Originally Posted by jad3675

Yes, the single rear cup can be a point of failure, but I think they take that into account. It would be easy enough to design a mount with two cups for a single bike, but they don't.
Even if it fails what's the worst that happens? There's still friction between the cup and the roof so it shouldn't slide around too much. If you hit a bump, maybe the rear wheel comes up off the roof?

John