A Mobile home axle is a good hub set in concrete. I have also used a rear end from a truck before. If they get real wide you need upper support on the arms such as our old 36 footer. I also made a 22' jenny before out of a solid piece of I beam with no upper support. Believe it or not, I had the thing out in back of my work, and I kept 1 or 2 dogs that I was conditioning at work, and got paid for shaping bulldogs. Those were the days!