All the dictionaries I’ve checked claim a motorcycle is a two-wheeled vehicle powered by an engine. But if you ask Wikipedia, a motorcycle is a two- or three-wheeled motor vehicle. And similarly, the National Highway Transportation Safety Administration (NHTSA) defines a motorcycle as ...
a motor vehicle with motive power having a seat or saddle for the use of the rider and designed to travel on not more than three wheels in contact with ground (49 CFR 571.3).
If you ask me (and even if you don’t), I’ll tell you a motorcycle is a motor-powered, two-wheeled vehicle where those 2 wheels are in-line. But don’t take my word for it, because these lines? They get grayer, my friend...
Let’s consider the conventional, two-wheeled motorcycle and its characteristics. It’s unstable at rest (you need to put a foot down). You turn it by countersteering. If there’s a passenger, they ride behind the pilot.
Now let’s consider the conventional three-wheeled vehicle. Wait, what’s a conventional three-wheeled vehicle? Actually, let’s focus that question: what is the wheel configuration of a conventional three wheeler?
Harley-style trikes have one wheel in front, and two wheels in the back. But Can-Am Spyders have the opposite wheel layout: one wheel in back, and two wheels in the front. And don’t forget about sidecar motorcycles. Urals and the like are - let’s say - one wheel in back, one wheel in front, and a 3rd wheel on the side (but in the middle-ish).
Okay, well, there is no conventional three wheeled vehicle. But maybe we can lay out some consistent characteristics. Are three wheelers stable at rest? Harley trikes, Spyder’s, and side car are indeed stable at rest. But leaning three-wheelers like the Piaggio MP3 or the Yamaha Niken are arguably not stable at rest.
Do you turn three wheelers by countersteering? Well, yes and no. Trikes and Spyders do not countersteer. Side car motorcycles countersteer when the 3rd wheel is off the ground, and leaning three-wheelers always countersteer.
How about passengers? Again, it depends on the three wheeler. And if we’re willing to include Vanderhalls and Slingshots in this discussion, I quit.