I'm still wondering about that third support arm. Thanks to ferball's "sidecar questions" thread for the link below...
Watsonian Sidecar Images - Gentleman Cyclist | Google Groups
In all of the above photos of Watsonian sidecars for bicycles I don't see any indication of a third support arm. I do know that Watsonian made mostly sidecars for motorcycles and I'm guessing that at low speed and pretty light weight there was no need for that third arm with the bicycle models, but at higher speed and larger sidecar it became necessary (not that I have any theory on why it might have been necessary). We are somewhere in between, going faster than your average pedal bike, but much slower than a motorcycle. Can anybody weigh in on this to help clarify what might be needed for our purposes? I wish I better understood the principles involved here. Once I'm able to do test runs I think I will put some weight into it to approximate the dog being there. She never signed up to be a test pilot, just wants to go for a squirrel spotting ride. Maybe a case of beer will do. A full case on the way out and if all goes well, one short coming home.