Big is better
By Mick Hamer
The difference between winning
the cycling gold medal at the
Olympic Games in Sydney in 2000 or settling f
or the silver could
depend
To save weight, world-beating designs of
bicycle all try to minimise the size of the two
sprocket wheels that carry the bicycle chain.
However, Stuart Burgess, a mechanical engineer
at the University of Bristol, has proved that
this design philosophy is wrong.
Tests by Burgess have shown that
larger sprocket wheels are more efficient than
smaller ones, because larger wheels reduce
friction in the chain drive, which is more
important than the marginal increase in weight.
"Designers have concentrated on minimising
losses from what they can see," says Burgess.
"They can't see the chain losses."
When Chris Boardman broke the world
distance record for cycling in Manchester in
1996, he completed 56·38 kilometres in
an hour--breaking the record by more than
1 kilometre. In a paper to be given to the
Engineering of Sport conference in Sheffield
in July, Burgess says that if Boardman had
doubled the size of his sprocket wheels, he
would have added 100 metres to his record.
Although the increase in efficiency is
small--equivalent to a saving of 6 seconds
over 25 miles--a few seconds can mean the
difference between first and second place, says Burgess.
Modern aluminium sprocket wheels are so
light, he says, that doubling their size is
relatively unimportant. "It is extremely
marginal, no more than fractions of a second
over 25 miles," says Burgess.
Far more important is the impact that larger
sprockets have on friction in the chain drive.
Competition cyclists typically have a chain wheel
with 52 teeth and a sprockets with 13 teeth on the
rear wheel. Burgess tested two different sizes of
sprockets to demonstrate his point. One pair of
sprockets had 26 teeth on the chain wheel and 13
on the rear wheel. He also tested double-size
sprockets, with 52 teeth on the chain wheel and
26 on the rear wheel.
At 50 kilometres per hour, the force on the
chain with the smaller sprockets is about 45 kilograms.
The force on the double-size sprockets is half that.
"The cyclists is still putting in the same torque, so
if the radius doubles the force would be halved," says Burgess.
The reduction of the force on the chain is accompanied
by a similar fall in the frictional force in the chain. Each
link of the chain is coupled with a pin that fits inside
a roller, and the chief source of sliding friction is the
pin sliding in the roller, says Burgess.
In his tests, Burgess showed that doubling the sprocket
size increased the efficiency of the chain drive from 98·8
per cent to 99·4 per cent. Even so, doubling the size of
the sprocket wheels may not be practical for all competition
cyclists, says Burgess, because there has to be clearance
between the chain wheel and the ground. But competition
bicycles could easily be made with sprockets that are up
to double the normal size, and racing cyclists could order
to larger sprockets tomorrow simply by going to a specialist
supplier.
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on something as simple as the size of the bicycle's sprocket wheels.