I agree with Bunk on the speed and keep grease in them. I use them that way here too and they work o.k.
Higher speed and long distance would need grease more often.
20 mph would be a reasonable speed for them. I pulled one of mine with a 3-wheeler and stayed down to around 20 with no problems.

"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands."