Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I would suggest that a perfectly cut "U" groove, sized to match the belting up to, but not beyond its sectional equator, would give the absolute best traction as it would provide the greatest realized surface area of contact, without the added friction of partial capture.

I'd agree and that's not shown in that diagram. A perfectly sized cut U groove to the belt diameter would have a good surface area in contact.