Wheels
Last updated: January 27, 2009.
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 alloy wheel on a small car. Thick spokes provide a great deal of strength while reducing the weight compared to a solid wheel of the same size.
Why we need wheels
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 travelling 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 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. By 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 for two reasons: 1) the ground is rough; 2) there's a large area of box in contact with it:
When you push the same box loaded onto a cart with four wheels, there's much less resistance. The cart wheels don't eliminate the friction, 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. There's also relatively little area of each wheel rubbing around its axle. These two things together mean 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. By providing leverage
But wheels on carts help in another way too: they work like levers. The rim of a wheel 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.
Turn the wheel at the center and the rim of the wheel goes further and faster. That's how you can use a bigger wheel to multiply speed.
Invention of 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.
Photo: Beasts of burden are still used today, but often more
for nostalgic reasons than from necessity. Photo by John and Karen Hollingsworth courtesy of
US Fish & Wildlife Service.
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.
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.
Wheels work more effectively when they have a smooth road surface to
travel on. The Romans pioneered road-building from the 5th millennium
BC 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 travelling 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 pedalling 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.
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-centre 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!
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.
