Updated: Jan 18
Tube selection is paramount to the build. You need to consider the weight that it will carry. Mainly you're choice of engine will influence that.
Build from scratch or choose a bicycle frame?
Bicycle frames are a quick and a simpler way to create the frame and there's nothing wrong in selecting one. However, you're going to add a considerable amount of weight. If you select a rigid frame extra strengthening should be a consideration. Bicycle frames are braized with lugs, so if you plan on modifying by welding, you need to know what tube it is. Certain types of tubing used in bicycle frames, such as 4130 Chrome Moly need a lot of preparation.
With scratch built frames you can choose the tube type, wall thickness and diameter.
First of all you can buy tube or pipe. They may look similar, but pipe has no place in frame building. Pipe is measured by inside dia and wall thickness. Outside diameter is not so critical.
Tube sizes are given by it's outside diameter.
There are a number of types to choose from:
Cold drawn (CDS) either Mechanical or Hydraulic
Drawn over Mandrel (DOM)
Electric Resistance Welded (ERW).
I decided to use CDS.
Main frame is 1.25" x 1.63mm wall and seat and chain stays of 1" x 1.63mm wall.
The correct way to bend tube is with a tube bender. You buy a die with follower for a given diameter and radius. Benders aren't cheap and for this reason people resort to other methods. If, like me you make the frames commercially, then it's of utmost importance that all the bends are uniform and without defect.
WELDING OR BRAIZING
This is personal choice I believe. A braized frame, with or without lugs can be as strong as a welded one.
As regards the type if welding, again it is down to your choice.
Whatever you decide, one thing is certain you will need some form of jig. The purpose of the jig is so all your tubing is aligned whilst welding or braizing. Tack around the joint to avoid distortion. Once all the frame has been tacked you can start braizing/welding. Don't do all of one tube before moving to the next one. The aim is to balance the heat out around the frame to avoid distortion.
The finished joints may or may not be of concern. For me I want it to look like an old frame.
The original frames contained castings at each joint and the frames tubes braized into them.
I offer a frame with a cast lug appearance. It's more work but I personally think it looks better.