Drafting & Wind Resistance

General Questions
#1

I am not sure how this topic ever came up, but it did over a dinner some friends and I were sharing. One friend told a story a woman told him. She had told him that she, in her VW Beetle, was able to get behind an 18 wheeler, shift her car into neutral and then draft if for an extremely long period of time, to the order of about 50 miles or so.

I argued them and said that it was not true and couldn’t happen. I tried to explain what I believed, that what would happen would be an extreme decrease in resistance for the car and that a vehicle would be able to travel at the same velocity with a greatly decreased need for energy [ie. pressure to the gas pedal] or that it would be able to travel further than without an object in front bearing the majority of wind resistance. (But even if it were possible in a frictionless, perfectly flat environment there’s still those factors that would never allow it to happen, right?)

They however argued that the wind would create some sort of force to essentially hold the car behind the truck and essentially tow it along. One argued that a system of high pressure and low pressure air could create essentially a force to maintain the car’s velocity.

After trying unsuccessfully trying to find a good answer to support either side, and the mass majority not agreeing with my position, I had to ask.

Which is it?

Can anyone please explain this as in depth as possible?

#2

I’m skeptical, too. It’s certainly the case that there is an area of low pressure behind a large vehicle like an 18-wheeler. And a small vehicle in that area will, as you suggest, require less energy than it otherwise would. Maybe substantially less energy.

But zero energy? I seriously doubt it.

It would also be extremely dangerous, since you would have to be very close to the truck and not have much time to react to any changes in the truck’s speed.

#3

There’s some discussion of drafting effects way back here: Drafting a Semi.
While you can increase your gas mileage by drafting, there’s no way you can coast for 50 miles, unless it’s all downhill. And if you manage to coast at highway speeds for 50 miles, you’ve stepped over into the twighlight zone, and are on the highway to hell. There are no hills that big.

#4

I suspect that this is something of an urban legend. My father had heard something similar, and tested it in his impetuous youth. He was on a cross-country road trip and spoke to a truck driver about it. The driver agreed to let him draft for the next leg of his trip to compare mileage, and while he did notice a large improvement in mileage, he still had to drive the car and work all the pedals. If he’d been able to coast, I’d still be hearing about it to this day.

#5

I’d have thought it would be more feasible to ride the ‘bow wave’ off to one side. Still sounds rather implausible that this could be maintained for any great distance.

#6

I missed this sentence on the first reading; ask your friend the woman’s name; I’m betting he will say “Ah, well, it wasn’t actually someone I know, but my friend knows her”.

#7

YEs, the infamous Ms. FOAF.

#8

The invocation of a low pressure region behind the truck, and the resultant suction, is causing some confusion here. It helps to keep in mind that the result of the suction here is wind, nothing more. Air flows quickly into a region of low pressure, and if a car impedes that flow, the car will be acted on by a wind; the truck’s tailwind.
The amount of forward force on the car will depend on how fast that wind is. So how fast can a tailwind behind a truck going say 55mph be? Well, the tailwind can’t have an average speed any faster than the truck that’s creating it, or it’d actually pass the truck. That’s absurd. That speed limits how fast a drafting object can go, and still pick up energy from the tailwind. How fast can a car coast in a 55mph tailwind? Hint: the answer is less than 55mph.
So drafter’s have got to use gas to keep up with the vehicle they’re drafting.

#9

Is this legal? It seems dangerous in the extreme, and could result in a careless operation of a vehicle cite.

#10

Isn’t it conceivable that the truck creates a circular vortex, and that part of that vortex is always moving forward relative to the truck (i.e. going faster) than the truck? Just like the top edge of the tire which is always moving forward at twice the speed of the truck.

I’m not saying this is true or even likely, but it’s not completely inconceivable.

#11

That could happen momentarily, but your vortex is transporting air towards the back of the truck. If it keeps doing that then you’ll build up a high pressure region trailing the truck. The wind from that would push the drafter back. There’ll be some oscillations in the air flow behind the truck, but it’s all got to balance out in the steady state.

#12

No, I meant a steady state circular flow. If a stable circular whirlpool forms behind the truck, then at some fixed (relative to truck) spot there is a constant forward wind faster than the truck itself.

#13

Oh Lord, can we avoid nasty fluid dynamics here and look at this another way? :slight_smile:
What would supply the energy to keep this vortex going?
If it really is back there, why don’t trucks come with a spar poking off the back end which holds a sail that can harvest the otherwise wasted energy of the whirlpool? That way the truck could draft itself.
-That’s not an answer, I know, but I think it points out the sort of logical problems you get into when you postulate a fast, stable eddy like that.

#14

In 1980, I bought a new Chevy Chevette, a sturdy little oxcart of a car. I did a lot of drafting with it. I found that I could increase my gas mileage from 28mpg to 32mpg. It was a damnfool thing to do, and I didn’t realize how dangerous it was.

Do not attempt. This stunt was performed on open roads by an amateur fool who was lucky not to be killed. :smack: