Greetings Martin,
I think that you will have to experiment, try to visit a trials event where sidecars are running and take plenty of photos and measurements. Tarmac road settings may not be quite what you require but as I say you will need to experiment.
What engine/transmission will you be using, frame type, suspension, wheels, the list goes on? Do you have any photos of the project parts so far?
Good luck, my first bike was a Matchless G3 350cc with a Watsonisan Bambino scooter sidecar, I had to have a modified central bar made after I cracked the first one. It was so much fun, I later made a Norton ES2 framed, 650cc Dominator Twin, Commando gearbox with a Watsonian Monza sidecar. The sidecar was a child adult, child car seat fitted for our first born son who really enjoyed the experience.
Perhaps Hitchcocks might be able to help as they may have set up the Enfield combinations of the 80's. Another source of info might be;
http://www.trialscentral.com/forums/for ... ar-trials/I am looking forward to knowing and seeing how you get on. Good luck.
TTFN
Hugh.