3/4" and 1/2" square tube would do the job as long as the pins are beefy enough. I wonder if the way it's done on the Hawthorne Valley model, 1/2" i d schedule 40 tube inside a down tube from a bicycle (perfect sliding fit) might be as good as any way to make telescoping tie rods. Also with most square tube there's that weld seam to broach out or notch around.
That go cart design is great - very much a Farmhack solution - and the other kinds that use a bicycle head tube and fork are too. Mine is based on the parts that are easiest to find and maybe modify (bend) a little, with the goal being tight steering, fast disassembly, and adjustability of the length of the steering column. Dan fixed the length problem brilliantly and easily at the Metro Pedal Power event, by cutting the top off a seatpost and welding it to the top of the steering tube: the quick release lever now holds the handlebar at the height you want. So anyway, the 30 degree steering column is about right to accommodate most people, and bending that knuckle is fast, but I'd agree the diygocarts way would be tighter, as long as the design allows fast disassembly.
I'm getting the drawings and build page ready to post this weekend - thanks,
Definitely, which opens up another question, what is the ideal weight overall in terms of balancing traction and effort? Maybe an elliptical sprocket too, oriented the way Sheldon Brown advised - http://sheldonbrown.com/biopace.html