The basic idea I’d like to convey with this post is that a skateboard always balances on the line that connects the two centers of the axles of the trucks. What is usually called “roll center” can only make sense on a skateboard as this line. I’d call it the “balance line” of a skateboard.
This is true of all trucks, including double-pivot trucks like all the so-called surf-skate trucks (which are essentially copies of Carver’s C7).
It is important to point out that skateboards shouldn’t be conceived of as four-wheel vehicles like cars are. A rider balances on a skateboard as on any vehicle with two wheels. In principle, bushings (or whatever the mechanism is, with which trucks resist turning, i.e., the restoring mechanism) are not necessary and a skateboard can be ridden without them. Conversely, an axle could have four, or eight, or any arbitrary number of wheels attached on it, but that wouldn’t (in principle) make the skateboard any more stable. Skateboards are to be conceived as two-wheeled vehicles; they are not cars.
In my opinion, the balance line is the most fundamental concept about skateboards.