- A little history
- Misaligned pivot pins
- KP/PA at an acute angle
The TKP template has been with us entirely unchanged for many decades, during which materials technology and consumer needs have evolved. While it has nothing significantly different to the RKP template (as I explained here), it is fascinating to me that the original form of the TKPs is preserved by manufacturers like an endangered animal species or some ancient ritual. Likewise, many in the skate community, including professionals such as Riptide, revere TKPs as much as they are mystified by them.1 RKPs, though superficially different, obviously share the same principle as TKPs and, if anything, they are a proof of concept that some things that are characteristic about TKPs are not also essential. In this post I want to examine two ubiquitous but not defining characteristics of TKPs, which truck makers ought to reconsider. Since I focus exclusively on TKPs, I will first take the opportunity and tell their brief history.
A little history
There’s conclusive evidence that the TKP design was first developed for and used on roller skates. Anecdotal accounts tell us that kids in the 50s took parts out of roller skate shoes and bolted them on wooden planks, thus kicking off skateboarding. That much is clear. But I cannot fully understand the evolution of roller skate trucks. I can only imagine that major limitations were size and weight; trucks had to be tucked under the shoes, two for each foot. Another consideration must have been the material used as bushing (“the advent of plastics” occurred later in the 60s); we could safely assume it wasn’t as durable as modern polyurethane compounds. It is apparent also that initially (after what is known today as the “TKP” design became prevalent) the pivot pin was actually meant to be spherical (picture 1), which would justify its unorthodox orientation – if it is spherical, it doesn’t matter much if it’s aligned with the pivot axis (PA). However, early on, pivot alignment considerations were ignored by careless commercial manufacturers (at least this appears to be the most plausible explanation for the regressed form we see today). Much later, there’s evidence that some skateboard truck manufacturers realized the mistake and tried to patent a correction.
At any rate, to substantiate the previous paragraph a bit, the first recognizable TKP design I found was this one from 1899 (without a spherical, but with a cylindrical pin that’s clearly intended to follow the PA – picture 2), while this patent from 1946 (picture 3) introduces a second bushing to hold the hanger (there may be more like this one, but it suffices for the point I am making here – check out the patents from that time yourself, you’ll enjoy the steampunk aesthetic of the documents and please be sure to send me earlier ones if you find any). Considering roller skating was already a developed industry (apparently it was all the rage back then), while skateboarding was at its infancy (if at all in existence), we can safely assume the design jumped from the former to the latter rather than the other way around. Thus, in the 1950s-60s, the roller skate TKP template was brought over to skateboards without much adaptation and remains as it is 60 years later.
Does this mean the design is perfect? As I have hinted above, I am not so sure. I’d like to discuss two things that are not easy to justify with what we know is possible to build today: the pivot pins and the angle between the KP and the PA. I cannot see why manufacturers insist on making them like that (except for catering to the irrational sentimentality of their clientèle, as this story on Independent shows, but that’s a topic for another day).
Misaligned pivot pins
As intimated above, I am convinced that it is only oversight of careless commercial manufacturers that can explain why pivot pins today are misaligned with their own PAs; the pins were meant to be spherical, but that may have been too costly for commercial manufacturers, and since the trucks weren’t meant to turn as much as we today would like them to, nobody really took notice (roller skaters tilt their skates less than skateboarders tilt their longboards, don’t they?). At any rate, I can see no possible advantage in this configuration. On this, I will now make my case step-wise, starting with definitions:
What is the “pivot (or steering) axis” of a truck?
Each and every truck (all RKPs, TKPs and “concept” trucks like Seismic G5, Original, Rojas, Revenge etc – exceptions are: Gullwing Sidewinder, the so-called “surf-skate” trucks like Carver C7, the Waterborne adapter+truck etc and other double-pivot trucks I may be missing, as well as seemingly fancier – but ultimately doomed to flop – designs like the latest fad, Major Arc) 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).
Where is the pivot axis on a TKP truck?
The two points where the hanger and the baseplate are in contact are shown with the magenta circles (picture 4). These are the only two points that hold baseplate and hanger together. With these two points in common, the baseplate and the hanger can rotate against each other. If you take the baseplate as your reference (which is more intuitive, although there is no reason not to take the hanger as a reference), the hanger rotates around the PA (shown in green) under the baseplate. If there is some axle offset, aka rake (OFST), like there is in all TKP trucks and some RKP trucks, then the axle is not exactly on the PA, but hovers over it at some constant, set distance (=OFST). It then rotates around the PA at that constant distance on a plane perpendicular to the PA. That’s all a truck does and all it can do (of course, there are imperfections; but in principle that’s how both RKP and TKP trucks work). That’s all any truck does, including the “concept” trucks. It’s a very simple machine, but unfortunately over-mystified.1
Which direction is the pivot axis of TKP trucks facing?
The problem with most TKP trucks is that the pivot pin and pivot cup are not facing in the right direction. Their axis (shown in the picture above in dark yellow) is at odds with the pivot axis (PA). When the hanger rotates, the pivot pin rotates with it but at a constant angle with the pivot axis, because it’s fixed on the hanger, while the pivot cup, which is fixed on the baseplate, is facing in another direction. The two parts are in conflict; they are forced to occupy the same space. Only when the truck is at rest are the pivot cup and the pin at a matching orientation with each other.
What happens when a TKP truck turns?
But what exactly happens with the interface pin – cup when the truck turns and the pivot pin unavoidably binds (gets stuck) with the pivot cup? Here’s my take:
1) At the beginning of the turn, the tip of the pivot pin rotates, as intended, at the bottom of the cup, while the upper part of the pin slides towards the lips of the cup. 2) Then, the pin’s upper part starts compressing the lips of the cup, while the tip of the pin starts sliding towards the sides of the cup away from the center. 3) Then, the “bind”: either the truck abruptly stops turning, or, more probably, the pin starts sliding out of the pivot cup (see consequences in picture 5). 4) Finally, the tip of the pin reaches the lips of the pivot cup and the truck can continue turning without pressure from the sidewalls of the cup. What are the consequences? The patent applicant from 1977 who I mentioned earlier is very concise:
“When the hanger pivots, forces are applied to the pin and to the hole in the base in which it is received which are lateral with respect to the axis of the pin. These forces cause binding and wearing of the pin, the hole in the base in which it is received and a bushing which is placed in the hole. In addition, stresses are placed on the hanger itself which can result in breaking” (List, 1979).
I believe the above is enough evidence for the hypothesis that pin/PA misalignment is mere oversight to have merit. Dear reader, let me know if you have another theory to explain this peculiarity.
KP/PA at an acute angle
Another persistent feature of TKP design is the angle between the kingpin (KP) and the PA (thumbnails, right). Unlike what appears to be a more straightforward choice, i.e. a right ∟ angle, the KP/PA angle is invariably acute ∠. I wouldn’t call this oversight, because I can guess possible reasons designers from previous times opted for it and also the KP/PA angle is a characteristic that gives individuality to a truck. However, I cannot explain why each and every TKP truck today has an acute KP/PA angle, especially when one considers the drawbacks this brings. I will explain what the KP/PA angle does and then what its advantages and disadvantages are, before I restate my case that manufacturers ought to reconsider clinging to acute angles.
What effect does the KP/PA have?
When the bushing is deformed, it pushes back. It’s not a bad approximation, I think, to assume it pushes back with a force perpendicular to the surface that deforms it. This surface is the hanger seat. When the board is tilted, so are the bushings (which are fixed on the baseplate) in relation to the hanger. The force with which the bushings push back is sketched in picture 6 & 7 as BushingResistance0. It can be resolved into two perpendicular components: Bushing Resistance1 and Bushing Resistance2. Bushing Resistance2 is parallel to the PA (SteeringAxis) and therefore it has no effect (except friction, but let’s assume that for a moment as negligible), while the force with which the truck actually resists the turn is Bushing Resistance1. That component’s measure is a fraction of Bushing Resistance0 which depends on the KP/PA angle (in Bennett Vector’s case, it’s approx 0.8 times the Bushing Resistance0). In other words, it is that component that is responsible for the force that the rider has to overcome to turn the truck and likewise it is with that force that the truck returns back to center. Its measure is the sine of the bushing’s force, not the whole of it.
What is so odd about an acute KP/PA angle?
It is not that odd in itself. Apologies for the rhetorical bait. But I’d like to enumerate a couple of things that make this choice in need of more substantiation than what we get from TKP makers.
First of all, although earlier we assumed friction to be negligible, it probably isn’t. When the KP/PA angle is acute, the hanger has to rotate around the KP too. The bushings can either stick to the hanger or to the baseplate, but they, or the washers, have to rub against the other part. It is also conceivable that they stick to both ends and actually twist. That would be interesting and if the reader has any evidence of that I would be extremely keen to see it. But if they do rub, they generate heat and wear out parts.
Second of all, Bushing Resistance2 represents energy lost: it is bushing deformation that has no effect in the system. I will try to step carefully here because my chemistry is not good. Logically, a significant part of the energy stored in bushings is spent on the ineffective component that pushes the hanger parallel to the PA. This must mean some decrease in the life span of the bushings, in comparison to the same bushing mounted on a right angle to the PA but with everything else being equal. Let me break this down, because your skepticism on this is justified, dear reader. My thought, more formally, is as follows. A) When a bushing is deformed, it deteriorates and also it releases heat in the environment. At some point the bushing has to be replaced with a new one. Just its entropy increasing, that’s all. B) The useless component, Bushing Resistance2, is a fraction of the whole force. If A and B are true, then that fraction of the entropy is wasteful. The smaller that fraction is, the greater the reduction in the increase of entropy. It could have been saved inside the bushing and keep it alive longer, if it were somehow possible to mount the bushing on a right angle and deform it less for the same hanger turn. I know that’s an impossible counterfactual scenario, but my point is not that the bushings get wasted literally faster (comparison is impossible), but that more of their deterioration from new to waste happens without it actually being useful. Which deterioration releases heat while part of it produces no other effect, except added stress to parts of the truck, reducing their life span as well (Edelstein, 2021). At any rate, deformation without practical effect has to at the very least represent energy lost.
What might be useful about it?
As I have explained in detail in this article, playing with the KP/PA gives different return-to-center curves (assuming of course deformation volume is directly proportional to the force with which bushings resist that deformation; which can’t be a bad approximation, at least for small deformations), as shown in picture 8. That, in my view, is an interesting trait and could also be exploited for simplifying the problem we all have finding the right bushings for the front and rear trucks (see for example Carver’s CX+C2 set which uses the same bushings front and back, with the CX’s KP/PA angle being a tiny 30°). On the other hand, and that’s speculation, it could be that it was useful, for manufacturers of the previous century who had to work with different materials, to place the bushings in such a way as to absorb vibrations and shocks. Additionally, an acute KP/PA angle certainly leads to a more compact truck: remember, dear reader, that roller skate makers had to put two of those trucks under each shoe.
With all of the above in mind, it seems to me odd that TKP trucks are only ever made with acute KP/PA angles. On the one hand, there is certainly enough room under skateboard decks for more spread-out designs. On the other hand, materials, both plastic and metal, must surely be more durable now than what they were decades or centuries ago. It seems then that the only valid concern truck makers might have is the severe shocks trucks need to endure from freestyle/street skating (picture 9). But in that case, it would also seem odd that no designer ever decided to go all the way and build the KP vertical to the ground, or at a right angle with the PA to follow the hanger’s arc at an impact. Rather, I think that under the invariability of the design each company sticks to is a firm but irrational devotion to the “good old” thing.
It should be clear to you, dear reader, that there is no need to attach ourselves to an antiquated design that could easily be updated for the needs of the sport as it has evolved. As a matter of fact, there is already a couple of designers who have corrected the pivot issue (Surf Rodz and Carver – along with the latter’s copycats). I am eager to see manufacturers also being more playful with the KP/PA angle, of both their TKP and their RKP models. This would arguably help them expand their range with genuinely discreet options.
All credit for discovering the “lost component” of the bushing force that does nothing to resist the rotation goes to Ben Edelstein; he pointed it out to me during our very fruitful email exchange on TKPs a few months earlier. I’m grateful also for his comments and ideas when I was writing this text.
1. It’s worth it to show here a screenshot (picture 10) of Riptide’s website, before they inevitably edit this out. Their page on pivot cups tells us that TKPs “exhibit a much more complex movement” due to their “design geometry;” a movement that is presumably more complex than rotation.