Safely follow a truck on the highway with Prius: 65 - 85 MPG

<div class='quotetop'>QUOTE(John in LB @ Mar 12 2007, 09:00 AM) [snapback]404153[/snapback]</div>

Oddly enough, that's entirely false. Vehicles being drafted actually reap a very slight benefit when being drafted vs. running alone. It turns out that the following vehicle modifies the all-important region of low pressure behind the lead vehicle, which is the source of a huge chunk aerodynamic drag. It's like a big vacuum trying to suck you backward. If you are close enough to increase the pressure acting on the rear of that vehicle, the drag force goes down and that vehicle's fuel economy improves slightly.

That still doesn't make it a good idea, but it's interesting how it works.

 

I guess, being an avid cyclist, I could have figured that out myself. Drafting is an important part of cycling, and while it's tiring to ride in front, it's not more tiring than riding alone... it's harder to quantify the difference from riding alone because you get a break by rotating back through the pack though, so it feels much easier.

 

Birds on migration surf the draft of the bird in front, as in the vee formation of cranes and of geese. They do this to travel effeciently. If the leader was being drawn back they would have to change position to keep up the pace. Racing cyclists use a chain gang system to keep a fresh rider up front sugesting the leader does more work than those close behind but it is not clear if they do more work than a lone rider.
The only time I have heard that the lead vehicle gets pulled back is in F1 racing but this is at extreme speed and the air exiting the back of the vehicle is being used to create a downforce so as to maintain traction. I am sure that this would not apply to an articulated lorry at 55mph. Indeed, if you have a lorry very close to the back of the car at highway speeds it will bring you into the disturbed air in front of its cab and push you along. This is very noticable on a small motorcycle.

 

<div class='quotetop'>QUOTE(Ichabod @ Mar 12 2007, 06:18 AM) [snapback]404165[/snapback]</div>

Reminds me of an incident about 40 years ago when I was cycling. A friend of mine (I wasn't with him at the time, but he told me about it later), got pulled over by a cop while drafting a truck. He apparently was going 45 mph. When the judge found out he was doing it on a bicycle, he dismissed the charges. The fastest I've every gone was 55 mph. But, that was down hill with a strong tailwind. My wife was clocking me and I was counting my rpm. They were in good agreement.

In any case, air resistance is a big factor for cycling (and a car).
Hey. So what if it's OT.

Dave M.

 

Having an articulated lorry (a.k.a. in the u.s.a., "tractor-trailer") riding your nice person while on a motorcycle sounds like a thrilling (in a bad way) experience!

55 mph on a bicycle sounds equally thrilling. My fastest speed has been about 45mph and I start fearing for my life at slower speeds than that.

 

<div class='quotetop'>QUOTE(John in LB @ Mar 12 2007, 10:00 AM) [snapback]404153[/snapback]</div>

Interesting Theory.

How do you reconcile this with the fact that two race cars together go faster than one alone.

Yes, the first guy in line is doing more work than the ones following, but he is still doing less work than if they other guys weren't there.

This is because the guy drafting behind breaks up the 'vortex' that forms behind you, and tries to suck you back and slow you down.

Drafting is, in fact, a benefit for everyone.

Truckers draft each other when it's practical btw.

 

i'm not sure drafting is beneficial to everyone. the lead is the lead for a reason. they seem to take the hit. we tried these wile traveling through Utah going 85mpg. i was following my fathers F450 super dully. he could feel when i started and stopped drafting him. it would take considerable amounts of fuel for him to keep speed when i was drafting.

I have however seen a few interesting things lately.

one being a group of 3 or more Prii going over highway 17 (long highway through the mountains.. plenty of twist and turns)... i wonder if they were drafting. they were traveling rather close. I've seen this with several other groups too. usually 2 cars. every once and a while 3.

let's set up a prii drafting group to see how it goes.

 

Speaking strictly of semi-trailers - four flaps ["unventilated, planar-sided cavity devices"] tapering off the back of a semi-trailer reduce fuel consumption by 8%, according to "Drag Reduction of Tractor Trailers" by Ken Visser of Clarkson University, Potsdam, New York; November 2005: (real world test)

"A full-scale prototype utilizing rigid composite sides with a flexible top and bottom, shown in figure 3, was road tested in 2000, exhibiting cross-country road fuel savings of about 0.5 mpg, or 8%, over a 10,000 mile trip. Estimated fuel savings for a typical 120,000 miles per year traveled were 1,500 gallons per truck."

you'll have to Google the title and author if you're interested, sorry.

Because, yeah, there's turbulence and vacuum and stuff creating drag as the air flows past the rear edges of the trailer.

This other study, by SAE using mostly models and wind-tunnels and computers, came up with a higher (theoretical) fuel-savings number:

"Aerodynamic Drag of Heavy Vehicles (Class 7-8): Simulation and Benchmarking".

There's a link to that article in this thoughtful and well-illustrated article about the aerodynamic benefits of the tapering, boat-tail shape:

http://www.metrompg.com/posts/boat-tail-prototype.htm

V8Cobrakid - ? -Maybe your dad's F450 isn't tall and wide enough to create the theoretical situation that's beneficial for both the car and the truck - ?

anyway, my 2 cents:

I think a close-following car probably smooths out the air turbulence behind an articulated lorry to the benefit of both vehicles as the air disturbed by the lorry re-converges more smoothly behind the car. (is the way I picture it)

oh btw, the 1st-mentioned article says: aerodynamic devices that carry no cargo, and stick out less than 5 feet behind the truck, are allowed by federal regulation.

btw I suppose 8% fuel-saving savings isn't deemed by the truckers to be "worth it"? I don't see trailers with aero-effects on the back, on the other hand,

trucks pretty much all have those aerodynamic wind shrouds on the front of the trailer (or on top of the cab) ... so what gives?

... ? Is it too inconvenient to fold away some flaps before backing up to the loading dock?? ? to save 1,500 gallons at $2.50 per gallon is $3,750 every year.

 

<div class='quotetop'>QUOTE(Tempus @ Mar 12 2007, 08:47 AM) [snapback]404223[/snapback]</div>

Boogity Boogity Boogity!!!
--Darrell Waltrip

You see this in NASCAR all the time, when the commentators talk about drivers "working together" to get to the front, or one driver getting a "push" from the driver behind them (not to be confused with "push" meaning understeer).

 

Perhaps he had 'drafting' confused with this latest invention from the Acme company:


BEEP! BEEP!

Bob Wilson

 

Build one of those tail-tapers out of fiberglass, design it with a small motor or pneumatic to open/close the flaps, and make yourself a million bucks selling it to truckers.

That's my free million dollar idea of the day. But if you make a million, consider sending me $50,000.

 

I suspect the money spent on new windshields from road debri kicked up by the truck tires and missed by the flaps, over time would negate any savings in gas!

 

<div class='quotetop'>QUOTE(brick @ Mar 12 2007, 06:15 AM) [snapback]404163[/snapback]</div>

Sorry for the delay in reply - however, I have to admit that I was wrong. Brick's response is correct.

The truck benefits slightly from you drafting him very closely. The Prius benefits the most.

Someone asked me for a reference. I did some searching, and found many sites that talk about this. However, the following site had the best explanation for our particular problem.

http://www.markmartin.org/drafting.html

In doing the research, a couple of things came out...

* To benefit, you have to be less than 20 feet away from the truck... so approximately 1 car length.
* A big rig is not that great of a vehicle to draft for a Prius because a lot of air is coming through from under the big rig. Drafting a cop car would be better (just kidding about that last part.... )

 

Thanks for the link. The "wreck" pictures might help slow folk down, as will the picture on Cleanmpg.com if anyone saw it yesterday.

My experience is that the draft effect starts much further back than 20 feet, it will be greater the faster the speed of the lorry. If you slowly approach the rear of a lorry at a steady speed you start to see the fuel consumption drop away almost as if it was linked to the distance. Just joining the slow and steady traffic will give more miles per gallon through the slower speeds and the smoother driving.

Drafting must work or why else would I always have a 4x4 right on my tail.

Stay safe.

 

Thanks for the info John

 

<div class='quotetop'>QUOTE(Ichabod @ Mar 12 2007, 08:14 AM) [snapback]404162[/snapback]</div>

Looks like an episode for Myth Busters is needed. B)

 

Dennis, cool movie. So have you been "busted" by one of your fellow motorists yet? You probably get some funny looks.

 

For one thing, truckers *hate* when you do that. They can't see
you. On the way out to Hybridfest last summer I tried some drafting,
and after some long-term data collection it turned out I was doing
better by *not* drafting. Being close to the truck caused my own
speed control to have to be much more manic-depressive than if I
was able to predict long-term, pushing me out of the sweet-spot
engine operation bellcurve far too often, just to stay at the right
distance. And if one of those big guys happened to throw an
"alligator" right then, I would have been *toast*.
.
You're better off with a *huge* gap in front of you, for any number
of reasons. Read this if you're not convinced.
.
_H*