Videre non videri
Join Date: Sep 2004
Location: Gothenburg, Sweden
Bikes: 1 road bike (simple, light), 1 TT bike (could be more aero, could be lighter), 1 all-weather commuter and winter bike, 1 Monark 828E ergometer indoor bike
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Aerodynamic drag (in terms of power required to counter it) on a bicyclist varies as speed cubed. Speed in this case is speed through the air (in your direction of travel), which is lower than your "road speed" if you have a tailwind, and higher if you have a headwind.
When a bicyclist rides through an airmass (with a speed not equal to that of the bicyclist), the air surrounding the bicyclist will be accelerated in the bicyclist's direction of travel - he is setting air in motion, pulling it along. The power required to set this air in motion is the power required to overcome the wind resistance.
Behind the moving bicyclist will be a mass of air moving along with him, but not quite at the same speed. Towards the sides of this rearward-pointing "cone" of dragged-along air, the speed of the air will be gradually lower until at some point, it will be the same as for the unaffected air. The same is true in the rearward direction, so at some point behind the bicyclist, air will return to its normal state, and be effectively unaffected.
If second bicyclist rides closely behind the first, he will be moving along inside that accelerated mass of air, and will see a reduction in relative air speed for himself. This translates to a lower power required to maintain any given speed.
Since power required to overcome aerodynamic drag varies as the air speed cubed, even a small reduction in relative air speed will make a big difference in the power requirement for the second bicyclist. Another factor to consider is also that the faster the first bicyclist rides (and/or the greater the headwind), the greater the reduction in drag for the second rider will be.
The cone of accelerated air behind the first bicyclist will be deflected to the side in a crosswind, so the actual sweet spot could very well be at an angle behind the first, requiring a staggered position for a trailing bicyclist.
The effect of drafting increases as the distance between the two bicyclists decreases. At a sufficiently low relative air speed, the drafting zone is too small for another bicyclist to fit into, and virtually no benefit comes from drafting.