I measured the wheel from the hub mounting surface to the inside of the tire, parallel with the axle. Mine was approx. (6.5"). I then determined the distance from the tire to the frame of the trailer (1 3/4"). Add that together, then add the distance from the frame to the center of the spring perch. Mine was (3.5"). I ordered mine from redneck trailer supply, on their website there is a picture depicting these dimensions that they require to get the axle made. It's very simple, you just need to know where you want your tires.