Bicycles
Last updated: June 18, 2007.
If you had to pick the greatest machine
of all
time, what would you say? If we were talking about machines that
helped spread knowledge and educate people, you'd probably opt for
the printing press. If we meant inventions that let people farm the
land and feed their families, you might plump for the plow or the
tractor. If you think transportation is really important, you could
go for the car engine, the steam engine, or the
airplane jet engine. But for its sheer
simplicity, I
think I would pick the bicycle. It's a perfect example of how pure,
scientific ideas can be harnessed in a very practical piece of
technology. Let's take a look at the science of cycles—and just
what makes them so great!
Photo: The bicycle—a brilliantly simple form of
transportation. Photo courtesy
of US
Navy.
The frame
A typical adult weights 60-80 kg (130-180 lb), so
the frame of a bicycle has to be fairly tough if it's not going to
snap or buckle the moment the rider climbs on board. Ordinary
bicycles have frames made from strong but lightweight tubular steel
(literally, hollowed-out steel tubes containing nothing but air).
Racing bicycles are more likely to be made from carbon composites,
which are even stronger and lighter.
The frame doesn't simply support you: its
triangular shape is carefully designed to distribute
your
weight. Although the saddle is positioned much nearer to the back
wheel, the rider leans forward to hold the handlebars. The angled
bars in the frame are designed to share your weight more or less
evenly between the front and back wheels. If you think about it,
that's really important. If all your weight acted over the back
wheel, and you tried to pedal uphill, you'd tip backwards; similarly,
if there were too much weight on the front wheel, you'd go head over
heels every time you went downhill!
Photo: The bicycle's inverted A-frame is an
incredibly strong structure that helps to distribute
your weight between the front and back wheels.
It helps to lean forward or even stand up when you're going uphill so
you can apply maximum
force to the pedals and keep your balance.
The wheels
If you've read our article on tools
and machines, you'll know that a wheel and the axle it turns around
is an example of what scientists call a simple
machine: it
will multiply force or distance depending on how you turn it. Bicycle
wheels are typically over 50 cm (20 inches) in diameter, which is
taller than most car wheels. The taller the wheels, the more they
multiply your speed. That's why racing bicycles have the tallest
wheels (typically about 70 cm or 27.5 inches in diameter).
Photo: Like a car wheel, a bicycle wheel is a speed multiplier. The
pedals and gears turn the axle at the centre. The axle turns
only a short distance, but the leverage of the wheel means
the outer rim turns much further in the same time. That's how a wheel
helps you go faster.
The wheels ultimately support your entire weight.
So if you weigh 60 kg (130 lb), there's about 30 kg (130 lb) pushing
down on each wheel (not including the bicycle's own weight). The
spokes are what stops the wheels from buckling. Since each wheel has
around 30-40 spokes, each spoke has to support only a fraction of the
total weight—in this case, less than 1kg (2.2 lb), which it can do
easily. Bicycles have spoked wheels, rather than solid metal wheels,
to make them both strong and lightweight. Spokes also reduce the air
resistance or drag on the front wheel when you're cornering.
The gears
A typical bicycle has anything from three to
thirty different gears—wheels with teeth,
linked by the
chain, which make the machine faster (going along the straight) or
easier to pedal (going uphill). Bigger wheels also help you go faster
on the straight, but they're a big drawback when it comes to hills.
That's why mountain bikes and BMX bikes have smaller wheels than
racing bicycles. It's not just the gears on a bicycle that help to
magnify your pedalling power when you go uphill: the pedals are
fastened to the main gear wheel by a pair of cranks: two short levers
that also magnify the force you can exert with your legs.
Photo: A gear is a pair of wheels with
teeth that interlock to increase power or speed.
In a bicycle, the pair of gears is not driven directly but linked by a
chain.
At one end, the chain is permanently looped around the main gear wheel
(between the pedals).
At its other end, it shifts between a series of bigger or smaller
toothed wheels when you change gear.
The brakes
No matter how fast you go, there comes a time when
you need to stop. Brakes on a bicycle work using friction (the
rubbing force between two things that slide past one another while
they're touching). When you press the brake levers, a pair of rubber
shoes clamps onto the metal inner surface of the front and back
wheels. As the brake shoes rub tightly against the wheels, they turn
your kinetic energy (the energy you have
because you're going
along) into heat—which has the effect of slowing you down.
Photo: The rubber shoes of this bicycle's brakes
clamp the metal rim of the wheel to slow you down.
The tires
Friction is also at work between the rubber tires
and the road you ride on: it gives you grip that makes your bike
easier to control, especially on wet days. Different kinds of
bicycles have different kinds of tires. Racing bicycles have thin,
smooth tires designed for maximum speed, while mountain bicycles have
thicker, more robust tires with deeper treads that can withstand
tougher terrain. The tires are not made of solid rubber: they have an
inner tube filled with compressed air. That means they're lighter and
more springy, which gives you a much more comfortable ride.
Photo: The world's favorite form of
transportation? Bicycles come in
all shapes and sizes. This is a three-wheeled rickshaw, common in
countries such as India. Photo courtesy
of US Department
of Defense.
The clothing
Friction is a great thing in brakes and tires—but
it's less welcome in another form: as air resistance (drag) that
slows you down. The faster you go, the more air resistance becomes a
problem. At high speeds, racing a bicycle can feel like swimming
through water: you can really feel the air pushing against you and
you use most of your energy overcoming drag. Now a bicycle is pretty
thin and streamlined, but a cyclist's body is much fatter and wider.
In practice, a cyclist's body creates
twice as much drag as
their bicycle. That's why cyclists wear tight neoprene clothing and
pointed hats to streamline themselves and minimize energy losses.
You might not have noticed, but the handlebars of a bicycle are levers
too: longer handlebars provide leverage that makes it easier to swivel
the front wheel.
But the wider you space your arms, the more air resistance you create.
That's why racing bicycles have two sets of handlebars to help the
cyclist adopt the best, most streamlined position. There are
conventional, outer
handlebars for steering and inner ones for holding onto on the
straight.
Using these inner handlebars bars forces the cyclist's arms into a
much tighter, more streamlined position.
Physics in action
All these things make bicycles incredibly
efficient machines. A bicycle is so efficient that it can convert
around 80 percent of the energy you supply at the pedals into kinetic
energy that powers you along. Compare that to a car engine, which
converts only about 25 percent of the energy in the gasoline into
useful power, and you'll see why I say bicycles are such brilliant
examples of scientific machines.
What makes it so hard to fall off a bicycle?
People often say that it's virtually impossible to
fall off a bicycle because its spinning wheels make it behave like a
gyroscope. Scientists have been puzzling over what makes bicycles
balance virtually since they were invented, back in the 19th
century. In 2007, a group of engineers and mathematicians led by
Nottingham University's J.P. Meijaard announced they'd finally
cracked the mystery with a set of incredibly complex mathematical
equations that explain how a bicycle behaves—and it turns out that
gyroscopes are only part of the story.
According to these scientists, who used 25 separate "parameters" or
"variables" to describe every aspect of a bicycle's motion,
there's no single reason for a bicycle's balance and stability. As
they say: "A simple explanation does not seem
possible because the
lean and steer are coupled by a combination of several effects
including gyroscopic precession, lateral ground-reaction forces at
the front wheel ground contact point trailing behind the steering
axis, gravity and inertial reactions from the front assembly having
center-of-mass off of the steer axis, and from effects associated
with the moment of inertia matrix of the front assembly".
Or, in
simple terms, it's partly to do with gyroscopic effects, partly
to do with how the mass is distributed on the front wheel, and partly
to do with how forces act on the front wheel as it spins. At least, I
think that's what they
said!
If you're feeling brave and your maths is top
notch, you can read more in:
'Linearized dynamics equations for the balance and steer
of a
bicycle: a benchmark and review' by J. P. Meijaard, J. M.
Papadopoulos, A. Ruina, and A. L. Schwab. Proceedings of the Royal
Society, 2007.
