Looking at the upturned handlebars in the plans and the comments about twist grip shifters being hard to use, I was not surprised. You would naturally grip these thumbs up. You have to grip the shifter with your 2 smallest fingers against your palms. Poor grip and leverage. Below is the photo of the handlebars from the Warrior plans.
I want to do a flatter arrangement so that I can reach down and have my thumbs toward the center rail and a normal arrangement of the brake lever and twist grip shifters. I had purchased an aluminum mountain bike handlebar, but it did not turn up far enough for my liking. So it made a number of passes through the Harbor Freight roller tubing bender and ended up in a decent upwards arc. It was a bit tough at first to keep it from turning as it went through, but then shallow grooves wore into the center section and it stayed centered. The rollers I used are for 1″ diameter tubing which matches the ends but he center is 30.8 mm (approx 1.25″).
To make the pivot, I used another of the same bearing sets I used for the front wheels. The center post is fixed. The post is 1 1/8″ steel tube and has a step ring brazed on as I did for the wheel bearings. This was then tack welded to the frame for a test fit to see how it felt. The distance from the tail end of the rail is 7″ rather than the 10″ in the plans. After testing, I solidly welded it all the way around.
The center section is 1.5″ OD steel tubing that is bored on the lathe to fit the bearing cups as before. To this, I welded 2 steel shaft collars. They held a piece of tubing while I did the welding to make the alignment right. The collars were a bit undersized for the handlebars (it would have been better to test fit BEFORE welding up the assembly). So I had to bore them out on the mill for a perfect fit. I placed some washers as shims between the halves before boring to make the opeing a bit elliptical for good clamping pressure. The soft Chinese steel bored easily. Here is the view from the underside.
Next comes the linkage from the handlebars to the front wheels and hooking up the brakes.