Tag: drafting

Paris 2024: Triathlon Swimming
Unlike the swimming competition, Olympic triathletes complete their swim legs in open waters. There are no lane dividers and no rules against drafting off a fellow athlete. Curious to see how draft positioning could affect swimmers, researchers experimented with swimmer-shaped models in a water channel and a numerical simulation. They found that the most advantageous position is directly behind a lead swimmer, where the follower could enjoy a 40% reduction in drag. Another good position is near the leader’s hip, where waves off the leader provide a 30% reduction in drag.
The worst place to swim, interestingly, is immediately side-by-side. With both swimmers neck-in-neck, drag is maximized, and each swimmer feels more drag than they would swimming by themselves! (Image credit: J. Romero; research credit: B. Bolan et al.)
Related topics: Drafting in each triathlon stage and drafting effects in nordic skiing
Join us all this week and next for more Olympics-themed stories.
July 30, 2024
Saving Energy By Following a Leader
Scientists have long suspected that birds save energy by following a leader — think of the V-shaped flight formation used by geese — but a new study captures that savings directly. The team studied starlings, flying singly or in groups of two or three, in a special wind tunnel. Each bird wore a tiny backpack with sensors and lights that captured its motion and helped researchers identify it individually in videos. And, using before and after metabolic measurements, the researchers could pin down exactly how much energy each bird used when flying.
They found that birds who spent most of the flight in a “follower” position used up to 25% less energy than they did when flying solo. That’s a major incentive to follow someone else. Interestingly, they also found that the most efficient solo fliers were the birds most likely to take on the “leader” position. The team notes that these “leaders” tend to use a lower wing-flapping frequency, but a full explanation of how they save energy will require a follow-up study. (Image credit: R. Gissler and S. Hao; research credit: S. Friman et al.; via Physics World)
July 16, 2024
PyeongChang 2018: Cross-Country Skiing
Cross-country skiing, also known as Nordic skiing, is a part of many longstanding disciplines in the Winter Games. Unlike downhill skiing, cross-country events typically involve mass starts, which allow athletes to interact, using one another for pacing and tactics. Drafting can be a valuable method to save energy and reduce drag. A following skier sees a 25% drag reduction while drafting; the lead skier gets about a 3% reduction in drag compared to skiing solo. Competitors usually wear tight-fitting suits to minimize drag, but unlike speedskating, for example, cross-country skiers don’t get much benefit from roughened surfaces and impermeable fabrics. Typical race speeds are 4 – 9 m/s, and most of these high-tech fabrics don’t provide tangible benefits until higher speeds. (Image credit: Reuters/S. Karpukhin, US Biathlon, GettyImages/Q. Rooney)
February 22, 2018
Schooling in Soap Films
In sports, flocks of birds, and schools of fish, we’re accustomed to thinking that the followers get an aerodynamic or hydrodynamic advantage over the leaders, but this may not always be the case. Here are two flags placed one after another in a soap film flowing from top to bottom. The flags are passive, meaning that their motion is entirely dependent on the flow around them; they cannot exert any resistive force of their own. In this case, scientists observe an effect known as inverted drafting. The lead flag actually experiences less drag – by as much as 50% – than the following flag. This seems to be a result of flow around the second flag having an upstream influence on the motion of the first. (Image and research credit: L. Ristroph and J. Zhang, pdf)
July 3, 2017
Sochi 2014: Downhill Skiing
Like the athletes who compete on ice, skiers rely on a film of liquid beneath their skis to provide the low friction necessary to glide. The moisture results from the friction of the ski’s base and edges cutting into the snow, and, depending on the conditions of the snow, different surface treatments are recommended for the skis to help control and direct this lubricating film. Similarly, skiers uses various waxes on their skis to lower surface tension and provide additional lubrication. Fluid dynamics can also play a role in tactics for various ski-based events. In endurance events like cross-country skiing, drafting behind other skiers can help an athlete avoid drag and save energy. When drafting, cross-country skiers have lower heart rates. Drag and aerodynamics can also play a significant roles in alpine skiing, especially in speed events like the downhill or super G. In these events solo skiers reach speeds of 125 kph, where drag is a major factor in slowing their descent. Between turns smart skiers will tuck, decreasing their frontal area and reducing drag’s effects. Athletes use wind tunnel testing to dial in their tuck position for maximum effect, and, like speedskaters, skiers may also wear special aerodynamic suits. (Photo credits: F. Cofferini/AFP/Getty Images, C. Onerati; h/t to @YvesDubief)
February 18, 2014
Reader Question: Drafting in Triathlons
Reader juleztalks writes:
I’ve just entered an amateur triathlon, and there’s a whole load of rules about not “drafting” in the cycle stage (basically, not sitting in other cyclists’ slipstream). However, there are no such rules for the swim or run stage; I thought the effects would be the same from drafting other swimmers and runners. Any ideas?
As in many endurance sports, it’s all a question of energy savings from drag reduction. Drag on an object, like a triathlete, is roughly proportional to fluid density (air for cycling or running, water for swimming), frontal area, and the velocity squared. Because drag increases more drastically for an increase in velocity, it makes sense one would worry most about drag when one’s velocity is highest – on the bike.
Drafting has major benefits in cycling and can reduce drag on a rider by 25-40%. Aerodynamic drag accounts for 70% or more of a cyclist’s energy expenditure, so that reduction can really add up. The energy saved by drafting during cycling can even increase a triathlete’s speed during a subsequent running leg. So it makes sense for a sport’s governing body to be concerned with it.
That said, there’s plenty of room for drag reduction in swimming as well. Even though the velocities are much lower, water’s density is 1,000 times higher than air’s, generating plenty of drag for an athlete to overcome. For swimmers at maximum speed, drafting can reduce drag by 13-26%, depending on relative positioning. Such drafting has been found to increase stroke length and may (or may not) improve subsequent cycling performance.
Although a similar reduction in drag is possible by drafting when running, drag on a runner only accounts for about 8% of his/her energy expenditure so such savings would matters very little next to the swimming and cycling legs. There could be some psychological benefits, though, in terms of pacing oneself. (Photo credit: Optum Pro Cycling p/b Kelly Benefit Strategies)
May 15, 2013
Reader Question: Drafting in Cycling
jonesmartinez asks:
As a cyclist, I’m curious about drafting. How fast do I need to be going for there to be a measurable benefit? Additionally, often in a time trial a single rider is often followed by the team car and I’ve heard the rider can be pushed by the air around the team car. Any truth to this rumor? Thanks, I love the blog.
Drafting plays a major role in cycling and its tactics (check out our previous series on cycling). In general, drag increases with the square of velocity and data show this holds for cyclists. The rule of thumb I’ve heard given is that aerodynamic drag doesn’t play a large role below 15 mph, but I have not seen the numbers that inform that claim. Moreover, you have to consider the resultant airspeed around the cyclist. For example, a cyclist moving 13 mph into a 15 mph headwind (28 mph effective) will be experiencing more drag than a cyclist moving 20 mph with a 10 mph tailwind (10 mph effective). With drag being reduced 25-40% by drafting a leading rider, it is almost always beneficial to get behind someone.
That said, I have seen no measurable benefit for a leading rider with a paceline behind him, even though this should, in theory, reduce the drag on the lead rider by closing out his wake. With a large object like a car behind a solo rider, there might theoretically be some benefit. However, the car would have to be driving extremely close to the rider–far closer than they do in reality.
That said, with the prevalence of power meters in the amateur market these days, I think it would be a neat project to go out and try a few of these things firsthand and see whether such tactics actually result in a measurable difference in a cyclist’s performance–though I don’t recommend riding a foot off the front or back of a car!
May 17, 2012
Flow Around Traffic
Flow visualization in a water tunnel shows what the flow around a line of traffic looks like. Note the progressively more turbulent flow around each car as it sits in the wake of the car before it. Turbulent flow is usually associated with increased drag forces, but because turbulence can actually help prevent flow separation it is sometimes desirable as a method for decreasing drag. In the case of these cars drafting on one another, it is clear that the cars further back in the line cause less effect on the fluid–and thus have less drag to overcome–than the front car. (Photo credit: Rob Bulmahn)
February 14, 2012
Vertical Axis Wind Turbines
Conventional wind turbines feature horizontal axis propellers which must be placed far apart from one another to avoid wake interference. Researchers have found that using vertical axis wind turbines specially arranged to utilize the wake of one turbine to improve the efficiency of its neighbor can produce far more energy per square meter of land. The inspiration for this arrangement came from fish, which also derive benefits from the drafting that occurs in their schools. #
July 21, 2011
Drafting Behind a Big Rig
For those who like the effects of drafting in cycling backed up by Mythbusters, here’s a comparison between riding a mountain bike at 20 mph solo and on the tail end of a semi. #
July 14, 2011