Welcome back to Sports & Building Aerodynamics, in the week on cycling aerodynamics. In this week, we're going to focus on the aerodynamics between a cyclist and a car. We start again with a module question. A team car is following a cyclist in an individual time trial. The distance between the car and the cyclist is 10 meters. What is the aerodynamic effect of the car on the cyclist? Is that A) The car will reduce the aerodynamic resistance of the cyclist. B) There is no aerodynamic effect. Or C) The car will increase the aerodynamic resistance of the cyclist. Please hang on to your answer, and we'll come back to this question later in this module. At the end of this module, you will understand how a car following a cyclist influences his or her aerodynamic resistance and you will understand how the presence of this car can change who wins, and who loses in individual time trials. In earlier modules of this week, I've actually focused on the mutual effect that cyclists can have on their aerodynamic resistance. And of course, it's very well known that the first cyclist provides a benefit for the second one that will be positioned in his wake. But we found that also the second one provides an advantage for the first one. The reason of that was that when there's a second rider, following the first one; that the overpressure area in front of the second one influences and reduces the underpressure area behind the first one. So the first one is actually sucked back less. And this we called subsonic upstream disturbance, to indicate this upstream disturbance of the wind-flow field. And then we saw also that this effect, although it was rather small, it is certainly not negligible; 2.7 to 3.1 percent. The question now is what happens if instead of a cyclist following a cyclist, we have a car following a cyclist. What would happen then? Well, a car is definitely a large body, a very large, much larger body than a cyclist. So it can give more subsonic upstream disturbance if, of course, the car is sufficiently close behind the cyclist. And then the question is, what is sufficiently close? Let's first have a look at a short movie to see how close this can actually get. Now what you see here is a fragment of the 2013 World Road Championships. And you can see that the car following Fabian Cancellara, is actually riding very, very close behind him. How close exactly is not possible to determine from this view, but this could be about 5 meters. So the question arises, how do following cars influence the results of time trials. So then to answer this question we started a detailed study with Computational Fluid Dynamics. I will not go in detail back to the computational settings and parameters; they are actually the same as in the previous modules in this week. So I will only focus on the results in this module. First, we can have a look at the wind speed contours, when we have a distance of five meters between the cyclist and the car. That is, between the front of the car, and the end of the rear wheel of the cyclist. And here you see the wind speed actually indicated in meters per second, for the car and cyclist moving at 15 meters per second, so at 54 kilometers an hour. And from this it is of course clear that the wake behind the cyclist also engulfs the car. But you don't see the upstream effect of the car on the cyclist. In order to see that, we have to look at pressure coefficients. And this is a view of pressure coefficients around the cyclist and the car, where red and yellow color is overpressure, blue color is underpressure. And you see in front of the car indeed the very large overpressure area. In this case expressed as pressure coefficient, so dimensionless. And you see actually that this overpressure area interferes with the underpressure area behind the cyclist. So definitely there will be an aerodynamic effect of the car on the cyclist. Based on these simulations, we can calculate it. And this is the result that we get for different distances in-between cyclist and car. You see here the drag reduction on the cyclist, so for the cyclist's body. Which is quite large, up to 14% for one meter distance between cyclist and car, but of course this is a very dangerous situation, such a close distance. And actually the rules by the International Cycling Union, indicate that the distance should be at least 10 meters. But that's a rule actually born out of safety precautions. That's not a rule taking into account this aerodynamic effect. And we will see later on, that actually this rule is not strict enough, and can change the outcome of time trials. So what does this mean in terms of seconds in an individual time trial? Well, what you see here is actually a calculation of time differences, that you can gain compared to the case where you would have no car following you, when you have a car following you at a distance of one meter, two meter, three and so on. And those are the different curves in this graph. So this graph is semi-logarithmic. You see the logarithmic axis on the vertical axis. And then the distance, total distance of the time trial, on the horizontal axis. And if you have a long time trial of 50 kilometers, and you would have a car following you at a distance of three meters for the entire duration of the time trial, you will gain 62.4 seconds compared to a cyclist that would do the same race without the car. However, three meters is quite dangerous and not very realistic. So let's look at five meters. The difference is 24.1 seconds, and then let's look at 10 meter, which is the minimum distance that you should keep according to the International Cycling Union. But we saw in the movie before that this is not always followed in practice, so that actually cars do drive closer to the cyclist, well then it would be 3.9 seconds. The question is, can these amounts, these time differences can these determine who wins and who loses in a time trial. And we can do that by looking at some recent races. This is the result table, the top ten of the World Championship in 2012. Where you see that the difference between Martin and Phinney is a bit more than five seconds. So here, the effect of the car could have been decisive. This is the time trial result of the Tour de France of 2013. Where also you see that the top three actually is within 10 seconds, so very, very small differences. This is the Tour de France 2010 time trial, where also we have a rather small difference between Cancellara and Martin. And then actually looking at general classifications. Sometimes also a general classification is won by only very small differences. Here you see the Vuelta of 2011, only 13 seconds between Cobo and Froome. And then in the Giro 2012, you see only 16 seconds in the final classification between Hesjedal and Rodriguez. So our conclusion actually is here, that the International Cycling Union should change its regulations, increasing actually, the 10 meter safety threshold to at least 20 meters. To such an extent that the aerodynamic effects of the car on the cyclist are negligible, and to avoid unfair competition. In this module we have learned about how a car following a cyclist influences his or her aerodynamic resistance. And how the presence of this car can change who wins and who loses in an individual time trial. In the next module, we will present to you an interview with professional cycling coaches, about the new insights in cycling aerodynamics that I have presented in this MOOC. Thank you for watching, and we hope to see you again in the next module.
