At a recent carting workshop, the presenter made a comment about
the single tree being important because it keeps the forces on
the lines equal when the dog is turning. On my current wagon, I
decided to just connect the lines to the poles, but because the
poles rotate with the front wheels (which are not far from where
a single tree would pivot) I don't see this as a problem.

The vehicle I'm building(/modifying) now is set up like a
conventional wheelchair - large stationary wheels in back and
castering front wheels. Instead of turning just a set of wheels
(as on our current wagon), the poles will turn the entire
vehicle, pivoting about the rear axle.

So...how beneficial is a single tree in this situation? There's
hardly any play between the guide poles so I'm having difficulty
imagining that the dog is going to be able to move significantly
away from the centerline. If even a tiny amount of change is a
problem, I think I can solve it by using shock mounts (which I
wanted to incorporate anyway). Is there more I should consider?

Oh...and here's something I picked up in researching this topic...
http://www.luckymojo.com/bluesiwhippedmywomanmjb.html

Thank you.

--kyler