I knew Gene McCall well. I threw a leg over his rig once and that was enough. I could not believe that he and Don Seedsman from Australia, rode that rig, towing a camp trailer behind it, left Murfreesboro, Tennessee and rode north to the Arctic Circle in Alaska, then across Canada and down to Pennsylvania to my location. After a rally near here it was back to Tennessee.
I would urge anyone that wants to attach a sidecar to an ST1100 to try to find one to try out first. You really need to have the front suspension altered to make it workable and it takes a considerable amount of money to buy, install and set one up properly. You will want an expert to do it.
In short, IMHO, you would be better off buying a car (with four wheels). I used to install Velorex side cars on CX500s. They were fun, especially flying the car with or without a passenger. But still, I told myself that if I had one, I would still want a regular two wheel motorcycle.
