Aerodynamics and drag
Dr Mark Jermy and PhD student Lindsey Underwood from the University of Canterbury explain what aerodynamics is and how it relates to competitive cycling. They introduce three different types of aerodynamic drag and describe some of the most relevant things that cyclists can do to reduce their drag.
Point of interest: What are the three types of aerodynamic drag? What types of drag are relevant to competitive cyclists?
Transcript
DR MARK JERMY Aerodynamics is the study of how air flows over objects, things, and how that exerts forces on the things and how those things exert forces back on the air.
LINDSEY UNDERWOOD And so it explains why a plane can fly, why a golf ball has dimples, why a racing car can go faster than a big truck.
DR MARK JERMY If you look at a competitive cyclist travelling quite fast – about 60 kilometres an hour – about 90% of the power they are producing through the pedals goes into fighting aerodynamic drag, and 10% is other things like inefficiencies, friction in the chain and rolling resistance. So aerodynamic drag is really the biggest force that they’re fighting against.
LINDSEY UNDERWOOD Drag is a force that opposes motion. There is pressure drag, which is caused by separation of the air flow around an object. So when an object is moving forwards, you have a region of high pressure at the front of the object and then a region of low pressure behind the object. The pressure difference opposes the motion.
The shape of the object affects the pressure drag. So the more streamlined a shape is, the lower this difference in pressure will be and the lower the pressure drag will be. So, for example, the wing of an aircraft will have a lower pressure drag than say a ball, because the wing of aircraft is a very streamlined shape.
You also have skin friction drag, which is caused by the actual air particles on the surface of the object, so a very smooth surface will have a low skin friction drag whereas a very rough surface will have a higher skin friction drag.
DR MARK JERMY Then there is another type of drag – it’s called induced drag, and wherever you have an object which is producing lift like an aeroplane wing, it’s travelling through the air, and the way the air travels over it produces a pressure difference and that produces an upward force, and there is an extra drag associated with that called induced drag. And that is particularly important if you are designing an aircraft, but for something which isn’t flying, like a cyclist, it’s not relevant.
So the rider really is the most important thing, and when a cyclist comes into our wind tunnel, we do most work on getting their position on the bike right. They have got to get their back as flat as possible. They have got to get their elbows in the right position. So the smaller the cyclist can make themselves by hunching down, the less drag they have. We have done tests with very loose clothing flapping around, and that produces a lot of extra drag.
Road cyclists always wear tight clothing – something like Lycra because it keeps its shape very well – so the tighter the clothing the better. If you choose a material which is very smooth – so something like rubberised Lycra which has been coated with something very, very smooth – you get a lower drag still.
We don’t make recommendations by ourselves. We always work with coaches and support scientists, so all the time we are learning about the science of cycling and they are learning about the science of aerodynamics. So there is an exchange there, and that makes it a lot of fun, but together what we are recommending is changes in body position, so there is no magic answer, it’s different for every person. Each person has their own optimum riding position, which might well not work for the next person.
Acknowledgements: Geoff Searle 123rf Free-Extras.com SKINS International Trading AG/Benjamin Fitzmaurice
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