Came into this thread rather late, but two points came into my mind, You don't have any suspension, as a quad doesn't have a natural stable position how does one wheel in the air affect the bikes performance? The steering position seem a bit awkward, why didn't you go for steering sticks?
Hello! One wheel in the air doesn't tend to affect performance too much- it doesn't happen often- only if a corner/slope (e.g. into a driveway) is taken a little on the quick side, but the BMX wheels take it in their stride. I'm generally reading the road ahead and planning the route along it so as to avoid any particularly nasty potholes, but it can cause a bit of a jolt if I get it wrong!
Steering is on the upgrade list, as it is a bit tricky getting into the seat if you're not used to it (although myself and Mrs IRW have perfected the art!) I went for this method as it (a) worked with all the bits I had lying around, and (b) worked around the rest of the frame.
With hindsight, I would have planned it a little better when I was planning the frame, but if you read the whole thread you'll see the whole build was a fair old learning experience for me!
A few more pictures of the latest modifications:
New chain guide pulley mountings, rear driver side pulley:
Forward driver side pulley and chain rings from above:
As you can see, the two chain ring pulleys (on each side) are now mounted on a length of M8 threaded rod. This allows them to move to line up with where the chain is coming from/going to. Because of the extra space they now have to move, the drive is now much quieter, and I hope I won't go through pulleys quite as much as I have been!
A view under the stoker's seat, showing the new slot/wingnut/washer arrangement:
I dismissed it as I felt sure after all that work the builder had thought of it. 
