Axle offset (aka rake): urban myths and skate companies

First published on a skate forum

Introduction

I guess it’s good that the skate industry is still relatively innocent and small. Big corporations have so far been kept at bay. But this also means there is no serious effort by anyone to check the veracity and consistency of various claims. When this does happen, the actual (as if this qualifier should be required…) truth holds no sway. Rather, it’s the more powerful actor who decides what is “fact.” Thus, through its innocence, the skate industry also is conductive to the perpetuation of myths about how a skateboard works.

The myth

Picture 1. Sabre’s graph

The other day (note: the article was written in January 2017 as a forum post) I was searching the internet about “rake” and speed wobbles and stuff and I stumbled upon Sabre’s webpage on “Truck Geometry.” It contained a couple of the misconceptions one sees in forums (to be fair to Sabre, their only “sin” is trying to communicate to consumers in more detail than other brands what -the general impression of skaters is about what- rake is and does). What was striking to me was the confidence they have to publish the graph (without the stamp, of course) in picture 1.

The most disheartening thing was the people commenting at the bottom things like “thanks for clearing it out for me” (since I wrote this article as a forum post on Pavedwave in 2017, the page has removed the Facebook commenting widget thing).

Picture 2. Tracker R-TX Offset; description on Sickboards’ website

I replied to one commentator his/her question about what rake is, and the company told me that if things weren’t as they claimed they are, then they wouldn’t have claimed it. The argument is not convincing because it is circular. I also pasted there the graph of the actual equation (picture 3). Sabre removed my otherwise respectful comment that the lean/turn function is universal (I didn’t keep a screenshot). Post-factual “truths” are thus established.

Sick boards, also is a victim of this myth. After reading the description of this truck (picture 2) , I sent them a message that there are unrealistic claims in that. I do hope they revise their page some day. (For more on this, check an article I wrote later.)

The reality

Picture 3. The actual graph of the lean to turn function (given also in the academic literature, but easily derived from basic euclidean axioms) for various pivot axis angles. The x-axis is the tilt of the deck (lean) and the y-axis is the rotation of the axle on the ground from rest.

Why is the function universal (the same for all single pivot trucks – RKP, TKP, torsion, whatever)? The geometrical explanation goes like this: the axle (however, wherever they may build it and at all times during skating) is parallel to the perpendicular line to the steering axis (or pivot axis – i.e. the imaginary line connecting pivot and hanger hole on traditional trucks) on the horizontal plane (btw since the steering axis cannot itself be perpendicular to the horizontal plane, there can only be one such line). Thus, the steering axis can’t turn more or less for the same truck turn; axis and axle are one piece. And if it did, then the deck would just have to lean more. Their relationship to each other is given, for a given steering axis angle.

I rephrase this, in simpler terms: the axle, by it’s construction, is solidly fixed to the steering axis of the truck. So (rather awkwardly put), it’s the wheels, as soon as they touch the ground that tell the axle how to rotate, then the axle tells the hanger how to rotate, then the hanger rotates too, and then the baseplate, fixed as it is on the deck, “looks” at the hanger rotate away below it. The wheels and the axle are together in this, while the baseplate and the deck are together (with you on top of them) doing their own thing, with the steering axis bringing it all together. Therefore, it’s the relationship between your feet and the wheels that matters. Or, seeing the whole scene from the frontlines, it’s the angle between the baseplate and the hanger reallythat matters. It’s all about how much the steering axis twists.
You might say: “but doesn’t the shape of the truck matter in any of this”? Sure it does, but ultimately the wheels call the shots, axle and hanger follow, baseplate and deck look at them go away as a unity.

Summary: The way to think about the lean/turn relation, is this: there are only two units: the things below the steering axis (or pivot axis), i.e. the hanger, axle and the wheels, and the things above the steering axis, i.e. the baseplate, the deck, the rider. Regardless of each unit’s shape (offset or not), their respective constituents are in static relationship to each other, they are fixed. So, how much the wheels turn determines how much the deck leans, and there’s nothing else to interfere with this relationship.

(I will try to clarify more if you want. Just let me know in a comment.)

Therefore, all the trucks and all the decks in the universe turn and lean in exactly the same way (for a given “truck angle” or, more accurately, steering axis angle). That’s just what skateboards are, no exceptions. That’s also how roller skates work.

There’s one exception to this: what if there are two (or more) pivots, like the Gullwing Sidewinder or Carver C7? Then it gets complicated: it’s like having two skateboards on top of each other. You can’t directly control the one at the bottom, it’s up to the “skateboard” that’s on top to push to lean the one at the bottom. Ask the top one’s bushings how much they can push to lean the bottom one. Find their respective leans and then, for known axis angles, you get your sum turn. But there’s no other exception: if you have one pivot, you fall under the universal law of the lean-to-turn relation, which is constant and known.

Obviously, different trucks with different rake (i.e, forward-offset, backward-offset, 0-offset), different decks (topmount, dropped etc) and wedges (flat and angled), result in different deck movements when turning. Some make the deck go towards the turn, some outwards, some make it drop, some lift it up. And this of course also changes according to the lean angle. But, no matter what happens laterally and vertically to the deck, any axle will, necessarily and for the same steering axis angle, turn the same amount for the same amount of lean, on each and every skateboard ever.

OK, with this out of the way, I must also clear Sk8kings of any insinuation that they claim their truck does something it couldn’t do. I read their ad carefully and it doesn’t say anything like this. They do, of course, imply they know exactly what each discipline requires, and I do doubt there are clear-cut specifications like that, but, meh, that’s social science and, even if they couldn’t/shouldn’t be too authoritative, I won’t try to disprove their claim either (although a case could be built against absolutism in things like that; I’ll try this another time, it’s perhaps too early now). Their ad is within the ethical limits of our consumerist culture, imho…:)

In sum

• Flipping the hanger, or in any way changing the axle position, does make a difference; though, sorry for repeating, it does so on everything except lean/turn.

• ‘Rake‘ is indeed not clearly defined in skateboarding and companies abuse it. If they borrowed the term from car engineering (as is my impression), it clearly is inadequate. Before we use it to talk about steering characteristics, we need to understand what the position of the axle actually does (and doesn’t do 😉 ). I think ‘offset‘ is far more expedient for now.

• Lean/turn is used by those manufacturers to talk about their trucks and that’s what started this thread. I made a joke above by mixing it with the circle radius of the skateboard, but that is a different and also more straightforward matter: wheelbase.

• Intuitively, I would say that when steering a skateboard lowers it (and keeps lowering it after the initial turn), it makes it more “divey” or, I propose, “turn inclined” (all other things -bushings, construction etc.- being equal). But this is just my hunch and I don’t trust those. We should totally look into this in more detail.

Conclusion

To the people at Sabre, if they happen to drop by these parts some day: it’s euclidean geometry from secondary education; we’ve all done that. You knew that at some point somebody would come along who would check the veracity of your text. I know it’s not fair to hold you responsible for widespread misconceptions such as this: you didn’t start it and you can’t be expected to stop it in a market where challenging your customers’ cozy preconceptions is riskier than fostering them. But at the same time you can’t expect not to be challenged from those who don’t run such risks. Ultimately, the customers will have to be challenged, one way or another, and if you’re not part of the solution…


Sabre did reach out to me some time in 2021. They took my criticism very gracefully after all and we had a nice exchange about the industry and the ways that are actually available to companies to communicate their products. I sympathized and I added a few sentences to reflect that. Ultimately, however, I don’t think I managed to make them understand what their factual mistake is.

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