Does a truck work extra to pull a drafting car?

March 10, 2007 9:11 PM

Does drafting a truck theoretically lower the truck's mileage?

There have been a few articles about the practice of hypermiling (most recently I saw it on MotherJones via kottke.) Hypermilers try to extend their gas mileage to extreme lengths, and one of their more dangerous practices is drafting a truck -- tailgating it to ride the vaccum the truck creates as it moves forward.

I blogged a little about this and mentioned that the drafting is technically zero-sum for the environment since the truck must use more gas to make up for towing the car. My brother disagreed, and suggested that the energy of the vacuum would otherwise be exerted on surrounding air.

Could this be one of those infuriating puzzles like the jet and the conveyor belt, or is there a simple answer?

posted by condour75 to science & nature (19 comments total)

It's a tough question. The only data point I can contribute is cycling. If someone is drafting you, there is no way to tell if they are six inches behind you or three feet. (At three feet there is no draft, at six inches there is a lot). I would expect if the truck was doing more work to pull the drafting car that you would feel it when cycling.

My physics guts say the same thing. The effect of the draft is not that it sucks the car along but that it reduces the drag... because there is a low-pressure area behind a speeding truck. Low pressure means less drag. The low pressure area is there whether or not the car is drafting.

Guts =/= proof however.
posted by sweet mister at 9:21 PM on March 10, 2007

No, it cannot. The truck is not "towing" the car, it is creating an area of low air resistance irrespective of what exists in that space.
posted by geoff. at 9:25 PM on March 10, 2007

In the blue, previously.
posted by Steven C. Den Beste at 9:49 PM on March 10, 2007

Bike racing data point. No. When you're out front, it's just like riding by yourself on an open road.
posted by |n$eCur3 at 10:59 PM on March 10, 2007

In this article about NASCAR drafting, some guy from the University of North Carolina Charlotte's motorsports and automotive engineering program said:

In car drafting, the lead car is also getting a benefit. Trailing cars fill in the lead car's low-pressure wake, thereby cutting down pressure drag. "The decrease in work for the lead car is substantial," Hill said

posted by jaimev at 11:46 PM on March 10, 2007

Two vehicle drafting is not the same as pack drafting in NASCAR, or the pelloton in bicycle racing. In a two vehicle draft situation, the lead vehicle's vacuum "shadow" is "filled" by the parasitic vehicle, and thus the vacuum shadow of the lead vehicle is extended by about the length of the parasitic vehicle (at normal road speeds of up to 60 mph), because the presence of the parasitic vehicle prevents the rapid collapse of the vacuum shadow by pressurized air pushed out of the way by the lead vehicle. In effect, the vacuum shadow becomes longer and more turbulent for the lead vehicle, costing it mileage (or in NASCAR situations, top speed). In the case of vehicles of very different lengths and aerodynamic cross sections, the effect will be somewhat proportional to the order in which the vehicles run, i.e. a truck followed by a car will get a marginally longer, dirtier vacuum shadow with increased parasitic losses, whereas a passenger car drafted by a 40' long trailer truck will lose its vacuum shadow entirely in the length of the truck's following aero-profile. So, for best mileage, what car drivers would really want is to get big semis to tailgate them very closely -- if you're a mileage freak, you want 72,000 pounds of semi, complete with full aero-package and low boy trailer, on your bumper, all the time. Of course, your car will be a little bit aero-loose in such situations, and the semi will run right over you if you make the least mistake, but you'll get fuel economy worth engraving on your head stone! The truck will gain a little fuel economy, too, in that configuration, although it is a very small improvement, perhaps a gallon of diesel fuel over 100 miles of level road. But when you reverse the situation, and draft trucks in passenger cars, you are creating additional parasitic losses for them, to get the aerodynamic benefit you seek for your own vehicle, because your vehicle length is comparitively short. In contrast, two trailer trucks running in draft don't create the same degree of parasitic losses for the lead truck, due to the greater length of the truck in trail. If you see bob tails and semi-trucks running together on the road, the bob tail will generally be out front, for this reason.

But in a multi-car draft, or a bicycle racing pelloton, something a little different occurs. The vacuum shadow of the lead vehicle becomes extended so far, that it effectively decouples partially, in an aerodynamic sense, from the lead car, (much like what happens when a longer trailer truck tailgates a car). Down force on all cars in a multi-car draft is decreased, which can lead to cars in the draft becoming aero-loose, but the net effort of pushing air out of the way is shared across all cars in the draft, and as a result, the cars can all go faster (or use less fuel to maintain a given speed). So this why you see long lines of cars at the NASCAR super speedways, trained up for hundreds of miles, waiting to get down to the last few laps to start making individual moves. And it is why in distance racing, the bicycle racer's pelloton is such a formidable beast. No one, on his own, can do the work being shared by the vehicles in the draft train, so long as the draft train can all run together. In NASCAR, the advantage is so great, that car trying to join a draft has to initially be in a favorable position as the draft line goes by him (low on the track, to a high draft line), and have some reserve power to pickup speed when the pack goes by, or he'll "miss the draft" by not being able to accelerate enough to keep up, and get in the extended vacuum shadow his own car will have to create as he adds its length to the collected aero-length of the cars already in draft.
posted by paulsc at 11:55 PM on March 10, 2007 [2 favorites has favorites]

I was lucky enough to work in a large wind tunnel facility.
Basically the trucks drafting on a freeway helps on speed and milage. It does become significant in a long runs for both front and any rear trucks.

The rear truck or trucks act similar to those aero wings attached to back of Hatch backs, minivans, suvs, etc....
The wings are basically trying to longate the wind as long as possible before it drops off from the vehicle.

The second truck and trucks after similary act the same way.

Those behind the first trucks benefits by not hitting the wind on their frontal area and by passing resistance.

Again... the first truck benefits by having its passing wind longated by the truck behind it... (if there is nothing to carry the wind after it passes the first truck, there will be a vaccum behind the truck pulling backwards....)

Usually the truck/car behind the first car benefits most... as you can see how NASCAR drafting at same horsepower can swing from second to passing the first car easier with better innercia.... (well... that is what was said in the movie "Days of Thunder"

That's my two cents...
posted by curiousleo at 12:17 AM on March 11, 2007 [1 favorite has favorites]

In cycling, the lead biker does have to work a little harder which is why the leader is switched every so often. The same thing happens with geese in a flock. For bikers to gain the benefit for the leader though, the echelon has to be extremely close. To gain the same benefit for the truck and the car, the following car would have to be within inches of the truck's bumper. The car has to be in the area of turbulence which even in tractor-trailers doesn't extend out very far. See some scientific analysis of truck turbulence.
posted by JJ86 at 5:36 AM on March 11, 2007

In cycling, the lead biker does have to work a little harder which is why the leader is switched every so often.

But they'd be working harder regardless of whether there was anyone behind them.
posted by odinsdream at 5:39 AM on March 11, 2007

Exactly, as odinsdream says, the lead cyclist is switched so that they can then slipstream behind the new leader, not because they have to work harder because there is someone behind them. They only work harder than everyone else because everyone else has someone to follow.
posted by opsin at 5:43 AM on March 11, 2007

True
posted by JJ86 at 5:50 AM on March 11, 2007

It sounds as though the car is not strictly parasitic, but there's still no consensus on whether the car's alteration of the slipstream is a benefit or loss. I'm guessing it's a slight benefit, and if truckers do hate drafters it's more because the practice is annoying and dangerous.
posted by condour75 at 7:41 AM on March 11, 2007

Geese have figured this out.

Also military jets.
posted by beagle at 8:22 AM on March 11, 2007

I can tell you when driving a cube van at highways speeds you can often feel someone who has sucked up so much you can no longer see them as a decrease in power. The effect is probably pretty small but when your powered by a mid 70's 305 of maybe 125hp it's noticeable.
posted by Mitheral at 9:10 AM on March 11, 2007

One of the subsequent pages at JJ86's link addresses some of this directly. It shows the drag effect on cab, and trailer as gap is varied (bottom of page). Quite bewildering. Also, it does not appear to be sourced from the article indicated.

Suprisingly (by now you'd think I'd learn), it appears to confirm paulsc's statement:

So, for best mileage, what car drivers would really want is to get big semis to tailgate them very closely

At least, for ridiculously small gaps. It is a highly idealized model though.. In a more practical situation, the car will experience a lot of its own wake effects no matter how small the gap, because the back end of a car is not just a box. Some (probably most) of the cars wake drag will appear in front of its back bumper.

For more reasonable, but still very small gaps, the cab does seem to "toe" the trailer, by a very, very small amount. And then there is that bump at a normalized gap of ~0.62, which looks really interesting. Perhaps you could build a device that would allow the lead cyclist to detect that bump as a trailing rider latches on - a mirror would work better though :P
posted by Chuckles at 10:45 AM on March 11, 2007

The question is whether the truck experiences a reduction in mileage efficiency if a car were to draft it. From the NASA link it would appear that while a drag would be measurable in real world conditions it would be so negligible as to be discarded?

I have a feeling if you did a "real world" experiment and put a gallon of gas in a semi, started it up and had a car follow it until the semi stopped the distance would be the same regardless of whether or not the car is behind it. You would probably need more than a gallon to get the semi up to highway speeds. I doubt there would be any meaningful mileage loss (not to mention a car is nowhere near the same in the dimensions of the trailer in the experiment).
posted by geoff. at 11:58 AM on March 11, 2007

Has mythbusters done this? Seems like a good one for asavage.
posted by srboisvert at 7:42 AM on March 12, 2007

Why would it be zero-sum? Zero-sum cancellations are miraculous coincidences unless there's a conservation law operating, and wind resistance is not a conservative force.

If you want a clear answer, you'll need a wind tunnel test. But I really don't see any reason to expect that the fuel economy of a large box designed not for aerodynamic sleekness, but simply to carry smaller, similarly shaped boxes would be substantially decreased by a smooth body trailing it under its own power. Of course I don't see any reason for it to be increased, either, but the point is that drafting doesn't upset some carefully engineered aerodynamic profile.

Paulsc, some references would be appreciated. What does extending the length of a vacuum shadow have to do with anything? How does this tell you how the pressure on the surface of the truck changes? Forgive my skepticism, but from the truck's point of view, a drafting car represents merely a slight change to the profile of the truck, without changing anything else, and qualitative arguments about aerodynamic profiles are typically of very, very limited usefulness. I'm just not buying that drafting will cause more of a difference than random variations in driving caused by the driver, weather, construction, traffic, gusts of wind, tire pressure, quality of gasoline, etc...
posted by dsword at 12:36 PM on March 12, 2007

as a few have mentioned the car will probably help a bit... similar to the concept in ballistics known as "base bleed"

a base bleed projectile releases a small amount of gas from the tail of the projectile while in flight to decrease the trailing vacuum. this has shown decreases in drag of up to 30%
posted by wackoacko at 9:29 PM on March 13, 2007