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From: (Jobst Brandt)
Subject: Re: The stability of bicycles
Date: Mon, 22 Jan 1996 00:08:08 GMT

Alan Walker writes:

> On the basis that one experiment is worth a thousand expert
> opinions, I invite readers to try an experiment.  Walk along next to
> your bicycle, wheeling it along with one hand on the saddle, without
> touching the handlebars.  With a little practice, this can be done
> with complete control at arbitrarily slow speeds.  If you go slow
> enough the gyroscopic force can be made as small as you like, but
> you can still control the bike without touching the handlebars.

Oh oh!  Riding no-hands and wheeling the bicycle as you suggest IS
affected by gyroscopic forces of the front wheel.  The ability to
steer a bicycle by holding the saddle, vanishes as speed approaches
zero.  Riding no-hands has no external forces and therefore no side
forces, although guiding the bicycle by hand can introduce side
forces.  Changing the lean of the bicycle when riding no-hands, causes
gyroscopic steering motions by which the bicycle is controlled.  Most
riders do not master this technique because the forces are so small.

> This control depends on the fact that the centre of mass of the
> wheel/fork/handlebar assembly is in front off the steering axis.
> The front wheel can be steered by leaning the bike, which causes the
> front wheel to fall towards the direction of lean.

When riding no-hands, all vertical forces are in the plane of the
bicycle and cause no steering forces.  To observe this, one can watch
track riders ride no-hands on a steeply banked track.  This is the
same and equivalent of cornering on a flat road.  The gyroscopic
forces furnish the steering forces.

> Stability against external perturbations is provided by steering
> trail, which returns the front wheel to the straight ahead position
> after is has been deflected.

I agree, and it is also this feature that makes a bicycle shimmy.  The
more trail, the more a bicycle tends to shimmy, besides having other
undesirable characteristics, such as generating unwanted steering
forces when pedaling standing.

> This is yet another in a 60 year history of attempts to debunk the 
> gyroscope myth.  Sigh

But be careful how much of the gyroscope you throw out.  Some of it
works.  The gyroscopic thread has been here as long as the bicycle.

Jobst Brandt      <> 

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