The Curious Design of Skateboard Trucks

There’s something wrong with skateboard trucks, and I don’t know why.


I’ve always been interested in skateboards – not so much the sport itself, but the design and engineering behind it. A skateboard only has a few parts, the most interesting of which are "trucks" – metal components that connect the wheels to the board and convert the rider’s lean into a turn.

traditional kingpin skateboard trucks
traditional kingpin skateboard trucks

Over the past two years, I've dived deep down the rabbit hole of skateboard truck design. Surprisingly, there aren't many good resources about this topic. With the exception of a few independent bloggers, most of the information online is misguided, oversimplified, or just straight up false.


A Quick Explainer


When you think of a skateboard, you probably envision something like this.

The truck design used here is known as “TKP” (Traditional Kingpin), and they're mostly used in the skateboard events you see on TV, like park, street, and halfpipe. TKP trucks will be the main focus of this article. There is another style of trucks, commonly used in longboards, called Reverse Kingpin (RKP).


TKP trucks came first, originally derived from the design of roller skate trucks in the 1950s, and popularized/standardized by the company Independent. RKP trucks came later, around the 1970s.

(TKP left, RKP right)


Both TKP and RKP trucks consist of the same basic components, they only differ in their configuration:

  1. The hanger is a large, T-shaped component that holds the wheels on its ends and rotates about a central axis (called the pivot axis).

  2. The kingpin is a thick bolt that fixes the hanger to only one rotational axis. It keeps everything in place.

  3. The baseplate bolts onto the wooden deck, connecting the trucks to the rest of the board. It has a little socket (called the pivot cup) for the hanger to slot into, and also holds the kingpin in place.

  4. Bushings are donut-shaped rubber pieces that provide a spring force to push the trucks back to their center position (for stability). Each truck has 2 bushings, one above and one below the hanger.

(TKP left, RKP right, but each truck has the same components)


When you lean on the deck, the hanger rotates about its axis so that both wheels continue to touch the ground. Since the hanger's axis is at a slanted angle relative to the ground, the wheels shift so that they're no longer facing directly forward, initiating a turn. For a satisfying visual explanation, check out the video below.


Much like bikes, skateboard trucks are a deceptively complicated mechanism. I wanted to understand at a fundamental level how they work, and why they look the way they do. The RKP design made perfect sense to me, but as I studied TKP trucks more closely, I noticed some peculiar issues. I researched around for any discussion on why these issues might exist. I posted on Reddit, scoured forums, crawled old patents – but nobody had a definite answer. Most of the attempted reasoning was technically imprecise or repeating old myths.


I finally came across a refreshingly technical and critical skateboarding blog, so I reached out to the author, Alex Georgoulis, to get his perspective on my research. Alex hadn’t written specifically about TKP truck design flaws, but when I brought it up to him, he agreed it was an odd problem. You should definitely read Alex’s post on this topic, where he dives deep into the history and evolution of the TKP template, in addition to covering the physics of these design flaws.


I'm still writing this article for two reasons: one is that I had a few directions I wanted to go in that Alex didn't discuss, and two is that this is just an interesting topic worth writing about.


Problem 1. Misaligned Pivot Cup

misaligned axes on a TKP truck

A truck’s pivot axis is the line between the center of its pivot cup (where the hanger slots into the baseplate) and the bushing seat. These are the two points that constrain the rotation of the truck. However, TKP trucks noticeably have the hanger oriented at a higher angle (in the photo above, the difference between these angles is 36.5°). This axis misalignment causes the hanger to grind against its pivot while rotating. People debate how exactly this affects the lifespan of the truck, but it certainly can’t be good.


Just glancing at a TKP truck, it’s tempting to think that the pivot axis *is* aligned with the hanger. Many online resources get this wrong, which might be why this design flaw is not well known. If we compare to an RKP truck, we can see that there is perfect alignment of the pivot axis and the hanger.

aligned axes on RKP truck

Problem 2. Acute Bushing Angles


To return to a central position, skateboard trucks use rubber bushings to spring back into place. Bushings are much more compact, light, and wear resistant than metal springs, but they still do a good job of resisting rotation.


For a spring to be maximally effective, its applied force must be in a direction that can do work. In the case of rotation, this direction must be tangent to the circular rotation path. If we look at TKP trucks, however, the angle between the pivot and kingpin axes is is significantly less than 90 degrees (72.5° in the figure below), which means that some bushing energy is wasted.

Only part of the bushing force, Fuseful, is doing work. The other component, Fwasted, is acting in a direction that the system is not constrained to move in – it’s wasted energy. This means that bushings wear faster, and that TKP trucks require harder, beefier bushings just to get the same spring resistance that they would otherwise need.


Again as a point of comparison, let’s look at the RKP design above. The bushings are at a near perfect 90° angle to the pivot axis, so very little energy is wasted.


A Truck's Purpose: TKP vs RKP


To understand why each truck is designed the way it is, it’s important to understand their uses. TKP and RKP trucks are each used for different types of skateboarding.

downhill longboarder leaning into a turn
downhill longboarder leaning into a turn

RKP trucks are used for longboarding. Longboarding has many sub-genres, but it’s generally focused on precise turning and handling. When you’re bombing down a switchback at 60 mph, you can’t risk turning on imprecise trucks, or you might die. The design of RKP trucks well-reflects this objective.


In skateboarding, trucks aren't just for turning, they're the interface for executing technical tricks on rails and curbs. It also has many sub-genres (street, halfpipe, big air, etc), but they’re all generally concerned with jumps, grinds, and other technical tricks. For these objectives, the biggest truck design considerations are weight, impact resistance, compactness, and ease of grinding. I believe the TKP truck prioritizes these factors over turning, since it just isn't as big of a priority. But why can’t it do that without the two design flaws I mentioned?


 

What’s the Catch?


As I researched deeper without finding a satisfactory explanation, I was tempted to think that these were just glaring, negligent design errors. But I hesitated to come to such a strong conclusion.


I love this tweet about a math professor who wrote a letter to Carnation, a canned cat food manufacturing company. He wrote to explain how they could optimize their can geometry for lower costs – it’s a classic calculus problem to find the optimal cylinder dimensions to minimize surface area-to-volume ratio. The company wrote back, but not to praise his insight. They kindly explained 5 other, obscure factors for the can’s design that he hadn't considered. Unlike calculus class, in the real world there are very few one-dimensional optimization problems.

I suspected my question was similar - I noticed these two glaring flaws, but there are likely other, less obvious factors that contribute to the TKP design. So here are my speculations.


Impact Resistance

TKP trucks are used more for jumps and grinds than tight turns. The design may be of advantage here. For one, the hanger and kingpin are oriented mostly vertically (at angles 74° and 70.5°, respectively). When a rider lands a jump, the truck redirects the impact force vertically into the bushings and pivot cup, which partially absorb the impact. If the hanger were lowered to be inline with the pivot axis, and the bushing were lowered to be at a 90 degree angle, then less of the impact force would be directed vertically through the trucks, and more toward bending them inward, leading to fatigue and shear failure.


Some designs [3] have attempted to solve for the axis misalignment problem by using a kinked hanger, but this likely makes the truck even weaker under impact. The kink is a point of high stress concentration, making it a target for shear failure.

Angle Tradeoffs

There are 3 relevant angles on a truck:

  1. the plate angle, which is the angle that the pivot axis makes with the board. This angle solely determines the turn-to-lean ratio of the truck [5].

  2. the kingpin angle, the angle that the kingpin makes with the board. A high angle here helps hide the kingpin and bushings down away from the hanger, so it doesn’t get caught on grinds. This is the main reason why you can't just use RKP trucks for all skateboards. And as I mentioned above, it may help with impact resistance.

  3. The bushing angle, between the kingpin axis and the turning axis. A perpendicular bushing angle means that no spring force is wasted.

These 3 angles are all dependent - they form the 3 corners of a triangle. So if we vary one, we necessarily need to vary another. It is hard to find a configuration of all these angles that does not sacrifice on turn responsiveness, bushing efficiency, grinding, and impact resistance.


Tradition, Cultism

This may be a case where cultural adoration triumphs over pure technological efficiency. Here’s a quote from a Jenkem Magazine article titled “A look at the Cult of Independent Trucks”:

And though Wilson acknowledged that with every new truck, 'there’s got to be some sort of evolution there. Skaters want that,” he also noted that Indy still, “mostly sells our standard silver, solid-axle, solid-kingpin, gravity-cast stuff.' [2]

As I mentioned earlier, Independent popularized the TKP design as we see it now. And as the title alludes, there is certainly a cult behind them. Because riders are so allegiant to their brands and specific trucks, it’s hard to make leaping changes without upsetting them. This may lead to a cycle of incrementalism, where companies are caught between appeasing current preferences and pushing the envelope of performance.


I also think that skaters’ loose understanding of and connection to truck design may not allow for precise experimentation, it’s more about vague “feel”:

'I’ve ridden Stage VII Indys exclusively since 2005,' he said. But when pushed to reveal why he’s a longtime member of the old-school Indy cult, the Englishman wouldn’t say. Luckily, Swedish artist, Insta skate favorite, and JK Industries rider Ludvig Håkansson was more forthcoming. 'The turn is more responsive,' he explained, offering at least some insight into the mind of an old-school Indy rider. Still, that info didn’t exactly settle anything. [2]

Many skaters will claim that TKP trucks have a more “divey” feel to them, but there isn’t clear evidence as to what mechanically causes that feeling. Other skaters claim that RKP trucks have a more linear turn-to-lean curve than TKPs, which is provably false [5]. So skaters’ preferences when it comes to truck geometry may not be completely scientifically grounded, leaving room for myths and personal preference. It seems there is not a strong gradient in the function of truck design → better performance, so there’s no clear path to optimized designs. Other factors dominate, like identity and brand allegiance.


I do think that culture and branding play a role here, but I have a hard time fully buying this theory. Although skateboarding is a more open-ended sport, it is still ever-evolving and highly competitive. Athletes are constantly hacking their equipment to eke out extra performance, and I don’t see how skateboarders would be different. Before truck designs adapted, skaters use to shave down the bolts of their trucks for easier tailslides [2]. Rodney Mullen used to tape ball bearings to his trucks to weigh them down for more control [7]. Even now, there's a culture of modding trucks for more niche skateboard disciplines like long-distance pumping [4]. Smart companies observe and productize these hacks, and although there may be resistance to a new design, over the span of decades a better product should eventually win.


Maybe it’s not that skateboarders don’t care about optimized technology, but turning is just a dimension on which they don’t care to optimize. Skaters would rather optimize features that directly impact grinds and tricks. They may say that they care about the turning feel, but in reality skaters don't care so much [6].


The counterexample of RKPs is poignant because that is a sport specifically about turning, so the trucks have been optimized in that dimension. There is also wild variation in designs of longboard trucks, whereas skateboard designs all generally follow the same TKP template. TKP companies seem afraid to experiment.

 

The Traditional Kingpin truck may be a time-tested compromise among many conflicting constraints, or it might just be a mediocre design that's waiting to be disrupted. I've been thinking about how I might design a better truck recently, combining the best aspects of RKP trucks with the toughness of TKP. That topic will need its own post.


All this being said, I am an engineer, not really a skateboarder. I don’t have inside industry information on why trucks are designed the way they are, I’m just speculating based on public information. If you know more about TKP trucks, please reach out! I’m happy to update this article.



Sources

[1] Alex Georgoulis's "Changing Angles" blog

https://changingangles.com/2021/05/05/tkp-trucks-history-and-quirks/

[2] Jenkem Magazine Independent Interview https://www.jenkemmag.com/home/2018/05/08/look-cult-independent-trucks/

[3] Kinked kingpin patent https://patents.google.com/patent/US4166629A/en

[4] truck mods youtube tutorial https://www.youtube.com/watch?v=a7HxYk6es9E

[5] Skateboard turn-to-lean ratio depends only on the plate angle https://nelsonlongboards.com/blogs/blog/80421639-different-wheelbase-same-turning-radius-dial-in-your-next-setup-with-the-power-of-math

[6] The focusing illusion, coined Daniel Kahnemann, "Nothing in life is as important as you think it is while you are thinking about it" https://fs.blog/focusing-illusions/

[7] Rodney Mullen Interview https://blog.slamcity.com/rodney-mullen-interview/