Hello Andrew,
I hope this helps, I bent my tubes in a similar manor as you. I tack welded a strip of metal to the top of my workbench then pulled the tube against it to form a continuous bend. The tube I was using was 1.31" OD galvanized top rail, 17 ga. I adjusted the form radius by trial and error, here is the results,
Form radius 134" resulting radius 313" (2.33 spring back)
Form radius 59" resulting radius 81.4" (1.38 spring back)
Form radius 99" resulting radius 166" (1.68 spring back)
Form radius 88" resulting radius 141" (1.6 spring back)
Between each attempt I used a new tube because I though a pre-bent tube would throw off the results.
My goal was 137" radius so I considered it good enough. What I learned from this is the amount of spring back is not fixed so you may need to use trial and error the way I did. If bending bar stock is any indication, then bending larger tubes should spring back more. So, I would guess 1.8 spring back for my first attempt. To get a 15' radius I would use a (15/1.
= 8.3' radius form.
Good Luck,
John
