Wheels are everywhere in our world
today—in very obvious places (on cars, trucks, and airplanes), but also hidden inside everything from
computer hard-drives and clothes washers to
electric toothbrushes
and dishwashers.
Six thousand years ago, there weren't any wheels at
all. The rise of the wheel, from a basic turntable that helped people mold
clay pots to a key component in hundreds of important inventions, owes
everything to the simple and effective way it helps us capture and
harness energy and transform forces. Let's take a closer look!
Photo: The Wheel, a neon-lit Ferris wheel in Orlando, Florida. Photo from Carol M. Highsmith's America Project in the Carol M. Highsmith Archive courtesy of Library of Congress Prints and Photographs Division.
In modern times, we assume there have to be roads for wheels to
travel on. But wheels were first used on carts precisely because there
were no smooth tracks to use for reliable transportation.
Before carts were invented, people dragged loads on sledges and frames
hauled behind animals such as horses and dogs. Sledges were an
effective way to move heavy loads before wheels were
invented, but friction slows them down. Frames, where a load is part
dragged and part carried, help to solve this problem. The A-shaped
dragging frame, known as a travois, is thought to have been invented
thousands of years ago and Native Americans used it up until the 19th
century. Even with animal power to help, friction between the rough ground and the frame made
the going difficult.
Photo: Friction isn't a problem when you're traveling on ice, like the occupant
of this dogsled. But sledges don't move nearly so well on normal terrain: that's why wheels
were invented. Photo by Jo Goldmann, courtesy of US Fish & Wildlife Service.
How do wheels work?
Dragging a load using a wheeled cart is far
easier than dragging it on the ground—for two reasons:
Wheels reduce friction. Instead of simply sliding over the ground, the wheels dig in and rotate, turning
around sturdy rods called axles. That means the only friction
the animals have to overcome is at the point where the wheel and axle meet—between the relatively smooth inner surface
of the wheels and the equally smooth outer surface of the axles around
which they turn.
Wheels provide leverage (in other words, they are examples of force multipliers or simple machines). A cart with bigger wheels is easier to push because its greater-diameter wheels
work like bigger levers, multiplying the pulling or pushing force and making
it easier to turn the wheels around their axles—in exactly the same
way that a long spanner makes it easier to loosen a nut.
Let's look at both these things in more detail.
1. Shifting friction to the axle
When you push a box on the ground, there's a lot of friction between the bottom of
the box and the ground below, because both surfaces are relatively rough:
When you push the same box loaded onto a cart with four wheels, there's much less resistance. The box no longer has to slide along
the ground so that part of the friction disappears. However, wheels don't eliminate friction entirely, as some people think—far from it! There must be friction between the four wheels and the ground or they'd simply slide along (like something being pushed on ice). Friction between each wheel and the ground helps it "dig in" so the wheel can rotate.
Carts are easier to push because the only real friction you have to work against is between the four wheels and their axles. As you push a cart, the relatively smooth inside surfaces of the wheels rotate and slide around the relatively smooth outsides of the axles. The important word here is smooth; the key to how wheels reduce friction is that they can slide more smoothly round their axles than an object can slide across rough ground. If the ground were always as smooth as ice, we wouldn't really need wheels and axles at all—we could just slip and slide everywhere! Sometimes wheels and axles are separated by ball bearings (small spherical balls made of hard metal, often lubricated with oil or grease), which help to reduce the friction between two surfaces even more by rolling around in the space between them. Without or without bearings, there's much less friction compared to pushing the box straight along the ground—and that's why the cart makes loads easier to move:
2. Providing leverage
Wheels on carts help in another important way too: they work like levers. That's a bit more obvious when you look at
the ship's wheel below.
Ship's wheel. Photo by Shannon Heavin courtesy of US Navy and
Wikimedia Commons.
If you turn the outside of a wheel like this, the axle at the center turns more slowly but with more force. In other words, a large steering wheel helps a sailor to turn a ship's rudder more easily than a small wheel. If you imagine each spoke is a lever, it's easy to see how this wheel works. Why isn't the wheel solid? Thick spokes provide a great deal of strength while reducing the weight compared to a solid wheel of the same size.
So what about the wheels on a cart?
The rim of a cartwheel turns a greater distance than the axle so, in the case where you're pushing a cart from behind or pulling it from the front, there is more force at the axle than at the rim. That means it really helps if your cart has big wheels because they give you more leverage, magnify your pushing force, and help you overcome the force of friction at the axles.
Turn a wheel at the rim and the force you apply (red arrow) is multiplied to give a bigger force at the axle (blue arrow). The bigger the wheel, the greater the effect, because the radius of the wheel works like a lever. The bigger the wheel, the longer the lever,
and the more leverage you get.
Turn the wheel at the center, instead, and it works the opposite way. Now the rim of the wheel goes further and faster. That's how you can use a bigger wheel to multiply speed. However, if you apply a force at the center of a wheel, the leverage works in reverse and you get less force at the rim, even though you're getting more speed there. Just as with gears, you can't increase both the force and the speed at the same time. If you increase one of them, you must reduce the other, otherwise you'd be using a wheel to make energy out of thin air (which violates a basic law of physics called the conservation of energy).
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Who invented the wheel?
People were using animals for transportation long before the
invention of the wheel and even before the development of human
settlements and agriculture in the Middle East around 8,000–9,000BCE.
Dogs are believed to been tamed and domesticated in China around
13,000BCE; horses were domesticated much more recently around 4500BCE.
Animals used for human transportation in this way are called beasts of
burden.
No-one knows exactly when, where, or how wheels were invented.
Potters wheels are believed to have been widely used around 7000 years ago in
Mesopotamia (a region of the Middle East now largely occupied by Iraq):
it's easy to imagine how a potter might have hit upon the idea after
repeatedly rotating a stool to work on a pot
from different angles. We don't know when the potter's wheel was
invented either, but some historians believe it may date from as far back as 8000BCE. In its early
form, it was little more than a turntable or "tournette" mounted on a
central support.
Photo: Making a rounded pot is much quicker and easier with a potter's wheel,
which can also be used for decorating a finished pot. Some wheels are turned slowly by hand; others spin around quickly powered by a foot-treddle. Photo by G. Eric and Edith Matson courtesy of
US Library of Congress, Prints & Photographs Division [LC-DIG-matpc-20729].
Perhaps someone eventually turned a potter's
wheel through 90 degrees to make a new kind of transportation, or
perhaps the wheel was completely reinvented for this new purpose, but
another 1000–1500 years elapsed before wheels were first used on carts.
Most likely, someone using tree trunks as rollers realized their job
would be easier if the logs could somehow be fixed in place underneath
the load, sliced up like salami so they would pass more easily over and
around obstacles. Such an effective idea was bound to spread widely and
the wheel found its way to Europe and Asia during the following
millennium.
Photo: Early wheels were made from rounded off slices of tree trunks or lumps of stone
with holes cut through for an axle. Solid wheels like this evolved into lighter and faster semisolid wheels,
with a large solid plank across the middle and a few spokes on the diagonals. Fully spoked wheels,
like the model cart wheel shown here, take the idea a step further, doing away with as much
heavy mass as possible without sacrificing strength. That made possible the invention of fast chariots,
such as those used in Roman times.
Wheels work more effectively when they have a smooth road surface to
travel on. The Romans pioneered road-building from around 300 BCE onward as a
way of linking disparate parts of their empire. Roman
roads were built in a similar way to modern ones from layers of
different materials, including large boulders to support weight, and
smaller stones, sand, and tiles to allow drainage. Often cement and
concrete (another important Roman
technology) were used to bind loose materials together. On top, there was a hard-wearing surface made of
flattened stones cut and pieced together like a jigsaw. Roman roads
were famously built in straight lines to minimize traveling time.
Development of the wheel
In terms of their basic science, the wheels that carry our vehicles
today are virtually identical to those first used in ancient times:
though built with more sophisticated materials, they are still
essentially flat discs rotating on solid axles. More interesting is the
way wheels have evolved in other ways in a range of increasingly
complex machines.
Photo: The gear evolved from the wheel and axle. Put lots of gear wheels
together and you can transform force and speed in a machine in all kinds of ways.
With the addition of teeth around their rim, wheels become gears,
capable of changing the torque (rotational force) of a machine or its
speed: gears enable a bicycle to go fast or climb a hill very slowly—with the rider pedaling at exactly the same rate in both cases. Extended into drums, wheels can be used as winches to raise water from wells, rocks
from mines, or anchors into ships: simple machines of this kind are
known as capstans and windlasses. Winches that use several wheels,
linked by multiple lengths of rope, become pulleys: powerful machines
that greatly magnifying pulling forces, allowing a person to lift many
times their own weight.
Photo: Water turbines (like this one from the
Grand Coulee Dam in Washington State, USA) also evolved from the wheel and axle. Photo by courtesy of US Bureau of Reclamation.
Wheels form the heart of turbines
(machines that capture energy from a moving liquid or gas): waterwheels and windmills, civilization's most
important sources of machine power in the Middle Ages, both evolved
from a basic wheel turning around an axle. Engines too rely on wheels
to convert fuel into energy and drive a vehicle: in a modern car
engine, for example, fuel burning in the cylinders pumps pistons
back
and forth, turning an off-center axle known as a crankshaft, which then
powers the gearbox and the road wheels.
In 7000 years, the wheel has gone far beyond its original use
as a pottery-making tool. By helping us to move loads, harness energy,
and transform forces, this simple but amazingly effective invention
literally made it possible for people to conquer the world!
Wheels: A Pictorial History by Edwin Tunis. Johns Hopkins University Press, 2002. A modern reissue of a classic 1955 book that charts the history of wheels from ancient times to the 20th century.
For younger readers
These are for ages 9–12 unless otherwise stated:
Making Machines with Wheels and Axles by Chris Oxlade. Raintree, 2015. A very good 32-page, project-led introduction for ages 7–9 that puts wheels in the broader context of simple machines.
Invention by Lionel Bender. DK, 2013. A tour of the classic mechanical, electrical, and electronic inventions we tend to take for granted. Rather dated and with very little coverage of modern inventions, but still a reasonable overview of ancient technologies, including various types of wheels.
All About Physics by Richard Hammond. DK, 2015. A lighter and more enjoyable introduction to physics aimed at a similar audience. (A reissue of an earlier book called Can You Feel the Force?.)
Wheel by David and Patricia Patricia Armentrout. CATS, 2009. A simple (32-page) introduction to wheels and how they work.
Articles
Simple introductions
A Salute to the Wheel by Megan Gambino, The Smithsonian, June 17, 2009. A whistle-stop tour through wheel history.
Remaking the Wheel: Evolution of the Chariot by John Noble Wilford, The New York Times,
February 22, 1994. A fascinating introduction to the development of spoked wheels and fighting chariots from the NY Times Archive.
Train wheel science by Svenja Lohner, Science Buddies. Why do train wheels have a tapering, conical shape?
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