A unicycle is a single-wheeled vehicle traditionally used during circus performances. It consists of a spoke wheel, pedals, and a tube shaped body attached to a seat. Unicycles are made like bicycles; individual parts are produced separately then pieced together by the manufacturer.
The unicycle's history began with the invention of the bicycle. Comte De Sivrac first developed bicycles during the late eighteenth century. His device, called a celerifere, was a wooden horse that had two wheels joined by a wooden beam. In 1816, wooden, riding horses like these had become improved by the addition of a steering mechanism. In 1840, Kirkpatrick Macmillan introduced a mechanism for powering the hobbyhorse with his feet. During the 1860s significant progress was made with the introduction of rubber tires, metal spoke wheels, and ball bearing hubs. During 1866, James Stanley invented a unique bicycle called the Penny Farthing. It is this vehicle that is thought to be the inspiration for the unicycle.
During the late nineteenth century the Penny Farthing was a popular bicycle. It had a large front wheel and a small rear wheel. Since its pedal cranks were connected directly to the front axle, the rear wheel would go up in the air and the rider would be moved slightly forward. This likely prompted riders to see how long they could ride with the back wheel in the air and the unicycle was born. Evidence for this theory of development can be found in pictures from the late eighteenth century that show unicycles with large wheels.
Since the unicycle requires a greater degree of skill to ride than a bicycle, many people that could ride them became entertainers. Over the years, unicycle enthusiasts have inspired manufacturers to create new designs such as seatless and tall, giraffe unicycles. During the late 1980s some extreme sportsmen took an interest in the unicycle and outdoor unicycling on rugged terrain was born. Today, the unicycle remains a relatively obscure vehicle however there are more people riding unicycles now then ever before.
A unicycle is a single-wheel vehicle. Typically, it consists of a seat attached to a frame that is attached to the wheel hub. Seats are available in a variety of sizes and shapes. Unlike a bicycle seat, they are often curved slightly downward and symmetrical. They are padded and may have "bumpers" on each end to protect them during one of the rider's inevitable falls. Some seats are designed with handles on the front to enable the rider to do various tricks.
The seat is attached to the frame via the seat post. The various types of posts that are available can differentiated by the way they allow for height adjustment. The most basic seat post is a metal tube with holes drilled at regular intervals. There is also a hole in the main body through which a bolt is placed to hold the seat post. To adjust the seat higher or lower, the bolt is removed, the seat is adjusted to the next hole and the bolt is replaced. A more sophisticated seat post has no holes but is attached to the frame through a clamp. The clamp is loosened or tightened with a blot or Allen wrench screw to adjust the height of the seat post. This design al-lows for a finer adjustment of the seat height. In addition to these differences some seat posts allow for angle adjustments.
The unicycle frame is a metal structure with two forks that attach to the wheel and a hollow tube that connects to the seat post. Different designs are available. The simplest has flat forks bolted together above the wheel and attached to the seat post. More sophisticated versions of this design have curved forks for better rigidity. A better design is the tubular one-piece body with a squared or rounded fork crown. These structures have less points of weakness and are more durable.
The frame is attached to the center of the wheel along with the bearings, pedal cranks and the spoke hub. The bearings are sealed balls of steel that reduce friction. The pedal cranks are attached to the bearings, and at the end of the cranks are the pedals. The spoke hub is the area where each of the wheel spokes are attached. The hub can have straight or angled flanges that attach to the spokes and have a cottered or cotterless shaft.
The spokes are thin metal tubes measuring in thickness from 0.08 to 0.125 in (0.2-0.32 cm). The number of spokes on a unicycle varies depending on the design and can be from 28 to 48. In general, more spokes are better. The spokes can be arranged in different patterns on the wheel. For example they can have a three or four cross pattern. Additionally, they can have an interleaved pattern that adds to strength and stability. The spokes are attached to the tire rim through a series of holes. These holes can be straight or angled to match the angle of the spoke.
The final component of a unicycle is the tire. A standard unicycle tire has a round cross-section and a flat or smooth tread. This design is ideal for riding on flat surfaces. Some outdoor unicycle tires have thicker treads that are better for rugged terrain. The size of a standard unicycle tire is 26 in (66 cm).
In addition to the standard unicycle just described, other designs are available. The Giraffe unicycle is a taller vehicle that is chain-driven like a bicycle. While standard unicycles have a height of about 3 ft (0.91 m), Giraffe unicycles can be 15-20 ft (4.6-6.1 m) tall. The record height for a Giraffe unicycle is about 100 ft (30.5 m). Another type of unicycle is called the Ultimate Wheel. This version has no seat or body consisting of only a wheel and pedals. The spokes are typically replaced by a plywood disk to reduce injury to the rider's ankles. The Impossible Wheel consists of a wheel and side posts that the rider stands on. The challenge to this unicycle is figuring out how to propel oneself. Finally, a rare type of unicycle is the monocycle. This design consists of a large wheel with the seat on the inside.
Numerous raw materials are used to create a unicycle. Since the main body must provide strength and rigidity but also remain lightweight, it is typically made of a steel alloy, aluminum, or titanium. Steel is a material made up of primarily iron. Other metals that can be incorporated include aluminum, manganese, titanium, tungsten, vanadium and zirconium. During the 1990s, composite materials such as carbon fibers were introduced and are now sometimes used to make unicycle frames. Depending on the frame material it can be protected with various coatings including a baked-on enamel, powder coating or chrome plating.
Metal alloys are also used in the construction of the rim, spokes, pedal cranks, hub, and seat post. The most basic rim is composed of chromed steel. More sophisticated structures use a steel alloy or chromed steel alloy. A typical spoke is made of zinc plated steel. The may be also made of stainless steel or chrome-plated steel.
Other components of the unicycle are made using various materials. Pedals are made out of rubber or plastic. A basic pedal has a solid rubber, block construction with no spindle adjustment. Better pedals are made of a solid plastic with an adjusting device to fit different sized riders. The seats can be made from various materials such as leather or vinyl. They can also have a polystyrene or polypropylene padding.
The production of a unicycle is typically done in two phases. In the first phase the individual components are made. Separate manufacturers that specialize in a specific component typically do this. In the second phase, the unicycle manufacturer buys the components and assembles them.
Creating the frame
- 1 Seamless tubes for the frame are made from solid blocks of steel. The steel is heated and molded into a cylinder shape. This cylinder is put in a furnace making it white-hot. It is then rolled under high pressure. The rolling stretches or draws out the steel and a hole forms in the center. A bullet shaped piercer point is pushed through the middle of the steel as its being rolled. After the piercing stage, the tube is passed through a series of rolling mills to correct any irregular shape or thickness.
- 2 To fashion the tubes into a unicycle frame, they are first heated until softened. This allows the tubes to be bent and shaped into the appropriate structure. In one type of unicycle design, the forks are flattened and crimped at the end to seal them. This end is also welded to provide greater stability and strength. A hole is drilled at the crimped end to provide a place for the frame to be attached to the wheel axle. This process can be done on a machine or by hand. The piece of the frame that attaches to the seat post is drawn out to the appropriate thickness using rollers and then cut. The three frame pieces are then joined together in a metal coupler that has two holes that attach to the forks and one hole on the opposite side in the middle that attaches to the third piece. These links are welded closed.
- 3 The frame is put on a jig and checked to ensure that it is aligned and shaped properly. Since it is still hot, subtle adjustments can be made. Excess flux and brazing metal is cleaned off and the welds are ground smooth. After the metal is cooled, other adjustments may be made.
- 4 To protect the metal frame and give it a more appealing appearance, it is typically coated. Coating may be done by hand spray painting or by passing the frame through an automatic electrostatic spraying room. In this room, frames are given a negative charge and sprayed with positively charged paint. The frame is rotated to get full coverage. A finishing lacquer is then applied. Some unicycles are coated with chrome plating instead of paint.
- 5 The seat is typically made by an outside contractor and shipped to the plant. The seat post is bolted to the seat's bottom. The seat post is then inserted into the unicycle frame and attached with a bolt or clamp.
Wheels, rims, and spokes
- 6 Rims are produced on machines that roll steel strips into circular hoops. The hoops are then welded and further shaped. Holes are drilled in the rim at set intervals and angles. A larger hole is also drilled in the rim to allow the tire's air nozzle to be accessible.
- 7 Spokes are attached to the rim in these holes. On one end of the spoke is a nipple that screws into the rim holes. The other end of the spoke is attached to the wheel hub, which is a metal disc shaped device. The spokes are placed in the rim holes and hubs in a specific pattern. The spokes are tightened in a uniform direction to straighten the wheel. A liner is placed on the inside of the rim to protect the inner tube from damage by the spokes.
- 8 A rubber innertube is placed around the rim and then the outer tire is attached. The air nozzle is pulled through the rim frame and the innertube is inflated to an appropriate air pressure.
9 To complete the unicycle, the various pieces are connected. The frame is attached to the wheel via a solid metal tube. The bearings are attached along to the same area, along with the pedal cranks. The entire assembly is locked in place with nuts and bolts. The unicycle tire is inflated and final adjustments are made.
A variety of tests are done to ensure that each part of the unicycle meets specifications. The first phase of quality control is provided by the suppliers of the unicycle parts. Visual inspections are done during most manufacturing processes. For the makers of plastic parts visual inspection can find things such as deformed pieces and improperly fitted parts. Additionally, other quality control measures are taken. For example, the steel tube manufacturers are required to form tubes at a specific thickness. To do this, they utilize a device known as an X-ray gauge. This instrument is attached directly to the production line and controls the rollers to resize the steel tube if a change in thickness is determined.
The second phase of quality control is done at the unicycle manufacturing plant. The incoming component pieces are physically checked to ensure they meet specifications. For example, the diameter of the tire is measured or the color of the frame is checked. After the unicycle is assembled a quality control specialist tests it for obvious defects. For instance, the wheel is spun to ensure that it is straight or the bolts are checked for tightness.
While the unicycle designs have changed little over the years, inventors have not stopped trying to produce a better cycle. Most of these attempts have been related to making the unicycle safer and easier to ride. For example, a patent issued in the United States during 1994 describes a unicycle that is designed to have a limited tilt. This invention uses a ground contact attached to the pedals to limit the amount of tilt that the rider experiences. Another patent issued in 1999 describes a unicycle that is equipped with handlebars that can aid beginning riders. In addition to these new designs, other improvements in unicycles will likely be in the form of new composite materials that make the vehicles stronger, more durable, and lighter.
Where to Learn More
Kirk-Othmer Encyclopedia of Chemical Technology. New York: John Wiley & Sons, 1994.
Johnson, R. C. "Unicycles and Bifurcations." American Journal of Physics (July 1998).
Martin, S. "Miyata Unicycle./Bicycling (April 1993).
The Unicycle Web Page. December 2001. <http://www.unicycling.org>.