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Old 11-27-2008, 09:24 AM   #1 (permalink)
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Benefits of drafting "dirty" trucks?

Forgive me if this has been covered before, but I couldn't turn up much with the search function.

While driving to work this morning, I started to wonder if there were any negatives associated with drafting trucks with aerodynamically-dirty loads (i.e., car carriers, flatbeds with equipment, class 8 tow trucks, etc.). I settled in behind an 18-wheel flatbed with an uneven load and did not notice nearly as much drafting benefit as with a normal covered trailer.

Are the currents behind trucks with uneven loads potentially detrimental to my FE?

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Old 11-27-2008, 10:05 AM   #2 (permalink)
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A flatbed or car carrier has less of a turbulence trail behind them compared to a van trailer. I can notice how drafting effects MPG's by monitoring my vacuum gauge. If I run a steady rate on the highway and then come up to a tractor/trailer traveling a similar speed, I can actually see my vacuum slightly increase when following behind.
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Old 11-27-2008, 02:02 PM   #3 (permalink)
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And of course, some of us will tell you not to draft at all, since, as I'm sure you know, it's dangerous,
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Old 11-27-2008, 02:07 PM   #4 (permalink)
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Mythbusters did a test on it.

100ft behind was something around a 20% gain in MPG, 50 feet was 30% about, and it scaled all the way up to 20ft behind with a grand total of about 43% gain in gas mileage. I forget what episode it was but I think it was this season, which is season 12. They used some big v8 american car.
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Old 11-27-2008, 03:58 PM   #5 (permalink)
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Quote:
Originally Posted by D_Bat View Post
Mythbusters did a test on it.

100ft behind was something around a 20% gain in MPG, 50 feet was 30% about, and it scaled all the way up to 20ft behind with a grand total of about 43% gain in gas mileage. I forget what episode it was but I think it was this season, which is season 12. They used some big v8 american car.
I saw this one as well. 100 feet is almost two big-rig-lengths--enough to easily be out of the danger zone. When I'm travelling the same speed as big rigs around me, I'll generally stay behind them by 100 to 150 feet. It seems to make a difference in my overall mileage.
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Old 11-27-2008, 04:47 PM   #6 (permalink)
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Originally Posted by Clev View Post
I saw this one as well. 100 feet is almost two big-rig-lengths--enough to easily be out of the danger zone. When I'm travelling the same speed as big rigs around me, I'll generally stay behind them by 100 to 150 feet. It seems to make a difference in my overall mileage.
The results from that Mythbusters episode are summarized here. I use this technique when it's available and likely to help. I ran some numbers on safety of the technique here. Finally, I estimated when it's likely to save fuel, i.e., when the fuel savings gained are not outweighed by having to go faster to keep up with the truck here.

Last edited by PA32R; 11-27-2008 at 04:48 PM.. Reason: faulty linking
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Old 11-27-2008, 05:31 PM   #7 (permalink)
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Quote:
Originally Posted by PA32R View Post
The results from that Mythbusters episode are summarized here. I use this technique when it's available and likely to help. I ran some numbers on safety of the technique here. Finally, I estimated when it's likely to save fuel, i.e., when the fuel savings gained are not outweighed by having to go faster to keep up with the truck here.
Read your blog post you referenced about reaction time. My concern is not with the truck braking so much as the truck seeing something in the road (like a tire tread.) He's high enough to clear it, but you may not be, and might sustain some damage.

If I did a lot of cross-country driving (where it would make sense to make the effort to draft a lot), I might get a CB and ask ahead of time if I can draft. Some truckers are more amenable to this than others, but all appreciate being asked first.

That said, I do draft at 75-150 feet (depending on speed) during my commute, for 2-5 minutes at a time.
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Old 11-27-2008, 05:39 PM   #8 (permalink)
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Quote:
Originally Posted by Clev View Post
Read your blog post you referenced about reaction time. My concern is not with the truck braking so much as the truck seeing something in the road (like a tire tread.) He's high enough to clear it, but you may not be, and might sustain some damage.

If I did a lot of cross-country driving (where it would make sense to make the effort to draft a lot), I might get a CB and ask ahead of time if I can draft. Some truckers are more amenable to this than others, but all appreciate being asked first.

That said, I do draft at 75-150 feet (depending on speed) during my commute, for 2-5 minutes at a time.
I can't disagree with your concern. I think it was in this forum where another poster mentioned that, for all the concern expressed about the safety of drafting and how trucks react to it, he was much more likely to see a truck at an extremely close distance in his rear view mirror than his windshield. I do understand that such a truck will see over my car and know about obstacles ahead in a way that I can't when drafting, but they sure do drive up my bumper. In other fora, truck drivers complain about car drivers who draft them, how unsafe it is, how it could affect their ability to earn an income should a drafting car hit them, etc.

In any event, I haven't found it to be one of the sharpest knives in my fuel saving drawer - finding the perfect truck at the right speed for any significant length of time is quite rare.
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Old 11-27-2008, 05:46 PM   #9 (permalink)
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Quote:
Originally Posted by D_Bat View Post
Mythbusters did a test on it.

100ft behind was something around a 20% gain in MPG, 50 feet was 30% about, and it scaled all the way up to 20ft behind with a grand total of about 43% gain in gas mileage. I forget what episode it was but I think it was this season, which is season 12. They used some big v8 american car.
they used a v6 dodge magnum
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Old 11-28-2008, 02:26 PM   #10 (permalink)
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don't draft, end of story.

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