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Monowheel science
 Gyroscopic stability
Power, braking and steering
Pros and cons
What? How? Why? All fair questions when it comes to one of Scrapheap's more unusual challenges ...
What? A monowheel, as the name suggests, is a large single wheel in which sit both engine and rider. How? The explanations below should shed some light. Why? Well ... why not?
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| Trying to master gyroscopic stability ... |
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| ... could do your head in |
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Gyroscopic stability
As you might imagine, riding a monowheel takes some practice. Balancing on one wheel is difficult, but it is possible because of gyroscopic stability. Gyroscopic stability or inertia also cropped up in the Blind Navigation challenge.
In order to understand the influence of gyroscopic stability on riding a monowheel, think about riding a pushbike. While stationary on a pushbike, if you take your feet off the ground you will fall over. And if you pedal very slowly, it's difficult to maintain balance. However, as speed increases, balancing becomes much easier. This is gyroscopic stability in action. Gyroscopic stability is created when a mass rotates – in the case of a bicycle or monowheel, the wheel rotates as you move forward.
Gyroscopic stability can also be demonstrated using a penny. It's quite difficult to stand a penny on its side and the penny can easily be knocked over. However, if you roll it across the floor, or spin it, a penny balances very well. See More Info for a link to a detailed explanation of how gyroscopes work.
In a way monowheels are a bit like roller-coasters. You can think of the outside wheel of the machine as circular piece of track along which engine and rider travel. Both teams use a system of guide-wheels or coaster-wheels to keep the inside frame, containing engine and rider, in place.
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| The Hammerlocks opt for friction drive using a scooter's engine and
rear wheel
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| The Tunnel Ratz use a moped as a power source, but transfer power to the wheel via a chain and sprocket |
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Power, braking and steering
The Hammerlocks opt to use a scooter engine and rear wheel to drive their machine. The scooter wheel is fixed so that it is in contact with the large outer wheel. As the moped wheel turns, the friction between the two wheels causes the large wheel to move forward. This is known as a friction drive.
The Tunnel also use a moped as a power source, but transfer power to the wheel via a chain and sprocket. They weld a long length of chain around the inside of their wheel and replace the rear wheel of their moped with a sprocket. This sprocket sits in the chain and drives the wheel.
When accelerating and slowing down on a monowheel, engine and rider have a tendency to start to climb up the back (when braking) and front (when accelerating) of the wheel. From a standing start, as power is transferred to the wheel and it begins to turn, friction between the coasters and the wheel cause the engine and rider to travel up the front of the wheel, and their combined weight gets the wheel turning (think of a gerbil on a wheel inside its cage). The centre of gravity has moved forward, beyond the contact point between the wheel and the ground. The centre of gravity is an imaginary point at which its whole weight can be considered to be focused or 'act'. The weight of an object acts straight down in a vertical line through the centre of gravity. So if the centre of gravity moves away from the contact point between the wheel and the ground, it will push down on the wheel beyond the contact point, causing it to turn.
At a constant speed, equilibrium is reached, and engine and rider assume a relatively stable position at the bottom of the wheel. When slowing down, the reverse happens and engine and rider travel up the back of the wheel.
Braking too quickly in a monowheel can be very dangerous. Brakes work by applying friction to a wheel. On a monowheel, if too much friction is applied, engine and rider can lock to the outer wheel and start spinning around with it. This rather unpleasant predicament is known as 'gerbilling'! In the event, neither of the teams reach a speed at which this is a danger.
Monowheels don't have a steering mechanism as such. To turn a corner, the rider must shift his or her body weight in the direction they want to go. The act of leaning moves the centre of gravity to one side, causing the wheel to fall away in that direction. If the wheel is stationary, it will fall over, but as it is rotating, it keeps its balance because of gyroscopic stability. Once a turn is made the rider must quickly centralise his or her position.
The stability of a gyroscope varies in proportion to its weight and the speed at which it is rotating. The faster and heavier the wheel, the more stable it is and the more it wants to go in a straight line. So a heavy wheel is more stable, but also more difficult to turn.
The relationship between speed and stability explains why it's much easier to ride slowly on a BMX that on a large-wheeled bike: the BMX wheels rotate more quickly at a given speed.
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| The Hammerlocks opt for a tractor wheel. This saves time. |
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| Fabricating a wheel from scratch is a high-risk strategy. It must be perfect. |
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Pros and cons
Hammerlocks
Using a pre-manufactured wheel saves build time.
Friction drive is a straightforward and reliable way of propelling a monowheel.
Having a 'Psycho' drive your machine seems to help!
On the other hand ...
The size and weight of the agricultural wheel the Hammerlocks find make their machine very difficult to steer, though they partially compensate for this by filing all the tread off the tyre, making it easier to lean the wheel over.
Tunnel Ratz
The Ratz construct a strong, light wheel, which is the right size for their machine.
They make good use of their tinkering time, changing their guide sprockets for rollers. This means they only have one sprocket mechanism to worry about.
On the other hand ...
Fabricating a wheel from scratch is a high-risk strategy. It must be perfect.
Using a chain and sprocket drive is also high-risk. A chain and sprocket system is a lot less forgiving than friction drive, because the sprocket can easily part company with the chain. Chains and sprockets have proved the undoing of more than one Scrapheap hopeful before now.
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