drafting semi's
 

okay, first off I know it is dangerous...

I recently discovered that you can be 50 to 100 ft back and still see a benefit?

I heard mythbusters found a pretty significant increase all the way back to 100ft?
Or did i hear wrong?

I am pretty confident that i can still be safe at 50-100 ft back, and was wondering if anyone with scangauges have done any informal tests with this?

In the past I've drafted within 10-15 ft off semi's with little problem.
However, i dont drive with that kind of risk level anymore...

but 100 ft is quite safe in my opinion

thoughts?

Well, 65-70 feet or so is about 1 of their length. Too close for me. I like to be 3 of their lengths behind and no more than 55 mph. I love slow trucks!

PS Yes, I use the scanguage to confirm this - AND it depends on wind speed and direction. Side winds - forget it. Head winds - nice benefit.

okay, so a full semi truck length back you are seeing an increase?

i understand about the side winds...been there done that

so 65-70 ft back, what kind of increase are we talking here?

I can get roughly 10-20% from drafting. Over 55 mph the benefits decrease. In my little car, I can feel the difference like night and day. I get back a little too far and I have to stay squarely behind him to stay in the draft. A little to the left or right can throw me back out of the envelope. I get way too far behind and I can feel the buffeting which tells me I am out of the envelope, plus the scangauge confirms this. After 65 mph, I give up and fall back to my grandpa driving. This is just my informal findings.

wow, well 10-20% is a lot.
Damn, I need to bite the bullet and buy a scangauge...with xmas time here, we have been spending and spending and spending, lol

I hate to sound like I'm beating a dead horse...but WHY do you think over 55 there is a decrease in benefit? If find most of the semi's in my area are doing 65 or so....
And how far, that you have found personally, is usually too far away? A full truck length? Less or more?

I will get a scangauge soon, and try and do some testing and report what I find.

I'm just so intrigued as I thought I would never draft again until I found that you can do it from so far away. To me, this is a safe enough distance...100ft that is...

So I saw this gasoline tanker and decided to give chase....

Last time I drove down the highway in the rain, I observed the spray in the air comming off the back of a semi in front of me.

I'm guessing it must be a rough indicator of the proportions of the hole they punch through the atmosphere and may give an idea of where the sweet spot is for drafting in other conditions. (Although I suspect the weight of the water vapor would tend to show a smaller clear pocket than in dry conditions.)

I drafted behind a Greyhound bus on a motorcycle once. The turbulence was like getting between two guys boxing each other wearing pillows for gloves. It was on one of those California 12 lane freeways with heavy traffic.

I did not save any gas, and got beat up in the process.

Over 55 the benefit of the cleaner air is overcome by all the other factors of increased speed.

I'm a big fan of minimum safe following distances. Two seconds is good three is better. The faster the truck is going the further back you should follow. 65-75 feet at 55 is way closer than what is safe. At 65+ that distance is plain stupid. I don't like people riding my butt, why would I do it to others?

Kirk

I agree. Safety is more important to me than miniscule increases in fuel economy.

If you are a REALLY slow driver like me, and look at 99 out of 100 cars in the fast lane that are "bunched up", you will notice that they are completely comfortable driving less than 1/4 of a second behind each other. Sometimes they are traveling 65+ mph and are less than one vehicle length away from the next driver. There might be a line of 20 drivers with this distance between them. This is why there are 70 car pile-ups.

Even my 2 to 3 lengths behind a 55 mph semi could be considered too close. At 55 mph we are looking at about 80 feet per second. Two seconds is about 160 feet. I am probably at the edge of safety on the very best of days. Something to ponder anyway.

well, believe it or not, I'm an ex cop and have handled a LOT of traffic accident investigations.
the VAST majority of them were failure to yeild right of way.
This means things like pulling out from an intersection into the path of an oncoming car, running a red light, changing lanes without looking, etc.
VAST majority.
Believe it or not, very few were from things like excessive speed or following too closely.

What you find with drivers following closely is that they have a heightened awareness...because they know the dangers...and as a result are not usually caught off guard.

Most accidents are from inattention, lazyness, etc... Not from people with heightened attention.

Also, unless some of you hypermilers have reduced the size of your brake rotors in an effort to save weight :D you should have no problem outbraking a loaded semi truck.

Unless you arent paying attention :D

Today I followed a semi from about 60 ft back at 61mph. i could feel the pull assist...although wont have hard figures until I get my scangauge...and felt VERY safe at that distance.

Also I have been accident free since my 20th b day, and I'm 41 now...so I think I have a pretty good handle on what safe driving is and what isnt.

obviously we all have a comfort zone when it comes to driving, and mine is probably a little skewed since I road raced for so many years, and am used to "close" driving at elevated speeds :thumbup:

When I was 18, I coasted on a bicycle, behind a fire truck at 35 mph for 3/4 mile. NO effort on level ground. This was 2 feet off his bumper. When I turned away it was like opening the parachute. Kind of like the movie "Breaking Away". I know, stupid things kids do!

and you are correct.
when I was younger I would get right up behind semi's....which is getting dangerous

I recently discovered that you didnt have to be all that close at all to get a mpg benefit...which is why I started this thread.

I recently started drafting semis and trucks and love watching my mpg shoot up! Going 40 mph I was getting 45 mpg (I usually get 38-40 at that speed) behind a truck. And going about 58 mph I was getting 40 mpg (usually 28-32).
I don't know how close I am to the truck, but I don't get any closer to where I can't see the trucks side mirrors, so the driver can see me as well.

hey its a free boost in mpg!
Cant beat that!
no aero fabrication time/money necessary ;)

I just ramble down the highway until I either approach a semi or get passed by one, then just file in behind at a safe distance...maybe 60 ft or so...and enjoy the fuel savings :thumbup:

You need to be able to see the mirrors on both sides of the big truck. Any closer is too close.


okay, first off I know it is dangerous...

Ya, truck drivers are cretins, kinda like anyone who joins the military. Just thank them for their service. Out of sight, and out of mind.


.

ive always drafted larger vehicles, mostly semi's, and not at recommended distances either, as matt stated, if you cant stop faster than a fully loaded, or even an empty, tractor trailer, then there is something wrong with you or your car.
as a long time motorcycle enthusiast, its also why you >NEVER< tailgate a bike, because they have about 1/2 the stop distance of you in most situations.

I see, on avg, a solid 20-30% benefit from drafting semi's. but the vehicle also plays a role, as in I saw a much larger increase with my old ranger than I do in the mazda protege5 I now own. although I do see a pretty decent increase with the protege :)

and its quite easy to gauge the "pocket", if your being severely buffeted then your not in it :)
for cross windy days, try being in the adjacent lane on the opposite side of the truck from the direction of the wind (IE if your heading N, the wind is blowing from the E, stay in the lane to the trucks left side, put your nose about at the end of his bumper)
but make sure to allow for faster traffic, if you need to slow down and change lanes to allow people to pass, do so.

Uhhh, motorcycles in general need the same stopping distance as a car.

Motorcycle Braking - Motorcycle Safety Information and Resources

I don't tailgate them anyway, because I don't want to run over the rider when they fall down. :p

I live next to a UPS center. I leave for work the same time they do and when i can i draft them at a safe distance at 45 - 55 all on single lane roads all the way to work.

I would think a UPS, or delivery truck like Frito with a much larger rear are much better drafting choices. The pillow is much lower to the ground with less turbulence than a semi's taller rear where the pillow goes over the car. Behind a semi your really mostly driving into the less ideal turbulence in the undercarraige and wheels. It's probably safer too.

I follow the two second rule (counting one one-thousand, two one-thousand) and it still provides a great benefit while being safe. Plus the faster traffic sees the semi truck in front of you and tries to avoid you both.

I tried drafting semis a few times. I had a real eye opener when the truck in front of me ran over a cardboard box.
I didn't see it until it was right in front of my car.

WHAM !!!

I was following at the point that his mirrors were just barely visible at the time.

BTW, He didn't like me there at all. He kept swerving and slowing down.

Not worth my life to save a few pennies ( or even $5 ) in gas !

some of them really dont like people drafting them, if I get behind one like that, I dont stay there and will go around him.
but some dont mind it at all, that is who hope for :turtle:

@WDB, trust me when I say 85% of cars on the road cannot stop anywhere near as short as a motorcycle can with an experienced rider.
when you get into some of your higher end cars luxury cars, they can stop pretty darn good. yet still I dont think quite as quickly as an experienced rider can. of course, my experience on a bike is mostly of the performance variety (crotch rockets) that are much lighter than you avg harley/goldwing :)

when my son/family is with me, I tend to follow the 2-3 second rule, but when Im alone I push it and will follow where I need to be to hit that pocket.

My own experience is that commercial rigs, (especially heavy construction trucks), are much more likely to throw up rocks than other vehicles. Just the potential for a cracked windshield alone is enough to make me not want to do it.

I generally keep 3 stripes between myself and a big rig. That's at least 129 feet of separation and closer to 140 feet. West of my home the speed limit is 70 MPH and if you want to go that speed then you better just drop in behind a drafting leader and that also has the benefit of convincing other drivers to just pass you in the left lane. In fact they will try to block you from passing the truck, which means they mover over earlier rather than climb up your rear end. I have found that this gives me the same mileage at 70 as 55 without a windbreaker ahead of me. It also allows for pulling over to the right if there is some debris in the road. If the trucker gives me any indication he does not like what I am doing then I back off or pass.

Traffic around here is usually heavy and the trade off is much greater attention to what might come under the truck, versus the constant aggressive tailgating passing on the right and general stupidity that seems to be the norm around here. The tandem (dual trailer) rigs also have a reinforcement extends down low where the trailers hitch together. Another good alternative is a bus or motor home.

regards
Mech

so I'm hearing some figures like anywhere from 10 to 30% increase in mpg's...
does anyone have any instantaneous evidence...like from a scangauge? Or are these estimates over a tank?
Anybody done any informal testing...say where they would draft a semi, record the mpg's off the scangauge...then change lanes or drop back and getting another reading, etc..?

Dunno about the US semis, but European rigs stop in a surprisingly short distance for their size and weight.

I've put up a link to a video between a semi and a VW Transporter van before - the truck stops as fast as the van.
More load on the rig is better, BTW !

That's a myth.

A mediocre family car (Toyota Avensis) will stop shorter than a sportsbike (Honda CBR900) or SportsTourer (R1150RS + ABS).
The myth has been tested by a local bike magazine years ago, and proven to be untrue.
To the surprise of quite a few bikers.
Add wet roads, lock-up fever, and it gets even worse for the average biker.

The tests weren't done using average riders, but by Belgian bike racing champions instead - Louis Wuyts won the Belgian title 16 times, I'd say he knows how to ride a bike.

In Dutch, but you can see the tables with the results :
Motornet.be
droog = dry
nat = wet
beton = concrete

Distances in meter and speeds in km/uur (km/h)

The effect is there all right.
Obviously, the closer you get the more significant the effect becomes.

You can try it by closing in on one, and see the instant fuel economy improve as you close the gap.

Trucks are limited to 90 kph / 56mph over here (some even down to 85 kph voluntarily by the hauling company), so most cars are faster.
I sometimes hitch on to a semi at a fair distance (so the driver doesn't get annoyed) and stay there.
On occasion, the instant fuel economy readout was over 100mpg @ 52 mph !


Definitely not at the 50' mark.
100' is more like it.

But I too get a lot closer on occasions. :o

I used to road race bikes with the CCS here in florida and agree that braking distance isnt any better than a car...

I also stay at a fair distance, as I've had the semi drivers get annoyed at me. If I stay a full semi truck length back, they dont seem to care.
I'm going to research and see if anyone has done any tests at different distances, speeds, etc.

The buffeting you will feel is the vortexes off the sides of the truck. Moving closer will get you out of the vortexes and it depends on the size of your car. You can also see it when it is raining if you use rain-x on your windshield. The drops will normally run almost vertically off your windshield in clean air (no draft). When you get in the draft they go crazy weaving and even going in circles sometimes.

I don't like getting real close, with 100 feet about my minimum. The stripes on US roads are right at 43 feet from beginning to beginning of each individual stripe. When you barely have 3 between you then you are right at 100 feet. Get right at 4 visible and you are at close to 140. Look for trucks with large mud flaps and especially those with the rear section closed at the rear down to about 2 feet off the road.

Best drivers I have seen here are Wal-Mart. Very consistent as a norm with UPS drivers close. Many will actually slowly lose speed climbing a grade and go to a higher speed downhill. This was the type of trucker I found on my summer of 08 trip of 300 miles in my old VX (sold) that got me 68 MPG for 300 miles, even including 2 hours of night driving coming back home. On one stretch there were at least a dozen trucks drafting each other, and I just followed the pack. Other trucks would pass me a pull over in front of me to pick up the group draft, and I let them in, since they were saving a lot more in fuel than myself. Nighttime was much more stressful, constantly keeping a vigil for the inevitable large piece of truck tire in the road, but I never had to swerve off the road.

I'll bet my Maxima would do 40 MPG under the same circumstances, but in mid 2008 the truckers were running 64 MPH average. They have since raised the speed limit to 70MPH on the same stretch of I95 south from DC to Richmond.

regards
Mech

My 2 pence (caution, only for european road conditions):

1) Road debris is not an issue over here. On the autobahn, nothing survives more than 30 seconds before being shredded and cast aside :-) If there is something substantial lying on the road, police will be quite quick to clear it. And if you want to anticipate every possibility, you need your whole stopping distance as a safety cushion anyway. This will only work at 3 a.m. though...

2) In my optinion, the old 3 second rule is for half-asleep drivers in mediocre 60s cars (you may call that worst case). A modern car can stop inside 40 m (app. 120 ft.) or better on dry road from 100 km/h. Given the fact that a trailer rig will stop AT BEST as fast as a normal car (in any weather condition), we only speak about reaction time which must be buffered by the distance between truck and car. For an alert driver, one second should be quite luxurious, which translates to 22 metres at 80 km/h.

3) Give the truck drivers some pause... neither are they idiots, nor do they drive as clueless as many car drivers do. The overwhelming majority of truck drivers I have encountered do not mind someone using their wind gap (though not many of them know they even save fuel when someone tailgates them as he fills up their energy-consuming low pressure drag pocket) and drive very anticipatory to conserve their fuel.

The only time I had to hit the brakes hard when trailing a truck was when that guy slammed the brakes because I was trailing him. It was a good adrenalin rush, but far from a close call. And that was one case in a whole year of rather intensive drafting.

Most hairy situations I encounter arise from people not looking where they want to drive but just driving there...

To provide some numbers:

usual trip to work yields around 4,0-5,1 l/100km depending on how much I can use trucks for drafting (scangauge numbers). The extreme numbers are more due to head- or tailwind and other weather and traffic conditions, I would say between 4,3 and 4,8 l/100km is the usual range. So 10% is a good estimate of the savings. Maybe less for a car with low cd value, my van has one near that of castle Neuschwanstein.

so long,

tinduck

With the FE savings I've never understood why truck drivers don't like tailgaters ?
Worried about a windshield ?
If rear ended, the rear driver is (normally) at fault.
For the drivers here, does it make the trailer dance ?

A big truck has significant blind spots, all around it. I've had trucks with as many as 13-mirrors, most with 8-mirrors to try and keep up with all that is around me. As what is behind me is at least as important as what is ahead of me in uncountable situations, knowing where all the other vehicles are is critical.

Truck tailgaters are assholes, pure and simple. Guys whose concerns are to be ignored even though we all share the road, little different than the false thank you to a veteran . . . geez, glad it ain't me hump'n it. Truck driving is a high fatality livelihood. No cop will ever see the combination of shortened lifespan and on-the-job fatality rate as do truck drivers (or, farmers, fishermen, etc) and he has a defined benefits pension after 20-years plus union protection throughout his career. So where is the truck driver memorial? The drivers retired at age 52? Few and far between. Almost non-existent. One of these is productive, the other sucks up tax money. Which do you pay more attention to while next to them in traffic?

Some tailgater suddenly comes out of the shadow into an adjacent lane as you're starting to change into that lane because of what is ahead, etc, well, guess who'll be held to a higher standard if there is a wreck? Guess who may be fired, or forced back into a backbreaking low-paid truck job, or lose all the years of a career as a result of an asshole. We're all humans and make mistakes, but taking advantage of another man in order to save some pennies is past unconscionable.

Stay back far enough that the tractor mirrors on both sides are always visible.

One can get the distance correct and the other traffic will flow around if the man following the big truck uses headlights/tail-lamps constantly. A little brake-light action can get the next pack of cretins the idea that staying left is beneficial. Not doing any lane-changing with the big truck breaking trail is worth more than getting in closer.

When you find that geosynchronous orbit back there, and know how to define it for others to get them around both vehicles then you have a good relationship going. Keep the distance perfectly constant if you want the Merit Badge.

Also:

Always pass a big truck with at least 5-mph speed differential. Get off the cruise and get on it. You want a noticeable amount of "positive throttle" to overcome the big truck bow wave. Change back into that lane about a football field ahead (no joke, you've no idea who's driving that truck, what shape it's in, or how heavy it is). This is not quite hyperbole. Smoothness will come with experience, and FE won't suffer noticeably.

Never, ever ever get stuck next to a big truck while behind another vehicle. Wait till the other vehicle is fully past the big truck (with room to move back over) before making your dash. Wait your turn, but jump on it to keep the one behind you from staying close to you as you pass. In the wait, cut off the other a-holes from moving from behind the truck to between you and the truck you as you wait for the first guy to get past. Defend that territory until you launch.

Stay out of packs, and you won't have the problem. Slow down a half-mile back to have them get around the big truck and you'll not have to do any more than maintain the necessary minimum speed differential while passing. You can learn to take advantage of terrain and actually be slowing as you pass the big truck to hit your intended mark ahead of him to get back to the right lane. If you can do it towing fairly heavy, the beer is on me.

Remember that if something goes wrong with that big truck when you're next to it then the Jaws of Life are a handy tool to help get the trailer unstuck so it can be turned upright and shoved out of the way. Their intended use is irrelevant over 4,000-times per year


http://theoldcardawg.com/wp-content/...ne_map_dc1.jpg



.

Amen to that! EXACTLY what I do all the time. I see 3 cars right next to a big rig and I think this could get real ugly. I back off these scenes to prepare to take defensive action if something does happen. Folks take huge unnecessary risks all the time and one day they will lose that 9th life.

Side note on not hanging out next to a big rig, ever see a tire come apart?

Thanks Slowmover !

Going about 60 mph, I see something like 3 lbs of boost. At 70, it can get up to 7 or more. If I get behind a semi at about 30 metres, I can see a reduction in boost. But because of the increased effort to try to stay at that distance, I sometimes find myself playing catchup and having to accelerate more than just cruising.

As far as truck drivers liking or disliking a drafter, I find that it is hit or miss. Sometimes they'll do nothing, and sometimes they seem to change lanes more than a semi normally would. When they start doing that I let them go cause I don't want to make them angry.

Now I just need to mount a strip of rare earth magnets on my bumper...

Welcome.

Next time any of you pass a tractor-trailer, be aware that each tire on your side of his vehicle is carrying near to 4,500-lbs. Times 9. The oldest tires are on the trailer, and they get younger as you go forward, axle pair by axle pair; second re-tread, to first re-tread, to new in standard practice. As you come up on a tractor-trailer look at the tires: see any wobbles, bulges or (yes) missing tires? Haul butt, boys, if you're feeling brave.

And to the above: Don't assume you can read the truck drivers mind based on truck movement. In most cases you're messing with someone with more driving skill than you've imagined is possible. You don't know that you don't know. Were your income dependent on it, your skill would also increase (conversely, trucks from Swift, Werner, Stevens, Schneider, Crete and some others are populated by low skill rookies; double-beware), so if that driver wants to F with you he can, before you're aware of it in many instances.


IOW, don't assume that because your rolling beercan will hypothetically stop sooner than a big truck that it infers invulnerability. The above example is only one way that big trucks could screw with you, highly unlikely though that is.

Unsaid are the mechanical problems that could cause a truck to rapidly decelerate.

Plus, it ain't because of ETD's that the ICC Bumper below the deck is now required:

http://primetrailerleasing.com/blog/...24-300x199.jpg


.

What are ETDs? I doubt it's Express Tire Delivery ...
End of Train Device?

Yes truck drivers have a tough job. That doesn't make them all good drivers. I've seen more than a few complete yahoos behind the wheels of semis.

What I'd really like to see is semis in the US designed more like the European semis. When I drove in Italy, the trucks were well equipped with aero aids, and generally designed to a have lower center of gravity than the trucks here. They also appeared to have pretty strictly controlled speed limits, significantly lower than the limit for smaller vehicles; every semi I saw in two weeks of driving in Italy was in the right lane, and moving more slowly than my little 1 liter Fiat - which was far from the fastest vehicle on the road!

http://www.youtube.com/watch?v=W-fMWpCokn0

Today as i pulled into the parking lot at work i saw a truck with one door open flailing into the lane over and closed again! He mustve just pulled out of the Pricechopper loading dock and forgot to lock it. I wouldnt want to be behind that when soup cans start falling into the road. Or maybe i would, as in a recent news story around here about $200,000 falling from a moving money car.

Okay, how much of a benefit do you get from drafting car, as opposed to being being drafted by a tail-gater? The most gas saving mod appears to be a boat tail, so would you benefit more by having that low pressure behind your car being filled by a tailgater? Or is it best to draft behind a car? Theoretically speaking of course.