The two crossword solutions for the term 'motorcycle add-on' are 'sidecar' and 'sidar'. A sidecar is a one-wheeled attachment to a motorcycle, typically used to carry a passenger or cargo. It is usually mounted on the left side of the motorcycle and is connected to the frame by a universal joint. A sidar is a type of sidecar that is designed to be attached to the right side of the motorcycle. It is typically used to carry a passenger or cargo, but can also be used for storage.