First Try af Drafting,

Discussion in 'Gen 2 Prius Fuel Economy' started by pardreamer, Mar 7, 2008.

  1. pardreamer

    pardreamer Member

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    Hi all,

    Today I tried the drafting technique for 10 minutes, it seems to work. I was following this truck for about 10 miles and the MPG was really high. To be fair, there was a long stretch where we were going down hill, certainly not for the whole 10 miles, maybe 5 miles of it is down hill but also some uphill.

    Check out the image, is this close enough for effective drafting? I think I was 1 second behind the truck.

    Hey, my MPG was killed this morning when I needed to go fast and driving around 75 mph for 80 miles. The average was around 56 mpg until yesterday, after the trip this morning, it went down to 52 mpg. I was able to get it back to 54 after I got home.

    Par
     

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  2. derkraut

    derkraut Member

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    Looks about right for drafting. I like to draft whenever the opportunity presents its self. But, you gotta be alert in case that monster truck does something unforseen.:eek:
     
  3. hyo silver

    hyo silver Awaaaaay

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    For really effective drafting, you have to get much closer. Back when I was young and invincible and attained motorcycle escape velocity on a regular basis, I found there was a fair bit of turbulence at about the following distance in the picture. When I got closer, the air became very calm, and I had to back off the throttle to avoid smacking the truck. Tricky and dangerous, but it was fun at the time. The only drafting I do now is with bicycles, and even then only with someone I trust. An inch apart is about right.
     
  4. bwilson4web

    bwilson4web i3 and Prime

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    I don't draft because I'm further back, the distance between power poles. I use trucks as a 'pacing' vehicles and keep the distance far enough that I don't have to feel stressed. Following trucks, especially the moving vans, allows me to run my Prius at efficient speeds while other traffic smoothly passes to either side.

    Bob Wilson
     
  5. patsparks

    patsparks An Aussie perspective

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    Looking forward to reading the thread you start about how Prius get more stone chips in the bonnet than any car you have ever owned.

    May be the "I ran up the arse of a truck" thread will come first?
     
  6. tochatihu

    tochatihu Senior Member

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    Pardreamer, your photo shows you certainly close enough to observe the effect. It also shows you too close to see the truck rear-view mirrors, and some of them don't like that. We don't not to want to make them too mad, do we?

    The Prius Screen makes the drafting benefit very obvious. But the closer in you go, the greater risk TO YOU if any little thing goes wrong. It is very easy to work out how many fuel $/hr can be saving by getting close. I would urge any drafters to do this math, and then decide the cost you assign to the added risk. Your calc, not mine.

    Strong request though, that nobody do drafting at night. Too many things are invisible then. When a saw giant chunks of tire tread flying over my Prius, I recalculated the risk.
     
  7. HardCase

    HardCase SilverPineMica, the green one

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    As a long-time long-distance motorcyclist, I'd strongly recommend against frequent "drafting" behind large trucks. You'd not believe the amount of crap that falls/flies/drops off of those monsters over any regular amount of time. Tie-downs, rocks lodged between dual tires (this is especially bad on logging trucks), large chunks of tire-tread.....ever wonder where those black 'snakes' you see along the side of the road come from?....liquids (from brakes, hydraulic systems), and the list goes on. Obviously, in a Prius (or any "cage") you are far less vulnerable than on a motorcycle, but do you really want your pride-and-joy taking those kinds of hits? I don't. I stay clear of big trucks as best I can, and pass them upon overtaking.
     
  8. KV55

    KV55 Member

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    The photograph looks to be about it, but going by the feel of the car is the way to be sure, as you slowly approach the lorry (truck) you will feel the car pick up and get drawn in. The bit about seeing the mirrors is good advice and given a choice I tend to pick coaches that have a rear view camera (as lorries should have if they want to see behind). Also coaches tend to go a little faster, an added benefit. I generally get a straight run of 75mpg (UK) when drafting on the motorway, typically at 56 to 60 mph. Don't use the cruise control when doing this!
     
  9. Prius 07

    Prius 07 Member

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    IMHO:


    evaluate risk / cost vs reward:
    risk: safety; flying debris damaging your car
    reward: X? (for me meaningless given the cost/risk)
     
  10. pardreamer

    pardreamer Member

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    Yes, I like to drive behind Trucks (usually 2 or 3 seconds behind) to keep my speed under 65 mph without other motorists pushing up my nice person. Also, since they are slow in braking and acceleration, it allows me to adjust to their speed without sacrificing much fuel.

    I tried drafting out of curiosity, but I wonldn't do this every day out of the concerns you guys have listed. Usually I get over 55 MPG without drafting any way.



     
  11. ksstathead

    ksstathead Active Member

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    25 years ago when I was young AND foolish, I close drafted semis up to 90mph in 55 psl days. Until a trailer BLEW a tire and the entire thing separated from the rim and came up right in my face. Luckily, it cleared my entire car. Had it hit the windshield, I was dead.

    You still get a little benefit a couple secs back without that danger. I was taught a 2 sec follow rule and it seems ok to me. I think a longer rule is recommended today.

    I also hate the rock chips, even at a distance. And remember, the truck can clear debris that will destroy your Prius, esp. after the turbulence sends the junk flying. You cannot see the debris.

    Truckers don't tolerate drafters as well today. Some violently object. Esp. from a Prius?

    One of the attractions of the Prius to me is that I can get great mileage just by slowing down, having a proper setup, warp stealth, super highway mode, etc. I.e., I don't need to FAS or draft to get great mpg.

    Drafting works. Close drafting is phenomenal. But Prius gives us other, safer tools to achieve mpg goals.

    It is always tempting, but I increasingly prefer to go it alone. I can relax, knowing neither my family nor my paint job are in danger.

    Just my perspective on this cost/benefit analysis.
     
  12. Stringmike

    Stringmike New Member

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    I recently had a truck tire explode violently just in front of me. The main carcass flew high in the air and landed in a field alongside the road - it probably weighed 100 pounds or more and went over 100-feet high. Other pieces shredded on to the road in front of me and I had to swerve to avoid them. I'm glad I wasn't closer!

    I always maintain 2 to 3 seconds between me and any vehicle in front - the potential savings are not worth the risk to me. I did all the stupid things when I was younger!

    Mike
     
  13. Steamboatsig

    Steamboatsig Member

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    From the picture, I would think you are doing more to harm to your economy than helping it. At that distance at highway speeds, you are where the most air disturbance is, where the air flowing around the truck reconverges.
     
  14. Chuck.

    Chuck. Former Honda Enzyte Driver

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    As others said, drafting works, but not a good idea.

    To a troller, this topic may be the tastiest thing to feed them.
    • In 2000, a Honda Insight drove about 100 miles inches behind an Explorer with a plywood barrier, getting 121mpg - and increase of 50% cruising at 58mpg....if this had been a Prius II, the results might be 90mpg.
    • With close drafting, my ScanGage indicated the coolant temperature increased 7F.
    • After all the publicity last year on hypermiling, a number of trucker sites have alerted their members to shake drafters off.
    • Even if you find a trucker that does not mind you drafting them on an open road, it's not likely they are going to be going 60mph.
    • More likely than getting hurt, a close drafter is doing to be going too close to see the exit they need to take and turn in time...unless you know the route by heart of have a NAV, you will miss turns.
    • At CleanMPG, we discourage close drafting and driving in the "No Zone" ...ever since summer 2007.
    • Intentional drafting is uncommon, but unintentional drafting is everywhere - it's called tailgating. ;)
    • In urban traffic, the congestion makes nearly everybody an involuntary drafter.
    There are few opportunities I get to draft that will benefit me. Again, this is very bad PR for hybrids and hypermilers, so please don't do it.
     
  15. abq sfr

    abq sfr New Member

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    I had some drafting benefit a few days ago from being about 40 feet behind a semi, in the lane to the right of it. That stretch of the freeway goes uphill, and I was able to back off the go-pedal a bunch, but it was only for probably a minute. Wind direction might have something to do with it too. That kind of benefit probably doesn't come too often though.
     
  16. berinker

    berinker New Member

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    I don't believe in drafting either for safety reasons but it is fun to experiment. To draft properly you must be about a car length or less behind the truck (you see trucks doing it from time to time in a convoy).
    Immediately behind the truck there is a reversal airflow direction that flips from side to side that will actually pull you towards the rear of the truck.
    The gas mileage is unbelievable but, as mention before, it is dangerous.
    When you get beyond one or two car lengths behind a truck you will be
    buffeted by the whirling side to side vortices which spoil the smooth laminar flow over your car that it was designed for and your mileage will suffer. A little further back as the flow smooths out but the air is still 'somewhat' following the truck you might see a small increase (which looks like about where you were) but smooth undisturbed air is the best. If you can find a truck moving at higher speeds (or into a strong headwind) the effect is multiplied many times over. Be careful.
    Bud (aerodynamic specialist).
     
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