[Richard Rowe]

Actually, now that I think of it, I think I will post one section from the Cruising chapter here. Just so ya'll can get a feeling for my personal style, and the book's. I picked this one because it's something that you're probably familiar with, and I'm sure you know about the part I cite toward the end. Unfortunately, I can't post the cool CFD pic of airflow going around the truck...that's gonna be publisher property, and they'll pitch a hissy if they find their paid-for pics on this forum later. Text, I can work around, but those people are frickin Nazis about their pictures.

Just so you know, this particular part is first draft, and parts of it are a little darker than I go for 99 percent of the book. But, I imagine you'll understand why when you read it...especially coming from an ex-trucker like myself. But most of the book is very light and positive...this part's pretty much one of two exceptions to the rule. The other is the section on kinetic energy gains from crossing the double-yellow while cornering. That's proviso'd in pretty much the same way. Anyway...enjoy, and let me know what you think!


Drafting – Vacuum Pockets for Airheads

How good are a tractor-trailer's brakes, exactly? How quickly can a truck slow down? These days, all company trucks have computers, and those computers record everything that the truck does. How fast it's going, when it stops and where, how hard the driver brakes and even whether or not he buckled his seat belt. Trucking company safety departments use the latter two bits of information to provide the human resources department with precedent to justify random terminations free of payment for unemployment compensation. Err...sorry. That's not true at all. They use it to ensure public safety. And to win lawsuits.

Most company safety departments define a “hard braking event” as a reduction in speed of more than 7 mph in one second – a rate far within the braking threshold of the average truck. Particularly when empty. Empty, the average tractor trailer can bleed off as much as 10 to 12 mph in less than a second. So, why is this relevant information? Three reasons.

The first is that the average human reaction time, while driving, is about 1 to 1.5 seconds. The second is that, at 65 mph, you'll cover between 95 and 142 feet in that 1 to 1.5 seconds. Lastly: hitting the back of a tractor trailer 1.5 seconds after the driver slams on his brakes will have about the same effect on your windshield as driving face-first, at around 20 mph, into an 80,000-pound, horizontal, steel I-beam. And, if you're very lucky, you and your passengers will subsequently live to be dragged under that I-beam at 45 to 50 mph, inside of a screeching, burning husk of twisted steel and broken glass, by someone who didn't even notice you were there.

So...BUNNIES!!

Now, let's talk about drafting.

Truck drivers didn't invent drafting, or even discover it – but they did master it, and show the public how the technique could be of use in everyday life. You've heard the phrase “wake vortex” a couple of times throughout this chapter, and saw a brief explaination of the phenomena earlier on. But what does a wake vortex look like, and what does the drag pocket behind a vehicle have to do with it?

(PICTURE)

This is what the airflow around a vehicle looks like from above; it's what you might see if you were in a helicopter, looking down on a tractor-trailer on a foggy morning. In this picture, you can clearly see the triangular pocket of empty space behind the truck; this drag pocket is the void created when a vehicle punches through the air, and it's what's pulling you backward as you go down the road. Inside the drag pocket, airflow is almost nonexistent. Aerodynamically, any vehicle trapped within the drag pocket might as well be idling along at 5 mph.

If you look just behind the truck, you can see how a certain amount of air, sucked inward by the powerful drag pocket, will form the inward-spinning tornadoes that we call wake vortices. These vortices push anything that enters the drag pocket inward, centering it behind the lead vehicle.

Truckers figured out decades ago that getting very close to another truck made their own trucks not only somehow easier to drive – owing to the vortices' centering effect – but more fuel efficient as well. By now, you undoubtedly know why; big trucks displace a lot of air, and eliminating air resistance can free up hundreds of horsepower's worth of fuel. And the trailing truck wasn't the only beneficiary; by taking up space in the lead's drag pocket and directing the vortices down the length of his own trailer, the trailing driver relieved the lead truck of much of its own drag penalty. Start adding trucks to the line and you've got the famous, high-speed “convoy,” where all but the leading and trailing trucks run as though operating in a nearly complete vacuum.

Convoying was a big part of why truckers adopted the CB radio. A CB allowed the lead truck to communicate with his followers, letting them know well in advance if traffic and slow-downs were iminent. And drivers would typically coordinate so that the trucks were arranged from fastest and most powerful to the slowest and least powerful, heaviest to lightest, in order to avoid pile-ups while ascending and descending hills. And all of this was a necessary part of convoying, because, in order to stay in the draft, drivers had to remain no more than five to ten feet from the lead truck's bumper. And, the longer a particular trucker's hood, the closer the lead truck he had to be. Ever wonder how those flat-nosed, cab-over trucks got so popular? Or why a lot of them had huge, tube-steel front bumpers going clear up to the windshield? Now you know. Hard-core convoy truckers would often run high-double or even triple-digit speeds, packed so closely together that little nose-to-tail love taps were all but inevitable. Of course, officially, cab-overs were popular because they had short wheel-bases, and were easy to maneuver and park. But try telling that to anyone who hauled freight during the 1960s, 70s and 80s.

So, trucker trivia aside, what does any of this have to do with you? First, so you know how close you need to be to stay fully within a truck's drag pocket; 10 to 15 feet or less, depending on your speed. But primarily to point out the fact that you're probably not driving a cab-over truck, and that you almost certainly don't have a tube-steel bumper going up your windshield. And, unless you're Burt Reynolds, you don't have a CB radio. And, additionally, even if you did, any trucker worth his salt would slow down to 20 mph and stay there before he would play Front Door to a four-wheeler. So, what are you doing without that vital communications link? Driving blind at 70 mph, is what.
And, before we get to the theoretical gains, it's worth mentioning that you'll never actually be fully within the truck's drag pocket. The trailer body does most of the blocking, and the bottom of the trailer ends about four feet from the ground. At car-level, only the trailer's tandem axles run interference between you and the atmosphere, and the axle set isn't solid like the trailer box. Furthermore, the tandem axles are adjustable, and may be ten feet or more further in from the truck's rear bumper. Just something to bear in mind as you read the following, completely theoretical fuel efficiency figures.

From the earlier chart, we know that aero losses at 70 mph (the standard truck cruising speed) for the average sedan come out to around 17.6 horsepower, and losses for the average truck or SUV ring in at 35 horsepower. Assuming a BSFC of 0.40 pounds/horsepower per hour, we can calculate that the sedan would save about 7 pounds of fuel per hour, and the truck/SUV would save around 14 pounds per hour. As a US gallon of fuel rings in at about 6 pounds per gallon, that's 1.12 gallons per hour saved for the car, and 2.33 gallons per hour for the SUV. Assuming that the car started out at 30 mpg and the SUV at 18 mpg, and converting gallons/hour to miles per gallon, we get...

56 mpg for the car, and 47 mpg for the SUV.

So, are those figures realistic? Possibly not, since they assume that the trailing vehicle is operating in a complete vacuum. Which it isn't quite, given airflow around and over the tandem axles. But, even if you're feeling pessimistic and want to subtract 20 percent for airflow over and through the tandem axles, this calculation still yields a 52 percent increase in fuel economy for the car, and a whopping 208 percent increase for the aero-challeneged SUV.

Now, do those numbers reflect real-world testing? Yes, they do. And then some. Back in 2007, the Discovery Channel's Mythbusters infamously performed this test using a Dodge Magnum (baseline 32 miles per gallon) and a Freightliner at 55 mph. At 10 feet from the truck's rear bumper, the Magnum's fuel economy jumped by a full 40 percent. But, you say, 40 percent isn't 52 percent. True, Grasshopper – but recall that the Discovery Channel performed its test at 55 mph, and our calculations assumed 70 mph. Given the difference in power consumption between 55 and 70 mph (about 10 horsepower for the car, and 20 horses for the SUV), it's almost certain that, at 70 mph, the blunt-nosed Magnum would have seen efficiency gains handily exceeding our fudge-factored 52 percent, and possibly very close to the original, theoretical 86 percent.

So, does drafting save gas? Yes it does, without a shadow of a doubt. Does that mean you should do it? You can if you'd like. But please don't have children before you do. First, because you don't want to leave kids to face this world without a parent. And second, because Social Darwinism suggests that they'd only wind up spawning a race of hairy-palmed Morlocks.