I rummaged a Schwinn tricycle from the dumpster outside my work. This was a spur of the moment project with an angle grinder and a welder. What cannot you do with an angle grinder/cutter and a welder?
Step 1: Remove Seat and Cut Frame
The seat was affixed with one hand tighten nut and bolt which came off easily.
Make sure to use safety glasses before the next part of this step.
I cut the frame with an angle grinder in the straight section closest to the ground as shown in the picture to make stretching easier.
Placing the frame in a vice to secure it while cutting is helpful.
Step 2: Butting Tube in Frame
Luckly, my tubing fit almost exactly inside the frame of the trike with some sanding on the edges. I used a hammer to drive the tube in about 1 1/2 in. I used 1.25 in OD aircraft tubing for reference.
To determine the length, I check fit it after butting the tube in the frame by sitting on the ground with the tube under me and holding the handle bars. It's not an exact science so just mark the tube where your bottom sits.
Step 3: Welding 1
Clean the paint off the part of the frame close to where you plan to weld.
Make sure to have proper safety gear.
Weld the butted tube to the frame. I used a Miller mig welder at 19 V and 188 Feed Rate
Let the part cool before touching it (duh)
Step 4: Welding 2
Because the rear tube piece was a slight oval shape, I had to bend the butted tube to fit. A vice worked well for this, make sure to align the oval with the rest of the bike, otherwise it'll be sort of "twisted"
Now that the tubes match up nicely, I used the same but up and weld method as for the other end
Step 5: Rear Wheels!
Standard 10 in wheels from Harbor Freight worked well for this. I was even able to use the original axle after machining a bushing for them.
Check out the pictures for comments on the steps within this step
Step 6: Front Wheel Fix
The valve for the front wheel was angled, which means the inner tube is not properly lined up with the rim.
Take all the air out and squeeze the sides all the way around to loosen the tire from the rim, then just hold the rim and slide the tire around until the valve lines up. This works for the small bicycle wheels but I wouldn't recommend using this method on larger adult bicycles.
Step 7: Handle Bars
This was fairly straight forward. More angle grinder cutting and welding.
Follow the comments in the pictures for steps
Something helpful is to cut the handle bar post in two so that you may still insert the cut end into the frame clamp.