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#1
Old 03-12-2002, 12:42 PM
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Physics of Race Car Drafting

What are the scientific explanations behind race car drafting? I know that it is supposed to help the cars behind it go faster, but why does this work and what would happen if a car tried to get out of this draft?
#2
Old 03-12-2002, 01:05 PM
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ok. Here goes. I'm never too good at explaining things, but I'll try, and hope I'm getting it right...

At 180 mph, the air becomes almost a solid force - stick your hand out the window of your car when you're going 60 on the highway some time, you'll see what I mean. It actually helps if you think of the air the way you think of water - in terms of visualizing.

So the front end of the race car has to "break" the air, so the air will flow up and over the top of the car. (I ought to have a nifty graphic here to show you what I mean...) The air flows over the top of the car, up off the spoiler... and behind the rear bumper the air is all disrupted and non-flowing and the back end of the car creates a little windbreaker.

The second car, if it were by itself, would have to be breaking the air too. But if it can sneak up and get its nose into the little windbreaker area behind the front car, then it can take advantage of the slipstream that's coming off the other car. It doesn't have to break its own air. (Please, no "breaking wind" jokes. It's the best word I can think of right now. Sorry.)

Having a car behind it also helps the front car; I'm not quite sure here, but I think it's because it cuts down on the disruption of the air behind it. This means both cars together can go faster than one car on its own.

I was going to explain the slingshot effect here but I'm not sure how it works, myself. I have a theory (the second car in line doesn't have to do as much work to go the same speed as the car in front of it, which has to work hard to create the slipstream, so the second car ought to have some speed in reserve), but I'm not sure that's how it works.
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#3
Old 03-12-2002, 01:43 PM
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Well the question was answered in thsi months Popular Science Page 75.

Racing had it right.

The first car has a "bubble" of air rushing past it causing drag. When the second car comes up the air current from the first car bounces up and over the second car. This causes the lead car to lose its vacuum and the trailing car to lose its "bubble"

ANyway Pop Sci explained it much better than I could and I am not going to type it all out.

[hijack] How about that honda concept car hybrid? talk about a sweet 400hp 42 mpg machine I gotta get me one of those! [/hijack}
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#4
Old 03-12-2002, 01:45 PM
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If there was ever a thread tailor-made for an Aerospace Engineer racing fanatic! Anyway, when two cars are drafting, the front car blocks the free airstream from impinging on the front of the drafting vehicle, so the drag is less. Also, the trailing car fills in the area behind the lead car, which reduces the vortex behind the lead car and the drag is less for the leader as well, but not as much as for the trailing car.

Drafting also effects downforce and handling. Sometimes you'll hear drivers comment about how their car handles a certain way in the draft, but a different way by itself. Since the air is no longer pushing on the front of the trailing car, front downforce is reduced and the car will start to understeer (or "push"). That is, the car will not want to turn as well. For the lead car, the effect is to lift the airstream off of the spoiler and cause the car to oversteer (also called "loose"). The back of the car will want to snap around and spin the car in a turn. (Drivers sometimes take advantage of this by getting the lead car a little loose, forcing the driver to check up a bit.)

The net result is that two cars running together on a high-speed oval (such as Daytona, for example) will lap faster than a single car running alone. If there is a group of cars drafting in line and a car steps out of the draft, it will generally fall back until it can get back in line. Interestingly, three cars will be a little bit faster than two, and four a bit faster than three, beyond which you get diminishing returns (the reasons are a bit complicated for this post, but it has to do with the ratio of overall length to cross sectional area getting longer). This is why you'll often see a lead pack of three or four cars breaking away from the rest of the group.

Sometimes a car can also use the draft to slingshot around another car. Basically, it is taking advantage of the reduced drag to run up on the lead car and build up enough momentum to pull out and pass the lead car before the increased drag of running in the full airstream slows it back down to it's normal speed. This requires careful timing to pull off successfully.

Then there is bump drafting, where the trailing car will actually run up and bump the lead car. This will give the leader an additional 1-2 mph boost and pull the trailing car along with it. (The danger is all this bumping might close up your air inlets and cause the car to overheat).

All of this assumes you're talking about NASCAR-style cars. For high-downforce cars, such as a Formula 1 or Indy car, the turbulence and reduced downforce upset the trailing car so much, they generally don't form the drafting packs you see in stock car racing. Usually they use drafting (which is generally called slipstreaming in European racing) just for slingshot passes.

Also, you don't have to be at race-car speeds to experience drafting. Even bicycles can draft, for example (which is why these races tend to have a large peloton).

(On preview, I notice racinchikki beat me to it, but since my post has some information hers doesn't, I'll post anyway.)
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#5
Old 03-12-2002, 01:47 PM
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When the front car is going say 180 mph as the air flows over the car there is a hole behind the car that the air must flow back into. This creates a low pressure area. If the following car can get into this low pressure area then the second car will be pulled along by the first car. Because the second car is not breaking wind (sorry I couldn't resist) the second driver can back way out of the throttle and maintain speed. This two car train actually has a little less air resistance than a single car so the "train" can go slightly faster.
As far as slingshoting goes since the second car is not using all his throttle to break wind (sorry again) the driver can get as much space as possible (without losing the draft) between the cars then floor the gas. In absence of wind resistance the second car will accellerate rapidly at the last instant a lane change is made and the second car is going about 5 mph faster than the first car.
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Old 03-13-2002, 12:34 PM
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Quote:
Interestingly, three cars will be a little bit faster than two, and four a bit faster than three, beyond which you get diminishing returns
Unless they're doing bump drafting, I don't see why this would be. The lead car has it's vortex reduced by the second car, and it's the lead car which is limiting the group speed (all cars being more-or-less equal). What effect are the third and higher cars having on the first car that lets it go faster?
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#7
Old 03-13-2002, 12:59 PM
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Quote:
Originally posted by Rick
As far as slingshoting goes since the second car is not using all his throttle to break wind (sorry again) the driver can get as much space as possible (without losing the draft) between the cars then floor the gas. In absence of wind resistance the second car will accellerate rapidly at the last instant a lane change is made and the second car is going about 5 mph faster than the first car.
Ah, good. That's pretty much what I had originally typed up to include... But then I remembered Buddy Baker (I think it was Buddy Baker) saying some ridiculous not-that explanation of slingshotting during a race on TV a couple years ago, and I didn't want to provide misinformation.
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Old 03-13-2002, 06:20 PM
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Quote:
Originally posted by ZenBeam


Unless they're doing bump drafting, I don't see why this would be. The lead car has it's vortex reduced by the second car, and it's the lead car which is limiting the group speed (all cars being more-or-less equal). What effect are the third and higher cars having on the first car that lets it go faster?
Welcome to the wonderful world of aerodynamics. Check your common sense in at the door, you won't be needing it here.

The trailing cars do effect the airflow ahead of the front car as well. The airstream in front of the car "knows" about the cars behind it and will deflect accordingly. One example I can find where you can see this effect is this photo of an airfoil in a smoke tunnel. The streamlines in front of the airfoil are clearly deflecting before ever reaching the leading edge. If that airfoil were to extend a flap from the trailing edge, you would be able to clearly see the airflow in front of the wing change (I wish I could find a video of that, it's really pretty cool). When several cars are all in the draft together, the airflow treats them as if they were all one object. The drag coefficient of a long, thin object is generally less than a short object, so the overall Cd of the three or four car group is less than one or two cars.

I hope this makes sense, I tried not to get too technical.
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