"classic MM style" Harr, ha! Never thought anything I built would ever be described as the classic design!
When I first started toying with trikes there were no blue prints, no CAD designs, no thinking things through really. Just me in a shed with some tubing & a bender. Most of mine were built by the Try It & See method. Others here have since put a lot more thought into it all, (including the multi talented Mr Olds), & refined the whole thing. I do believe they can sometimes over-think & over-complicate things though. Mine were always intended as basic, simple -& fun machines. Fortunately I moved on to other vehicles before they all got too good & showed me up.
My first couple of hardtail frames were bent up in single lengths, one each side, all the way from the headstock to the axle clamps, but on later ones I decided it was just as strong, & a damn sight easier, to do it in 2 sections, cut, sleeved & welded as you say. The first ones also had the coolant flowing through the bottom frame rails to rear mounted radiators, so doing them in uncut lengths meant less leaking welds. Making them in 2 halves means you can tweak them until they're just right. It's also very easy to forget one side needs to be a mirror image of the other -I've ended up with 2 left or 2 right sided sections several times! As long as your frame's properly triangulated & gusseted, with decent sleeves in the joints, it should be fine.
My sleeves were usually about 6 inches long, (no comments about short arms please), made from the same tubing as the frame, cut down their length & squeezed in a vice until they slotted inside the frame ends. Chamfer the ends of the frame tubes so they form a V when pushed together over the sleeve & leave a few mill gap between them, so you have a good weld area. I also drilled through the frame into the sleeve & "button welded" through the holes. Long sleeves will help to keep the tubes straight so you don't end up with a kink where the joint is, but I clamped the tubes into a length of angle iron to make sure they sat true as I welded them.
The ERW tubing we all use is just basic "steam pipe", available from plumbers merchants such as Plumb Centre. I used the 1" bore, medium or thick walled variety, (somewhere between 3.5 & 4.5mm thick). It comes in a red oxide finish, which would be the primered look you mentioned. It's a basic grade tube, but bends nicely in a Clarkes type hydraulic bender & welds nicely with a MIG. It's as strong as you need for a 40bhp Reliant trike frame but soft enough to have a little flex to soak up the vibrations without stressing or cracking.
If you're not sure of your welding abilities, then please, find a mate who can, & take your time -don't worry if you mess up, the tubing's reasonably cheap & you can always re-do things -but above all, enjoy it! Oh, & post photos -we like photos.
