Drafting Behind a Semi

It’s not so much that the car is caught in a vacuum behind the truck. It’s that the semi-truck is breaking up the wind that the car would normally be pushing in front of it. The car is pushing less air because it is in the wake of the truck. You can see this same thing as two boats travel together through water. The truck’s fuel effeciency is not affected though because it has to push the same amount of air whether a car is behind it or not.
Another point I would like to make is that you do not have to be two feet from the truck to experience this. An episode of Myth Busters had actually addressed this issue. They found that the car can be as far as 100 feet from the truck and still get an increase in gas mileage. That would have to be a great deal of suction if vacuum is the explanation.

Typical car: under 3000 lbs, about 12 feet long and 4.5 feet wide and 5.5 feet tall 25 mpg.

Typical 18 wheeler: 80,000 lbs, 70 feet long, 8 feet wide, 13 feet tall 5.5 mpg

Whatever the savings or cost to the truck is going to be calculated 3 or 4 places to the right of the decimal. In terms of significant numbers, it’s not.

Skip

I am an engineering student
and have been studying recently in one of my classes. The
caller stated that he thought that slip streaming cars were
costing him money because he was giving them energy.
They replied by saying that they thought the car drafting was actually
saving him money (in fuel) but that They did not know
exactly why. They were correct and this is why. A normal
semi traveling down the road at a typical speed, lets say
70 mph, would have the air hitting the front of the truck
would be pushed to the side at relatively high pressures
and would travel along the side until it reached the end of
the trailer. At the end of the trailer there would be a low
pressure “bubble” where the air would swirl around. A
simple way of thinking about it is to consider the air
pushing back on the front of the truck and the lack of air
in the rear sucking the truck back. This is creating a
phenomenon known a pressure drag. There is also friction
drag but thats a topic for another day. A normal car has
these same effects, just not as bad because they are more
streamlined. When the car gets in behind the truck within
the area that the air is swirling around, it looses the air
pushing on the front, and because of the shape it creates
when combined with the truck it also looses some of its
“sucking” on the rear. This means less drag for the car
and thus much better gas mileage. As for the truck. To
put it simply, the car is making the truck more
streamlined and this the area that would normally be low
pressure and sucking the truck back is not because the air
is flowing around the car. I hope this helps, it can be a
very complicated subject but those are the cliffs notes.

Also, someone said that drafting works better at higher speeds.
This is not necessairaly true. It would depend on the shape of
the semi and trailer. You would have to determine if it the air
was in a laminar, transition, or turbulent flow regieme and it
would take a lot of research and testing for a particular vehicle
and could even change with the weather. The blanket statement that
faster speed means better drafting is not true.

The Car Talk guys were right about drafting. The second car improves the aerodynamics of the first car and allows both vehicles to travel together either getting better mileage or going faster. It doesn’t cost the first car (truck) anything.

Road & Track magazine proved this a while back. They had a Porsche 928 that could do about 145mph and a Corvette that could do about 135 (top speed without drafting). When the Corvette drafted behind the Porsche, the pair of cars could do 150, faster than either of them could do on their own.

And how far behind the Porsche was the Corvette?

There are lots of interesting comments here ahead of me on the technical aspects, I?ld like to make a social comment. Everyone agrees that the caller was right about the conservation of energy but that led him to a false conclusion. Saving energy by reducing drag is not the same thing as creating energy.

Socially we have the feeling that the piper must be paid. If one person does not pay then another must. But the fact is that once I hire a band, we can all dance and the band actually plays better for having a larger audience.

Love your show.

I like Rocky’s definition from the RV Now website . . . “As to drafting truckers, you are in a vortex behind the truck that is there because of his poor aerodynamics. That vortex actually tries to pull him backwards. So, breaking that vortex up by drafting is as much of a help as anything. The real issue is safety; keeping a safe distance between you and the truck. I use the wind deflector on the hood of my truck to tell when I have entered the back draft - it actually stands up and even bends forward a bit in a good draft. If I do that, I can stay at the far end of the back draft, keeping a safe distance behind the truck.” I think that there are several experiments that would be worthy of you that can be tried. For instance, a 50 foot length of PVC pipe slightly larger in diameter than a beer can. Next, start water moving through the pipe. Feed a nylon string (shot line) through with the water. Attach the bitter end of the line to a beer can. on the other end, attach a fish scale to the reel of line and start pulling the beer can through against the water. After it is through, and you have recorded the pounds of effort, push it back through, and attach two additional beer cans at about 2 inch intervals. . . preferably full. You can empty them later. Now, pull the tandem cans through the pipe and measure the pounds of effort. You can also re-run the experiment by placing the cans approximately 3 feet apart and measuring the effort at that configuration. Good luck with the steak dinner . . . maybe it should’ve been a stake dinner.

Special aerodynamic rear doors have been developed for trucks that claim a significant reduction in drag. I don’t know if they have reached the market yet. Those who claim that the savings come from the reduction of energy loss to turbulence are correct. MIT is sending out a recall notice to Tom and Ray.

The caller is right in that the energy a drafting car “gains” is “lost” somewhere else. But that lost energy isn’t from the fuel of the semi — it is from the drag behind the truck. And Ray is right that the truck benefits also — but not because the car is pushing the truck (an air cushion couldn’t be formed in front of the car as there is nothing to trap the air and the relative speed between the two vehicles is small).

A low pressure area is created behind the truck that tries to “drag” or pull the truck backward (the more square the back end, the more drag is created). The drafting car reduces the drag as it acts like a faring (making the rear of the truck more streamlined). This low pressure area “traps” the car (to some degree) and pulls it forward (thus helping the car) while the car reduces the drag on the truck which lessens the force pulling the truck backwards (benefiting the truck).

So the energy gained by the car would have been lost in drag and turbulence anyway — drafting essentially increases the efficiency of the truck.

Yes, drafting does help the car or truck in front. But it isn’t quite correct to say that the car behind is giving a push to the truck in front. In a way it is, but the visual doesn’t work.

A drafting car helps the truck in front because it improves the truck’s coefficient of drag. Normally, a semi has a huge squared-off rear end, a shape that causes large eddies and a lot of turbulence to form in the wake, all of which is a big source of aerodynamic drag. In fact, a lot of the drag on a semi is created by the blunt tail end of the trailer and the air rudely spilling off of it.

Putting a car back there helps straighten the flow off the back of the truck, making the airflow more laminar as it departs the truck, which reduces its drag.

The car benefits because it doesn’t have to break wind. I’ll say it another way: the car doesn’t have to punch a hole in the air by itself–the truck is doing that for it. Yes, there is a bubble of negative pressure behind the truck, and that’s another way to look at it. But all of this is just another way of saying that the car’s coefficient of drag is improved (from it’s point of view), by the presence of the truck close in front.

And for both vehicles, reduced drag means improved fuel consumption, or in the case of a pair of NASCAR stock cars (or a whole string of them), increased top speed.

I have nothing more to say that hasn’t been said. A moving vehicle has a large cushion of air in front of it, slowing it down. It also has a large vacuum behind it, slowing it down.

Two vehicles, one drafting the other, merge the lead’s vacuum with the rear’s air surplus, reducing the drag on both.

Thus, fuel is conserved.

This is how traffic may be organized when autodrive systems become prevalent (and networked).

Dear Ray & Tom:

It was with great interest and amusement that I listened to your discussion with the truck driver concerned that cars drafting in his wake might be increasing their fuel economy at the expense of his.

Although I have a degree in aerospace engineering and have been an aircraft structural design engineer for over a quarter of a century I make no claims as to being an expert aerodynamicist. Admitting that I know nothing, of course, puts me light years ahead of you bozos.

However, I will say that although I generally agree with your conclusions, the drafters are indeed not adversely impacting the truck, and may even be helping, I feel compelled to assist you in improving your infantile thought processes.

Firstly, if you have a piece of ply wood standing erect on a large flat surface such that a wind (subsonic flow) is blowing directly against its largest face you will have high pressure on the forward side of the plywood, and low pressure air on the back side, as the wind tries to converge back to an uninterrupted stream. The force resulting from the difference in pressures is called drag.

If you were to map the pressure in the region behind the plywood you would find the lowest pressure immediately on the back side, and gradually increasing pressure as you move further downstream from the back side (ignoring effects of turbulence). This is scientifically referred to as the Hersey?s Kiss (HK) effect in honor of the shape of the low pressure region in back of our imaginary plywood (not really, but just trying to help you guys understand).

Hence, a flat piece of plywood is said to have high ?form drag? which means it?s not a very aerodynamic or efficient shape, as opposed to an airfoil which has low form drag (it?s streamlined stupid). The airfoil?s reduced drag is largely the result of having a rounded leading edge and a converging trailing edge which allows the wind flowing around it to quickly re-converge back to its uninterrupted state (that is it gives the HK a hickey). Also note that the same piece of plywood laid flat so that only the edge was facing into the wind would have much lower form drag.

Regardless, a large, more or less square, truck attempting to bludgeon its way through the atmosphere will have high form drag and a correspondingly large HK behind it. Automobiles, at great personal risk (as you noted), can take advantage of this effect simply by utilizing the decreased pressure in the HK region to reduce the pressure on the forward surfaces of the car. This results in significant, easily measurable, improvements in the car?s gas mileage.

Now, to digress briefly, if you turn a cup upside down and immerse it in water a pocket of air will be trapped in the cup and keep the water from entering. This has absolutely nothing to do with the above discussion. But, analogizing the car in the Hersey?s Kiss region of the truck to a ping pong ball placed inside the cup, the car has no more effect on the HK than the ping pong ball does on the pocket of air (or a similar very limited effect). Hence, one may deduce that the car does not create additional drag for the truck.

Where the above analogy starts to fail is that the car, due to complex aerodynamic interactions (which blow right by you guys), may in fact be decreasing the HK effect in a fashion similar to having a fairing on the back of the truck.

A fairing would tend to streamline the truck?s shape which would reduce the form drag. Similarly, having a spoiler which deflects air downward into the HK region would also diminish its effects. Or, you could shoot high pressure streams of air downward off the top of the truck on to a curved spoiler, or fairing, mounted just below the trailing edge and take advantage of the Coanda effect. But, perhaps that?s going too far for this discussion (and especially for you morons).

Regardless, and finally, although a car, even in the HK, may well have higher pressure on its frontal surfaces, relative to the diminished HK pressures, to conclude that the car is ?pushing? the truck (using your simplistic terminology), is a bit of a reach. It is unlikely the cushion of air on the leading edges of the car would have sufficient force to actually push the truck (although, again, one can?t rule out unforeseen aerodynamic interactions in a complex 3D subsonic flow field). But, even if the car is pushing the truck, it is probably a very insignificant effect, somewhere on the same order of magnitude as your understanding of aerodynamics.

So, I conclude by stating I am certain a drafting car (parasite) isn?t hurting the (host) truck, and feel there is a distinct possibility it may be helping. The help which may be provided is much more likely the result of improved streamlining, rather than any physical force imparted from the car to the truck. And, I suspect the less the disparity in size between the truck and the car (or other vehicle), the greater the potential for the car to be actually assisting the truck.

Note that conservation of energy is irrelevant to this discussion. Any beneficial effects for either the truck or the car are the result of reduced pressures and or streamlining, allowing both vehicles to operate more efficiently. There is no energy transfer (stealing) between the car and the truck.

I hope I have been able to assist you in your feeble efforts to explain this phenomenon, its ramifications and consequences.

Sincerely,

kadini
Bothell WA

Don’t forget that wild geese have been doing this for millennia. Natural selection has shown itself to be pretty good at developing efficient energy utilizing behaviors and systems.

I can see how both vehicles would benefit. But I think the Tappet Brothers are a bit off the mark over why this is so. The semi does not suck the car along, and the car does not push the semi.

The boxy shape of the semi trailer is aerodynamically inefficient, and creates a partial vacuum in the space immediately behind the trailer. As the trailer moves it displaces air, and air spills back into the area just behind the trailer, but it can’t keep up with the motion of the semi.

The faster the semi goes, the greater the vacuum generated behind the trailer. This lower air pressure pulls back on the semi, reducing its fuel efficiency. But this lower air pressure can help the car.

The low air pressure at the back of the semi reduces the drag the drafting car encounters, thus enabling the car to move at the same speed while burning less fuel. The car benefits.

Having a car behind the semi reduces the void of displaced air that needs to be “refilled”. This means the vacuum behind the semi is reduced, so there is less of a pull behind the trailer. The semi benefits.

When the car is drafting behind the semi both vehicles need to be considered as a system that overall is more aerodynamically efficient the separated vehicles are when they travel far apart.

As far as race cars mentioned in some other comments are concerned, the lead car is at the greatest advantage, because the air it drives through is less turbulent. It has the most downforce, and can go through corners at the highest speed. All the trailing cars are driving through turbulent air that the lead car stirred up. The trailing cars encounter less drag, but they also generate less downforce so they must corner at lower speeds.

First look at only the semi. As the semi travels forward air flows over the front to the rear of the semi. The air separates from the surface of the semi at the rear. So the air pressure at the front is higher than at the rear. The pressure differential pulls (exerts a force) the semi from high pressure to low pressure creating drag.

Along comes a car to the rear of the semi. As the car nears the low pressure region near the rear of the semi, the car experiences a lower pressure towards its front than at its rear. The car is pulled forward requiring less energy to and conserving gasoline, diesel, ethanol, or recycled french fry oil–depending on the engine configuration.

The semi now will also conserve energy because the car fills in part of the low pressure area reducing drag on the semi. The semi-car system now presents a longer flow surface reducing the low pressure area behind semi rear surface. The semi will expend less energy saving on diesel. Eventually the semi-car system will reach an equilibrium that benefits both vehicles.

The semi gives up no energy to the car because they are not attached. Likewise the car will give up or take no energy to the semi. The caller correctly stated that energy cannot be created. The energy transfer takes place in the surrounding air. The pressure differential is what makes each vehicle benefit the other. The movement of the air pushing and pulling on the vehicles will normally experience a temperature change.

In reality the semi will benefit very little because of its mass as compared to the car. The car will benefit most the lighter it is. Now if a semi is drafting on the rear then the benefit to both semis will be nearer to equal.

John Trinkle
Troy, VA (near Charlottesville)
434-589-2126

How about if the car, with the smaller profile, was in front? Would the truck see a decrease in fuel consumption due to the more streamlined frontal surface or would the lack of a more streamlined rear substantially increase the rate of fuel use? Would the car benefit by having the truck drafting?

drafting behind a large truck

A large vehicle sets the air into motion as it moves. As a
result, the wake behind the large vehicle moves in the same
direction. A smaller vehicle located in the wake moves into
an air mass moving in a favorable direction (like a tailwind in
airflight). The overall drag of the second vehicle is reduced
because the relative average airspeed is reduced compared
to the ground speed. Drag reduction implies fuel savings
because the engine does less work to maintain the ground
speed. A small decrease in airspeed causes a proportionally
larger decrease in drag because the drag is proportional to
the relative speed squared (D~V*V)


I’m late weighing in on this, but here are some simple explanations.

Why does the car benefit at no cost to the truck? Imagine the vehicles are travelling through a tray of marbles (an analogy to the molecules in air). Once the truck pushes the marbles out of the way, the car has less drag. But the work the truck has to do to push them out of the way is no greater, whether or not the car is following behind.

But why does the car actually help the truck? After the truck moves the molecules of air aside, they have to fill in behind the truck as it moves forward. That creates lower pressure behind the truck than in front of it, which effectively adds to the work the truck has to do to move forward. Having the car directly behind fills part of that space, which reduces turbulence and the drag on the truck.

So Ray is correct, although his reasoning that the car is pushing the truck makes no sense

Guys! I listened through that entire discussion (twice, Saturday and Sunday) in urgent anticipation for you to EXPLAIN the dynamics of the thing. But you never uttered the words “aspect ratio” and “Reynolds number.” The two vehicles close together behave aerodynamically as one with a larger length-to-width ratio than either vehicle alone. That amounts to a better streamlined unit. That’s why a racing skull CAN go faster easier than a fat canoe. Part of the difference comes from a slightly longer interval of time between the parting and reuniting of the air they pass through.

They could do even better to get a van in front of the truck, a van behind it, then the automobile.

I have this to say about that.