 2. if left untapered and allowed to scrub as you describe, the logs would quickly take on the proper taper from the scrubbing just as lorry tyres would wear on one side if they were constantly turning right but never left.

hope you enjoyed the new videos. clyde