How do truckers feel about drafting?

Because of the Prius's CD of .26, improving fuel mileage by drafting is an exercise in futility. I had seen the Mythbusters episode about drafting and I decided to give it a try.

I was on the NY Thruway on a light traffic day and following a big semi. I decided to try an Ad-Hoc test to see if there was any improvement by drafting.

I drifted closer and closer to the semi's tail while watching the instantaneous fuel mileage on the MFD. The number hovered between 45 and 55 MPG (70 - 75 MPH) no matter how close I got to the semi's tail. Basically, I didn't see any change no matter how close I got to the truck. The only measurement that increased was my blood pressure.

BUSTED!

 

Anytime, oh good news, I just got my call from the dealer. My Prius is in. Picking it up NOW!!

 

Congrats!! And keep a safe distance behind all trucks on your way home.

 

Bumper sticker spotted on a young lady's car the other day: "If you're going to ride my nice person, at least pull my hair!" LOL

 

I'm not a psychologist, so I won't speculate on what it means about the size of their schlongs, but my frequent experience here in the (fairly-large-city) Milwaukee metro area and the (even-larger) Chicago and St. Louis metro areas is that if I drop more than 2 sec behind another vehicle, someone will pull in between us. Then I feel I need to slow down to put at least 2 sec between me and the new car, and then someone pulls into this new gap. If you haven't observed this where you live, it must be a very peaceful place filled with very patient and polite people.

If you have some statistical facts about the direction retread gets thrown, please provide a link; otherwise, please provide an argument to refute my theory instead of simply telling me it's bunk.

My theory is based on physics. Of the eighteen wheels on a standard semi, only the back four have an unimpeded shot at the vehicle behind the truck. The remaining fourteen cannot send retread directly backward, because it will hit the next wheel behind it. The forward surface of the next wheel is moving down, but the piece of retread was thrown from the rear surface of its wheel and is therefore moving upward towards the underside of the semi trailer. Unless the sheared edge of the retread is perfectly perpendicular (in which case it might get sucked straight under the next wheel), it will get some lateral force vector and spin off sideways.

Douglas

 

Man, don't do that stuff around the Harriman exit (Toll)! Guys with the Purple Power Ties are really cracking down on truckers in that area! I'm sure you've heard about all the accidents around that part of the Thruway, mostly trucks, but when they get out of control they seem to take out Mini Vans... Could be these folks have been to close to the trucks, who knows...

 

I'm on board with Pat on this one. I don't know how many lanes are in your direction, but on the 6 or 8 lane highways I commute on, the truckers are typically in the far right lane (unless they are passing someone). If you are 2 seconds behind a truck, the only people who will pull into that gap are the ones taking the next exit off the freeway.

People typically pass the big rig and then go into its lane (thus reducing HIS following distance to the vehicle in front of him) because there is a nice big gap between the trucker and the vehicle in front of him. Then they get to drive faster than the other traffic (ha ha! I win!) until they tailgate the car in front of them (who is driving ridiculously slow in the far right lane, sheesh, the nerve!)

People don't like to drive behind big rigs because you can't see the traffic in front of them (not like you can behind several cars....all tailgating each other). They can't "see" which lane is moving more quickly, where the gaps are etc. It's a control thing. I'm not a psychologist either, but that is my layman's guess at it.

Pat's my hero on this topic.....

 

Roger on that! The section I'm talking about is between Albany and Rochester - mostly 2 lanes, fairly light traffic weekdays, and very flat.

 

I live in an area known pretty well for aggressive driving and I'm here to tell you, if you leave enough of a gap, cars will come and go from your gap and not impact your drive time in the slightest.

I have the same instinctive reaction you describe from driving too much in this area: I think to myself that I'm not gonna let some *#@$*! take my spot, but then I take a deep breath and let them do what they want. More often than not, the cars that "cut" in front, just cut right out again pretty quickly.

It takes some patience and you have to force yourself when you're used to the aggressive driving mentality, but if you force yourself to do it for a week, you'll see the difference. But hey, safe and courteous driving isn't for everyone, I guess.

 

Your ad hoc test results seem to defy the laws of physics.

I think even with a low 0.26 CD -- compared to most cars' 0.30 - 0.35 CD and most SUVs' 0.35 - 0.45 CD -- a Prius would still have significant mpg gain from drafting. 0.26 is low, but it's something -- specifically 58 to 87% of the CD of most cars and SUVs.

 

How do you think they feel? Come on now.

Drafting is not only illegal it is dangerous.

 

Some thoughts:

- Prius is not exempt from air resistance simply because it has a decent Cd. If it were you would get almost the same mpgs at 75 as you do at 35. The resistance force is smaller than in a less aerodynamic car, but the Prius' small engine also has to work harder to overcome that force. Since the power requirement goes up with the cube of velocity it gets big fast.

- 55mph = 80.67 feet per second, so the Mythbusters 11% improvement for 100' at 55mph would be about 1.25 seconds. At higher speeds, you'd have to get farther back to maintain this following time but the trucks aero "shadow" should also extend.

- The stopping distance for a semi is easily 3-4 longer than that of a normal car. Rear-ending a semi should not be an issue for anyone who is sober, alert and not distracted. Of course, we are not always all of these things. A semi tailgating you is a much bigger problem.

- Assuming a trucks mirror extends 1' from the side, and the truck is 8' wide and 55' long (from mirrors back) driver can see your rear end at > (trailer width - prius width)/2 * truck length or > 65'

- Kicked up road debris and tire separation are certainly a very real concern.

- Trucks do make a nice big sign that says "slow moving vehicle" to the Escalade doing 80 that might otherwise run you down.

Rob

 

That is right Syclone.

Box trucks give no advantage at all. The box is high up, and the air coming underneath is all turbuelent.

And the bigger semi's need more air speed than is common in metro areas, during humid and/or summer temperatures to have a noticable advantage. In the winter on the open road, with a 20 mph head wind (80 mph plus airspeed) --- hold on to your horses! I do not do this deliberately, just have noticed it happen to my car during maneuvers on occasion.

You will also notice a decrease in mileage with a truck along side, all the way back until the nose of the car is even with the tail of the truck. Then there is a very narrow zone of slight improvement back to one car-length behind the semi. Trying to manuever to stay in that zone is useless, it will just use up more gas than the improvement. Its rare to find a trucker on cruise-control in the metro area. One can steer farther from the truck, when its along side, and then closer as the rear of the truck passes to minimise the loss, and maximise the gain.

When the truck is just behind and/or to one side, there is a slight improvement as well. This is a side benefit of reverse tail-gaiting (using the truck as a traffic block, by holding position a reasonable amount in front of it). The bow wave of the truck fills in the wake of your car. Again, not something I do, just noticed the effect. Its sometimes dangerous in that fast cars coming along side the truck will not see you there until the last second, when they intend to cut in front of the truck - after having starting to cut in.

 

Good Question, and one that was asked by a Trucker on Click-and-Clack on the radio last Saturday. Listen in next Saturday, and we will hear what all the physicists from Harvard and MIT say.

 

A couple of interesting things I've read in this thread, and some inspire questions and comments. Please note that none of these questions have anything to do with what I feel about the safety of drafting. Drafting is a regular part of my daily routine. But I'm usually on a bicycle behind other bicyclists. This is 100% expected behavior, and everybody understands what is going on (except those crazy guys on recumbents who cheat).

1. Many folks have made a HUGE point about the trucker not being able to see a car close in behind them. My guestion is: What would a truck driver do differently if they *could* see that car behind them?

2. The truckers on the list seem to worry about the safety of the people in the following cars - but still use retreads, and apparently run tires down to failure as a matter of course. The truckers also say that it is silly to give up safety to save a few bucks on gas by drafting. Yet my guess is that using cheaper retreads and running the tires down to failure is completely about saving money - at the expense of the safety of everybody else on the road.

3. The responses are overwhelmingly anti-drafting for SAFETY. Most here say they won't do anything that decreases their safety. Yet every one of the people posting is apprarently driving on the freeway, and is driving a gasoline car amongst a bunch of other gasoline cars. More people are killed from gasoline exhaust than are killed in auto "accidents." But we don't seem to ever account for *that* sort of safety. My quick back-of-the-envelope calculation shows me that overall safety would be *increased* if we all drove EVs and drafted.

 

Great utopian vision, except that you won't find your typical
drivers pointing down and calling out "hole!" for each other.
.
I fooled with a little drafting on a long road trip a couple
of years ago, and found that I did better on MPG by backing off
and concentrating on ICE sweet spots rather than someone else's
DOT bar. Dodging a fresh alligator some time later by seeing
it well in advance [because I was quite far back], and sailing
gracefully around it after it had come to rest using a tiny
and smooth steering correction sort of clinched it for me.
Hopefully that little story will clinch it for the dear
reader, too. It's all part of predictive driving, which
requires long forward vision, and is one of the pivotal
high-MPG techniques wherever you are.
.
_H*

 

Not so utopian... the only vision you have is of the nice person of the guy in front of you. Not so bad if the "guy" isn't a guy, of course...

Ah, another rider!

 

Another drawback to drivers drafting vs. cyclists doing it is that few drivers on the road would be courteous enough to take their turn pushing air at the front of the line.

Also, if someone's "drafting" me when I drive, I wouldn't point out holes or other obstacles even if I could because I enjoy seeing their car go *bump* or *swerve* because they can't see far enough ahead to avoid the obstacle that I just drove past.

 

I have tried it but now that i'm in my 50's i live a little different .
I have one of the best mpg's cars on the road why take the risk
of getting killed ,stone damage ,a pitted bumper ,hood and windsheild .
Just does not make sense.Also to get a good draft you need to taligate.
I know when i get tailgated i don't like it .So why do to others for a few mpg's .

 

I'd love to see the back of that envelope you have. I wonder if it looks anything like the napkins upon which the original Clean Air Act criteria air pollutants were written? :biggrin1: