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Well, maybe you're saving tons of gas. Maybe you're saving teaspoons. And if you have any sense at all, you're probably fooling yourself entirely. That's because to benefit from the truck, you have to follow it very, very closely. If you approach with any timidity, if you possess one atom of caution, you'll miss the free ride. |
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Where is the free-ride zone? That requires an exploration of aerodynamics. . |
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Air, for starters, isn't nothing. Think of all those air molecules as snowflakes to get an idea how air behaves when confronted with a
thundering truck. These snowflakes are just hanging out, suspended peacefully above the highway, when the truck comes along. Its flat
face bangs into a wall of snowflakes. They stagger backwards against the snowflakes behind them, packing harder and harder. When the
pressure gets too great, they squirt out the edges -- over the truck's roof, under its belly, around its cheeks. They stream along the
sides of the truck.
Meanwhile, what's happening at the back of the truck? It's much like the clear road behind a snowplow. There's a vacuum, or what aerodynamic researchers call a "low pressure zone" in the spot the truck vacated. And the snowflakes around this low pressure zone take a while to settle back into it. So in front of a truck is a block of high-pressure air; behind the truck is an envelope of low-pressure air. If you can fit your car (or at least the blunt, air-plowing parts of it) into that envelope, you'll have to push less air and your gas mileage will improve. Furthermore, you'll be doing the truck driver a favor: Even within the low-pressure zone, the front of your car is creating a little high-pressure zone, and giving the truck a little push. Race cars lined up nose-to-tail all benefit from this. Each pushes the one in front. Each except the leader enjoys a freshly plowed path through the air. A train of race cars goes faster than a single car can. |
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Sounds like a good deal for everybody. (Truck drivers, for the record, don't dig tailgaters; nor do cops.) . |
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I ask Jim Ross, a wind-tunnel researcher at NASA's Ames Research Center, how big the free-ride envelope is.
"It might be one or two truck widths," he says. I try to picture this. "Eight to 16 feet," he says. "Then the wake-flow closes up." I try to think what 8 to 16 feet looks like at highway speed. For a better view, I fire up my car. (I hang up the phone first.) Sixteen feet from the rear of a truck, at 60 mph, I look for some reference point: Can I read the fine print on the license plate? I look for the plate, and am suddenly only 10 feet from the truck, which must have slowed a tiny bit. I back off the gas, and fall 20 feet behind. I accelerate to catch up. ... Staying within 16 feet seems to require a lot of gas-wasting speed adjustments. And it's stressful, navigating from behind this greasy steel wall. I can't check my rearview for cops and reckless drivers because I'm driving so recklessly that it would be perilous to take my eyes off the truck. This isn't a low-pressure zone, it's a blood-pressure zone. And Jim "killjoy" Ross isn't done with me yet. "Your car is too low," he points out. "A lot of air comes under the truck because it's so high." Drafting, as this aerodynamics-inspired form of tailgating is known, works best between same-sized vehicles. You might also recall that the cooling system is in the front of your car. Less air to the radiator means less cooling. (Race cars poke their noses out of line from time to time to avoid frying their engines.) |
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But how about those fabulous fuel savings? . |
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One aeronautics researcher guessed it could be 5 or 10 percent. If your car gets 25 miles per gallon, you might save 3.5 to 7 ounces of
gas in a 50-mile tailgating session. So that you may put the value of your neck in perspective, that's six to 12 cents. For me, at least,
following a truck at less than 16 feet is going to cost me at least 13 cents in Rolaids. (Really. I figured it out.)
In fairness, many people end up behind trucks not because they're free-loaders, but because they're afraid to pass. And one reason is that invisible brick wall you encounter at about the cab door. That's the rush of compacted air ripping around the corner of the cab, like water squirting from a hose when you put your thumb over the opening. Ross says that wall of air can increase the wind drag on your car by 20 to 40 percent. Fighting drag accounts for half the energy cost of driving, so that's a 10 to 20 percent hit to your fuel economy. If it takes you 10 seconds to break through the wall, you could be looking at about one cent in extra gas. A small price for freedom. |
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Hannah Holmes traffic-tests her reporting in and around Portland, Maine. A fresh "Skinny On ... " gets posted here every other Friday, adding to her extensive work for Discovery Online. Hannah's writing also appears on the pages of Escape, Outside, Sierra, Backpacker, Eco Traveler and Women's Sports and Fitness. Send her a note at skinny@online.discovery.com.
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