GBomb’s Torsion Tails: a closer look

Introduction

Diagram 1. Torsion Tail (TTX) parts (source)

The Torsion Tails are a line of rear trucks made by GBomb specifically for platform decks. They are the most experimental product that GBomb markets and they are a work-in-progress. It is difficult to design a truck like that and be sure beforehand how it will behave in practice, but the first iterations were apparently successful and promising enough for GBomb to continue development. At the time of writing, the latest model is the TTX (picture, right).

The Torsion Tails consist essentially of three parts: the base-plate, which is attached to the deck; the axle, on which the wheels are attached; and a pair of arms, which connect the base-plate with the axle. The arms are long and slim and their flexibility constitutes the truck’s restoring mechanism.

Diagram 2. TTA (source)

There’s a number of misconceptions online about the Torsion Tails that I’ve been wanting to address for quite some time now (on “zero degree” trucks in general, see here). The following text is, I admit, not very ambitious and doesn’t go far beyond that goal. The assumptions I needed to make in order to complete the model, on which I based this text and illustrations, render the model highly abstract; reductive, even. However, I think it’s sufficient for demonstration purposes. What follows is a couple of observations based on this model, as well as a few other things collected from the internet.

Background

The Torsion Tails haven’t been around for too long. I think GBomb started marketing them around 2016, give or take a year. GBomb hadn’t entered the truck-making business before marketing the Torsion Tails, presumably because other makers do a good job at it already. However, I feel the primary reason is to not upset other players, with whom GBomb wants to stay in good terms. (If this topic interests you, you may want to listen to an interview GBomb’s boss, Mark Groenenboom gave to the IDSA‘s LDPCast podcast. At 52:10 in particular, the interviewer jokingly alludes to the possibility GBomb avoids marketing trucks so as not to “piss off” Dan Furrer from Dont Trip. Mark partially confirms that “he works well with Dan” and he doesn’t want to “get too much into” Dan’s market. In any case, the interviewer’s joke is revealing: everyone is scared of Dont Trip’s boss.)

GBomb’s interview to the IDSA’s LDPCast

Whatever the reason may be, I believe that if GBomb decided to have a go at truck-making, it would do a good job and it would help the sport progress technologically. What it has produced so far has been genuinely innovative and thought-through and it will be beneficial for everyone, if the circumstances permit it, to create its own line of trucks.

Not a 0° truck

Screenshot 1

The TTX, or any of the Torsion Tails for that matter, does not have a 0° pivot axis angle (PAA). That’s by design: if it were exactly 0°, then the base-plate would scrape the ground, because, as we will see in the following section, all TTs lower (they drop) when they are tilted. The TTX specifically, according to the company website (see screenshot), has a PAA= -6°, or at most -5°, considering flex due to rider weight and the centrifugal force.

Torsion Tail tilt/turn
Picture 1

I’d like here to explain what it means that the TTs are negative PAA trucks and to show that their axle turns when the board is tilted. First, I need to state some assumptions/simplifications: even though the arms of the TT bend (flex), otherwise the wheels would lift immediately, they are not noodles: their straight-line, end-to-end length is at all times not too far from their length at rest. So, the main assumption here is that the straight-line length of each arm remains approximately constant. Another simplification is that we will consider bending only due to tilt/torsion, not due to centrifugal/weight. More about the latter in a following section.

Picture 2

So, what happens when the rider tilts the board? When the deck is tilted, so does the base-plate and therefore also the base (roots) of the arms. Assuming no wheel-lift, both the arms’ ends (tips) remain in contact with the ground and, to do so, they must rotate against the base around their pivot axis. Their pivot axis is the line that connects the midpoint of the roots with the midpoint of the tips, which is the midpoint of the axle, of course (the dashed line in picture 2, right). As mentioned, that line is approximately 6° (or a bit less) with the ground. So, the axle of the TT has to rotate in the same direction as the front truck’s axle. It can’t not do that; the arms of the TT don’t magically shrink.

Torsion Tail axle turn when deck is tilted
Picture 3

This is not a 0° truck; its axle turns (picture 3). This turn is not as predictable as on regular single-pivot trucks (it depends on the PAA which changes during the turn, as well as on the bending and twisting of the arms in directions and ways not examined here), but it exists and the consumers should not be mistaken about this. I guess GBomb avoids being explicit about that on its website for marketing purposes; as I wrote elsewhere, “zero” carries lucrative hype.

When it turns, it lowers; the arms also bend inwards

Picture 4. The image shows how I measured what I describe in the text. α and β are the deflections from rest of the two arms. I just tried to find another position for the left arm (α) so that the sum would be smaller, but there wasn’t any, really. I don’t know what the locus of the positions that minimize sum deflection is, but it seems the arm that is higher needs to stay at its original position. This is consistent with logic: once the rider’s weight moves to one side (and centrigual force is added in the mix) to tilt the deck, the other side returns to its original state (indeed, by slightly lifting the deck up on that side).

In the previous section I talked about the PA of the TT. This might have left the impression that the TT is a simple single-pivot truck. It isn’t. The truck lowers when it is tilted and this changes its PAA; the PA gets closer to being parallel with the ground. So, unlike regular trucks, its PAA changes while turning. But let’s just talk about this lowering: how and why it happens.
One extra assumption in this section is that each arm’s deflection from rest is directly proportional to the force required for that deflection. If the arms were springs, it is a fact that compression is proportional to the force, but I’m not sure what is the deal with elastic bars, plates etc. I’m confident though it’s not a bad approximation, for our purposes here, to assume deflection is proportional to force.
Keeping in mind that, for a given tilt from rest, the arms of the TT must necessarily be deflected as little as possible (this is required by logic: the arms can’t be deflected in a way that their total deflection is more than needed to counter the force that deflects them; applying a force to tilt the deck will tilt it as much as the total deflection permits), we conclude that the least possible deflection happens when the arm that is higher from the ground (the one outside the turn) remains at its original, unweighted position, while the other arm bears the brunt of the tilt. This means that each time the board is tilted, the rear end lowers towards the side of the turn.
Another point that is relevant here, is that the arm that is inwards the turn is deflected not only upwards from its original position, but also inwards. Why? The arms must bend inwards, towards each other, because of the TT‘s built-in constraints (i.e., the two tips must remain at a constant distance to each other). This necessary inwards deflection, however, happens only to the inward-the-turn arm, because, again, for total deflection to remain minimal, the outward arm must stay undeflected.

Torsion and bending

Video. Bogbugboards’s footage and on-screen analysis. Lo-fi capture from Instagram. Go ahead and check out the original post for a higher definition video and the user’s analysis.

Fascinating footage taken by Instagram user bogbugboards shows that the arms of the TT not only bend, but also twist, when the TT is tilted. This means that the bushings that the TT is equipped with don’t always engage. It seems to me from the footage that the arm that doesn’t bend (the one outwards the turn), twists more than the other one. It’s a bit too chaotic to derive a theory about what’s happening (and I bet not even GBomb knows; Mark just goes by intuition and incremental corrections), but I guess that the outwards arm is less tense, so it is easier for the bushing to twist it.

Bushings

Screenshot 2. Melonenkacke’s post on the topic.


One short point about the bushings: since the tips of the arms are constrained to remain at a constant distance to each other, while due to the geometry of the turn they tend to go away from each other, the middle bushing is actually unnecessary. This point was proven in practice by melonenkacke (screenshot), who thinks it actually made her TTX easier to tilt (which is highly plausible, considering that the inwards-the-turn arm, which apparently twists less, would squeeze the middle bushing more; but the middle bushings is gone, so it is possibly easier for the arm to bend even more and avoid squeezing the outside bushing as well). Notice however that you can’t do this on previous TT versions, because the square bushings will slip out (check Bogbugboards’s video). The TTX addressees in this way an obvious weakness of its predecessors.

TTX

The TTX has one more thing that is different to its predecessors: its arms are closer together (please compare diagrams 1 and 2). Therefore, they don’t have to bend as much. The closer together the arms are, all other things being equal, the less hard to tilt a design like that. Of course, not all things are equal. The TTX‘s arms are redesigned and look like they are rounder and thicker that other TTs. That would make them harder to bend and twist.1 GBomb maintains, however, that overall the TTX is indeed less stiff than its predecessors.

Conclusion

It is out of my reach to model this design with fewer simplifications and to predict what actually happens with more certainty and detail. In reality, the rider’s mass, muscle and centrifugal forces bend the arms in other directions as well (in addition to “torsion,” i.e., rotation around the PA, which I examined above). Riding style and conditions must also play a part in this. Generally, I believe that the lighter riders have to make do with shorter pumps, because they lack the force to bend the arms more. That has got to be somewhat limiting. That said, to see the glass half-full, it might also account for such riders developing fast-paced techniques that could actually be conductive to long-distance skates. Given the size of the company and its resources, however, one can’t seriously believe that the product was developed with a wide range of riders and applications in mind. It is a happy accident that it actually works for many people, though once again this might be testament to the power of peer pressure and confirmation bias, rather than the product’s (lacking) versatility.


1. Thanks to Bogbugboards for pointing that out.

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