Wind Tunnel Testing - The science
Pure physics ...
The power we produce when riding a bike is used to overcome frictional losses of the drive train, rolling resistance of the tyres and gravity when we climb hills and wind resistance. As the speed increases the wind resistance force becomes by far the largest force, and above 25 MPH, 85% of your power is used to overcome wind resistance.
We can train hard to increase the size of our engine and the power it produces, so that there is more power available to overcome the resistance forces. However, once you get to your peak performance it is difficult to improve beyond a few percent a year. An average TT rider or tri-athlete will generate around 2 Kg of drag force at 40 KPH which takes around 220 watts to overcome. If they are lucky enough to ride at 50 KPH they would need 415 watts and have 3 kg of drag acting against them. Just imagine being pulled back by a 3kg weight as you try to ride forwards at 50 KPH!
The majority of the drag force unsurprisingly comes from the rider. For an un-optimized riding position, 85% of the drag will come from the rider, in a good position the rider drag will drop to around 70%. It is not difficult to see what you should start with when looking to go faster!
So what is aerodynamic drag?
In simple terms, when air particles hit the body and equipment they will impart a small retardation force that slows you down. Reducing the frontal area will help to reduce this force, but unfortunately this isn’t even half the problem. The flow that passes over the surface needs to stay attached and follow the contours of the bike and rider. When they don’t ,the flow separates and causes a suction which will pull you back – this in very simple terms is drag.
DRAG2ZERO – BECAUSEDRAGSUCKS
There are many theories on optimum riding positions for minimizing drag, and a lot of conflicting ideas. To a point, reducing frontal area will reduce drag, but once you are using tri-bars the gains will come more from body shape rather than simply frontal area. Your shoulders, back, neck and arms form a unique set of surfaces and only wind tunnel testing will determine what the optimum riding position is for you. Careful testing and experience can help to derive the most efficient aerodynamic position by finding the best compromise between power, comfort and aerodynamic drag.