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How does a skateboard turn?


LanghamP
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The answer is obvious; you look thataway and it goes there. Same with any other vehicle.

Yesterday I was watching a younger guy on the One-wheel contraption trying to guide his wheel up a curvy bicycle path going uphill. And he was not easily able to. The One-wheel rode like the Titanic; it wasn't turning. Only by a series of jerking movements could he get it turning.

We who ride Wheels know that tipping the wheel over or countersteering the wheel is how to steer the wheel quickly and easily, with very little energy.

But then how do skateboards steer? I think people look at One-wheels (I've never ridden one) and treat them like skateboards. I think they look like skateboards but they are not; they must be EUCs and therefore using skateboard techniques on a One-wheels isn't going to work.

I think.

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41 minutes ago, LanghamP said:

But then how do skateboards steer?

The actual mechanism is of little concern really, the result is leaning turns it. 

In the case of a skateboard, it is the design of the ‘trucks’ (the mechanism connected to each wheel axle) which causes those axles to turn when the board is leaned http://www.exploratorium.edu/skateboarding/skatedesigntruck.html effectively giving you 4 wheel steering.

With a one-wheel it really depends how close to the original design the one you saw was. The actual one-wheel has a wide, golf buggy like, tire. Leaning presses down on one side of the tire, causing extra drag on that side, plus some gyroscopic forces (see below) causing it to turn. If the tire pressure is too high it will dramatically reduce that turning ability.

Some one-wheel clones have a narrow wheel in the middle of the board. Those will turn more like an EUC, and strike me as being much more difficult to balance and turn than the original.

As far as EUC’s and bikes are concerned it is a mixture of forces including gyroscopic forces that turn it. Basically a gyroscope acts 90 degrees further around the circle so if you lean a wheel over to the left (think of it as pushing the top of the wheel to the left) then the force acts 90 degrees further I.e. at the front of the wheel and pushes the front of the wheel to the left turning the whole thing left. This is why at slow speeds, when the gyroscopic forces are much reduced, on either a bike or EUC, The wheel has to be twisted to the left, not leaned.  Think of riding and balancing a bicycle at speed, you just lean, and do not even need to have your hands on the handlebars. At very slow speed even to balance you need to be quickly moving the handlebars left and right to stay upright. That is equivalent to the twisting hip action you need at slow speed on an EUC.

Additionally, leaning over, say to the left, on a rounded bike/EUC tyre means the left hand side of the tyre in contact with the ground has a smaller diameter, and hence circumference, so one revolution of that left part of the tyre will be shorter then the middle of the tyre, also pulling the wheel to the left. 

Edited by Keith
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I have a OneWheel and i have to say that in some aspects its superior to what we have an in some other aspects its inferior. My guess is that you probably saw someone who was either new to OneWheeling or was riding a knockoff which there are numerous now and they are not well built at all. The way they turn is how we accelerate and decelerate. Their acceleration and deceleration is much more weight shifting than what we have because they cannot "push with the toe" otherwise they risk shutting off the gyroscope due to the sensor not reading a full foot on the board.

I ride skateboards and the OneWheel rides like a skateboard. However, it should be noted that it turns better than a regular skateboard or an electric skateboard (I've owned those as well). 

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Like a skateboard, the OneWheel and twin-wheeled EUCs (wheels on the same fixed axle) turn by sidewards leaning/tilting. Yet the mechanism for the skateboard is different and as described above. By sidewards leaning/tilting a OneWheel or a twin-wheeled EUC, the circumference of the inner wheel or the inner part of the wide tire becomes smaller. That is, the inner part travels over less distance than the outer part for each revolution, hence the wheel turns. That also means that tyre pressure will have a relevant influence on turning behaviour.

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1 hour ago, Mono said:

By sidewards leaning/tilting a OneWheel or a twin-wheeled EUC, the circumference of the inner wheel or the inner part of the wide tire becomes smaller.

I was actually VERY careful to NOT say that as that theory is actually quite controversial.

There have been innumerable discussions on this, usually with respect to the idea that lowering or raising the pressure in your car tyres will dramatically change the reading on your speedometer as the radius of the tyre where it touches the road can change by a significant degree.

The argument is that the outer tread of a car tyre (and that would also go for the one-wheel tyre) is actually a fixed circumference and therefore in one rotation of the wheel that tyre circumference must also do one revolution and the wheel must travel that fixed distance. I.e. regardless if the  tyre is under inflated that tread must still rotate the same distance, it should be thought of more like a tank track and absolutely not as 2xPi x tyre radius.

I, personally think that this argument has validity.

Edited by Keith
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On 12/5/2017 at 12:09 AM, Keith said:

There have been innumerable discussions on this, usually with respect to the idea that lowering or raising the pressure in your car tyres will dramatically change the reading on your speedometer

The amount of expected change is quite simple to calculate geometrically, and I doubt it will be a dramatic change, 5%-ish seems a good ballpark number, I doubt it can ever be 10%. That's just an exercise in how good my intuition is, as it just can be checked and (either way) refuted easily.

On 12/5/2017 at 12:09 AM, Keith said:

The argument is that the outer tread of a car tyre (and that would also go for the one-wheel tyre) is actually a fixed circumference and therefore in one rotation of the wheel that tyre circumference must also do one revolution and the wheel must travel that fixed distance.

Interesting point. The argument seems to assume, and require, that the tire material cannot expand or shrink. What is also suspicious to me is that there is an argument at all: it is just too easy to actually measure how far the wheel travels with a single circumference under different load and/or different tire pressure. That means, we know what the truth is, it just can't be controversial, we only may not know why this is so. Almost certainly I will make a little experiment, even though I am convinced Google would reveal the truth as well.

Edited by Mono
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Reporting back a small experiment on a 14" CST tyre: reducing the tyre pressure to 20PSI reduced the distance traveled under very moderate weight (~30kg) roughly from 110cm to 107cm. Less weight gives lesser reduction. That is, tilting a two-wheeled EUC will make the inner tyre travel less distance. The resulting radius is, when I am not mistaken, roughly the gap between the wheels divided by the ratio between the larger and the smaller travel distance minus 1. With a 9cm gap it would be 9cm / (110cm/107cm - 1) = 321cm.

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Very wide and poorly as I found out first hand from riding with Boosted Boarders. Usually on a regular skateboard you would kick up the front end and rotate on your back wheels, but you can't do that with these longer and heavier electric longboards. So the boosted boarders had to stop, get off, and turn the skateboard by hand every time we went from the street to sidewalk.

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