Drafting - don't try this at home

Discussion in 'Gen 2 Prius Fuel Economy' started by mfa-prius, Jun 4, 2005.

  1. mfa-prius

    mfa-prius Old member

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    While following an 18-wheeler on I4 a few days ago, I felt the buffeting off the back end of him, and wondered what would happen if I got a little closer. We were doing about 59 MPH, and I was getting about 56 MPG. As I moved closer, the buffeting stopped at about 50 feet behind him, and my mileage went up to slightly over 75 MPG and stayed there. I reveled in this for about 30 seconds, and then drew back so that I wouldn't have to worry about replacing all the airbags in the car should he suddenly stop. Too bad we can't exploit this safely. Otherwise, I'm averaging 49.1 MPG since new (2200 miles so far), which is OK by me.
     
  2. cybele

    cybele New Member

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    You're wise to pull back because it is a big safety issue.

    Also, though you're saving fuel, the truck is actually using more (and probably polluting more) when you add the extra drag of another vehicle in its stream. So you're doing us all a favor by allowing the truck to be as efficient as possible.

    Cool experiment though, I was curious what the difference would be.
     
  3. RonH

    RonH Member

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    Drafting trucks!! When I was young and dumb and commuted to work on a bicycle, my route took me through an industrial area and I could occaisionally hook up behind a truck. When you hit that sweet spot, it was like you got lifted off the ground. 30-40 mph without breaking a sweat!!

    As to the relative efficiency, although its true the lead vehicle does more work, under ideal conditions, the efficiency of the convoy as a whole improves. Bicycle racers can approach this ideal condition. Watch the pack chase down a breakaway in a tour de france flat stage and observe the lead rotate to even out the work. Its usually inevitable ending in a mass sprint, but sometimes they miscalcuate or let rivalries mess it up and the leaders stay away for the win.
     
  4. Emilyjohn

    Emilyjohn New Member

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    mfa-prius, if you were all of 50 feet in back of the truck, you wouldn't have to worry about him stopping; because those vehicles can't stop. Of greater concern to you would be debris flying off the wheels (there are 18 of them that are capable of catapulting debris). You were wise to back off. Avoid 18 wheelers. I know. I drive them.
     
  5. efusco

    efusco Moderator Emeritus
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    Not a Nascar fan are ya?

    Drafting helps both vehicles. They can go faster and use less fuel...it's a mutually beneficial process.

    But yea, at 50 feet that's darn tight.
     
  6. Emilyjohn

    Emilyjohn New Member

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    Doc, that may be true for vehicles of close to equal mass. The Prius, with occupants, weighs around 3300lbs. An 18-wheeler, with any kind of a load in the trailer, is going to weigh 20-25 tons. An 18-wheeler empty will get about 7-7.5mpg; closer to 6.5mpg loaded. Drafting will benefit the Prius, but will go unnoticed in the truck. At highway speeds, a loaded 18-wheeler needs almost 100yds to stop; when empty, considerably more distance. Stay away from them! If one of them is following you too close, find a way to let him pass; he's playing with your life, and he's too stupid to realize it. Get away from him!
     
  7. efusco

    efusco Moderator Emeritus
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    Well EmilyJohn, while the benefit may go unnoticed, and, indeed, may not be measurable, it none-the-less is real. In any case, it certainly isn't a harm to the mileage as was asserted earlier.
     
  8. prius04

    prius04 New Member

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    Actually, I think someday drafting will be built into our cars. Drafting does help all vehicles, and with proper synchronization, the use of radar and computers, it can be made safe. Without the computers and the radar and the synchronization via technology, it's just too dangerous for humans to do it acapella.
     
  9. jchu

    jchu New Member

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    I saw a piece on one of those science shows 2 or 3 years ago on a test track that involved a radar system that did just that and more. It allowed merging traffic to work itself into the queue while hardly breaking the draft. The system also included permanent magnets buried in the pavement providing steering guidance. So basically, once the merging operation was started you could sit back read your paper and drink your coffee (if you were trusting enough of the system!!!)

    The Pros stated for this system in addition to the improved mileage was that highways could handle a higher volume without being enlarged.

    The associated Con was that they had no answer for the increased traffic being dumped off the off ramps!!
     
  10. jayman

    jayman Senior Member

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    <div class='quotetop'>QUOTE(jchu\";p=\"95728)</div>
    If I had a choice of trusting an embedded system written to CMM Level 5, or the Wetware behind the wheel, I'd trust the software no questions asked.

    First of all, there is *no* such thing as "zero" error and probability analysis will never allow "zero" error. Depending on how much money we as a society are willing to invest, we can easily have Six Nines. Compare that to the Wetware behind the wheel.

    Commercial software and embedded systems are so expensive - compared to a Windows home system - because they must be certified to a certain reliability level. As an example: Capability Maturity Model or CMM as developed by Carnegie Mellon University for DARPA.

    As an example: The primary reason why commercial scheduled aviation has become so much safer, especially over the past 20 years, was the application of fully-coupled autoflight systems.

    The pilot enters a destination, the Flight Management and Guidance System (DGPS, laser inertial reference, EGPWS, etc) suggests the most commonly assigned waypoints, and the pilot presses Enter.

    Once normal climb has been established and the landing gear and flaps/slats retracted, the autoflight system is usually fully coupled. From that point on, the system has full authority over pitch, roll, yaw, engine power, trim, etc. The priority is usually for economy in flight, so lower power settings are used. Of course, manual mode is still available.

    I find it ironic that people fear flying because they have no "control" over the aircraft. That's why when a large passenger aircraft crashes, it makes the headlines. Car crashes - no such thing as "accidents" - are so common only the really spectacular fiery ones make the news.

    Yet these same folks will get s*** faced and get behind the wheel of an automobile, because they have "control." So I'm all for an integrated and automated highway system.
     
  11. Emilyjohn

    Emilyjohn New Member

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    Doc, I stand by what I said: do not use your Prius to "draft" with an 18-wheeler that can gross at 35-40 tons. The stuff that can kick off of any of those 18 wheels can be lethal. You cannot see what's going on in front of that rig. You cannot see the debris he didn't avoid. You're driving blind. You certainly won't get a summons for drafting, but you could be seriously injured.
     
  12. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(Emilyjohn\";p=\"95799)</div>
    Yea, ok, fine. I never encouraged drafting and, in this post specifically warned against it. I can assure you I know first hand and see first hand on a regular basis the consequences of vehicular damage caused by 18-wheelers.

    But that's not relevant to the physics of drafting which is all I was trying to correct.

    To summarize for those confused.
    1)Drafting is benefitial in terms of speed and mpg to both (or all if more than 2 vehicles involved, including the lead vehicle).
    2)Drafting is potentially dangerous to all vehicles involved.
     
  13. hdrygas

    hdrygas New Member

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    <div class='quotetop'>QUOTE(efusco\";p=\"95802)</div>
    Yea, ok, fine. I never encouraged drafting and, in this post specifically warned against it. I can assure you I know first hand and see first hand on a regular basis the consequences of vehicular damage caused by 18-wheelers.

    But that's not relevant to the physics of drafting which is all I was trying to correct.

    To summarize for those confused.
    1)Drafting is benefitial in terms of speed and mpg to both (or all if more than 2 vehicles involved, including the lead vehicle).
    2)Drafting is potentially dangerous to all vehicles involved.[/b][/quote]
    Click and Cack the car guys did a segment on this some time ago and had one the MIT Physics professors on about this subject. He contended that the speed and mpg would continue to improve with each additional drafting truck in a convoy. Interesting. Kind of describes a train does it not?
     
  14. V8Cobrakid

    V8Cobrakid Green Handyman

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    quote="jchu";p="95728"]
    I saw a piece on one of those science shows 2 or 3 years ago on a test track that involved a radar system that did just that and more. It allowed merging traffic to work itself into the queue while hardly breaking the draft. The system also included permanent magnets buried in the pavement providing steering guidance. So basically, once the merging operation was started you could sit back read your paper and drink your coffee (if you were trusting enough of the system!!!)

    The Pros stated for this system in addition to the improved mileage was that highways could handle a higher volume without being enlarged.

    The associated Con was that they had no answer for the increased traffic being dumped off the off ramps!!
    [/quote]

    I was going to comment on that exact same thing.

    Since all traffic basically drafts during rush hour. i was behind a small truck with a roof rack type of item attached. my mpg improved a fair amount. i can average between 55 and 60 mpg on the highway. Drafting with any car or truck allows me to achieve higher. If only there was a safe way to do it *shrug*
     
  15. BobA

    BobA New Member

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    <div class='quotetop'>QUOTE(mfa-prius\";p=\"95666)</div>
    I have tried drafting in the '51 Ford with a trailer and it works very well.. there are some things that should happen before you do this..
    1) be on a major highway
    2) make sure the truck knows you are there
    3) best to have a CB radio and tell the truck what you are doing
    4) have good breaks!!

    Bob Andersen
     
  16. Emilyjohn

    Emilyjohn New Member

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    Doc, yea, okay, fine...
     
  17. DanMan32

    DanMan32 Senior Member

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    "4) have good breaks!!"

    You meant have good brakes, right? Or did you really mean stop someplace and have a good rest?
     
  18. BobA

    BobA New Member

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    <div class='quotetop'>QUOTE(DanMan32\";p=\"96068)</div>
    I was in the printing industry for a long time... guess that's why there are proof-readers and the EVER-FAMOUS... spellcheck... :oops:

    Bob Andersen..should have stayed in the restaurant business..:mrgreen:
     
  19. DanMan32

    DanMan32 Senior Member

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    Spellcheck wouldn't have helped here. Actually, that might even have snuck by grammer checks also.
     
  20. bruceha_2000

    bruceha_2000 Senior Member

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    <div class='quotetop'>QUOTE(Emilyjohn\";p=\"95799)</div>
    I disagree with this claim. If you are driving too close, you can be ticketed. I was pulled over in CO and warned for following too closely. I wasn't even intentionally drafting.

    I had a 17' Ryder rental truck and pulled out to pass a semi on a hill. Unfortunately the AH that was doing about 80 MPH when I pulled in behind him at 65 MPH saw the state trooper's car on the side of the road as he passed the semi and slammed on his brakes dropping to 60 MPH forcing me to hit the brakes as well. :cussing:

    At first I thought maybe I wasn't supposed to be out of the right lane in a 'truck' on the hill. The officer believed me saying he didn't see the speed change, only how close I was to the guy. I didn't exactly have a lot of choice did I?

    I expect if he had seen the 2 guys with CA plates in KS who were clearly intentionally drafting each other (not enough traffic to even hope to claim otherwise), they would have been spending money on a citation. Maybe they were NASCAR wannabes. 50 feet? Ha! I doubt the guy in the front could have even seen the headlights on his buddy's car.
     
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