Drafting a big rig
 

I mentioned it in my intro post, but my clearest success story was on a trip that normally requires I stop for gas halfway through.

I made the entire trip on one tank, mostly due to sitting behind a big rig, 5 miles below the posted limit, for about 2 hours. Fortunately he was going the same way I was and never exited, and kept a steady pace.

It doubled my mileage on that trip, at least in theory. I don't have the exact numbers, but I remember being so surprised when we were back to our home city and hadn't needed to stop (it's a long trip we make frequently, and we know how often we normally stop).

I figure...those rigs are guzzling enough gas to power them along the highway...I might as well share some of that power in the form of a draft!

Thats a wind up right? WE DON'T DRAFT!

Huh?

In the absence of a crosswind, you get a FE benefit even three or more trailer lengths behind a tractor trailer. As long as you keep a safe distance between you and the truck, drafting is a great idea.

Here are your choices.

I95 north between Richmond and Washington DC. Speed limit is 65 and three lanes all the way with more when you get close to the metropolitan areas.

Left lane-if you don't average 80 MPH you have some SUV 6 feet from your rear bumper and they will pass you and cut you off with less separation than you could measure with a yardstick.

Center lane-if you don't average 75 MPH you have cars and big SUVs passing you on both sides with the potential of a simultaneous merge right in front of you and a nasty pileup.

Right lane- lots of big rigs going between 65-75 MPH where you can find a good drive doing the speed limit or slightly over and stay about 100-150 feet behind him, with the shoulder (paved and wide enough to drive on easily) as your escape route if something happens with the idiots driving stupidly in the other lanes, or a large piece of tire debris appears under the truck in front of you.

If you allow more separation between yourself and the truck in front of you, you invite the idiots to pass the car in the center lane by cutting you off by a couple of feet separation and passing the car in the middle lane on the right (illegal) using the separation distance you leave between the truck in front of you as a passing lane.

Your last option is to take another route that takes an additional 20 minutes in the 2 hour drive, and you run the risk of falling asleep at the wheel, or getting hit by a deer if its dark outside and killed.

I choose the right lane on the Interstate when its dark, or early in the morning when the traffic is less insane. I use the slower route when its light enough for me to see the deer before I get nailed.

In either the right lane at 65 MPH average of the alternate route at 55 MPH the mileage is basically the same. The alternate route is like driving on a different planet, with so little traffic that you have a hard time staying awake if you have had a long day.

The draft option (right lane) is good for an additional 10 MPG in the CVT Insight, with separation of 125-150 feet (more than anyone around you in any lane).

regards
Mech

The problem w/ a ~125-150ft interval IMO is that it can take ~130ft before someone even starts to slow down.

So...maybe 'draft' is the wrong word.

But I'm pretty sure that someone can be behind a slow-moving tractor trailer (ie: 60 in a 65) in the right lane, follow at a safe distance, and still be able to benefit from the wind break.

And because this particular truck was going below the speed limit, even with space in front of me, no sane person would jump between me and him, since he was so pokey.

:)

I read the 101 tips after I posted this, and noticed the 'no drafting' policy, so if I upset the apple cart, I apologize.

the potential for an accident is always there regardless of if you are drafting. i figure... if something is in the big rig's path, it will probably plow through it(unless it's another big rig).

people also argue that the trucks can stop abruptly but its highly unlikely that they could slow down faster than whatever you are driving(passenger vehicles)

i once made a 92mile round trip drafting and achieved 46mpg (calculated), whereas my average mpg is around the mid to high 30s.

in CA the big rig drivers go through A LOT of PAID training to get their license which includes behind the wheel.

the risk is on you.

People are also wrong sometimes. :)

The only semi rig that can stop faster than a car with functioning brakes is the semi rig that plows into a reinforced brick wall.

Now another rig BEHIND you might be a safety hazard.

Actually, there are risks that go beyond braking distances.
#1. Trucks have a huge draft, and they suck up/throw up a lot of trash.
#2. Trucks can drive over road hazards that'll take your undercarriage out. So if it's heavy enough for the truck's draft to leave on the ground, it's gonna break something going under your car.
#3. One word: "Gators". Semi tire treads.

When riding motorcycles, (no windshield or fairing) drafting behind a semi just made the trip easier. Until I saw a gator take out a Crown Vic's front fender. Maybe with a very "aero" car one could expect minimal damage, but what about the undercarriage?

Now, if traffic is bumper-to-bumper, and you have to be behind something, and that just happens to be a semi...

I'd like to meet the brick wall that can stop 20 tons at 65mph. But I would not like to meet that wall at 65mph.

The problem with the concept that a car can stop quicker than a truck is that by the time that you get the first indication that the truck is going to stop/slow quickly the truck driver already has his foot on the brake and is applying pressure.

A truck does not have to come to a complete stop to create a large difference in speed between it and the car following. Depending on how much room the driver of the following car gave himself to see, react to, and avoid the developing hazard will go along way in deciding if he needs a tow truck.

to draft a semi you need to be at nearly the same speed.

bestclimb; i could see an accident happening in that situation if you really weren't paying attention but otherwise, i think reflex's would be enough to make you hit the brake once you see the semi's lights turn brighter red.

k.civic.f4i -

I try to follow the 3-second rule (in the early 1980's I used to quote the 2-second rule). I could probably "push it" closer to that when I am in hyper-vigilance mode (aka right foot hovering over brake pedal), but that's too tiring for long term driving.

CarloSW2

Kinda depends on how close you are.

Most of the time, it's hard to find a big rig going slow enough to justify following it. I can almost always get better mileage going slower. On those rare occasions I find a really slow truck, I'll follow it 2 seconds back. At that following distance, we're often passed by an endless conga line of cars following so closely, three of them can often fit in the distance between me and the truck in front of me.

If you are 130 ft behind a vehicle that begins braking and it takes you 130 ft to react and begin braking as well, you will begin braking at approximately the same point as the vehicle in front of you, if you have better braking capability or shorter stopping distance you will still be well behind that vehicle when stopped as the vehicle in front of you will be incapable of an instantaneous stop (except in the instance of something influencing the abrupt stopping of the vehicle in front of you such as the already mentioned reinforced wall, car hitting a stopped big rig, etc).

Assuming that average human reaction time from seeing a situation to applying brakes takes 130 ft, I'd say that 130+ ft is safe for a following distance. Of course that distance will vary with speed, so basing your folowing ditance on something fixed like time will allow your distance will vary appropriately. 3 seconds following distance generally considered safe (the "three second rule"). By my calculations, at 70mph you can cover 100 ft per second, putting your safe following distance at 300 ft or 100 yards (a football field length minus the field goals). At 55mph that is reduced to 240 ft or 80 yards.

I have drafted big trucks before, and have seen significant gains in efficiency. Once I drafted a typical big rig with a box trailer, can't say the exact following distance as I'm not good at guessing distance in numbers but a good number of car lengths back. I had a 2000 Honda Insight at the time, 5-speed, A/C off, driving from west Texas into New Mexico drafting the truck at 55 mph for half the drive, the other half solo and I average 74.6 mpg and easily put around 600 miles on a single tank. I was stopped at a border checkpoint station where most cars were simply waved through and suspicious ones stopped, just so the guys could check out the car and ask questions about fuel economy. In normal everday driving I would typically get 52-56 mpg and when really trying being able to get up to 60-62 mpg per tank.

The Insight was replaced with my current 2006 Prius, which I have drafted an big rig that had an oversized load, it was just the perfect drafting truck, it had some odd shaped object on a large and very low to the ground flatbed that was all tightly wrapped in white plastic and the back end of the object had a nice taper and because the trailer was so low the ground there was little chance of any rocks or other objects getting thrown and it was going rather slow at about 55 mph heading east on I-10 from San Antonio to Houston, I was able to draft it for nearly the last half of the drive into Houston (mind you the entire distance is gradually downhill to the gulf coast) averaging just over 60 mpg where I normally average about 56 mpg.

+1 Braking is the least of your concerns

Something like that.

http://www.youtube.com/watch?v=8NA3D9faEKM

That's why i almost never do any drafting. Pretty much all the trucks around here go well over the psl on the highways.

Over here(UK) it's often called Slip-Streaming, but I know it more as Drafting.
Also, over here, it's the 2 second rule(At least), which is quite irrelevant when following a Truck(Rig), because there's a saying over here on the back of Trucks, "If you can't see my mirrors, I can't see you", well, that'll have to be about 4-5 seconds back, so pretty much blows out the idea of Drafting.
But, I have tried it, followed a few Trucks for about half an hour, before I/they turned off, and there's a big difference in engine revs, when up close behind them.
I have got really close(IMO), that is about 1.5 seconds distance, with left foot hovering brake pedal...It really didn't feel safe, so if I do draft, it's more just following with a decent safe-ish distance, where I can just about see their mirrors/whatever happens to be in front of them.
Thanks,
Tom.

Traffic Cameras | HamptonRoads.com | PilotOnline.com

First point. I do not actively seek a drafting situation.

Second point, if you check some of the traffic cameras on the above posted link, especially the bridge tunnel to the right (Hampton Roads Bridge Tunnel), which was built (first tunnel) in 1957, during rush hours, you will better understand that you really have no choice under certain situations.

In fact, inside the tunnel itself, where there is no lane changing and no escape route, notice how close the average distance between vehicles can get at the peak traffic volume times.

In summer (tourist season) the density is something like 95,000 vehicles per day for east or west bound lanes. I think that is 95k in each direction. That's 45,000 per lane, per day, with a day at 84,600 seconds.

Do the math. That's less than 2 seconds average, for every vehicle, for 24 hours. When the peak congestion periods come, it's much less than 2 seconds per vehicle.

You can try to leave a 3 second (football field length) space between you and the car in front of you, and all you will do all day long is hit your brakes as someone pulls over 20 feet in front of you, and if you are in the center lane of a 3 lane section of road, you risk as simultaneous merge by drivers on both sides, with a collision right in front of you at 65 MPH, with a half a dozen cars within 300 feet behind you times 3 lanes.

It's not uncommon to look in your mirror and see nothing but a pickup truck grille in your mirror.

I try to avoid the Interstates here, because people are stupid and don't know how to drive. You would think that they learned by watching a NASCAR race.


regards
Mech

Please, please don't draft!

I can't tell you how many hypermiling stories imply that "all hypermilers draft 18-wheelers and roll thru intersections" :mad Of course it's seldom practiced, but the media enjoys spreading this misinformation. The effect is the general public is turned off by hypermiling, even when most techniques are nothing like this.

Aside from obvious safety issues, you are likely not to see an exit until it's too late. Most truckers know when they are being drafted and will shake you off. And this is outside of the city - the only place you can get an opportunity to do this.

Completely agree. The media is all about trying to attack hypermilers and say that what we do is "dangerous". I also have a friend who is a trucker driver. He is mainly referring to the aggressive/road raged drivers, but he said that he absolutely hates when people follow too close.

The problem there is that if the average is 130 ft before someone starts to brake, then some people will be above average and fine, and another large chunk will be below average and tap the semi on the rear. Having a safety margin is never a bad idea AFAIK, which is why we have the three second rule like you mentioned instead of a two second rule even though on average may be able to respond in two seconds.

Me and the Mrs travelled to Florida from canada, 1800 Miles. I drove the RV, Ford F250 with a 15000Lbs 5th wheel trailer in tow. My mrs had the Toyota Matrix 1.8L engine 5 Speed.
I averaged 13.4 MPG on the trip. But the best was my Mrs with the Matrix. She averaged 45MPG on the 1st 2 tanks following me. I then told her to back off a bit from me until she felt the need to lift the accelerator a bit and stay there for most of the rest of the trip. The next 2 tanks of Gas were at 49MPG. I drove at 100KPH or 63MPH most of the way and all the traffic just passed us and left her behind me. We did the trip back for Christmas and we did average 40MPG, that was the car alone the RV is still in Florida. We travelled faster on the way back because any day I can do 45MPG with the Matrix at 100KPH. The MPG are in Imperial which is 6/5 of a US Gallon.

Driving very close behind a big rig with Honda Insight at 80 mph you can get well over 80/85 mpg on the instant dash board computer. At night with no big rig in front same highway MPG sucks alot.

How close it's safe to follow all depends on your reaction time and how attentive you are. I'll be one of the first to say drafting as a habit is probably a bad idea.

However, considering the difference in stopping distance between a fully loaded semi and any car with decent brakes, an intensely alert driver with decent reaction times can follow significantly closer than 3 seconds.

To me the question is more about other risks, (like tires coming off) and whether you're willing to stay on edge to keep it safe than it is about stopping distance.

I hate being behind Semi's period. In NM I guess they don't use enough tar when they pave, so seems every Semi throws an ungodly amount of rocks up. Windshields are expensive.

I'll either be WAY behind one, or pass em. Period. Drafting is fun when it's your buddy driving infront of you, but drafting a stranger is a HORRID idea.

Here ya go: Robot drafting: Follow the leader: SARTRE road train technology successfully demonstrated

The images show it being used in the slow lane, which would be very dangerous for someone realizing that they need to take an exit that the "train" is blocking. That should be used in the far left lane only.

So far, it is only on a test track. It may need steering enhancements for long trains to keep them in the lane, and many other enhancements, but it is progress. While it is easiest to deploy in the left lane, most places, it will probably acquire a way for drivers to create gaps for merging or crossing lanes. Even that might be automated by surrounding sensors, breaking trains into short sections as necessary.
The great potential here is to add smart speed controls, without which heavy traffic can't maintain a steady speed. Merging traffic can be neatly added by accelerating beside a "moving" LED display generated to synchronize with the train.

Stay away from trucks period.


I have driven trucks for many years and try to operate them as safely as possible for you and me, this includes keeping a safe distance from all vehicles in the event I have to react to a situation.

If you draft (tailgate) a truck, even though you're behind it, you can cause the driver to pay more attention to you than required, instead of whats in front of them and thats not good for anyone.

The rule to follow is

"If you can't see my mirrors, I can't see you"


Remember...

In order to get respect, you have to give respect.

>

If you aeromod your car to drop the Cd into the mid 0.10's, then drafting at normal highway speeds offers very minimal mileage benefit. The only use I have for drafting is that it can give me a quick snapshot into how much aeromodding would improve the mileage of whatever I am driving. When drafting, the only thing that has changed is the wind load on your car since engine rpm and rolling resistance remain unchanged. So if your drafting car is experiencing an air speed that half of your ground speed, then the mileage you are getting is the mileage you would be getting without drafting if you cut your Cd in half. Ideally you need an airspeed indicator in the nose of your car to determine airspeed if you want to quantify the drafting mileage benefit. Also the air speed indicator can let you find the best position for drafting and let you know if you were driving into a headwind or tailwind (two other things that a mid 0.10 Cd minimizes the effect of).

Basically the effects of dropping my Cd down to 0.16 is that (compared to the OEM car) it is like I am always driving in a tailwind or in a draft since my aeromods provide the same mileage benefits as drafting or driving in a tailwind without having to do either.

I don`t draft behind any trucks, but what is it that bothers a trucker if a car drives behind him?
The car˙s headlights can`t bother him because he can`t see them, also he cannot be concerned about safety distance because the car can brake much faster than a truck can.

I`m thinking of this because adaptive cruise controls (automated braking) are now becoming common and it`s quite safe to use them to draft behind trucks and save on fuel.

Travelling too close to something in front is dangerous for obvious reasons. Even if you could stop in time there is always the unexpected - in fact everything is unexpected because you can't see. If you pull back abour 5-7 car lengths you get most of the benefit and the truck (or whatever) in front fills less of your vision. If you don't and the truck switches lanes to pass something and you go - bosh.

Truckers also don't like people going too close because they don't want the hassle of an accident - you might be OK but others may not. Plus your lights may not look like they would be bothering the driver but it also may mean he can't see other vehicles passing so its harder for him to judge when he can pull out.

There have been folks on here tapping about truckers switching to the sides to kick up dust and cr@p to discourage tailgaters / drafters.

Plus of course dropping 5 mph or P&G on inclines will gain more and be safer all round.

EDIT : Sorry to be a pooper but I think safety is more important.

As you just said, drafting at a reasonable distance is key. It still gives benefits, but not quite as much. It's a heck of a lot safer though.

It may sound good in theory, but lets look at the European reality :

You'd be tailing a professional driver. Sounds good, doesn't it ?
In reality, you may very well be drafting an outsourced, often overworked and underpaid new-age slave : an eastern European truck driver without a clue of local traffic regulations.

If he messes up, you'll be messed up, with no chance to intervene - there simply won't be enough time to react.

Check the UN product code over the wireless internet to decide wether you really want to draft that cargo.

Check the shredded rubber on the side of the road.
It usually isn't from a passenger-car, it's too big for that. Way too big.
You could well be drafting a trailer with a few bald tyres about to blow out. Only a few meters ahead of your own car.

What's going to happen if such a train passes an on-ramp to the motorway ?
It'll be pretty hard to get on the motorway.


No thanks !

interesting thread, and I guess some of my opinions may be swayed not being in the US (I know some europeans have commented)

TBH I follow trucks when on the motorway (I dont draft ;) ) at a set distance. I find it helps me keep a steadier speed, and also seems to make me less of a hazard to other drivers (they see a truck up ahead, they automatically are starting to move over into the middle lane to overtake) rather than assuming Im doing the same 85mph they are (people leave it much later to pull out when there isnt a truck in front of me)

I always thought I was far enough back that I wasnt a problem (a good 4 or 5 car lengths at the very least)

Never had a trucker try to "shake" me. Im at a distance that any stones / etc fall to the ground before they get to me.
If it bugs truckers to have a car close, how come they regularly (and I MEAN regularly) spend their time about 10ft from the truck in front? with another close behind?

Like so....
http://img.photobucket.com/albums/v3...a8a2aa07_z.jpg
Very common site in the UK to see a row of 5 or more that close, or very nearly.

I am interested in the lorry tyres ripping off thing. That would obviously be bad, never seen it happen though, and rarely see stripped rubber at the side of the road. Seen plenty with flats / flappy tires, none where they have properly delaminated.

Thing is, people who arent hypermiling (average joes) have to follow trucks all the time on single carriage ways, at speeds upto 60mph, and it doesnt faze them? Are we thinking about this too much?

I dont think theres anyone here who would agree with sitting 10ft off a lorries bumper! :)

maybe they keep the motorways cleaner in the UK than the states (easier job, theres a heck of a lot less of them!) so the debris thing isnt an issue here (aside from potential delam)

Lorry's mate, Lorry's ;)

There was a post in here about the lights of their car could not bother a truck because they were so close the truck wouldn't be able to see them :eek:

And ShadeTreeMechanic described a scenario where the truck did kick up much to shake him off - or so he thought. I think its in the You Know You're an Ecomodder When... thread. So it has happened.

Not common I grant you.

And yes, truckers should know better.

Tapping a big rig is not an issue. Just be that far that your front just doesn't cover the brakelights. If you are close enough the rig won't slow down much before the collision so it will be much like bumping a stopped rig at 5 mph. If the rig isn't doing a panic braking you'll be just fine because there is that 1 second delay between the lights going on and the rig starting to slow down.

What makes drafting a big rig dangerous is the stuff coming under the rig or from the rig. A car muffler won't bother the truck driving man but will probably still go thru your car's ac, ic and radiator before splicing the hydraulic lines :p Retreaded tires can also loose their tread which usually takes the fender around it with it when it leaves and that is about the size of an average car windscreen. That is why I don't draft.