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 Aerodynamics Page: 2 of 2


 

Drafting

Drafting is an important technique in road racing. Exploratorium Senior Scientist Paul Doherty explained, "The bicyclist, as he moves through the air, produces a turbulent wake behind himself. It makes vortices. The vortices actually make a low pressure area behind the bicyclist and an area of wind that moves along with the bicyclist. If you're a following a bicyclist and can move into the wind behind the front bicyclist, you can gain an advantage. The low pressure moves you forward and the eddies push you forward."

Paul Doherty
RealMedia Clip
The Exploratorium's Paul Doherty talks about drafting.

Suprisingly drafting not only helps the bicyclist following the leader, but the lead cyclist gains an advantage as well. Paul explained, "The interesting thing is by filling in her eddy you improve the front person's performance as well. So two people who are drafting can put out less energy than two individuals (who are not drafting) would covering the same distance in the same time." While the lead cyclist gains some advantage in this situation she still needs to expend much more energy than the cyclist who is following.

In road racing, bicyclists group together in a pack known as the "peloton" or a pace line called an "echelon." Cyclists who are part of the group can save up to 40 percent in energy expeditures over a cyclist who is not drafting with the group. To be effective drafting, a cyclist needs to be as close as possible to the bicycle in front of him. Many professional cyclists get within inches of the the bicycle in front of them. The shorter the distance the larger the decrease in wind resistance. 

Le Tour de France: The Great Race

Aerodynamics play a huge role in the biggest bicycle race in the world, the Tour de France. Covering over 4,000 kilometers in three weeks of daily racing, "The Tour" is an all-out test of riders' speed, strategy, and heart. Cyclists travel mostly in a pack, called the "peloton," which creates a huge draft. In each day's "stage," the racers test themselves against each other and against the mountainous terrain of the Alps and the Pyrenees. Stages can last for six to eight hours, covering 90-150 miles (145-240 km), and racers can burn over 10,000 calories a day. The winner is the rider with the lowest total time for all stages. As 1988 Tour champion Pedro Delgado put it, "This is the war of the cyclists. That's what we call it."

Though the climbs in the Alps and the Pyrenees are arduous, the vicious crosswinds that play across the fields of France mean that the flats offer little respite. A lone cyclist can be battered to a standstill. In these conditions, the peloton strings out into a diagonal paceline formation called an "echelon." Like a paceline, this diagonal line of riders is designed to let the lead riders fight the wind, while following riders are sheltered, awaiting their turn at the front. When an echelon forms, its length is limited by the width of the road, resulting in a fierce battle in the "gutter" for the final places in the echelon line. Since the riders in the echelon have a 15-30% advantage in the wind, a failure to make the first echelon can be the difference between a chance to win the stage and a long futile chase.



In mountain biking, drafting seems to be less important. U.S. cross-country champion Ruthie Matthes explained, "In mountain biking, drafting doesn't tend to be a factor. The speeds are slower (than road racing) and the rolling resistance is greater. It helps to draft for the mental aspect, for keeping pace with someone ahead of you. But as far as using less energy, it's not really a big factor." Besides the rolling resistance and slower speeds, the twisting up-and-down nature of most mountain biking courses would make drafting extremely difficult.


Ruthie Matthes
RealMedia Clip
Ruthie Matthes discusses drafting in mountain biking.

 

Recumbents and HPVs

Drafting is not always an option and its benefits are somewhat limited. The easiest way to overcome wind resistance and reduce drag is to become more streamlined. In recumbent design, the cyclist pedals from a seated position, which gives the bicycle a lower profile and makes it aerodynamically more efficient. Recumbent bicycles have been around for over 100 years, although they have never enjoyed the popularity of the upright safety bicycle, which remains the design people associate with bicycles.


Human-powered vehicles became popular during the the 1970s. During that decade the popularity of the bicycle reached a new high and two OPEC oil embargos increased public awareness of alternative methods of transportation. Most HPVs use a recumbent design and a lightweight outer shell to make the vehicle more streamlined and to reduce skin friction. HPVs can travel at very high speeds. The world record holder in the 200 meters traveled at over 68 miles per hour (110 km).

Recumbent
A custom-made recumbent cycle.
Recumbents hold many speed and endurance records and are quite comfortable to ride. Recumbents are so efficient that many races do not allow recumbents to enter for fear that the cyclists on the traditional safety design will be at a disadvantage. There are a few disadvantages to the recumbent design. One is the cost; recumbents are not mass-produced and cost more than safety bicycles. In addition, recumbents are harder to see on the road--most use an orange safety flag so automobile drivers can more easily avoid them.

 

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