# Drafting Behind a Semi

When a car drafts behind a semi truck, just who is saving who gas? That was Tim’s question, on this week’s Car Talk. Tim, a semi driver himself, thinks the drafters are benefiting at his expense – after all, someone’s gotta create the energy the drafters are getting, right? “Au contraire, Piston Pop”, say Tim’s kids – they think it doesn’t hurt his mileage at all.

A steak dinner hangs in the balance.

Who’s right?

Ray posited that – ready for this? – a car drafting behind a semi actually saves both vehicles some fuel. No kidding!

What do you think? Could Ray be right, despite all those years of flunking classes at MIT?

Share your thoughts right here. And thanks!

Both, already proven by Mythbusters : http://www.treehugger.com/files/2007/06/drafting_behind.php

But dodgy…

As you say, the suction behind the truck pulls it back, hurting it’s mileage. The presence of an object, the car, in that space breaks up the suction and improves the truck’s mileage. The car is like a tail fairing for the truck. Same thing for NASCAR, drafting helps the lead car. Although much less than it helps the drafting car. sheesh, do you guys know anything about cars?

If you think of the car and the semi as a single system then there is no conservation of energy problem with both of them saving gas. If the car and semi system has less drag when the car is slip-streaming then the saving will be distributed between them. How the savings get split isn’t obvious to me. They could both save some or the car could get more than the total savings (and the truck has to spend a bit more gas to make up the difference).

Drafting causes a reduction of drag for the drafter and the draftee.
Bob

The reason both vehicles get better mileage is because the aerodynamic drag on the individual vehicles, added together, is greater when the vehicles drive separately. The sum of “the suction” between the cars and the drag on the final car is less than the sum of the drag on the individual cars driving separately. In a train, the drag between the vehicles is much smaller than the drag on the last car. Lining them up allows all the vehicles to share the load caused by the aerodynamic drag of the entire train.

It’s all about aerodynamic drag. The back of a trailer is aerodynamic nightmare, so it would seem to be ripe for assistance by a following vehicle.

The confusion was introduced when Tim supposed that the very real “conservation of energy” law of physics could be reformulated as a sort-of “conservation of road vehicle power” law. Trouble is: there is no such law. The confusion was exacerbated by thinking of the truck “pulling” the car behind. It is doing no such thing.

Vehicles require power to move down a flat road at high speed primarily to push the air out of their way (and some to overcome wheel rolling resistance and other mechanical friction). As a truck moves through the air, it builds up a positive (over atmospheric) pressure in front of it and a negative pressure behind it because the air tries move out of the way from the front and tries to fill-in where the truck just moved out from in the back.
For the sake of argument, let?s suppose the pressure in the front is 20 psi and the pressure in the back is 10 psi when no car is present. The difference (20 ? 10 = 10 psi) produces a force that the truck?s engine must provide a counter force against to keep moving at a constant speed.

The car has the same basic problem as well with its own pressure difference, 20 psi in front and 10 psi in back, so its engine must also counter this force. But what if it comes up close behind a truck? The 20 psi high pressure zone in front of the car will now get pushed by the car into the truck?s 10 psi low pressure zone, resulting in a hybrid zone of pressure that is now different from the 10 psi that the truck would otherwise feel. Now it may be, say, 12 psi between the truck and car, while it?s still 20 psi in front of the truck, and 10 psi behind the car.

The result? The truck now feels only 20 - 12 = 8 psi of pressure difference, and the car feels only 12 ? 10 = 2 psi, whereas they both felt 10 psi of pressure when by themselves. Thus, the truck saves a little and the car saves a lot. BUT THEY BOTH SAVE!

?You can?t create energy out of nothing?, Tim said. True, you can?t. But that?s not what?s happening. What is happening is that the car by drafting the truck is using less energy in the first place by staying out of the wind and this is not the same thing as creating energy out of nothing. The energy savings represented by doing something more efficiently does not violate the laws of physics.

Here’s a really easy way to look at it. Suppose that Tom is shoveling the sidewalk of snow on the north side of the street, and Ray is shoveling the sidewalk on the south side of the street, both traveling west. How much energy does it take each of them to “plow” through the snow? Quite a bit! But what if Tom puts down his shovel and simply traverses the sidewalk on the south side of the street instead, just behind where Ray has already shoveled? Tom will use much less energy to travel the sidewalk! THAT MEANS RAY MUST BE DOING MORE WORK! RIGHT?! RIGHT?!

Now then, instead of plowing through snow in their galoshes, what if the scenario had them plowing through air in their cars?

Clear now?

It works, but you have to be so close that you need a death wish to engage in it.

If you’re going with a snow shoveling analogy, a better one would be this: if both Ray and Tom were shoveling 10 inches of snow, imagine the difference if instead of each person shoveling their own path, Tom were to shovel the first 7 inches and Ray were to follow right behind and get the last 3 inches. The end result is the snow still gets shoveled, but both put less effort into it.

A truck and trailer punch a huge hole in the air and the hole has to be filled with something. Its better for the hole to be filled with a soild moving object like a van, followed by a car. The hole in the air closes more gradually and there is much less vaccuum behind the truck.

Look at any situation where drafting occurs; Daytona 500, Indy 500, Tour de France bicycles, speed ice skating. Drafting clearly benefits both the drafter and the leader.

The answer is the aerodynamics of the system (2 cars, 3 cars etc together) has been improved. The two cars drafting together are aerodynamically superior to the sum of the aerodynamics of the two cars separately. When ever you improve efficiency you are doing the same work with less energy - you are not creating new energy

For the semi under normal driving conditions, there is a tremendous amount of turbulance created by air coming around the trailer and filling the space behind. A drafting vehicle fills some of that space, allows some of the air to stream around the car and reduces the volume of turbulance creating drag behind the truck

Ray is right (maybe because of all these years flunking classes at MIT) But not because the drafters somehow are pushing the truck along (as he implied on the air).
I think the easiest way to think about it is the combination of the semi and the drafter has less drag than the total drag faced by each of them when running separate. So the car is facing less drag when running in the slip stream of the semi and therefore spends less energy, and the huge area of turbulence behind the semi which causes a lot of drag on the truck is reduced by the drafter therefore saving the truck energy.
No energy is being created here, just less energy being wasted on fighting drag by each vehicle.

Here’s a pretty good explanation.

I am surprised that you guys BOTH managed to go to MIT and missed the class in aerodynamics that covers these matters.

Drafting works better at higher speeds; and may not “work” (change mileage) at all at lower speeds. That is why race car drivers are famous using it to conserve fuel during a race. But they have lots of safety gear on. It is not clear that a small truck and a car will both save gas.

The amount of gas needed for any given vehicle at any particular speed is dependent on, amongst other factors, the frontal cross sectional area of the vehicle and its length. The effect is summarised in the coefficient of drag which is unique for each vehicle. This Cd is also dependent on the smoothness of the vehicle in question (can the air get around it easily (those Top Hats on 18 Wheeler Cabs, and those spoilers on the ends of many expensive cars. When two vehicles draft; they become one longer vehicle for aerodynamic purposes. However you can can imagine “suction” not working very effectively at 15 mph.

Mow, since Cd is vastly different for trucks and cars; it is not clear what the “new” composite Cd is; and whether the altered temporary Cd of each vehicle at speed while drafting is high enough to improve the mileage of both vehicles in a measurable way.

So I think your final answer should be: unless you guys want to instrument the two vehicles to get the answer at any one speed just “Split the Bill”

Best regards, A physicist from Phoenix ( I love your show )

In general, what you are doing when drafting is increasing the length of the “wing” moving through the air, and that reduces overall drag. However, as Armando noted, the vehicles are dissimilar. The truck, while not gaining any drag, may not be losing much drag either, while the car directly behind the truck may be losing a good bit of drag as he’ll be in the low pressure area behind the truck.

So, if the bet is that the truck is “paying” for the drafting car - the bet is lost, because the car behind the truck will not increase the drag on the truck, period.

But the truck may not be gaining much, if anything.

-A pilot in Northern Virginia

It is a matter of the disturbance of the air being the same for the profile of the vehicle regardless of length and lengthening the vehicle (by drafting of the second vehicle) spreads the drag over a longer plane by eliminating the profile of the second. With sailboats, the longer the waterline the higher the potential hull speed.

Drafting = driving into a vacuum so essentially there is no air cushion to push the lead vehicle. The energy expended by the lead vehicle is just as usual, but, the trailing vehicle benefits from the vacuum.

So here is how conservation of energy applies to this situation. Let’s model this problem as two stationary vehicles in a tunnel. There is a rope attached to the from the first vehicle to a spring and between the first and second vehicle. The vehicles are in neutral. There is a 50 mile per hour wind blowing into the tunnel.

For a first test, the length of rope between the vehicles is long (say 50 feet). The two vehicles together are pushed backward and the spring extends. The air leaving the tunnel is somewhat slower than it was entering (say 45 miles per hour).

For a second test, the length of the rope is shortened to only 2 feet. We would have to test this to be sure, but we would expect the vehicles close together would not exert as much force on the spring and it would not extend as much. (Engineers would say that this configuration has a lower coefficient of drag.) The air exiting the tunnel would also be faster than the first test (say 48 miles per hour).

So energy is conserved between the amount of energy stored in the spring and the amount of energy lost in the speed of the air.

So to extend this to the real situation…

The two drivers are moving less air out of their way when the second one drafts the first. It’s pretty hard to visualize less wind on the trees by the side of the road because of the drafting, but that energy is what is being traded between the energy created by the two engines.