From: [Permission pending]
Subject: Re: Bicycles and torque (an irrelevant comment)
To: rusin@math.niu.edu (Dave Rusin)
Date: Wed, 8 Feb 1995 12:37:46 +0000 (GMT)

> In article <3h7nhe$eds@eldborg.rhi.hi.is> you write:
> >Wrong.  My physics teaches also used to explain the easiness by which
> >one can ride a bike by the wheels' angular momentum.  The fact
> >is, this momentum is small.  It's just as easy to ride a bike
> >with aluminium or plastic wheels than with steel wheels.  The reason
> >one can ride a bike is the same reason that one can walk:  One
> >learns to respond to minute changes of balance with an adjustment
> >of ones centre of gravity.
> 
> This is an interesting point. But would that not imply that it's just
> about as easy to ride a bike at one speed as at any other (including
> holding still)? That is, after, part of the usual defence of the
> angular momentum theory, and indeed I have a much harder time staying
> stable at low speeds than high ones (particularly if carrying an
> unusual load such as a child).
> 
> dave
> 
> 
> 
> 

Maybe.  But perhaps the reason is that at low speeds, the
response time to inbalance condition drops (it takes longer
to turn). Let's do some figures for fun.
Let's say that each wheel has a 20" diameter. Its mass is 1kg
and is all at the rim.  You are riding at 20km/hr.
This gives an angular momentum of 0.72N*m*s. (both wheels)
If your 70kg mass is centered at the height of 1m, then
if it is 2cm from the vertical it is applying a torque of
14Nm to the bike pependicular to the angular momentum
vector.
You can see how miniscule the ang.mom is compared to the
torque, which if applied differently would stop the wheel
in 0.05 seconds.


These calculations may be riddled with errors, thoug.

[sig deleted -- djr]