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The effect of geometry on the drag coefficient of some bicycles is shown below. The upright position frequently used when riding a commuter bike gives a frontal area of 0.5 m2 and a drag coefficient of 1.1, close to that for a flat plate! By crouching the frontal area of the bike-rider system is reduced to 0.36 m2 and the drag coefficient also decreases to 0.88 due to the improved "streamlining" of this position. When two cyclists ride in a drafting configuration they behave rather like a single, longer streamline body. The frontal area remains at 0.36 m2, but the drag coefficient drops to 0.50 giving a 43% reduction in the drag force. The final streamlined configuration has a frontal area of 0.46 m2 but an even lower drag coefficient of 0.12. Prone, streamlined bikes hold world speed records due to this drag reduction advantage. Don't forget that cooling of the human becomes a problem with this design unless air is ducted to the rider. The needed ducts and air exhaust will increase the drag of the bike.

From: Munson, Young & Okiishi,
"Fundamentals of Fluid Mechanics," Wiley (1998)

From a Zipp advertisement. Is this a good representation of flow over the bike and rider? What about the lower part of the system?