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On bicycles.se a question came up about whether one cyclist drafting another causes the lead cyclist to be slowed down. A contributor suggested that the opposite might be the case, that the leading cyclist would be 'helped' too. Clearly, in the real world of cycling there are winds, potholes, traffic and other variables, plus the lead cyclist would not necessarily notice a small bit of extra help. Therefore 'probably not' is not the answer I am looking for, a bit of theory would help.

Another 'drafting situation' happened at Monza today in the qualifying for the F1. The Ferrari team had one of their cars give the other an aerodynamic tow along one of the straights, this helped their number 1 driver get to 4th on the grid, a position he would not necessarily have achieved otherwise, in a race where fractions of a second do matter. If you can better visualize F1 cars than bicycles, then today at Monza is another situation where drafting went on. We know it helps the guy behind, but does it also help the guy up front?

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The effect that I think you are referring to when you say "help the guy up front", is one of a bow wave (or bow shock for very fast moving objects). You can see these types of waves at work on the bows of boats and ships, and they can indeed effect upstream objects. However, for something like a car or bicycle in air, this phenomenon will have an insignificant effect on other such objects. Moreover, in these examples the chasing object is moving into fully turbulent air which will have a dissipative effect on such waves and could even prevent their production altogether. – Killercam Sep 12 '11 at 17:16
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I think the question isn't totally clearly written - the idea is: in an ideal case, what is the effect of the object behind on the object in front, and are there notable dependencies on variables like shape, speed, turbulence? – Jefromi Sep 13 '11 at 16:07
I think Camus hinted at what I was looking for, at modest speeds, in air, does an object behind another object help to increase the speed of the object in front? I think you do get 'bow waves' with big trucks on the road, if one of them follows another immediately behind, will the truck in front go quicker/use less fuel? – ʍǝɥʇɐɯ Sep 13 '11 at 16:16
The aerodynamics of F1 is a lot more complicated than bicycles: part of the limits on the straight-line speed is by available friction between the tyres and the road, which is effected by the downforce generated by the car body and the rear wing. Small changes in the air flow around the rear wing can lead to large effects (think the McLaren F-duct of yesteryear). So I am not so sure that the "observed" effect on an F1 circuit can be "intuitively explained" without referring to some wind-tunnel tests. – Willie Wong Sep 15 '11 at 13:12

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Following closely would speed up both vehicles in certain cases. Consider two cylinders following each other, end to end, at 100 mph, 1000 feet apart. Each cylinder would have a lot of air resistance.

Now consider the cylinders following one another at 1mm separation. The wind resistance will be less for the second cylinder because it's not getting hit in the face with all the wind. There will be less resistance for the lead cylinder because there is no suction to speak of pulling back on it. So the two cylinders are more efficient when they're 1mm apart.

The same thing would apply to cars, depending on shape, separation, and speed.

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