Sidecar rigs actually began in the late 1800s as bicycles with a passenger compartment attached to one side. In the teens, companies like Triumph and Harley-Davidson began producing and selling a three-wheeled sidecar motor-bike.
During World War I, Western Front soldiers used sidecar rigs for dispatch duties because of their stability and carrying capacity. After the war, automobiles became much less expensive, and many sidecar companies disappeared.
A few motorcycle brands, such as BMW, Ural and Harley-Davidson, continued to make sidecar rigs for people devoted to their unique style.
For more information, click here for an interesting article about sidecar history.