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January 3, 2007 10:44 PM   Subscribe


 


This guy reminds me of my dad.
posted by Dizzy at 10:52 PM on January 3, 2007


That guy's an idiot. The only way you can "draft an 18 wheeler" and gain from doing so is to follow it so damned close that if it hits its brakes you're guaranteed to pile into the rear of the truck.
posted by Steven C. Den Beste at 11:18 PM on January 3, 2007


That guy's an idiot. The only way you can "draft an 18 wheeler" and gain from doing so is to follow it so damned close that if it hits its brakes you're guaranteed to pile into the rear of the truck.

Given your history of numerous factual errors, I'm just going to discount that.
posted by delmoi at 11:22 PM on January 3, 2007


howto

A couple things looked interesting.
posted by IronWolve at 11:36 PM on January 3, 2007


It's true that the ideal drafting distance on an 18 wheeler is like 5 feet, but that doesn't mean you don't get ANY effect from following somewhat further back, as SCDB implies.
posted by Justinian at 11:40 PM on January 3, 2007


On reflection, however, I admit I don't have the background or mad skillz to determine what kind of effect you'd get by following a couple seconds behind an 18 wheeler, so SCDB could be right, I just don't know.
posted by Justinian at 11:44 PM on January 3, 2007


SCDB is pulling "facts" from his ass. And by "facts" I entirely mean "poop".

I will concede, however, that the driving style of this guy is probably inherently (moderately) dangerous - but that's probably more due to the fact that everyone else on the road drives like an idiot and/or asshole, with their hard, pedal griding starts and stops that waste fuel, wear and tear and also endager everyone around them.

And if you're going to tailgate for fuel economy, tailgating a trucker is probably the best target. Truckers are total fuel economy whores themselves, and not at all prone to sudden starts and stops. They very intentionally and consciously drive in a fuel-efficient manner.
posted by loquacious at 12:19 AM on January 4, 2007


If you really want to get a sense of what it is like to draft an 18 wheeler, ride a motorcycle on Interstate highways for a few hundred miles, and just do it. What you'll learn is:

The "quiet pocket" of low pressure air behind most semi-trucks running at 65 mph is a couple of car lengths long, at most. It is stable, for a given speed, but highly influenced by both truck speed and configuration of the trailer and its wheel trucks. A normal 108 inch internal clearance 40 foot trailer has a lot more turbulent air in its wake than a comparable 40 foot drop trailer, due to the better exclusion of ground effect air by the drop trailer. In a similar fashion, a semi-tractor with maximum air management devices, pulling an overlength trailer, won't give any bigger "drag" profile, than a less well equipped tractor-trailer combination pulling a shorter trailer. So, opportunistically "picking" your draft target by conformation can improve your mileage, all other things being equal.

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 shit 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, 2007 [2 favorites]


If you're traveling five feet behind a truck, at 95 feet per second, and the truck begins to decelerate at 19 feet per second per second (giving it a generous 60-0 time of less then five seconds), You'll still be 3'8" away in a quarter of a second, which is the normal reaction time for a person. If it takes you a half second, you'll still have three inches of clearance.

Anyone know what the actual breaking time of a semi truck is?
posted by delmoi at 12:34 AM on January 4, 2007


Delmoi, the problem is that when you reach that 3'8" point and start braking, you're still moving faster than the truck. You'll start decelerating, but the truck is also decelerating, so you still have a positive relative speed, and you'll hit him.
posted by Steven C. Den Beste at 12:44 AM on January 4, 2007


Delmoi, the problem is that when you reach that 3'8" point and start braking, you're still moving faster than the truck. You'll start decelerating, but the truck is also decelerating, so you still have a positive relative speed, and you'll hit him.

I'm assuming you can break more quickly then the truck.
posted by delmoi at 12:52 AM on January 4, 2007


He recommends a lot of elaborate fiddling about for small increases, but there are a couple of good basic ones. For instance, take your foot off the gas when the light up ahead turns red and coast up to the stop, even if that lets one of the dimmer folk cut in front. Speed Racer will just spend the extra time in a gas station down the road.

But he doesn't include ride-sharing in his calculations. Two people going the same direction (leaving one car at home and splitting the gas costs) decreases the mileage of car A a little (the cost of carrying your neighbor in the passenger seat) but reduces the fuel consumption of car B to zero for, effectively, making the same trip. The next day, you both take car B out and let your neighbor have a turn at driving while car A sits in the garage getting perfect mileage. If you share a ride, I bet you can take turns driving two average cars like a maniac and still get better mileage per traveler than a "hypermiler" traveling alone every day on the same route.

But what is the most fuel-efficient way to drive in a traffic jam? For many people, that's a big part of every day's commute.
posted by pracowity at 1:00 AM on January 4, 2007


In the article he says that mandatory fuel consumption displays in cars would save 20% on fuel overnight. I tend to be against adding more weight to vehicles because of government mandates, but this suggestion is a good idea.
posted by SteveInMaine at 3:45 AM on January 4, 2007


I'm assuming you can break more quickly then the truck.
posted by delmoi at 12:52 AM PST on January 4


Absolutely no question about that.
posted by weapons-grade pandemonium at 4:10 AM on January 4, 2007


I've tail-gated tractors on my pedal bike before and gone into the back of one when it stopped suddenly. It was travelling about twenty five miles an hour, so there wasn't much of an issue other than shock.

I know this experience is not exactly comparable, but I'd guess that doing the same sort of thing with a truck and a car is pretty dangerous.
posted by johnny novak at 4:18 AM on January 4, 2007


But what is the most fuel-efficient way to drive in a traffic jam? For many people, that's a big part of every day's commute.

This is where a hybrid should really show improvement (from regenerative braking). Basically you should do what ever prevents you from using the brakes. Or you can do like I do and ride a motorcycle between the lanes (I'm getting 53 mpg out of a $4000 used harley).
posted by 445supermag at 4:47 AM on January 4, 2007


He may not be an idiot, but he is a fucking show-pony. All this effort into 'energy conservation', and he has a TWO HOUR commute.

What's the fucking point to that? Wanker.
posted by pompomtom at 4:54 AM on January 4, 2007


Anyone know what the actual breaking time of a semi truck is?

Depends on the load. Tractor-Trailer rigs use air brakes. When they push the pedal, there's a .25 to 1 second lag between pushing and brake application, since it is a gas (air) not a liquid (hydrualic) that's involved.

So, even if everything is equal, you get an extra half second of stopping time.

Secondly, no truck can get maximum force from all the brakes -- air brakes don't lock up like drum brakes, and you don't have the hydraulic boost of disc braked. Unless they are perfectly adjusted, which lasts until you use the brakes a couple of times, trucks deal with different braking forces on each wheel.

Why air brakes? Trailers. If a hydraulic brake system has the hose between the tractor and trailer fail, you lose all brakes. If the hose fails on an air brake system, the brakes *apply*. Air hoses are much easier to deal with than hydraulic connections (and small leaks aren't a problem, as they are with hydraulics.)

Air brakes don't fade like hydraulic brakes do (there's no fluid to boil) though if you put enough heat into the brake, it will lose braking ability.

Thirdly -- Surprise! A loaded truck stops quicker in the dry. Empty trailers mean those eight wheels in back are just floating, almost any braking force means they skid, and in maximum braking, the trailer wants to come around. So, you *can't* use maximum braking, you have to feather so that the rear set keeps rolling.

On the wet or ice, the loaded truck will stop longer -- wheels are going to lock, you're going to have to feather the brakes, and now you are fighting inertia.

A fundamental rule is you never cut off a semi, because they can't stop as fast as you can. If you're behind a semi and paying attention, between the air brake lag (the light comes on immediately) and the stopping distance difference, you would stop well behind the tractor trailer.

Cut in front of one and slam on the brakes, and the truck will plow into you -- even if the drivers reaction time is perfect, the brake lag means you'll be hit before the truck has developed any real braking.
posted by eriko at 5:32 AM on January 4, 2007


All this effort into 'energy conservation', and he has a TWO HOUR commute.

Yeah, I noticed that too. If he were really serious about saving gas, he'd move into the city and take a job downtown that he gets to by electric train.

Still, I assume he has a lot invested in his career and his house was bought before he got religion, and he probably wouldn't care at all if his commute weren't so much of his life. So I'll give him that. I know when I had a lot of driving to work I'd get very serious about starting times and so on (five minutes later, for instance, and you'll spend fifteen in a jam). And as a driver, my whole life, I've preferred a defensive posture (like that Hobbit guy). There's no value to pissing off every other driver on the road, and you don't want them to start fucking with you when you're playing a no-engine, no-brakes game ...

I do this because "conforming to traffic" reduces my headaches about driving, even if it does little for my fuel economy.

The Drive It Forever guy, Bob Sikorsky, fields frequent letters about fuel economy strategies. I think the FCD proposal would cater to a lot more people but I'm not sure that many people would really pay attention, because as above, there are other factors that play into whether you care about your fuel economy. If you're driving a Hummer, probably a FCD would have very little point (although, it should be noted, the H3 is much more in line with other big SUVs wrt MPG -- it's built on the Tahoe platform).
posted by dhartung at 6:19 AM on January 4, 2007


Ummm, y'all, estimating braking distance is too imprecise and not terribly useful.

What you should do is this: Let the vehicle in front of you pass a fixed reference point -- a roadside sign, a prominent tuft of grass in the median, or an arbitrary dashed lane-divider line. Count "one-a-thousand two a thousand" etc. until you pass said reference point. That's how much reaction time you're giving yourself in the event that the driver in front of you causes a Drama In Real Life (tm).

I try really hard to maintain a two-second interval at anything over about forty MPH.
posted by pax digita at 6:32 AM on January 4, 2007


Of all the neat things to fetishize, he had to pick MPG?
posted by Burhanistan at 6:59 AM on January 4, 2007


i agree that the fuel consumption gauges encourages driving behavior. I remember my last car had one and it was always a game to see how high i could keep the number. putting them in every vehicle would be a great thing, although I suspect auto manufacturers wouldn't want to do this because most people would not get the mileage promised on the car's sale tag.
posted by daHIFI at 7:00 AM on January 4, 2007


Fantastic article, I need to send it to the folks I know who get 30mpg on their hybrid. Thanks.
posted by Shutter at 7:02 AM on January 4, 2007


Or you can do like I do and ride a motorcycle between the lanes

That's fine (aside from breaking the law, I suppose), until someone opens a door and you recycle yourself.

But I was really wondering about operation of a regular car -- how to idle-move-idle-move efficiently. Lower the idling speed of the car? Move only when the car ahead has moved a car's length? Secretly hook a rope to the bumper of the car ahead?
posted by pracowity at 7:19 AM on January 4, 2007


But what is the most fuel-efficient way to drive in a traffic jam? For many people, that's a big part of every day's commute.

This is where the Prius wins big. The gas engine will be off most of the time, so it's running mostly on electric power, plus using regenerative braking will keep the battery charged.
posted by mike3k at 7:49 AM on January 4, 2007


Wins big until after an hour or so of stop and go. Then it starts to oscillate between starting the ICE to charge the battery, and shutting it off. A real unintuitive, and frustrating situation.
posted by DesbaratsDays at 8:19 AM on January 4, 2007


Don't get me wrong, I'll never buy a "regular" car again...
posted by DesbaratsDays at 8:21 AM on January 4, 2007


There's so much confusion in Wayne's brain. If he really wanted to reduce fuel consumption, he'd ditch the two hour commute (as noted above) and the wife's SUV, and his car, and buy a scooter. This is just a dude with a hobby that is both dangerous and boring, which is a heck of a combo.
posted by Nahum Tate at 9:13 AM on January 4, 2007 [1 favorite]


Despite their reputation, taxi drivers practice fuel conservation techniques like this more or less constantly (in Philadelphia any way).

Many times I've seen a taxi cruising toward a green light at around 15 MPH while morons surge by doing 40 or even 50 (speed limit is 25 on most city streets). The cabbies know that they will not make the green, no matter how fast they go, so they coast up to it. Saves your brakes and saves gas. On other streets, like Market East, they never go above mid-20s despite green lights ahead, because they know the lights are timed to roll with you if you maintain 25 mph or so. Going faster just makes you slow down as you arrive before the light turns green. I am amazed at the number of people who drive in the city every day and haven't figured out these basic things.
posted by Mister_A at 9:19 AM on January 4, 2007


On the other hand, the lights in some stretches in my downtown grid are timed to encourage speeding. Because if I bust ass out of two lights in particular, I make all of the rest. I kinda wish that they would retime them so I would stop doing that. I can't help myself.
posted by intermod at 12:56 AM on January 5, 2007


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