Drafting Tractor Trailers for mpg gains.

[Best F100]

Question of the week:

Here is something I was wondering about, so I'll throw it out for this weeks mental exercise for everyone.

Question 1) When a passenger car drafts a Tractor Trailer for extended miles at freeway speeds, does it result in a mpg loss for the lead truck? Is it a zero sum gain for both vehicles?

Question 2) When the same passenger car is tailgated by a Tractor Trailer Truck, (under the same conditions as example #1), will it get better or worse mpg?

I don't think the answers here are the same as NASCAR drafting or Cyling Peleton drafting, so I would be interested in the responses. Especially if anyone has some wind tunnel calculation experiences. Like they say, for safety reasons "don't try this at home".

 

[Anduril]

Why would it effect the lead vehicle? All the lead vehicle is doing is pushing the air out of the way, which it would still do regardless of what's behind it.

 

[rriddle3]

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Question of the week:
Question 1) When a passenger car drafts a Tractor Trailer for extended miles at freeway speeds, does it result in a mpg loss for the lead truck? Is it a zero sum gain for both vehicles?

Question 2) When the same passenger car is tailgated by a Tractor Trailer Truck, (under the same conditions as example #1), will it get better or worse mpg?

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1.No difference on the lead vehicle.
2.No difference on the TTT. Its frontal area is so great, you aren't going to displace enough air with a car to help it.

Please don't get close enough behind a semi to draft it. You are putting yourself at risk because you are in the area they call the "no-zone". He can't see you in his mirrors and you can't see around him.

 

[huffmuds9320]

I remember Myth Busters tested that. Had a car follow a trailer truck. They tried something like 100ft, 50ft, and 25ft.

1) The passenger car will gain mpg because less wind resistance. The truck in front will not notice anything different. Is is dangerous to follow a truck closely of course.

2)If the truck is following behind a passenger car, it will most likely not notice any difference. The car is not high enough to change the wind resistance to the truck.

 

[Shannow]

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Why would it effect the lead vehicle? All the lead vehicle is doing is pushing the air out of the way, which it would still do regardless of what's behind it.

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The back end design of a vehicle is still quite important to its aerodynamics.

If the following vehicle were to fill some of the void, and reduce the turbulence, it could affect the lead vehicle.

 

[Chris142]

Draftin a truck is a fast way to get a busted windshield.

 

[GROUCHO MARX]

Attach a tow chain to the tractor trailer for greatest increase in MPG.

 

[Best F100]

Touche' Shannow you get 1 point for realizing the back end of the vehicle is important to aerodynamics.

I have to confess that this is a "loaded" question. I have heard the real reason that: (argument A) truckers hate tailgating cars behind them is that they can "feel" a parasitic draft behind them - not so much the visability issue. Thus, the trucker is paying for a tailgaters fuel savings. Drafting a truck is literally "sucking the diesel fuel out his tank." Now if this not true, any truckers please let me know. In Argument A, the lead truck sucks the tailgating car behind it. There is a low pressure area behind the lead truck. A truck followed by a car will result in a longer more irregular vacuum behind it with increased parasitic losses.

On the other hand, (Argument B) I have heard that for optimum automobile mpg, you want to have a Tractor Trailer on your bumper. The reason is, a car drafted by a truck will lose all of the car's vacuum behind in this situation. Now I just got, finished watching the effect of being aero loose at the Brickyard, watching the NASCAR boys around the turns. Just the thought of this example on the freeway, is enough to scare me.

For those of you who think you can out brake a truck while drafting one (you probably can), the thought of blown truck tire should be enough to discourage the practice. Thanks for your comments guys. Just something to talk about.

 

[dwendt44]

You may be able to outbreak a semi, but by the time you realize the truck has hit his brakes, you've hit him.

 

[Tornado Red]

You might be able to outbrake the semi, but can you dodge the big chunk of tire tread that he straddled?

 

[Lonnie]

The best time to draft a semi is when they are going from areas with ice and snow to warmer areas or just starting out after an overnight stop. Often times there will be big chunks of ice that as it warms up will drop from under the trailer right into your path. If you drive real close you may not only gain a few more miles per gallon but you can also catch some big chunks of ice. If there is no cold weather you can always hope for a big rock. They make neat noises as they bounce off your windshield. Don't worry about a $400 windshield when your talking an extra mile or two for every gallon of gas. Don't worry, windshields are really tough and it takes a really big rock to actually go though one. If you're worries, be smart, wear a full coverage helmet and keep the visor down, can't be too careful.

 

[Dominic]

rofl Lonnie,

 

[Shannow]

I've seen one semi where he missed a turn, buried the pedal, and locked everything up. (seen another where the airhose came off between trailer and cab).

I doubt that I could ever outbrake that machine (being a safe distance behind it, rather than a side by side braking contest where both drivers have the same signal to start braking.)

 

[rpn453]

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I doubt that I could ever outbrake that machine (being a safe distance behind it, rather than a side by side braking contest where both drivers have the same signal to start braking.)

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A semi? Modern cars typically have a 70-0 mph braking distance of around 170 ft. Some sports cars can do it in about 150 ft. I'm guessing that semis are at least as bad as cars from the 60's and 70's, so around 400 ft. Anyone have the actual numbers on this?

But you're contradicting yourself because if you can't stop in time to avoid a full-on panic stop in front of you, then you weren't at a safe following distance to begin with.

 

[Matt_S]

I can see where the drafting car would affect the semi. IIRC, on a vehicle with a flat, vertical rear end, the air currents flowing over the vehicle will curl back around due to the vaccuum produced by the moving vehicle and actually produce a little bit of a push on the rear end. If you draft a semi, you disrupt that push and cause the semi to use more power to move through the air.

 

[rpn453]

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IIRC, on a vehicle with a flat, vertical rear end, the air currents flowing over the vehicle will curl back around due to the vaccuum produced by the moving vehicle and actually produce a little bit of a push on the rear end.

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So it's a vacuum that is also a high pressure zone?

 

[CBDFrontier06]

I've heard for years that this is a way to get greater MPG, but it's at the expense of all the paint on the nose of your car, you can likely kiss your windshield goodbye and your general safety is a definite concern.

 

[Matt_S]

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IIRC, on a vehicle with a flat, vertical rear end, the air currents flowing over the vehicle will curl back around due to the vaccuum produced by the moving vehicle and actually produce a little bit of a push on the rear end.

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So it's a vacuum that is also a high pressure zone?

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Depending on which specific spot you're talking about, yes. The air rushes in to fill the vaccuum left by the moving vehicle and pushes on specific points on the rear end.

 

[badtlc]

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IIRC, on a vehicle with a flat, vertical rear end, the air currents flowing over the vehicle will curl back around due to the vaccuum produced by the moving vehicle and actually produce a little bit of a push on the rear end. If you draft a semi, you disrupt that push and cause the semi to use more power to move through the air.

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Uh, no. It doesnt work like that.

 

[Shannow]

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A semi? Modern cars typically have a 70-0 mph braking distance of around 170 ft. Some sports cars can do it in about 150 ft. I'm guessing that semis are at least as bad as cars from the 60's and 70's, so around 400 ft. Anyone have the actual numbers on this?

But you're contradicting yourself because if you can't stop in time to avoid a full-on panic stop in front of you, then you weren't at a safe following distance to begin with.

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I agree that I contradicted myself, but I didn't hit it, so I was still safe.

Just never seen anything coming towards me so quickly.

Given that he had smoke pouring off all 18 wheels, I think his deceleration was pretty close to 1G, and certainly not a rate that makes sense to your brain for the first second or two while you work out what's really going on.