Those Frey Sprints will break your heart. They're such delightfully lovely carts, but oh, the expense. Driving sure can be an expensive aspect of the horse world, you can obviously get saddles that range from $10-$1000+, but with driving, you have to get a cart and a harness and oh the money needed! I'm dearly madly in love with the Frey Stingrays. I love how they look, but if I ever had enough money, I'd hardly be able to use it.
I guess the easiest thing for me is to just upgrade what I have and be happy with that for now. I saw a nifty thing about an add-on suspension system for the easy entry carts that I'll look into more, and someday, with some curved shafts, I might just get closer to having something that kind of sort of looks like a Frey. Like having a Sorny instead of a Sony.
The hubs on my monster motorcycle wheels are 5". I think. I'd have to measure to be sure.