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

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.

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).

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.

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.

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

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

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. ;)

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:

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?

I would also disagree with the parasitic draft theory - and I also note that lorries (semis) in the UK draft each other a lot - often less than 1 car length between the truck behind and the one in front!.

In this case, the truck behind is 'pushing' the vacuum of the truck in front, meaning that the truck in front gets a lot less drag at the back!. The truck behind gets less frontal drag, but identical rear drag. With a car behind, it is just less extreme but the same idea.

It is true that staying back does still improve MPG - an easy test is just to put your hand out of your window a bit. You can feel the point where the wind blowing your hands slows down - that is the start of the drafting zone - and it starts several car lengths behind the truck!

Drafting them improves their drag coefficient. They just don't like you sitting behind them, just like most people driving any old car would not want you sitting behing them... If you stay a decent distance back they won't care too much and you can still get a mileage boost.

yes as an owner of an old car it pisses me the hell off that people will pass me by getting 4 feet from my bumper then whipping around me.or when they stop at red lights their so close i cant see thier headlights anymore.(which is within 2feet):eek:

but yea truckers like to know where everyone is thats around them. since they have HUGE blind zones they have to relaly pay attention to whos near them and if thier drivign along at 65 and can just barely see your side mirror in thier mirror thier prolly getting a bit paranoid.but if your a tiny car ridign thier butt they probably figured out that your just tryign to draft them. i was doing this once and a semi swerved a few times either trying to see me or trying to get me away from him.

Apparently the blind spot ends about 300 feet behind the box but the roads around here are curvy enough that you can't get directly behind them unless really close. If I draft I alway stick out to their side so they can see me and keep pretty far back anyway - plus I watch the road ahead and behind me so I know when they need to change lanes and keep in sight when that happens. If they pass me and need to pull back in I also flash my brights when it is clear for them to pull back in - they really appreciate that and usually flash their running lights in reply. Gonna have to bring my 2-way radio on my next trip to have ears on to see if they actually talk on the radio while they drive.

drafting is actually SAFER when...
 

On one trip on I35, my engine developed troubles. One of the cylinders went bad. Only a 4 cyl engine, too. Drafting was the only way I could stay with the traffic. Otherwise, I didn't have the power to do the safe thing which is go about the same speed as everyone else. Whenever I lost one truck, was a short wait for another truck to pass me, and then pull in behind. Only time I've really noticed how much drafting helps. Without the assist, best the car could do was about 10 to 15 mph slower than the traffic.

Well, I suppose the really safe thing to have done was get off the road and get the car fixed or buy a new one. What price safety? But I made it. When I got home, went to work on the car and found one of the cylinders had a hole in the top!

:eek: Thats running a bit too lean!

Truckers dont like it when you do a close in draft. And it can be quite scary. When you pull the car thru there wake and your only a few feet from the rear of the trailer,,, its mpg heaven.

So if I want to draft say on a day with a headwind. I will stay behind there wake in a distance draft. A small low pressure area develops in this area. I find this area by easing up to the wake. The car start buffeting. At that point I ease out of the wake about half a car length. Really helps with these okie winds that alway seem to be a headwind,, dag nabit.

i just get close sheesh

I think VetteOwner summed it up perfect as they have a blindspot. They do still know you're back there, but also consider what they kick up onto your car. What if they have a blowout? Something that is a commonplace for a truck driver. I've got a CDL but maybe 10,000 mile at most of wheel time as it isn't what I do everyday and I've still had 2 blowouts - I'm fanatical about tire care on a 18 wheeler. The people that worked for me back when I drove more regular had MANY more blowouts simply because they didn't take the same precautions. One day you might be behind one of those people.

I did once have a trucker happy to allow me to draft. I was on my motorcycle in TX driving from Tyler to Abilene. I drafted a truck from Ft.Worth all the way into Abilene due to a serious head/cross wind (45+knots). He let me draft and was doing a cool 55mph anyway. When I took my exit he even waved with all five finger - no fist or single finger.

As far as them being jealous - Bullhockey!! I don't believe that one for a moment.

i flashed my lgihts once but he had his signal on and it was raining. i guess from all the spray from his wheels he couldnt judge distances so i flashed my lgihts and he moved over. gave me a nice wave when i passed him a few miles later.

yea ive been near a truck that had a blow out. it was the scaryest thing ever. just driving along half in a daze then all of a sudden a shotgun blast noise came from in front of me and chunks of tire started flying everywhere.

Sort of like blowouts. I had a set of duals come off of a truck approaching me on a 2-lane road. The wheels crossed in front of me and went out in a field. It could have been ugly. You maybe heard that a fellow was recently killed up near Seatle this way. Not quite the same as a blowout but stuff happens.

The diplomatic draft
 

I didn't think about this, but it makes sense -- takes energy to make energy. Perhaps that's how they can tell you're back there when "invisible".

At the same time, I realized today on a drive, that some truckers won't think twice to break your momentum on an uphill by abruptly cutting you off if they need they need to pass a slower truck. Hitting the brakes is great for FE :mad: -- and I was in plain view for the entire pass (driving a rental).

On the daily drive, supplementing "diplomatic" side-draft or letting a truck pass you on a hill to "pull you along" seems to work to bump FE without raising a fuss. Following closely has too many safety points in the opposite. I'll still admit that if they cut into my lane, the room they give is the room they get :cool:

RH77

How was that boring? That sounds like fun to me!

One cut in front of me. He knows how long his truck is, and I had about one car length. I figure if he can maneuver in front of me like that, thats enough room...

Some things are more important than fuel economy. Blown tires from driving through a semi's blown tire debris (and many of them drive on recaps), rocks in the glass, sudden stops because someone cut them off, the risks are too great to me. 200 bucks for a new windshield will buy a lot of gas when you are looking at the difference in MPG for drafting vs not drafting.

The drivers don't like it because some companies will fire the driver if he's involved in an accident whether it's his fault or not. Even if the cops give you 100 percent of the blame, he may be out of a job.

I know when people try tailgating me, I start riding the brakes close, and I'll jam down hard on them for a split second from time to time, every time I estimate their speed will bring them to the point that they will have to floor their brakes or hit my back end if I stay in them. A couple of times is usually all it takes to get some breathing room. You never know when you will have to jam on the brakes and stand on them, and I don't want to be the meat in a metal sandwich because some jack@$$ can't maintain safe spacing. The ones that are usually tailgating me aren't doing it for drafting purposes, I doubt they even know what drafting is.

Other times, when I'm travelling at speed, I find that someone is using me for a radar block, which I also hate. I use others for it, but don't allow myself to be used for it. I've found that a couple of brakelights just after I go over a hill to be enough to get them over a mile behind, and at that distance they don't bother with me anymore.

I will admit to being a little paranoid about being followed.

Talked to a trucker friend of mine...
 

And here is what he said:

Pisses me off! I won't let them stay back there at all. It is a very dangerous place to be.

A truck can straddle objects much larger than a car is capable of straddling. Objects such as tire casings, buckets, ladders, pipe, suitcases, and objects you have no idea what they are can come out from underneath that trailer and really lay a hurting on a car (and on another truck also).

You can't "feel" them at all.

Myself, I wouldn't believe anyones results obtained by following another vehicle at an unsafe distance.

He has been driving for about 40 years and seen a lot of stupid and strange things happening.

I used to drive OTR and city buses, and know 1st hand that you cannot even feel the impact of a car on the rear of a bus unless it is at pretty high speed.

Also, I don't like to be anywhere near a truck during warmer and hot weather. I have lost body panels due to truck tire casings. I have had a couple tires shred on my bus, and even had one detonate taking out the floor and fender of a bus.

Just my $0.02 and some loose change from my buddy.;) :D

Ok just to thow my .02 cents, My father is an owner operator of two big rigs and here is what i know about this subject. Of course the blind spot is one thing and the chance of debis is also a major factor. But here is a couple of other things. Like one person said you cant feel when a car slams into you in one of those things the driver pulls away from the scene and now he is at fault for leaving the scene of an accident. another thing i saw happen is a car hit the back of a moving truck and got hung up, what would have been just a bad incedent ended up being an ugly incident because the truck drug the car for several miles, before another truck passing the opposite direction saw the sparks and radioed the driver to stop. a little history is police used to get right behind the trucks where they couldnt be seen and wait for the driver to break the law in some way...that is why a lot of older drivers tend to get nervous about cars being right behind them.

i know drivers who will squirt babyoil out the window and let it run down the side of the trailer until it gets all over the car, i watched a guy do a "bump and run" with a car one day. bump and run is when the truck slow down to the point where the car decides to pass when the car gets next to the trailer's dual wheels the driver does a slight swerve causing the trailer to whip out and bump the car off the road. this was before cell phones....