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October 20, 8:30 a.m. This is not a normal day at the wind tunnel in Sindelfingen. It’s Saturday, but that’s not the reason why. Our wind tunnel is currently in great demand because of the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) laboratory test, so it’s also being used on weekends, with supervision done by employees on a voluntary basis. I basically never volunteer, because I keep my weekends free for my wife and three children whenever possible.
The reason why it’s not a normal morning is that today we are hosting a special guest that I’m especially happy to meet: Jan Frodeno. He’s the first triathlete to win not only an Olympic gold medal but also the championship in the world’s most famous triathlon, the Ironman in Hawaii. On top of that, for over two years he has held the world records for the shortest times in the long-distance competitions: an incredible 7 hours and 35 minutes and 39 seconds for 3.86 km of swimming, 180.2 km of cycling, and 42.2 km of running. As a brand ambassador for Mercedes-Benz, he very credibly represents the slogan “The Best or Nothing.”
The central part of an Ironman triathlon — as well as the longest and thus the decisive part — is the 180-metre-long individual time trial on a bicycle. After the swimming segment, well-trained triathletes are sure to have enough power for the cycling segment. But the amount of energy they have left for the final marathon when they dismount from their bikes after over four hours of cycling depends to a large extent on their air resistance.
So Jan and I will have at least one topic in common to discuss inside the wind tunnel in front of the camera team. I’ve always been an enthusiastic cyclist. I used to ride my racing bike on the road, but today I mostly ride my mountain bike between Warmbronn, Sindelfingen, the Bärensee lakes, and Solitude Palace. Sometimes I also go cycling in the Black Forest or in the Alps.
A personal meeting with an athlete who performs at such a high level and also cycles is a very special honour. And the opportunity to engage in shop talk with him about aerodynamics in the wind tunnel makes our meeting even more appealing. Jan will ride his bike there in order to show how he defies the wind beating against him, whereas I have brought along the new CLA as a demonstration aid.
Our shared topic: Achieving more with less energy
The driving resistances are the same for a bicycle as for a car: rolling resistance (which is basically friction and the flexing of the tyres) and the incline resistance when you’re going uphill. Both of these factors largely depend on the weight of the cyclist and the bike. And of course there’s also the air resistance, which increases together with the square of the velocity — unfortunately for the cyclist. If you drive or ride twice as fast as you did before, you have to use four times as much power.
And if you’re cycling as fast as you can for 180 kilometres, the air resistance will be the crucial factor. For reasons of fairness, riding in the slipstream of another athlete is forbidden. This is the least respected rule in the triathlon. Violations of it result in the largest number of warnings and time penalties. Participants are obliged to maintain at least a twelve-metre distance from the next cyclist.
Because air resistance is so important, Jan Frodeno always calculates the air resistance in watts — in other words, the power he must summon up in order to maintain a set speed against the headwind. And he knows very precisely how much power he can generate during a period of just over four hours.
Air resistance is a combination of area and shape
The physical unit of air resistance is m² — in other words, an area. An area? Of course: The greater the cross-sectional area a body presents to the wind, the higher is the air resistance. The second criterion is how smoothly the wind can glide around the body, which is expressed as the familiar Cd value. When these two factors are multiplied together, the result is the air resistance.
Here’s an example: A square panel standing upright has a Cd value of 1.12 (it’s actually 1, but the edge vortex adds a bit more). At first glance, a truck seen from the front looks very similar to this square panel, but it actually has a Cd value of less than 0.5, because of lots of detail work and because the wind can stream past it lengthwise (as shipbuilders know, long and narrow boats go faster). Thus in spite of a frontal area measuring about 10 m², a truck has an effective air resistance of just under 5 m².
Today’s cars have a frontal area measuring between 2 m² (sedans) and 3 m² (big SUVs). In order to make the vehicles’ interiors comfortable and safe, narrow limits are set for frontal area optimisation. And that’s why we work so hard at Mercedes to reduce the Cd value. In addition, now and then we receive various requests from the design department and other development units. That’s what makes our work so interesting.
Enough about physics — back to cycling
Cyclists who ride fast instinctively lean forward in order to offer the wind as small a frontal area as possible. If you’re sitting upright on a touring bike, you’ve got an air resistance of approximately 0.7 m² (frontal area: about 0.7 m², Cd value: about 1 because of the many vortices). A mountain biker who is leaning forward offers the wind only about 0.6 m² of resistance. A racing cyclist, who leans even further forward and has a more streamlined bike, has an air resistance of less than 0.5 m².
Jan Frodeno tells me he has worked on his posture for hours in a wind tunnel. He also has a specially built racing bike that enables him to hold this optimized posture as long as possible with a minimum of muscle cramps. In addition, he wears a close-fitting racing suit without any seams or creases, shaves off the hair on the parts of his body that are still exposed (which is also an advantage during a massage and after a fall), and wears an especially streamlined helmet. His bike, which was built by Jan’s outfitter, Canyon, is designed to be as streamlined as possible in every detail — tyres, wheel rims, frame, gears, even the clickies.
Making yourself small leads to big results
Jan’s success confirms his strategy: He tells me that on his time trial machine he has an air resistance of 0.21 m². That’s about half as much as a person sitting in a natural position while riding an ordinary racing bicycle — in spite of the fact that Jan’s height of 1.94 metres and his athletic shoulders are something of handicap when he’s on a racing bike.
Jan tells me that he even does special physiotherapy exercises that help him press his shoulders together to make them especially round and narrow, just so he can reduce his frontal area. For me, this is very fascinating information. The efforts he has to make remind me of the work we do on cars. But altering the human body to such an extent costs so much more personal pain and sacrifice than developing an improved wheel spoiler!
One world champion meets another
At the moment, our most streamlined series-produced car is the current world record holder in this group: the new A-Class sedan. Its Cd value is only 0.22. Its frontal area measures 2.19 m², resulting in an air resistance value of 0.49 m². That’s about as much as a normal racing cyclist and somewhat more than two Jan Frodenos on time trial machines.
However, the A-Class has space for two people sitting side by side and even for two or three additional occupants on the back seat. None of them have to go to a physiotherapist to make their shoulders round. In fact, each of them can even take along a suitcase.
Before the new A-Class won the streamlining record, it was held by the predecessor of our new CLA. However, because of its wider track the new CLA can no longer claim the title — it has a Cd of 0.23. That’s still an excellent value, but it shows that in the field of aerodynamics it’s gradually getting harder and harder to make each new model better than its predecessor. The deck is constantly being reshuffled.
However, we’ve managed to ensure that all the versions of the new CLA have values lying very close together. The old CLA had a version that held the record, but the other versions were not quite as good. It’s a good thing that this is different for the new CLA, because in the calculation of fuel consumption and CO2 certification according to the WLTP, the precise air resistance value of each individual version and each set of equipment has to be taken into account.
Aerodynamics has its limits, for bikes…
The comparison between the racing cyclist and the car illustrates the potential of aerodynamics — as well as its limits. The shape of the human body is a given, and it’s not practical (and in the case of sports, it’s prohibited by the rules) to alter it by means of extensions or appendages. Jan has to sit on his bike comfortably enough so that he can deliver his top performance for more than four hours with a minimum of cramps and as few risks of injury as possible. What good would it do him to have even less air resistance while cycling if it makes him so tense that he can no longer run with ease?
…and for cars
The situation for cars is similar. The frontal areas have not grown any smaller for years now, because of the many other demands placed on the design. Car lengths can’t be changed arbitrarily, and for Mercedes in particular, the comfort and safety of the occupants is priority Number One. And because the proportions of cars are more or less fixed, aerodynamics sooner or later reaches its limits.
From our perspective today, a Cd value of 0.2 is more or less the “sound barrier” for a car shape that still has practical utility. All the same, for years now we’ve been trying again and again to push it back. The ideal car shape would be a long teardrop tapering toward the back, without any wheels — but that would not be a very practical vehicle.
The most efficient route to efficiency
Every new vehicle faces us with the challenge to get as close as we can to the optimal aerodynamics. Aerodynamics is the most efficient route we have to achieve energy efficiency. Nothing makes movement lighter and more energy-efficient than a streamlined shape. And that’s why it’s always satisfying to achieve an improvement, even if it’s the result of small changes. Aerodynamics saves energy, and thus fuel or battery capacity, and it increases a vehicle’s range. It enables us to travel farther or faster.
Jan knows this as well, and he will continue to work on improving tiny details. After our meeting in the wind tunnel, I hop on to my bike once again, and while I’m cycling I do some more thinking about efficient streamline optimisation. Hmmm…should I try to find out how streamlining applies to swimming? Jan would also have lots to say about the science of water resistance. We could meet at the swimming pool on a Saturday to talk about that.