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-   -   Drafting 18-wheelers....revisited (https://www.fuelly.com/forums/f33/drafting-18-wheelers-revisited-4583.html)

Brian D. 05-20-2007 06:17 AM

Drafting 18-wheelers....revisited
I'll start by saying that I DO NOT make a habit of this. Typically I don't want to drive behind "semi's" as much as they don't want me behind them. The few occasions which I've ever drafted a large truck doesn't seem to last too long anyway. With that said, I'd like to know...WHY is it that they mind so much? I would liken it to having someone tailgating me, but that just seems different to me for a few reasons.

If someone's tailgating me I know it's because I'm definitely not moving fast enough, or should no longer be in that lane.

If I'm behind a truck with a GVWR of...I don't know...89,000 LBS or something, what difference is it to the driver that there's an 1800lb car close behind him? His stopping distance IS SURELY much greater than mine, even with him starting to react to the potential crisis in front of him before his taillights alert me. My stopping distance is...normal. I also doubt that I'll cause any real damage to the back of his truck if there is an impact. My car would probably slide somewhat under the rear of the truck.

I'm not trying to say that folks should grab onto an 18-wheeler for all it's worth anytime they see one on the road. I'm just trying to figure out why it seems to piss the drivers of these trucks off SO badly (as the few enough occasions of doing so has taught me). If the answer is "just because", I'd just like to hear that. Does anyone know of a trucker site where I can post this question? lol

Snax 05-20-2007 06:27 AM

I can think of a few very good reasons that they mind:

The first reason is that it increases their chance of being involved in an accident by being rear-ended. And even if it is considered 100% your fault, it still ends up on their driving record and can affect employment.

The second reason is potential road hazards. A tire carcass that they freely pass over may end up your hood ornament, and the hassles that creates for them is likely something they wish to avoid.

Lastly, it just creates one more thing that they have to watch. Particularly on the freeway where they may need to switch lanes to avoid losing speed on a grade, I know they need to be able to do it confidently. If somebody is drafting them up to the base of that grade, there is no guarantee that they won't attempt to dart out into the intended lane at the last second, reducing the trucker's options to preserve momentum or change lanes safely.

So if you DO draft a truck, do it respectfully. They still may not appreciate it, but keeping these things in mind helps in understanding their irritation.

Brian D. 05-20-2007 06:38 AM

New (old) reason, newly found...
Well, I guess I should have Google'd this long ago, but I just found an interesting bit that I never thought of:

"Second, truckers hate paying your fuel costs for drafting. They know all about this, and they can actually feel a truly parasitic draft. If you were part of a NASCAR style multi-vehicle draft, where your pushing effort, as part of a line of vehicles materially reduced their parasitic air losses by pushing air back into the collapsing tail of their drag profile, you'd increase their mileage, without giving back all of your own gains, and they might tolerate you doing this; but, if you're doing what the hypermilers advocate, you're literally sucking gas from their tanks, in that what additional mileage you gain is paid by them in increased parasitic losses to fuel economy on their vehicles, and you are increasing their likelihood of being involved in an accident. Expect experienced truck drivers to shake you, or scare the living **** out of you, if you are parasitically drafting them for any length of time. And expect that your license plates are being radioed ahead several hundred miles.
posted by paulsc at 12:23 AM on January 4"

...funny, and here I always thought drafting was optimal for all drivers in the pack (until an accident occurs, of course).

northboundtrain 05-20-2007 06:59 AM


Originally Posted by Brian D. (Post 51954)
. . . I also doubt that I'll cause any real damage to the back of his truck if there is an impact. . .

Hmm, there's a word in this sentence that I think answers your question. Can you guess what it is?

You are greatly increasing the chances of an _______ by drafting a semi. Truckers (and anyone else on the road) would generally like to avoid an _______, particularly at highway speeds, for the reasons already mentioned plus several more.

Snax 05-20-2007 07:37 AM

I don't buy the parasitic drag argument one bit. The airflow off the rear of a big box style trailer is anything but smooth and controlled. Your vehicles presence within the vortecies of airflow that they create more likely reduces and cleans up the overall airflow. Smaller vortecies = lower drag. My bet is that if anything, it improves their drag coefficient, not the other way around.

trebuchet03 05-20-2007 07:42 AM

Without going into all the details just yet..... I found a significant change in FE by staying back ~300 ft. (of course, getting closer was better - but that's around where I felt a difference).

Changes are much easier to see when you're getting 20mpg due to a large sail :p

Hockey4mnhs 05-20-2007 08:04 AM

i draft all the time. i drive alot of highway and i draft pickups and anything that is bigger then me. the wind noise cuts so much its amazing. the longest ive ever drafted a truck was about 65 miles. when i had to turn off i gave him a head nod and a little thanks wave and he gave the no problem sighn. i think its really up to the driver if there not trying to get rid of you the dont care that much

Snax 05-20-2007 08:16 AM

I would agree that a draft can be taken advantage of as far back as 300 feet. I experience this on my bicycle everyday I commute. In fact, due to my offset to the side, I usually don't experience this effect until at least 50-100' behind passing vehicles - unless I move over to take more advantage of it.

Of course I'm not above tucking in 10' behind a city bus from a stop to get a rapid bump up to 30 mph or so before diverging back into the bike lane. ;)

kitcar 05-20-2007 09:43 AM

Since I limit my speed on the highway to 65, I'm always around trucks just poking along with them. I get best effect just at the point where I start to feel the wake from the truck and back off 10 feet or so. I always keep the trucks mirrors in sight so they know I'm back there or give them a quick bark on the CB to tell them I'm at the back door. And believe it or not, I only follow two types of trucks - Walmart (Schneider has their contract) or milk haulers. Here in Michigan those are usually the safest truckers to be around.

Not that I would ever drive behind a semi. :rolleyes:

minic6 05-20-2007 10:36 AM

I have only had one bad experience drafting. I-94 is hilly in spots and the driver kept slowing down. I thought it was because of his load, no when he exited he let me know how angry he was. I stopped doing it for quite awhile. I don't agree with parasitc lose due to drafting either. Years ago at the proving grounds we were testing chains for lube in automatic trans. Opels were the test vehicle. I was a test driver at the time (very boring job) The test consisted of driving 100mph for the entire shift. Cars were ballasted to keep speeds down to 100. Well get 8 test drivers and add boredom, and you get some serious drafting going on! So much for their 100mph top speed. Long story short the lead car never needed gasing before any of the other cars. Also wouldn't the lead car in NASCAR have to fill sooner?

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