Those of you who live in England miss out on one of our best radio shows on NPR. I haven't heard of anyone who doesn't find it entertaining: "Car Talk." Yes, I know it sounds boring, but the 2 hosts are quite funny and entertaining.

Today they talked about "slipstream," a new word for me. It is an area of reduced pressure produced by a fast-moving car as it moves through air, thus reducing the resistance for the car in back.

The question for the hosts was whether the car in back affects the car in front in any way. The hosts admitted that they did not know physics, but they guessed that it would affect the "molecules" and therefore would affect the car in front. I just don't see that, though I could be convinced by a more cogent discussion than they had!

The turbulent air rushing around the car creates a vacuum in back that reduces resistance for the trailing car. The vacuum would also tend to pull the rear car along, much the same way you "pull" a milkshake up through a straw. Both cars can't get a free ride. The rear car would produce a drag on the leading car, thus slowing it down a slight amount. How much, I don't know, but it would depend on how close and how large the rear car was. I imagine it would be an insignificant amount, but I don't know. Perhaps one of the physicists in our group can give a better answer.

I've heard that bicyclists will sometimes ride in the slipstream of a large semi. The semi blocks the wind and creates a drag to pull the bicyclist along. If the bicyclist gets too close, though, he will be caught in the vacuum and won't be able to break free.

Tinman

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I've heard that bicyclists will sometimes ride in the slipstream of a large semi.

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Slipstreaming plays a large part in track cycle races. It is sometimes quite funny to watch the cyclists all fighting to avoid being in front.

The one who ends up at the front finds the others tucking into his or her slipstream and getting pulled round by the leader's efforts. On the last lap they pull out, and, because they are fresher, usually overtake.

In fact, although it's not a completely free ride, there is an overall reduction in resistance. The reason is simple enough.

Most of the drag is created at the front and back of the moving body where the air is turbulent. The friction of the air against the sides of the body is less important, especially at slower speeds (less than sonic).

So regardless of the length of the body, there will only be the two areas of severe turbulence - the front and the back. So three separate vehicles each 20 feet long will create thrice the drag of one 60 foot vehicle. If the three 20 foot vehicles travel in close convoy, then the effect is similar to that of one 60 foot long venicle.

This is why birds tend to travel in flocks and observation has found that the lead bird changes from time to time so that the main effort is distributed

Richard English

I listened to that episode of Car Talk, and was surprised that the crazy Magliazzi borthers, one of whom has a PhD, did not know the correct answer, which Richard has nicely explained above.

While I'm not a track cyclist, I have ridden in many a "paceline," as a group of tightly spaced cyclists is usualy called, and assure all that the lead, or "pulling" rider benefits from those "sucking his wheel," though not as much as the others. Just as a flock of birds rotates the lead, a group of cyclists in a paceline rotates the lead, so that all in the line can maintain a good pace - usually over thirty miles per hour - for many miles.

If at first you don't succeed, you'll only go skydiving once.

Quote "...I listened to that episode of Car Talk, and was surprised that the crazy Magliazzi borthers, one of whom has a PhD, did not know the correct answer..."

It has long been a source of irritation to me that the various media gurus tend to know very little. Their frequent inaccuracies are quite inexcusable since, not only are the questions that are asked usually fairly simple, but also they often get notice of them in advance and could easily research the answers.

Their real talent (which I confess I do not share) is the ability to get themselves into the public eye until eventually they are famous simply for being famous and are thus always being asked to appear in or on the media!

Sadly I have never found any way of correcting the inaccurate information they give since the media that use them never answer my letters and certainly never admit their fault.

Richard English

Richard, the wonderful thing about the crazy Magliazzi brothers is that they actually welcome correction! I'll wager that come next Saturday they'll read a correction of their answer on the air! They'll cry, "Mea Culpa" - or "Nostra Culpa!" Then the brother with the PhD will blame the one with "only" a Master's degree, and the mayhem will ensue - all in good fun. Car Talk is a combination comedy/car repair show that is one of the very few really entertaining programs produced here in the USA. It is NOT just another of the typical pedantic ravings of an ego-inflated redneck, but a program that can stand head-to-head with the good stuff produced in the UK. Go to the "Car Talk" section of Cars.com and check them out!

If at first you don't succeed, you'll only go skydiving once.

To add to that, Asa, the brothers both freely admitted that they didn't know for sure. In fact one ridiculed the other for having to take freshman physics a number of times before finally passing, with an A at that! Asa is right, Richard, that this is not one of those know-it-all shows.

Am I to assume then that the front biker, bird, car, whatever, has less speed because of the cars in back? Or is it just that the vehicles in back have more speed because of less resistance?

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Originally posted by Kalleh:
Am I to assume then that the front biker, bird, car, whatever, has _less_ speed because of the cars in back? Or is it just that the vehicles in back have more speed because of less resistance?

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The front biker, bird, car or whatever will have as much speed as he, she or it can generate BUT will have a harder time of it since they are all pedaling, flying, driving or whatever for two. Picture an invisible towline between the back of the lead bicyclist and the leader of the pack. Brummmm! Brummmm! (No, wait. That was the Shangra-Las, another kettle of fish entirely.)

Once when I was young and foolish (as opposed to today when I'm old and likewise) I drove my little Japanese sub-compact some 200 miles by extremely tailgating a tractor-trailer truck doing some 60 or 70 m.p.h. By bringing my car within a foot or so of the big rig I found I could completely take my foot off the gas and be pulled along in that slipstream you're talking about. For most of the trip I didn't have the nerve to maintain that ridiculously short distance since it was, after all, on an interstate but, in any event, I kept my foot hovering over the break pedal and my eyes glued to the truck's break lights.

My thinking was that in the event we had to hit the breaks, it would take the semi longer to come to a full stop (not to be confused with a period) and that I would be safe. Turns out I was lucky no emergency stopping was required. I later bragged to a friend about the astoundingly high gas milage I had logged on that trip, so high that I boasted that I might never have to buy gas again, and was informed that those big trucks, with 18 wheels on the road compared to my measly four, can stop more quickly than I had realized. I could have very easily been decapitated.


I'm 52 now. How I got here is nothing short of a miracle.

Please don't do that again, CJ! I have seen too many massive traumas in the ICU!

It's an attractive notion but it's not actually true. The additional effort required by the lead cyclist, or other vehicle is little or nothing (and might even be reduced).

The drag is caused by the turbulence of the air at the front and the back of the vehicle and another vehicle entering the turbulent zone at the rear of the first vehicle will not increase the drag. Indeed, if the rear vehhicle were to be well-streamlined it might even reduce it.

It's not getting something for nothing and thus breaking the laws of physics; all that's happening is that the loss of energy through turbulence is being reduced. There is still a loss, though; no extra energy has been created.

Richard English

As we all know (except, perhaps, C.J. Strolin,) the recommended distance between your vehicle and the one ahead of you is one car length for every ten miles per hour of speed.

"Following too close" is a legal charge that's easy to prove. If, as a result of the truck driver's having tapped the air brakes, you end up as a client of the ICU, you were following too close(ly).

At 60 or 70 mph you should have been looking at an empty space the length of six or seven cars. Having achieved the ripe old age of 52, you apparently learned that lesson some time ago. Right, C.J.?

60 mph = about 90 feet per second.

[This message was edited by jerry thomas on Mon Jan 12th, 2004 at 14:52.]

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Originally posted by tinman:
The turbulent air rushing around the car creates a vacuum in back that reduces resistance for the trailing car. The vacuum would also tend to pull the rear car along ... Both cars can't get a free ride. The rear car would produce a drag on the leading car, thus slowing it down a slight amount.

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Are you sure of the last sentence?

Agreed that the lead car is slowed by air resistence. Agreed that the rear car benefits from having that resistence blocked by the car in front of it.

But I would think that the lead car is not "dragged" by the rear car: it suffers from air resistence, of course, but the suffering is the same regardless of whether its wake contains a car, or is empty.

The front biker/car/bird/etc WILL benefit from drag reduction, but not to the degree that those following will. Google "fineness ratio" for an explanation.

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Originally posted by shufitz:

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Originally posted by tinman:
The rear car would produce a drag on the leading car, thus slowing it down a slight amount.

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Are you sure of the last sentence?

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No, I'm not sure, but it makes sense to me. The wind rushes around the first car at an accelerated speed, thereby reducing air pressure (Bernoulli Effect) around and immediately behind the car. As C J has pointed out, you can ride in the slipstream created with very little expenditure of energy. If you're close enough behind, the leading car will, in effect, be (at leastly partially) towing the trailing car. This would cause the leading car to slow, or to increase its energy expenditure to maintain speed. The drag may be neglible. It would depend on several variables: the size and weight of each vehicle, the speed they're going, how close they are to each other, and probably others. At least that's the way it seems to me.

By the way, "drag" has several meanings. I'm using it in the sense of 3 b. I think others are using it in the sense of 4 b (1) or 4 b (2).

Tinman

[This message was edited by tinman on Mon Jan 12th, 2004 at 22:48.]

Not unless magnetism or gravity are doing the job. "Suction" works only when there is an area of low pressure on one side of a body and an area of higher pressure on the other. In turbulent condtions this does not happen - and the area behind an unstreamlined vehicle is full of very turbulent air. Any suction created will be transitory and uncontrollable. Think of the way in which small particles behave when thaey are raised by a passing vehicle - some will follow briefly in its wake but they soon fall away.

The leading vehicle's energy is used in overcoming the turbulence and the motion imparted by the turbulent air to other items is not taken directly from the lead vehicle - it has already been given up by that vehicle.

In fact, as I mentioned briefly earlier, a body placed behind the leading vehicle can actually reduce the turbulence drag by allowing the air to change direction more slowly and thus reduce the vacuum created. This is why aircraft (where drag is a major factor) are long and thin and taper off at the rear.

Richard English

Interestingly, I found a bulletin board discussion of "slipstream" on www.cartalk.com. Here is what they said. By the way, in another post, Richard, they mentioned the V-formation of birds that you mentioned:

"Depending on the aerodynamics involved, the lead car could see a benefit or detriment from the trailing car. A train of semis is not the same as a train of Taurii.

If the lead car is very aerodynamic, and the airflow around it rejoins smoothly at the back, then a car following closely enough to enjoy any slipstream advantage will only create turbulence in the wake, which would tend to drag down the lead car.

If the lead car is very un-aerodynamic, then *in theory* a trailing car that tucked into the low-pressure zone behind it and created a teardrop-tail to smooth the airflow's reconnection could benefit both vehicles. It's hard to imagine actual roadworthy cars that would satisfy this, though.

Car designers design cars to be aerodynamic on their own. I have a hard time imagining that they design them to be aerodynamic in a train. Designing a train of uncoupled bodies to maintain laminar flow over the whole thing sounds pretty far-fetched. My guess is that in the real world, a trailing car will tend to disrupt the airflow in the wake of a lead car, creating more turbulence."

I know there is a vacuum created at the rear of a moving vehicle due to the Bernoulli principle, which explains, in part, CJ's incredible gas mileage while he was tailgating a truck. I also know that turbulence, as Richard has pointed out, destroys this vacuum. But I didn't know how to reconcile this conflict. I mentioned this to my son, and he told me to look up "drafting". So I surfed the net for "drafting racing" and "drafting physics of racing" and found several sites explaining drafting, and I'll list four of them. I never was much interested in car racing before, but this has piqued my interest.

The first is a relatively (comparatively?) short article (four pages) by Dr. Joseph Yoon, an electrical engineer with degrees in aerospace engineering and electrical engineering.

second is a fifteen-page article by Mark Martin, NASCAR driver.

The third is a twenty-one page article by David Ronfeldt, a senior social scientist in the Internationsl Studies Group at RAND in Santa Monica, CA. The first part is a reprint of Mark Martin's article, but the last part uses the term "drafting" metaphorically.

The fourth is Brian Beckman's
The Physics of Racing a several-part series of articles.

In general, as I understand it, a moving vehicle has a layer of dense air in front of it and a slight vacuum behind it. These two forces create drag. if a second car pulls up close enough behind the first car, it breaks the vacuum, decreasing the drag on the lead vehicle, while the lead vehicle decreases the wind resistance of the second car. Thus both cars can go faster. But it's more complicated than that. Read the articles (the first three, at least).

Tinman

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