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Old 12-02-2008, 07:00 AM   #1
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Lightbulb Drafting: stream of cars vs semi

I've been driving about 132 miles per day on a long stretch of highway for about a week now and I've noticed that drafting behind a stretch of normal consumer vehicles (cars, trucks, suvs) may be helpful for increasing fuel economy. Consumer cars are also generally going faster than semis so drafting has a larger effect. I've found that even being 3 seconds behind any vehicle is better than hitting the air alone.

Any thoughts on what is better for fuel economy? I admit that while driving behind other consumer vehicles I tend to pulse and glide more behind a stream of cars due to the large variation of speeds, which may attribute to the increase in fuel economy. I realize this will depend a lot on the type of car driven (larger cars probably have more to gain behind a semi) but if all cars were equal in size and drafting in both scenarios were at the same speed, would it be more beneficial to go behind a semi or a stream of cars?
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Old 12-02-2008, 07:36 AM   #2
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I've wondered about that before. Specifically, I wonder if a whole line of cars causes a corridor effect, where the cars sort of carry the air between them. That would seem different from a single vehicle drafting another single vehicle, which breaks the wind and then allows it to reassemble behind.
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Old 12-02-2008, 08:37 AM   #3
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I've noticed the corridor effect on my commute. The freeway I travel has 4 to 5 lanes going my way, so there are more cars pushing the air than on a two lane backroad, for example. I noticed the effect when there are fallen leaves or debris on the road. I noticed the leaves are still being blown in the draft when I'm 6 or 7 seconds behind a group of cars. That's a big gap.

The tradeoff to drafting behind a group of cars is that they tend to change speeds too unpredictably, which makes me hit the brakes sometimes, which wastes any gas I save from their draft.
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Old 12-02-2008, 12:57 PM   #4
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Also think about ride height of the semi's vs cars. Semi's have a lot of air going underneath them only to hit the nose of the average car. Unless you are driving a lifted jeep cherokee with 33" tires then maybe a Semi will offer a better draft. I think that you are right with saying that a steam of cars on the highway offer a better draft for your situation.
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Old 12-02-2008, 02:45 PM   #5
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A truck has a much bigger, squarer back end. The wake it leaves is much larger.

You may have a point about the under-truck airflow, though.

Personally, I prefer to go a little slower than the flow, and catch a momentary boost every time someone passes me.
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Old 12-02-2008, 03:50 PM   #6
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My impression is that a column of cars produces a smaller, less turbulent, and possibly faster draft stream than a lone truck. If there is little crosswind, I'd prefer the cars. With greater crosswinds, I think the bigger hole left by the truck is more effective (depending on drafting distance, etc.)

Now... if you can find a column of trucks, you'll be cooking!
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Old 12-03-2008, 08:00 AM   #7
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Yeah and even a couple of trucks side by side is really great. They actually get the air moving in their direction behind them. I find it really interesting how as a big vehicle comes up beside me how my car first turns into their lane as their bow wave air pushes my cars back end sideways. Then as they pass my cars nose gets sucked into their wake. The force is strong with trucks . . . master.
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Old 12-03-2008, 09:17 AM   #8
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I like to be in somewhat of a "v" formation with a truck. I like to be beside them, with my front bumper just about even with the front of the trailer. I am easily visible in their mirrors, and I'm not tailgating. My SG reports that staying in that location is just as good as following 2 - 3 stripes behind.

-Jay
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Old 12-03-2008, 11:34 AM   #9
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I've noticed since I put a vacuum gauge in my '88 that I get some benefit from semi's at 100-200 ft. behind them. It actually seems better at approximately 100 ft than if I was 2 or 3 car lengths behind them. I think that you should benefit from a string of cars. I remember as a child riding a bicycle and when a car would pass it was almost like someone behind you giving you a push.
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Old 12-03-2008, 12:27 PM   #10
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Quote:
Originally Posted by Ford Man View Post
It actually seems better at approximately 100 ft than if I was 2 or 3 car lengths behind them.
2 or 3 car lengths is very close. Even 100 ft is very close. That's a 1 second following distance at 70mph.

Have you experimented with longer following distances? Others report 2 to 3 seconds distance as having the strongest draft pull.

Come to think of it, now that I've done the math, when I count off a 3 second follow at 70mph it sure doesn't look like 300 feet but the calculator (and google) doesn't appear to be wrong...
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