Drafting Tractor Trailers for mpg gains.

[Sven]

I should add that I now have a 5.7 Hemi in my little Jeep, so head winds should no longer be a problem when pulling the camper!

 

[swalve]

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Having walked on a shoulder and passed by 18 wheelers I can report there is not only the "big" wind, but, a couple seconds later, a gentler "after-wind". I feel good when I'm the requisite 2.5 seconds following distance, and the car's rocking a little. Figure I'm given a few MPH headwind. Science? None.

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I've noticed the same thing, there's a sort of buffetting that happens.

I've also noticed that I seem to get a big draft advantage when I'm following a truck and there's one next to me as well.

 

[gtx510]

Was Mythbusters mentioned yet?

http://mythbustersresults.com/episode79

http://kwc.org/mythbusters/2007/06/episode_80_big_rig_myths.html

 

[TallPaul]

Come to mention it, I have experienced the buffetting. In my motorhome, when a semi passes me and pulls in relatively close in front of me, my motorhome will shake left and right until the truck gets farther ahead.

 

[TallPaul]

Maybe it has been mentioned above, but the other side of this coin, as I see it, is that while the drafting vehicle gets a fuel mileage boost, the trucker has a reduction in fuel mileage. That is probably what made that trucker get so upset with me that he slammed on his brakes.

 

[Gary Allan]

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Come to mention it, I have experienced the buffetting. In my motorhome, when a semi passes me and pulls in relatively close in front of me, my motorhome will shake left and right until the truck gets farther ahead.

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I experience this in my jeep all the time. When I'm even bothered to do this, I just stick the nose of the jeep into the vacuum. You can feel ..and even SEE the chaos of the collapsing air trying to slam back together. A canvas top helps with the visuals.

As far as the economy loss to the semi ..I kinda think it's along the lines of how much Jupiter loses when a probe does some gravity acceleration maneuver.

 

[TallPaul]

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As far as the economy loss to the semi ..I kinda think it's along the lines of how much Jupiter loses when a probe does some gravity acceleration maneuver.

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Maybe so. If a 3000 pound car gets a 4 mpg boost off a 40,000 pound truck, then the truck's reduction is probaby at most 0.3 mpg. The bicycle drafting a semi would definitely be the space probe effect.

 

[Palut]

Can someone please explain to me the physics that cause the truck's mileage to drop when it's being followed? I honestly can't see it.

I can however see how a misconception like that could arise:

Truckers drive in caravans, but the truck in front doesn't get any mileage boost. Compared to the other rigs, the lead truck is getting worse mileage, but compared to running alone there is no change.

Then someone who doesn't really know about drafting hears about how the lead truck gets "worse" mileage and assumes that the mileage is actually dropping, not just lower in comparison. That person tells other people who don't know any better and BOOM, urban myth.

 

[TallPaul]

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Can someone please explain to me the physics that cause the truck's mileage to drop when it's being followed? I honestly can't see it.

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You have a good point. It really depends on what the following vehicle does to the aerodynamics of the truck. And chances are it won't do any thing harmful. More likely perhaps that it could improve the aerodynamics by smoothing out the airflow off the back of the truck. So maybe the truck being followed would actually get a slight mileage improvement, though likely insignificant.

The only way that one could readily assume the truck loses mileage is if one assumes that the following vehicle is being sucked along by the truck, in which case the following vehicle would be a burden much like as if it were hooked to the truck by a tow bar.

 

[SR56]

I drive a tractor-trailer. Drafting me will not affect my MPGs whatsoever. It does bother me, the same as most drivers I know, to not be able to see you.

As others have posted, it is very dangerous to tailgate a big truck like that. It's not uncommon for a tire to blow and launch debris into your car. Or then there are the times like when there was a 6' ladder in the middle of the road, and I couldn't do anything but straddle it. ---- help anyone that riding my ---- when something like that happens.

You should try "drafting" big truck to big truck sometime. The combined wind velocities are pretty wild.

 

[Gary Allan]

Well, can you discount to notion that "no one rides for free" using physics? I've never taken a formal physics course ..so I'm ignorant about many things. I don't think anyone was really pushing this notion ..but I'd like to figure out why I have the wind knocked out of my sails when a semi passes me ..that is, where that energy went ..and who gained it when I lost it. Not the same thing, necessarily, but I would imagine that the volume of the low pressure pocket is a specific volume for the prevailing conditions in allowing the disrupted air to reform at normal pressure after slamming back together ..I also can't imagine that void being filled with something ..and not effecting the outcome in the recombination of the disrupted air flow.

Again, I'm ignorant of many things.

 

[Tornado Red]

This afternoon I was passing a semi towing two gravel trailers when I heard a very loud "boom". At first I thought he blew a tire, but then guessed that he blew a brake line. Whatever it was, he slowed quickly in my rear-view mirrors.

How closely you want to follow a big truck might depend on how much damage his blown tire might cause to your vehicle. Then compare that cost against the fuel savings.

 

[TheTanSedan]

I drafted a few on the way back and forth from college nearly 30 years ago. The amount of grit in the air behind a semi is impressive, and it increases the rate at which the windshield is sandblasted.

As a truck driver it was ****ed irritating; manipulating the cruise and then the brakes usually shrugged them off.

The effective air pocket is close, quite close.

 

[wantin150]

Someone mentioned bicycles and semi's. I've drafted big trucks to 35 mph, flat and level, while on my bike. For comparison, I can sprint to about 32 for 1/4 mile and ride steady @ 22-23 for a few miles. I can only keep up for a mile or two but it's a blast. Extremely dangerous, yes, but still fun. Usually, I stay just to the right of the rear of the trailer and try to maintain eye contact with the driver (they play along) so he knows I'm there. I can also see lights that way. Like I said, it only lasts a short time, usually interupted by traffic lights.