Forming the Hoops
By A. W. Burton
Yes, curves everywhere (even if slight) to keep ALL the skin loaded—as soon as an oilcan develops, there are higher than normal tension stresses in that area.
Bending the hoops is fairly simple, and filling the inside with sand, etc. is not necessary. You have to build a curved wooden form out of bits of 2x stock and mount it on some plywood on a flat surface (wall or table). At the top centre you have to place another block 1" above the form so that the tube does not lift off the form in this area of modest curvature when the forming is done. Note that the form will have to be "over-curved" a bit to take care of spring-back (the adjustment doesn’t have to be too sophisticated). Remember that the exact shape is not important, just that all hoops are identical or very nearly so.
Mark the center of the tube and position it on the form at a registration mark placed at the form top. Now comes the exciting bit. With one person on each end of the tube give a mighty and (this is important) CONTINUOUS heave to bend the tube around the form. Do NOT relax the pull, as taking a second go at it could introduce some extra work hardening and produce a hoop with differences in the amount of bend. Try to repeat the "dance" for every hoop.
When all are done, stack the hoops to see if any are significantly different—expect to reject 1 or 2 -- any badly off could be used in other places like the door. Some slight differences may be corrected with shimming when the skins are added so that the skin does not change curvature when riveted & epoxied to the hoop. Remember that the hoop is there to provide the shape and curvature to the skin (and a place to join them) -- they provide nothing to the strength of the monocoque structure. If there are minor differences in the curve, it is possible that the cause is that the tube itself is not 100% similar along its length.
My trailer hoops have a small reverse curve at the very ends (3") so that the hoop is square with the floor angle for bolting them on—these bends should be done first.