Supercomputers and terracotta cyclists reveal peloton's aero sweet-spots.

An array of supercomputers crunching the fiendishly complex windtunnel data of 121 terracotta cyclists has revealed that a rider in the sweet-spot of a peloton experiences up to ten times less air resistance than previously assumed. 

It's long been known that an average person on a bicycle is a more efficient translator of energy per gram per kilometre than any other machine or animal, but this new research suggests that a cyclist shielded from the wind is even more energy efficient than, say, a hang-glider.

Drag in the mid rear of a densely packed peloton goes down to 5 to 10 percent of the drag of an isolated cyclist riding at the same speed. That's a huge difference from all previous studies which have reckoned on reductions of 50 to 70 percent. 

The new research, facilitated with ANSYS fluid flow software run through Cray supercomputers, has also mapped the most energy-efficient positions in the pack – this is critical information for anybody racing or riding in large groups. 

The research has been led by Belgian go-faster-against-the-air specialist Bert Blocken, based at the Eindhoven University of Technology in the Netherlands and at KU Leuven in Belgium. Professor Blocken assembled a large team from a variety of academic and commercial concerns. The work done for the multi-disciplinary "Peloton Project" shows that solo breakaways are even harder and more impressive than previously thought.

The research has been published in the Journal of Wind Engineering and Industrial Aerodynamics. Professor Blocken was aided in his research by riders from pro teams LottoNL-Jumbo and BMC as well as terracotta scale models of tucked-in cyclists made by modellers and assembled in the Eindhoven University of Technology's wind tunnel. 

Professor Blocken can be heard discussing his work on the latest Spokesmen Industry Roundtable podcast

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"Some teams use mathematical cycling models to calculate when exactly a rider should escape to stay out of the grasp of the chasing peloton," Blocken has written

"These models assume that the riders inside the peloton have a resistance of 50 to 70 percent that of an isolated rider. These values result from old tests on small groups of up to four in-line drafting cyclists that showed reductions for the third and the fourth cyclist, both up to 50 percent. This has led researchers to believe that also inside a peloton, this 50 percent would apply. This [statistic] can be found in many books and articles on cycling science. However, for a cyclist in the mid rear of a tightly packed peloton with multiple rows of riders providing shelter from wind, a larger drag reduction can be expected than obtained in simple in-line configurations of only a few riders."

Professor Blocken added:"Drag in the mid rear of a densely packed peloton goes down to 5 to 10 per cent of the drag of an isolated cyclist riding at the same speed. These values are about ten times lower than the values of 50 and 70 per cent used in state-of-the-art mathematical models of cycling. These models should be improved.

"In the mid rear of the peloton, the equivalent cycling speed is 4.5 and 3.2 times lower than the peloton speed. This corresponds to the experience expressed by professional cyclists and practical cycling experts that riding in the belly of the peloton requires very low pedalling effort."

(While following a peloton on the first day of this year's Tour de France, Dimension Data's Tim Wade told New Scientist: “You can see immediately, from the car beside the riders, that some are hardly pedalling at all.") 

In the Spokesmen podcast, Blocken said some pro cyclists have been upset that his work appears to suggest amateur riders could therefore easily keep up with pro riders if they were all in the same pack. The professor disputes this, acknowledging that his work was based on ideal conditions – a flat, straight road; perfect peloton; no wind – that would only occur minimally during a real race.

"The results [of this work] do not imply that a regular person can cycle along with the pro peloton for a whole race. For a few kilometres, yes, as long as ideal conditions hold. As soon as turns are taken and [an] accordion effect sets in, the air resistance to be overcome becomes much higher. So there is no reason whatsoever for less respect for the exceptional efforts by pro cyclists."

Pack positioning is also key, said Blocken, but he stressed that the most energy efficient position isn't necessarily the best one at all times:

"While the study shows that the lowest aerodynamic drag in the pelotons that were studied is found in the last four or five rows, this position is not necessarily the best position considering all aspects in a cycling race. Riders at the end of the peloton will be less likely to see and react to attacks from competitors. They might miss a breakaway. When the front part of the peloton accelerates, riders at the end of the peloton will have to generate a larger acceleration than the others to close the gap between them and the others. As a result, the best position in the peloton will generally not be in the last four to five last, but more in front, but nevertheless still sufficiently shielded by others, and where the leaders will generally be surrounded by members of their team."

Blocken's work doesn't refer to cycling's general energy efficiency but it's clear from his research that riding in a pack makes cycling even more efficient as a means of locomotion.

scientificAmericanMarch1973

Cycling was famously lauded as more energy efficient than any machine or beast in a Scientific American article published in 1973. English academic S.S. Wilson wrote that neither engineering nor evolution was any match for a person on a bicycle. 

(This Wilson is not the Wilson who wrote the seminal Bicycling Science of 1974; that was David Gordon Wilson. The two were not related.)

“It is worth asking why such an apparently simple device as the bicycle should have had such a major effect on the acceleration of technology," wrote Wilson in his influential 1973 article. "The answer surely lies in the sheer humanity of the machine. Its purpose is to make it easier for an individual to move about, and this the bicycle achieves in a way that quite outdoes natural evolution.

“When one compares the energy consumed in moving a certain distance as a function of body weight for a variety of animals and machines, one finds that an unaided walking man does fairly well (consuming about .75 calorie per gram per kilometer), but he is not as efficient as a horse, a salmon or a jet transport. With the aid of a bicycle, however, the man’s energy consumption for a given distance is reduced to about a fifth (roughly .15 calorie per gram per kilometer).

“Therefore, apart from increasing his unaided speed by a factor of three or four, the cyclist improves his efficiency rating to No. 1 among moving creatures and machines."

Wilson's famous chart, Scientific American, 1973

Wilson's famous chart, Scientific American, 1973

Wilson used cycling's efficiency to point to a cleaner, brighter, more energy efficient future:

“For those of us in the overdeveloped world the bicycle offers a real alternative to the automobile, if we are prepared to recognize and grasp the opportunities by planning our living and working environment in such a way as to induce the use of these humane machines.

“The possible inducements are many: cycleways to reduce the danger to cyclists of automobile traffic, bicycle parking stations, facilities for the transportation of bicycles by rail and bus, and public bicycles for 'park and pedal' service. Already bicycling is often the best way to get around quickly in city centres."

Wilson’s article was later picked up by philosopher Ivan Illich who, in his 1978 pamphlet Toward a History of Needs, wrote:

“Man on a bicycle can go three or four times faster than the pedestrian, but uses five times less energy in the process. He carries one gram of his weight over a kilometre of flat road at an expense of only 0.15 calories. The bicycle is the perfect transducer to match man’s metabolic energy to the impedance of locomotion. Equipped with this tool, man outstrips the efficiency of not only all machines but all other animals as well.

“Bicycles let people move with greater speed without taking up significant amounts of scarce space, energy, or time. They can spend fewer hours on each mile and still travel more miles in a year. They can get the benefit of technological breakthroughs without putting undue claims on the schedules, energy, or space of others. They become masters of their own movements without blocking those of their fellows."

However, as the aptly-named aero professor has shown, cyclists who are blocked by fellow riders in front of them take the understanding of cycling's energy efficiency to new levels.

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