Tom, we get our rope from an industrial supplier of safety and climbing equipment, the rope has to meet specs.
The 1/4 inch 3 ply twisted poly meets specs for FAA weak link requirements for most gliders. The Janus C requires a stronger weak link, so we use 5/8 ind dia for that. We also use 5/8 inch for auto tow. We could use 5/8 for aero tow, but it does not stand up very well to the abuse.

BillT