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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.