An excellent question. Answers certainly differ on this one.
As a preface, I consider scooters to be a type of motorcycle - in the same way that dirt bikes, mopeds, cafe racers and goldwings are also all motorcycles of one stripe or another. Occasionally I run into the view that scooters are some sort of inferior alternative to a motorcycle, but my definition of a motorcycle is broad enough to include any motorized two wheeler (ie. motorcycle = motorized bicycle).
So what defines the 'scooter' class of motorcycle? It can be a challenging subset to separate from other classes of motorcycles such as mopeds and other small bikes. There are a number of criteria that are commonly mentioned, most of which don't work:
- Small wheels (ie. 10")
- Small engines (ie. 50cc)
- Step thru frame design / flat spots for the riders feet in front of the seat.
- Weather protection from some sort of a legshield
- Automatic (Twist 'n Go) CVT transmissions
- Engine comprises part of the swingarm (as opposed to being mounted to the frame)
Small wheels, small engines and CVT transmissions obviously don't work. CVT's didn't even exist in vintage scooter and scooters come in a large gradient of wheel and engine sizes so you can't put a clear cap on it.
Having the engine hinged/mounted as part of the swingarm is almost universally true in the world of scooters (and perhaps separates them from mopeds well) but one could easily imagine a scooter that doesn't use this design and I think Derbi made a few back in the 90's, so I'm hesitant to include it in any definition.
Of the above-mentioned traits, only two are consistently present in virtually all scooters and IMO embody the essence of a scooter. These are step thru frames and some sort of a legshield. Some 1930's Cushmans may not have had a legshield, but whether or not Cushmans were 'scooters' is debateable and one could easily argue this was simply the formative period where the scooter subset of motorcycles was evolving off.
I know of no vehicle that would be considered a 'scooter' that has zero provision for some degree of 'stepping thru'. Similarly all scooters place the riders feet in front of the seat in sort of a chair like seat position. Some mopeds also enable 'stepping thru' but the riding position is different. Many scooters do not have a full floorboard (ie. maxi's, Honda PCX) but they are still step thru designs and have some flat areas for the riders feet instead of pegs. The presence of some sort of a legshield is also a key trait. Even non-traditional scooters like Honda's Ruckus have this to some degree.
In essence, some degree of a step thru frame is the most core characteristic of a scooter. This step thru frame needs be accompanied by either a floorboard, or some sort of flat area(s) for the riders feet that provides a chair-like seating position. A 'scooter' should also have some sort of a leg shield appendage, but if lacking a case may still be made if the floorboard is particularly well developed and other common scooter traits are present such as a swingarm mounted engine.
Some mopeds have a heavily sloped top tube that allows for stepping thru, but they do not have any sort of legshield or a chair-like seating position. A moped will typically use foot pegs (or pedals) instead of flat spots for the feet, these pegs will often be located further back and wider than a scooter normally would place the riders feet, there won't be any sort of a legshield and the engine will likely be mounted to the frame instead of on the swingarm. Bigger wheels are the norm, but not the rule.
There have been a few bikes made that really blur the line between scooter and moped (Yamaha made a few in the late 70's/early 80's, called the V-series I think). They were essentially mopeds with legshields (ie. no floorboard at all).
Scooters Owned: 2003 Ruckus, 2006 Ruckus, 1983 Yamaha Beluga 80, 2007 Vespa LX 150, 2009 Yamaha BWS 125, 2008 Ruckus