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Old 06-11-2009, 02:15 AM   #1 (permalink)
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Side-Drafting

On the interstate, when being passed by a big rig, if I pull closer to the side of it while it passes, I find that, in its tail draft, my mpg (Scanguage II) goes up by about 5 mpg for a few seconds. I take it I am "side drafting." My question is this: is the turbulence of riding the side lesser or greater for fuel economy than the draft you get at the rear?

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Old 06-11-2009, 09:40 AM   #2 (permalink)
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The drafting effect you see along the side of a semi-trailer is noticeable as you mention but it is not as strong as directly behind the trailer.

Aside from the fact that to effectively side-draft a truck you'll be riding in its sizable blind spot I do think that this is probably safer for you as the drafter, but will probably tick off the truck driver more that you're impinging on his freedom to change lanes.

Point of interest: in my college Vehicle Dynamics course we had a class project to perform an experiment and test an hypothesis. A group that one of my friends was on went out on I-81 and drafted semi trucks at varying distances behind the trailer while recording instantaneous MPG readings from the vehicle's OEM display. They recorded copious data and determined that even at following distances of up to 2 seconds at 65 mph there was measurable MPG benefit. Slightly more than 2 seconds produced an MPG *decrease* and somewhere around 2.5-3 seconds MPG was the same as without drafting.

So keep in mind all the knee-jerk "never draft you reckless fool" types; ever slightly closer than the standard 2 second following distance will improve your MPG via drafting without endangering anybody. Try counting out a full two seconds on the highway and you'll see the following distance invites people to enter the space.
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Old 06-11-2009, 10:48 AM   #3 (permalink)
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some people call it side drafting, some call it surfing, either way geese figured it out long before any of us, I tend to do this only while going up hills as they aren't going to get upset at you for not passing them on an uphill grade.
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Old 06-11-2009, 10:53 AM   #4 (permalink)
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Or you could have a CB radio and just talk to them. Tell them what you're wanting to do and some won't mind as long as you talk to them once in a while.
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Old 06-11-2009, 03:28 PM   #5 (permalink)
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I'd say that I feel safer 3 feet beside a truck than 15 feet behind. In my bigass F350 with a HUGE tall 20x8x7 cube, I can raise my right foot when I'm beside a transport truck. As long as you're well back from the cab, you're not in a blind spot. I find that I need to be back from the cab before I feel the pull anyway.
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Old 06-12-2009, 12:41 PM   #6 (permalink)
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Quote:
Originally Posted by MazdaMatt View Post
I'd say that I feel safer 3 feet beside a truck than 15 feet behind.
Then don't follow that close behind. At 60 mph a 2 second following distance is 176 feet. At that speed at 150 feet following distance you will be receiving some benefit via drafting. Probably about as much or more as you would see riding beside the truck. I won't draft anybody closer than 1 second and try to exceed 1.5 seconds. On the 65mph roads I travel that's 143 feet (and +~20% on my overhead instant MPG) and I do get people cutting in. My truck is longer than most passenger vehicles on the road and it's shorter than 20 feet (by inches).

In a side draft you do have to be behind the front of the vehicle to the side to receive benefit. The closer to the rear of the trailer you are the more benefit you'll see, but you should at least be behind the tractor (nose even with or behind the tractor's rear wheels).
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Old 06-12-2009, 12:51 PM   #7 (permalink)
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PURELY observational - I feel that I get a better boost cutting in close to the side of a truck than I do at any following distance that I feel safe. Maybe I just feel safer very close beside than I do at a reasonable following distance. I'm entirely judging by my "right foot reaction". I feel that I lift more when i tuck in beside. I have no instrumentation.
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Old 06-12-2009, 01:07 PM   #8 (permalink)
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Thats the whole point, MazdaMatt, Why don't you have any instrumentation?
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Old 06-12-2009, 01:31 PM   #9 (permalink)
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I'm a bad bad man.

really, though... life costs are murdering me right now... maybe by xmas I can get one.

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