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| Drafting Question | |
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| Tweet Topic Started: Jun 19 2006, 10:44 AM (378 Views) | |
| kenny | Jun 19 2006, 10:44 AM Post #1 |
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HOLY CARP!!!
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Some crazy hypermilers draft, tailgate, trucks to improve their mileage. (I don't). The idea is to travel in air that is already moving in the direction of travel. It works. Question: Shouldn't this *lower* the MPG of the truck being followed? You can never get something for free. But how can it lower the trucks MPG if you are behind it?
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| Aqua Letifer | Jun 19 2006, 10:51 AM Post #2 |
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ZOOOOOM!
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Nope, shouldn't. The car is just riding along in the air tunnel the truck creates. Its wind resistance would not change, and I suppose neither would its MPG. |
| I cite irreconcilable differences. | |
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| ivorythumper | Jun 19 2006, 10:54 AM Post #3 |
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I am so adjective that I verb nouns!
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Drag is drag, you are right, nothing is free and it goes to show that some hypermilers are unethical since they steal energy from others. Might as well just siphon someone else's tank. It also goes to show that some hypermilers are stoopid, since a risk/benefit analysis would indicate that the increased possibility of death is not worth shaving a few tenths of a gallon. |
| The dogma lives loudly within me. | |
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| Aqua Letifer | Jun 19 2006, 10:57 AM Post #4 |
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ZOOOOOM!
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Well, I guess the much slower deceleration of the truck compared to the car would make it slightly more safe than, say, tailgating a motorcycle, but regardless, it's stoopid. |
| I cite irreconcilable differences. | |
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| ivorythumper | Jun 19 2006, 11:00 AM Post #5 |
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I am so adjective that I verb nouns!
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Ok, I'm no physicist, so I'll defer to Aqua's explanation. I suppose it is sort of a sunk cost -- the air tunnel is there anyway. But it still seems rather risky for the benefits. |
| The dogma lives loudly within me. | |
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| JBryan | Jun 19 2006, 11:03 AM Post #6 |
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I am the grey one
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There is a cost to the truck. It is small but the addition of the car to the slipstream increases drag slightly. |
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"Any man who would make an X rated movie should be forced to take his daughter to see it". - John Wayne There is a line we cross when we go from "I will believe it when I see it" to "I will see it when I believe it". Henry II: I marvel at you after all these years. Still like a democratic drawbridge: going down for everybody. Eleanor: At my age there's not much traffic anymore. From The Lion in Winter. | |
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| Aqua Letifer | Jun 19 2006, 11:06 AM Post #7 |
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ZOOOOOM!
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Well yep, you're right. The truck's paying for it, whether or not the car is behind it, so it's a sunk cost. I've heard of really bizarre accidents from people doing this. The latest one is that a block of frozen ice on top of the tractor trailer came off on the highway, broke through the windshield of the car behind him and killed the driver. Not to mention the fact that most truckdrivers get as much use out of their tires as physically possible before they replace them. Those things are not to be messed with, that's for sure. I mean, terminators use those.
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| I cite irreconcilable differences. | |
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| Dewey | Jun 19 2006, 11:21 AM Post #8 |
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HOLY CARP!!!
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I've actually had a similar accident. I wasn't tailgating, I was a good piece behind the guy. Big ice chunk blew off the top of his trailer & crashed into the hood of my car - left a huge dent. It happened just as I was turning onto an exit ramp, & he was continuing straight on (so I couldn't get a license number ). I thought for sure it was coming through the windshield. I even ducked down below the dash expecting it to come through. I don't know how I stayed on the road. |
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"By nature, i prefer brevity." - John Calvin, Institutes of the Christian Religion, p. 685. "Never waste your time trying to explain yourself to people who are committed to misunderstanding you." - Anonymous "Oh sure, every once in a while a turd floated by, but other than that it was just fine." - Joe A., 2011 I'll answer your other comments later, but my primary priority for the rest of the evening is to get drunk." - Klaus, 12/31/14 | |
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| Jolly | Jun 19 2006, 12:06 PM Post #9 |
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Geaux Tigers!
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NASCAR. It's a common tactic to tuck in behind the leaders and conserve fuel for the final few laps, when you can let the engine take advantage of the extra fuel you have conserved... |
| The main obstacle to a stable and just world order is the United States.- George Soros | |
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| John D'Oh | Jun 19 2006, 12:11 PM Post #10 |
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MAMIL
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Funnily enough, in a lot of open-wheeled racing this tactic has the opposite effect, since the turbulent air behind an F1 car, for example, messes up the aerodynamics of any car following it. This is the main reason why overtaking is so rare in F1, and is also why they should ban wings, and go back to cars that look like this: ![]() Although preferably without the 40% death-toll that was associated with the fuel-tank on wheels approach. |
| What do you mean "we", have you got a mouse in your pocket? | |
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| kenny | Jun 19 2006, 12:37 PM Post #11 |
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HOLY CARP!!!
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So, besides stoopid, what is it? Does it lower the MPG of the truck in the lead? It would seem it would be messing with the ability of the air to get away from the truck, which would increase its drag. |
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| Aqua Letifer | Jun 19 2006, 12:41 PM Post #12 |
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ZOOOOOM!
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No, I don't think so. The drag comes from the wind resistance from the front of the truck. As the truck plows through the air (which takes energy, pushing those air molecules out of the way), it creates a bit of a low pressure area directly behind it (where the drafting cars try to reside), so maybe filling that space up with the car doesn't allow it to be filled back up with air, and the very slight push the truck woud get as a result gets taken away, but that would be incredibly slight. Probably couldn't even measure that accurately. No, I'd say the truck isn't much affected. As for "there's nothing for free" in the physical world, the car is benefiting from an unnatural event (the low pressure area) the truck is creating from its engine. The engine burns fuel, makes the truck go fast, and breaks up the air. Whether or not the car is there would have nothing to do with this. |
| I cite irreconcilable differences. | |
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| JBryan | Jun 19 2006, 12:50 PM Post #13 |
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I am the grey one
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According to this there may actually be a benefit to the truck although the relatve size and mass of the two vehicles would make it very slight indeed.
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"Any man who would make an X rated movie should be forced to take his daughter to see it". - John Wayne There is a line we cross when we go from "I will believe it when I see it" to "I will see it when I believe it". Henry II: I marvel at you after all these years. Still like a democratic drawbridge: going down for everybody. Eleanor: At my age there's not much traffic anymore. From The Lion in Winter. | |
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| ***musical princess*** | Jun 19 2006, 12:56 PM Post #14 |
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HOLY CARP!!!
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It would depend on the speed they were travelling at. The effects would be much greater (and more obvious) at higher speeds. x |
| x Caroline x | |
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| phykell | Jun 19 2006, 01:01 PM Post #15 |
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Senior Carp
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You would know that wouldn't you! It's certainly true though, even for something as small motorcycles. A bike can steal a win from behind another rider by judging the slipstream effect just right which is more than just having saved fuel of course.
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The greatness of a nation and its moral progress can be judged by the way it's animals are treated. - Ghandhi Evil cannot be conquered in the world. It can only be resisted within oneself. Remember, bones heal and chicks dig scars | |
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| Qaanaaq-Liaaq | Jun 19 2006, 06:38 PM Post #16 |
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Senior Carp
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Air flows from a higher pressure zone to a lower pressure zone. The air pressure directly behind the truck will be lower than in front of the truck. Generally speaking, the longer a moving object, the higher the coefficient of drag because there's more possibilities for air turbulence given the increased length. A car following a truck will reap the benefits of drafting. As far as I know the truck will not experience less fuel economy because of it but I'm not a physicist so I could be wrong. It's not a wise thing to do though because the stopping distance for a truck is much greater than the stopping distance for a car given the same speed. Also a car driver driving behind a truck can't see up ahead of the truck and be prepared for the unexpected. |
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). I thought for sure it was coming through the windshield. I even ducked down below the dash expecting it to come through. I don't know how I stayed on the road.

You would know that wouldn't you!
It's certainly true though, even for something as small motorcycles. A bike can steal a win from behind another rider by judging the slipstream effect just right which is more than just having saved fuel of course.


4:49 PM Jul 10