Swim Draft Efficiency vs. Bicycle Draft Efficiency

Isaac Newton's first law of physics is that an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. What slows a cyclist and swimmer down is the drag forces cause by air and water, respectively. Drafting can reduce the drag forces faced when swimming or biking, making you more efficient.

  1. Swim Drafting

    • To gain the most efficiency swimming, draft right behind the feet of the swimmer in front. A study published in the July 2003 journal "Medicine and Science in Sports Exercise” examined the best swim draft positions. The study examined both drafting behind and beside the lead swimmer. They found that drag was reduced the most, by 20 to 21 percent, when swimming 0 to 50 cm behind another swimmer. Lateral drafting, swimming beside another swimmer, was most effective when you’re 50 to 100 cm behind the hand of the lead swimmer, but only reduced drag by 6 to 7 percent.

    Bicycle Draft

    • Drafting in cycling benefits the whole pack, but the reduction in drag felt by a rider depends on the position in the pack. The second rider in a draft line feels a reduction in drag of 21 percent. The third and subsequent riders face drag reduction of 27 percent. Moreover, the lead cyclist actually gets a benefit from being drafted. The difference is small, changing the drag coefficient from is 0.277 to 0.285. No such benefit has been reported in swimming.

    Translation to Speed

    • Swim and bike drafting lead to drag reductions of up to 21 and 27 percent, respectively. This does not equate to times that are 21 to 27 percent higher, however. Drag increases with the square of the velocity, meaning as you go faster, the drag you’re feeling increases exponentially. In the 2004 Tour de France, Armstrong was able to complete an individual time trial with an average speed of 49.39 kph. In comparison, Armstrong rode on a nine-rider team with an average speed of 53.71 kph. That's only a six percent gain in speed, despite a 27 percent reduction in drag.

    Triathlon

    • Triathletes, who compete in an open water swim before cycling and running, have the option of drafting during the swim. A second 2003 study published in the journal of “Medicine and Science in Sports and Exercise” reported that drafting during the swim portion of a race improves a triathlete's performance during the bike portion of the race. The study found that cycling efficiency increased by 4.6 percent after drafting.