The unicycle is a human-powered vehicle with one wheel. It is a descendant of the penny-farthing bicycle. Originally used for performances, unicycles are also used for pleasure. People work on various unicycle tricks, play games like unicycle hockey and basketball, and do mountain unicycling.

Definition:

A unicycle is an HPV¹ (human-powered vehicle) with only one wheel in contact with the ground. It is the wheeled equivalent of the pogo stick.

A standard unicycle has a wheel of 20 or 24 inches in diameter. Often there are more spokes than a bicycle wheel, as more weight needs to be carried. In the centre of the wheel is a hub, and unlike a bicycle wheel, a fixed spindle. The bearings appear in the forks, rather than the hub on a unicycle, and the cranks and pedals are attached to the square-cut ends of the spindle.

The forks meet to create a downtube, into which a saddle stem is placed. The saddle is not a standard bicycle seat, as this would too easily slip out behind the rider; it must have some structure in front of the rider also, and typically resembles a banana.

History:

The history of unicycles has not been recorded fully, but it is thought that trick cyclists on penny-farthing bicycles would ride with the small rear wheel lifted off the ground. This is made possible by the fact that the driving wheel is fixed to the cranks.

It would then be a simple step to do away with the rear wheel altogether, and simply ride this single wheel. Circus pictures from the past show this kind of device.

Variations:

The most common variations are in wheel or crank size. Larger wheels (26 inches for 'mountain unicycling', 40 inches for distance travelling) allow a much smoother ride, making bumps less noticable; but if enough leverage is required, the cranks must be longer also.

Short cranks are fitted where speed or distance is the main objective, and the route is known to be smooth and flat.

Giraffe:

A Giraffe unicycle, is like its animal counterpart, rather tall. The pedals and hub are linked by a chain. Since the pedals must be above the top of the wheel, the minimum height for a giraffe is approximately 5 feet from floor to saddle. The upper limit is only bounded by practicality, and Steve McPeak rode a 31 metre-high example (with safety equipment).

Multiple wheels:

Multiple-wheeled unicycles tend to stack wheels on top of each other, the most common being a three-wheeled example. In this case, three wheels are built in line, with the rider powering the top wheel as they would normally ride. The middle wheel is driven by contact between wheels and rides backwards. The bottom wheel is thus driven forwards. Apart from the extra effort to overcome friction, this is much like riding a standard giraffe of the same height.

Other examples include a four-wheeled unicycle, with the wheels in a diamond pattern, and thus 2 'intermediate' wheels running backwards.

As we get into harder territory, we see a two-stacked-wheel example; the driving wheel must be ridden backwards to drive the bottom wheel forwards.

Then as we enter the ridiculous, we see a 1.5 wheel version. The driving wheel is only half a wheel, and so the wheel on the floor is driven only half of the time, and the half-wheel must be rapidly pedalled back round into contact, lest the rider fall off.

Eccentric wheels:

It's possible to build an eccentric wheel, where the hub seemingly bounces up and down as the rider travels along. When this is combined with turning one crank, so that they both point the same way, you have a kangaroo. The rider pushes on both pedals to drive the wheel half-way round, then the weight of the rider on the hub completes the cycle.

Care:

Generally ridden indoors, unicycles require little care and attention, other than cleaning moisture or dust off occasionally. They are a very simple beast, and the only truly mechanical attention should be greasing the bearings occasionally. If ridden outside, then care should be as for a bicycle.

Due to the fixed wheel and the nature of riding, the tyre can wear in a single spot, or pair of opposite spots. To avoid this, the tyre should occasionally be deflated slightly, rotated on the rim a few degrees (taking care not to disturb the inner-tube), and reinflated. This is often unneccessary on a giraffe, as the gearing between the pedals and wheel is often chosen as close-to, but not quite 1:1. A 19:20 gearing will feel like 1:1, but the hotspot of the tyre will change.

Activities:

Among the activities enjoyed by unicyclists are:



¹ Actually, a motorised monocycle was built, in which rider and petrol engine were suspended inside a huge wheel.

Sources:
Many years of personal experience.
http://www.unicycling.org/unicycling/btdt/unique.html

The unicycle actually predates the "safety bicycle", the basis of most bikes today. Penny Farthings had a somewhat unnerving tendency to flip over forwards easily, because of the rider being so high up and with little weight over the back wheel.

Some riders learned to lessen the chance of being launched in such a dramatic manner by riding it with the back wheel off the ground; when braking the rider retained the "buffer zone" of being able to lean back, unlike riders using both wheels. Inevitably this led to people removing the back wheel altogether, and simply riding the remaining wheel by standing on the pedals and holding on to the handlebars.

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