- Balance line
- Axle offset (or rake)
- Wheelbase and pivot axis angles
- Angle between kingpin and pivot axis
In what follows, a “truck” is any single-pivot mechanism that allows the wheels to turn when it is tilted. Each and every truck (all RKPs, TKPs and “concept” trucks like Seismic G5, Original, Rojas, Revenge etc – exceptions are: Gullwing Sidewinder and other double-pivot trucks) on the market essentially consists of two parts that rotate against each other: the baseplate and the hanger. All the other parts of the truck are attached firmly on either of these two main bits. I call all these trucks “single pivot trucks,” because, in principle, the rotation between the two main parts is defined by one, linear pivot axis. Regardless of which of the two parts you take as reference, the other part rotates against it on a plane perpendicular to the pivot axis. Simply put, there is one and only one (in principle) degree of freedom: the angle of the rotation from the state of rest (which we can arbitrarily consider to be the state of a resting skateboard).
Disclaimer on the terminology: most of it is entirely arbitrary. Why? Because to my best knowledge, a standard terminology has not been established for skateboarding. Terms used by skaters and companies alike are loans from car/bicycle engineering. I have adjusted and/or invented a few terms for two reasons: a. to reflect more accurately the particularities of a skateboard, and/or b. just to make them more relevant to me.
Using logic, one can realize that every truck (with a single pivot) turns the same (i.e. its axle rotates the same angle from rest) for a given pivot axis (PA) angle and deck tilt, regardless of rake or any other characteristic. That’s physical reality, nothing more, nothing less. You tilt a truck and its axle turns to keep both wheels on the ground, as much as it is constrained to by its -fixed- PA (and by its PA alone). Truck height or offset (rake) does not change that. Hopefully the illustration makes that clear. For more on this, check here and here.
[I do not see what a “carvey”, “divey”, “progressively-turning”, “fast-turning” truck is, or what “a truck’s ride” might signify. Nonsensical skate forum paraliterature…]
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” (see picture) 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).
Axle offset (or rake)
There should be no confusion what rake means. Wikipedia on bicycle geometry explains fork offset (or rake) clearly. However, I prefer the term “axle offset”, i.e the perpendicular distance of the axle from the pivot axis. I have seen this term used in skateboarding to connote the distance of the axle from the kingpin in a RKP truck (see picture – not to be taken as correct here!), but this, I find, is not particularly relevant (more here and here) and the term might well be reclaimed for more pertinent things.
In the following example you can see a model of a skateboard with 10cm (well, units are again arbitrary) offset. In relation to the size of the skateboard depicted, this offset is indeed exaggerated, but it is helpful in discerning offset’s effects. By “positive” axle offset (or “backward” rake), I mean the axle is offset backwards in the direction of the board. I prefer the word “positive” because as I explain below, it tends to give the truck a positive trail. More about all this below.
A positive axle offset tends to move the deck inwards the turn. That is, towards the side where the deck leans. The opposite is true for a negative axle offset. What does this mean? In practical terms, it changes how the other end of the deck faces the turn. Look at the two pictures and notice how the direction of the deck has already shifted inwards the turn on the top picture and outwards in the bottom picture (in the pictures, the pink lines are the pivot axes and the green lines are the axles; the thick, yellow line is the deck’s direction and, perpendicular to that, its tilt). It does seem logical (although more evidence is needed here) to claim that the turn of the board does not change. Riding just feels different. (There must be a reason why TKP trucks, and Bennett Vector in particular, have been so popular in slalom skateboarding, and the way a negative offset makes it more intuitive to avoid cones when turning could be the answer. Then again, it could just be hype. More on the Bennett Vector’s popularity here.)
A negative axle offset tends to move the deck upwards when the rider leans, but that’s only if (and as long as) this offset brings the center of the axle (the green points in the pictures) forward the point where deck plane and pivot axis intersect (the yellow points in the pictures). In other words, only when the following conditions are satisfied: negative offset, high pivot angle, low deck, first few angles of lean. Then the deck starts dropping again. What does this mean? In practice, not much. It has been argued that the initial rise could provide a natural resistance to the turn and a return-to-center force, but I believe that this force cannot be enough to be effective or even perceptible. Unless and until somebody actually bothers to do the physics on all this, it will remain a debatable conjecture. In sum, it’s plausible but improbable that the rise is a return-to-center force of a magnitude comparable to bushings or springs. But it does provide a different feel to the ride. (That might account for what TKP aficionados call the “dive,” since the deck starts dipping down again once the yellow point reaches the green point. However, my preferred theory for what “dive” means is explained in this post. In any case, the meaning of “dive” is moot and too esoteric for my taste.)
Trail is perfectly well defined in the wikipedia article on bicycle geometry. For skateboard trucks, the adaptation could be: trail is defined by the pivot axis and the contact points of the wheels with the ground. If this axis intersects with the ground on a point (the purple point in the picture, where the skateboard’s direction is leftwards) ahead of the point where the wheels touch the ground, then the trail is positive. The explanation why trail is important is trivial. A positive trail helps with stability, a negative trail doesn’t. Is it perceptible or even important on a skateboard? The jury is out until someone does the actual physics on this. Also, if you imagine the extreme, where the pivot axis is far behind the wheels’ contact patch (e.g. inverted rear trucks), does it make sense to think there’s a high risk of unpredictable turning? Not really.
The only question that I can answer is: is positive axle offset enough to secure positive trail? No, it’s not. A sufficient amount of positive offset is needed for a positive trail. The picture will make this clearer. Even though the offset is positive, the trail is negative.
Wheelbase and pivot axis angles
The PA angles of the two trucks along with the deck’s wheelbase determine the board’s turning radius for a given tilt of the deck (with a minor influence by the aforementioned lateral and vertical motions and also the small change in wheelbase when they turn). I’ve written about this in this article, because just a paragraph here wouldn’t be enough.
Angle between kingpin and PA
Finally, there’s some significance to the angle that relates the kingpin (KP) with the PA of a truck. However, this angle has no impact on the geometry of the truck (i.e., insofar as only the way that the deck moves is concerned), but on the physics (i.e., it has an impact on the amount of compression of the bushings for a given turn of the hanger). I talk about that in this post, so I will finish this article here.
If needed, a more technical version of this article, incl. formal proof and formulas, is here.