How Does a Bicycle Work ?
How a Bicycle Balances
.. the effects of
centrifugal
force are what actually keeps the bicycle
upright at this point.
August 3, 2018
Why does it become easy
to balance a bicycle when in motion, but not so easy – or
impossible - when not in motion ? How does a bicycle
manage to balance itself just because it is moving ? The
fact that a bicycle balances more and more as it picks up speed
should be thought as a blessing in disguise. This means
the bicycle works for us the more we try to ride it. The
answer to how a bicycle balances just because it is moving is
not always a simple one, but one that we should understand.
First, we don’t really
know how to ride a bicycle from the viewpoint of our birthright.
We have to learn how to do it. When observing kids, you
might see that they go through a stage using training wheels
before they develop “a sense” of how to stay up on their bike
and maintain a continuous motion with only slight steering
corrections. This means balancing on a bike is related to
learning and experience which involve processes within the
brain. Therefore, part of the reason a bicycle stays
upright and balanced is because it is a learned skill.
From the viewpoint of
physical mechanics, the first thing that keeps a bike upright is
the rather fast and reflexive steering movements that “catch”
the bike before it falls over. So when a bike begins its
journey and starts to lean to the left, for example, a quick
steering motion to the left prevents it from tipping over too
far and because the bike is now beginning to roll – and doing so
along a slightly curved path – the principle of
centrifugal force is also in effect and forces the bike over
to the right (causing it stay upright). This sequence of moving
the steerer towards the side which the bike is leaning to - with
the effect of centrifugal force pushing to the opposite side
(correcting the bike to stay upright) - repeats itself
continuously, going from one side or the other and in varying
amounts, until the movements become so slight the bicycle
appears to be merely balancing on two wheels. The bicycle
has to begin rolling for this to happen, and while it begins its
slightly weaving path, the effects of centrifugal force are what
actually keeps the bicycle upright at this point.
From a dead stop, a bicycle is unable to stay upright.
When a bicycle starts in motion, it will weave slightly,
steering towards the side it is leaning to. Although only
slight, the curved motion will cause a centrifugal force in the
opposite direction helping the bike stay upright. As speed
begins to pick up, the weaving pattern will diminish until it is
undetectable and the principle of angular momentum goes into
effect. As speed further increases, the angular momentum
increases and stabilizes the bike's forward motion resisting any
side-to-side forces.
Now, as the bicycle
continues its path and the slightly weaving pattern diminishes
to nearly nothing, a different principle goes into effect which
is related to angular momentum, and specifically the
conservation of angular momentum. This is the principle
that explains how a
gyroscope works and also explains how a
bicycle balances when it starts moving along a straight path,
sometimes called the “gyroscope effect”. The concept is
that a wheel rotating at a constant speed tends to resist any
orthogonal external forces1 and wants to maintain its
original bearing. This is why gyroscopes are used in
navigation. However, if we apply the steerer2
of our bike – which is an internal movement or force
– we can obviously change the course of the bicycle.
The gyroscope effect does
not take over completely. You are still free to apply the
steerer and the principle of centrifugal force will still go
into effect. It is that when angular momentum begins to
build up, your bike will want to go straighter and straighter
and resist any side-to-side movements or leans. To make
your bike turn now, you will have to lean a bit more because it
is being resisted by the conservation of angular momentum, or
slow down considerably so that the angular momentum becomes
less.
So
far, we have considered three principles (one human, and two
mechanical).
There is some debate over
what principle actually maintains the balance of a bicycle.
This debate is most likely caused by more than one principle to
consider. So far, we have considered three principles (one
human, and two mechanical). All three will be taking place
in overlapping phases, sometimes all three at once, and at other
times perhaps only two, or one. As one becomes adept with
their bicycle, they may pass thru the first phase (slight
weaving pattern) rather quickly such that you may not even
notice any side-to-side movements as one starts up – they will
be ever so slight. But all cyclists, no matter how
straight and smooth they first appear, will eventually steer
their bike to one side or the other, either to compensate a bump
in the road, overcome a slight fatigue, or just to vary their
riding path. In doing so they will be relying on the above
principle involving centrifugal force. So nobody really
escapes any of these principles, only the amount of each
principle will vary per person.
To test the gyroscope
effect, you can take an accurately made bike (such as a quality
road bike) and strip off all the items that cause it unsymmetry:
derailleurs, sprockets (front and rear), cables, levers, etc.
until you have a completely symmetric bicycle with two
“freewheeling” wheels. Take a straight running start,
running alongside with the bike, and then let it go. It will
proceed on its own in a straight line (steerer and all).
If you are running alongside, give it a nudge and it should
resist this side-force and stay upright. It is staying in
balance due to the conservation of angular momentum, or
gyroscope effect.
This test in part
explains why “connoisseurs of bicycles” or competitive cyclists
insist on having their bicycles made accurately. They will
benefit more from the gyroscope effect with an accurately
aligned bike. Less of their energy is required to balance
on their bike so that they can concentrate more on pedal stroke
and cadence. At a competition race when crossing the
finish line, you might see the winner throw up their hands and
continue to ride for many more yards with no hands. They
are continuing to ride on the gyroscope principle alone, their
weight balanced over the bike overcoming any small unsymmetry of
gears and derailleurs, especially if the weight of these
components are held to a minimum.
What makes a bicycle
balance requires a combined explanation, and for some reason the
phenomena of the bicycle - which appears simple at the outset -
requires us to open-the-book of science to fully understand how
it works.
1.
Orthogonal forces are
essentially at right angles to the bicycle. For example, a
bike heading North into a moderate wind from the Northwest
experiences an orthogonal wind component from the West, which
will not upset the bicycle due to resistance from the principle
of angular momentum. The component of wind from the North
will slow the bicycle’s rate of travel.
2.
The steerer is
a term describing a bicycle’s steering mechanism. It
appears simple in form and function, yet not taken for granted.
It consists of the handle bars and stem, passed through a head
tube comprised of accurate bearings, then connected to a fork of
specific dimensions to which the wheel is fastened by way of
fork-stays or dropouts.
