PDA

View Full Version : Riding a cycle



Geoffrey.walsh
07-30-1999, 03:36 AM
How do people ride bucycles ?

The dynamics of a bicycle are complex.. There is an interesting account
of some experimental bicycles in

Jones D. E. H. (1970) The Stability of the Bicycle. Physics Today 23
(4), 34-40.

The riding characteristics of different machines vary widely, but almost
anyone I suppose can almost at once ride any bike.

What are the controls the riser need to use ? The simplest explanation
is that by steering into a fall centrifgual force is generated which
brings the rider upright again.

But cyclists also lean. What are they accomplishing by doing this.

Has anyone ever made any measurements of movements of the limbs and
trunk when riding and perhaps compared novices with experience riders.

I have found an account in A Sharp’s famous book

Bicycles and Tricycles. This was published in 1896. It was reprinted
some time ago by MIT but the reprint is now out of stock.

I am pasting in below a bit from this book and would be glad of relevant
comments

Geoffrey Walsh



I62. Balancing on a Bicycle.

A bicycle has only two points of contact with the ground, and a
perpendicular from the mass-centre of machine and rider must fall on the
straight line joining them. If the bicycle and rider be at rest, the
position is thus one of unstable equilibrium, and no amount of gymnastic
dexterity will enable the position to be maintained for more than a few
seconds. If the mass-centre get a small displacement sideways, the
displacement will get greater, and the machine and rider will fall
sideways. In riding along the road with a fair speed the mass centre is
continually receiving such a displacement. If the rider steer his
bicycle in an exact straight line this displacement will get greater,
and he and his bicycle will be overturned, as when at rest. But, as
every learner knows, when the machine is felt to be falling to the
left-hand side, the rider steers to the left—that is, he guides the
bicycle in a circular arc, the centre of which is situated at the
left-hand side. In popular language, the centrifugal force due to the
circular motion of the machine and rider now balances the tendency of
the machine to overturn; in fact, the expert rider automatically steers
the bicycle in a circle of such a diameter that the centrifugal force
slightly overbalances the tendency to overturn, and the machine again
regains its perpendicular position. The rider now steers for a short
interval of time exactly in a straight line. But probably the
perpendicular position has been slightly overshot, and the machine falls
slightly to the right-hand side. The rider now unconsciously steers to
the right hand, that is, in a circle having its centre to the righthand
side. If the track of a bicycle be examined it will be found to be, not
a straight line, but a long sinuous curve. With beginners the waviness
of the curve will be more marked than with expert riders; but even with
the latter riding their straightest the sinusosity is quite
apparent....The bicyclist and his machine may thus be roughly compared
to an inverted pendulum. The time of vibration of a simple pendulum is
proportional to the square root of its length, a long pendulum vibrating
more slowly than a short one. In the same way, the oscillations of a
high bicycle are slower than those of a low-orie i.e. the time taken
for the mass-centre to deviate a certain angle from the vertical is
greater the higher the mass-centre; a rider equally expert on high and
low bicycles will thus be able to keep a high bicycle nearer the exact
vertical position than he will a low bicycle.

The track of an 'Ordinary' will therefore be straighter—that is, made up
of flatter curves—than that of a 'Safety,' both bicycles being supposed
ridden by equally expert riders.

>From the above explanation of the balancing on a bicycle, it will be
seen that the machine and rider are continually performing small
oscillations





--
Geoffrey.Walsh@ed.ac.uk
http://www.ed.ac.uk/~gwalsh
Phone (0)131.664.3046

64, Liberton Drive,
Edinburgh
EH16 6NW
UK

---------------------------------------------------------------
To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomch-l
---------------------------------------------------------------