# Skateboard physics: wheel lift

At the time of writing, wheel-lift is shrouded with mystery in the skateboard community. Some seem to think it’s due to hard bushings (partially correct but misleading, as we shall see), axle length (barely relevant), or truck make (entirely irrelevant). I will argue it’s essentially the difference in stiffness between the rear and front truck that causes wheel-lift. A shorter axle makes wheel-lift more probable by placing the possible axis of rotation of the wheel-lift closer to the middle of the board and therefore the rider’s weight further out from that axis. However, balanced front-rear truck stiffness would make wheel-lift impossible, by placing that axis entirely out of reach for the rider’s weight.
In what follows I pursue this point step-wise using basic mechanics. It may seem tedious, but please bear with me – or skip a few paragraphs.

## Definition:

Wheel-lift is the event when one or two wheel(s) of a moving skateboard lose contact with the ground when the skateboard turns (see right).
Let me break it down:
â€˘ A skateboard turns when it is tilted (i.e., when its rider leans towards the direction of the turn).
â€˘ The wheels that lose contact with the ground are on the outside of the turn (i.e., in the opposite direction of the lean & turn).
â€˘ “The wheel loses contact with the ground” essentially means that the hanger of the truck that the wheel is mounted on has stopped rotating as intended around the truck’s pivot axis. In other words, the hanger remains motionless in reference to the truck’s baseplate and the deck that the truck is mounted on. Consequently, beyond this point, as the rider leans and the deck tilts further, the wheel lifts off the ground.
So what exactly happens is:
As soon as the wheel lifts off, this wheel, as well as the whole closed system {deck, baseplate, hanger, wheel(s)}, does a circular motion with a certain axis of rotation. What’s that axis? This is where I want to distinguish between the only two possibilities.

## Two cases:

1) If only one wheel lifts off, then the axis of rotation is the imaginary line that connects the point where the opposite wheel of the same truck and the ground touch and the center of the axle1 of the other truck. On the picture on the right, the pink line is the axis of rotation.

2) If two wheels lift off, then the axis of rotation is the imaginary line that connects the two points where the wheels that remain on the ground touch the ground. On the picture on the right, that is again colored pink.

Which of these two cases occurs? I’ll talk about this right after the following bit on the sufficient and necessary conditions for wheel-lift.

## Conditions (or, what is happening just before the wheel lifts off):

1) During a turn, the truck(s) stops yielding to the deck’s tilt, either because the rider is not applying enough force to compress the bushings more (therefore, the bushings bring the hanger to a halt), or because the truck has reached another mechanical limit. (Incidentally, the force the rider is applying is the rider’s weight and the centrifugal force. Weight W is vertical to the ground, while the centrifugal force Cf is parallel to it and perpendicular to the direction of the deck; see image on the right). In any case, its hanger simply won’t pivot any more, but instead and of necessity the whole truck starts to tilt (with the deck) as one.
This is key to understanding what the necessary (but not sufficient) conditions are that result in wheel lift. Correct me if I’m wrong, dear reader, but I see no logical way for wheel-lift to occur before the truck stops yielding to the rider. If the truck keeps on yielding, the wheel necessarily remains on the ground.

2) The rider’s weight is applied on a point (sufficiently) beyond the potential axis of rotation I described in the two cases above. Notice again in these two cases, that the pink point at the bottom rail (which represents a possible point where the weight is applied) is further from the potential axis in the first case than in the second case.

## Observations

The above two conditions need to be met to result in the wheel(s) lifting off, but they need not be met simultaneously for both trucks. Indeed, most commonly, only one of the two pairs of bushings stops its truck from turning (when compressed enough during a turn), before the other one. At that precise moment, and if additionally the force from the rider is applied sufficiently far from the axis of rotation (1st case), the two conditions are met and the wheel lifts off. In other words, wheel-lift from just one truck is more common for two reasons:
a) The two pairs of bushings are rarely perfectly balanced with each other so that they reach maximum compression simultaneously.
b) Consequently, when the first of the two pairs stops being compressed, the axis of rotation in this case is located perfectly for the wheel to lose contact with the ground.

Conversely, to get wheel-lift from both trucks:
a) both pairs of bushings need to be balanced with each other so that they reach their limit simultaneously and
b) the axles of the trucks need to be narrow enough for the rider’s weight to be applied significantly outside the axis of rotation, which in this case is the two innermost wheels.

## Conclusion

To remedy wheel-lift it should be enough to balance the two pairs of bushings so that they reach maximum compression simultaneously, in view of the distribution of the rider’s weight on the deck (rider stance). In practice, that’s probably an impossible task, given the very rough measurements of bushing hardness, and the different rates with which bushings compress for different trucks. However, there are other things that work in favor of not abandoning this effort. Deck flexibility offers a bridge between small differences in maximum compression, by yielding correctively to the truck that keeps turning. More importantly, the closer to perfect balance one gets, the less probable 1st-case wheel-lift becomes.

Axle width, on the other hand, is far from significant. In the 1st case, the axis of rotation for a wider axle would move (in comparison with a narrower axle) only a few millimeters towards where the rider’s weight is, but still centimeters away from it. In the second case, it would indeed make more of a difference, particularly if both axles were wide, because the rider’s weight is applied millimeters away from the axis. Notice however that, precisely because the axis of rotation is anyway located so close to the rider’s weight, it is improbable that the wheels would lift off at all. It rather seems more probable that the deck will simply stop yielding to the rider’s lean with all wheels firmly on the ground.

1. That’s assuming a 0 axle offset, but generally it’s the point where axle distance from PA and PA intersect.

## 6 thoughts on “Skateboard physics: wheel lift”

1. Rich Austin-Berry says:

Really interesting read! Something Iâ€™d love to analyse in more detail and write a paper on. I think the video of wheel lift off picks up on the point that lift off is biased to the riders stance. I think naturally that weight distribution is heel bias. I would not expect single wheel lift off to occur all the while the system (board and rider) centre of gravity (CoG) occurs midway about the front and rear truck. As the CoG shifts, the force increases through the most local truck compressing the bushes, and decreases at the furthest, relaxing the bushes and causing lift off. However, despite more heel weight bias, more lift off would occur turning toe side. We can lean further forward than backward, thus shifting CoG further out and reducing the force through the bushes. And then, thereâ€™s riders skill which can re-check all of the above!

1. I see where you are going with this; it’s very interesting and I’d love to see a write-up from you! You can indeed make a model more complex and talk about more things. The important thing that I wanted to convey here with this simple model (in which the rider is just a single dimensionless point) is that wheel-lift always, necessarily, happens when the two trucks have unbalanced bushings (or springs, or Torsion Tail legs). If they were balanced (for any given rider stance), the rider wouldn’t even need to adjust stance.

2. dranoel says:

Hi,
I have been reading your articles and I really love them! Thanks for putting the time and effort into the LDP scene with documentation!
Might I suggest writing about wheels and how it affects pumping in your next article?
How thicker wheels affect pumping like Boas vs Speedvents. How diameter affects it, and lastly hardness.
Thanks !

1. Thanks for the comment and your kind words! Thank you also so much for the suggestion! And you are absolutely right: a thorough post on wheels is sorely missing from this site. I know and it annoys me too, but unfortunately I don’t have the confidence to write one. I lack the physics and chemistry knowledge that it probably requires.
With the confidence I do have, I can only suggest the following:
1) Try to be sceptical about the hype around “special,” or “advanced” or “homegrown” PU formulas. I’m pretty sure (though not 100%) that all the companies buy their first materials from a handful of factories. Build quality, processing, handling imperfections etc etc, sure, there are important differences. But hi-tech formulas? Nah, that’s probably BS. See also: https://changingangles.com/2019/10/02/polyurethane-pu/ and https://changingangles.com/2019/06/19/about-bushings/ .
2) I don’t see any serious disadvantage in bigger wheels (in dimensions and in mass), except weight. If you can afford them and if they don’t cause wheel-bite, get the biggest ones.
3) I don’t see how width has to do with traction. I’d like to say this with more confidence, but science says that wheel softness (well, “friction coefficient”) is the only factor relevant. Not area of contact. See here and judge for yourself: https://en.m.wikipedia.org/wiki/Friction#Laws_of_dry_friction
4) I’d like to say that harder wheels always go faster, but, as they recently discovered in cycling after decades of being convinced that harder is always faster, that’s not always the case (because energy and therefore speed is also lost in vibrations). Depends on the surface you are skating on and your weight. What’s more, it’s really hard to figure out what softness of wheels would be faster on your surface (and your own weight). I’d say just get the harder ones that don’t annoy you with vibrations. I guess you just have to try a few different ones.

That’s all I got I’m afraid đź™‚
Thanks and take care!

1. dranoel says:

Thanks for your reply, I presume in terms of watching loads of f1, the wider the wheel, the more the contact with the road and hence increasing grip.

2. It would seem like that, wouldn’t it? Intuition versus reality…
Like I said: read the science carefully (see link to accessible Wikipedia article above) and judge for yourself. I can’t answer you with any more confidence; this is unfortunately beyond my comfort zone. You could also search why F1 wheels are like that. Please let me know if you find something different!

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