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Science, Maths & Technology

Leg science

Updated Friday 2nd June 2006

How fast can you go using leg power? Adam Hart-Davis investigates the science involved.

Kevin Keegan pedals for his life on Superstars Copyrighted image Icon Copyright: BBC

Adam's beloved bike doesn't really compete with bikes designed for world racing because it has not been designed to be aerodynamic. It will therefore encounter more air pressure and friction than a streamlined bike.

A blunt irregular object disturbs the airflow around it that creates a region of low pressure behind it. With high pressure at the front and low pressure behind, the cyclist is literally being pulled backwards. A streamlined design helps the air travel more smoothly around the object reducing the areas of high and low pressure.

Friction also occurs when the wind comes into contact with the outer surface of the rider and bicycle. To reduce this force cyclists often use low handlebars known as "drop bars". These help cyclists reduce their surface area and therefore reduce the friction created. By wearing streamlined clothing, Lycra for example - and shaving their legs - they can further reduce this resistance.

If Adam trained hard he would be able to ride faster as he would be using his muscles more efficiently. His blood would transport oxygen around his body more quickly, his muscles would work more efficiently and he would be less likely to get cramp.

With Adam's bicycle and his current athletic ability it seems unlikely we're going to make it into the record books. But is there some contraption we can devise to speed Adam up?

Using his legs and feet he could also pump or wind up some sort of contraption to give him an extra push. Pop bottle rockets are currently his favourite option. You half fill them with water, pump them up with a bike pump, and watch them disappear over your neighbour's house. Great fun! The thrust from a two litre pop bottle could lift 100 pounds or 45 kilograms into the air. Compressed air pushes against the water, forcing it out of the bottle. An equal and opposite force then pushes the bottle in the opposite direction to the expelled water. Weight and friction from the wind acts against this speeding rocket and slows the bottle down till it runs out of power.

If we attached a very large pop bottle rocket to Adam's bicycle it should go a lot faster. We think a 100 litre water tank would be enough to accelerate Adam by about 30 mph. If he was cycling 30-35 mph at the time he might just be able to reach speeds of 60-65 mph.


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