[Archive: 21 March 1998]


Big is better

By Mick Hamer

the difference between winning the cycling gold medal at the Olympic Games in Sydney in 2000 or settling for the silver could dependChain reaction on something as simple as the size of the bicycle's sprocket wheels.

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.

From New Scientist, 21 March 1998


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