I expect the original poster has finished his project by now, but I was seeking similar information when I found this forum page. Here is a link to the best information I've run across yet-
http://www.metalgeek.com/archives/2005/05/01/000047.php
A chunk of 1 3/8" 17 ga. tube should be close to the 1 1/4" 14 ga. tube this fellow has bent. For a 30' hoop (15' radius), he is suggesting an 8'6" radius jig, very close to the 8.3' radius suggested above by John Steel.
My need was also to determine the radius of a jig to bend fence top rail for a hoop house. I intend to use a series of 2x4 chunks placed in the appropriate radius screwed onto a hayrack deck to make my jig.......... Yeah, I know there are commercial jigs for sale but what fun is that??
