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Drafting.. range effect

Discussion in 'Model S: Driving Dynamics' started by Steve3008, Oct 30, 2016.

  1. Steve3008

    Steve3008 Member

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    I was bike riding today and needed a break so I drafted for about a mile. I got to wondering how much of an effect this would have on range for a tesla and what autopilot setting would you have to use? I would only want to draft a tractor trailer on an interstate when the volume is very light. Does anyone have power consumption numbers say at 75mph drafting vs non drafting. How many more miles could one squeeze out this way?Also using autopilot will it keep you close enough to draft and if so what is rhe minimum number to get a substantial benifit.

    Thanks for the info

    Steve
     
  2. hacer

    hacer Member

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    With TACC set to 6 I dropped my wh/mi down to 220 behind a big truck that was going 68 on a mostly flat highway. Pretty much all of the other traffic was going faster than the truck so I was able to draft for about 15 miles. (My CC was set to 73 so it regulated the distance nicely) At 68 mph on my own in similar conditions my car does about 265-270 wh/mi so it was a good range extender.
     
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  3. dhrivnak

    dhrivnak Active Member

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    Just be careful as it is illegal as I found out through a ticket and points. It can also be dangerous.
     
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  4. hacer

    hacer Member

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    At a setting of 6 there is about 5 car lengths between you and the truck. I can't see any police deciding that is illegal drafting, but there is still substantial benefit. If you want to get up close I'm sure the energy benefits are much greater but then there are a lot of down sides too.
     
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  5. Naonak

    Naonak Member

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    Huge benefit to drafting in a Tesla. It's made the difference between running out of juice and making it to a Supercharger for me on at least one occasion.

    Also, a setting of 6 is crazy far back for drafting - you aren't getting much benefit. The closer you get to a truck, the more benefit, but it is indeed dangerous and not recommended. At highway speeds, though, the TACC setting of 1 is actually a setting of like 5 at slower speeds. It widens the gap as you increase speed, regardless of your setting, which seems perfectly reasonable to me.
     
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  6. RDoc

    RDoc S85D

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    1 is pretty close in my experience. At 70, setting the distance to 3 gives about a bit under 2 seconds of separation which seems pretty reasonable to me.
     
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  7. RDoc

    RDoc S85D

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    How close were you?
     
  8. dhrivnak

    dhrivnak Active Member

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    I was 20-25 ft back but on a lightly traveled section of interstate. I have a hard time seeing where my little Roadster could have done any damage to an 18 wheeler
     
  9. steve841

    steve841 Active Member

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    Not worth the stone chips and debris kicked up ..... no way.
     
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  10. supratachophobia

    supratachophobia Active Member

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    Go watch the mythbusters on this. It's pretty accurate and relevant
     
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  11. rpo

    rpo Member

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    In my Prius, I could consistently get 90 MPG when drafting at 65 MPH. That's about a 1.8x improvement.
     
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  12. bhzmark

    bhzmark Supporting Member

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    specifically, they found that the drafting improvement was best at 10ft behind (saving 40% mpg), but even 100ft behind saved 10% mpg.

    • 100ft: 35.5mpg, 11% improvement
    • 50ft: 38.5mpg, 20%
    • 20ft: 40.5mpg, 27%
    • 10ft: 44.5mpg, 39%
    • 2ft: 41mpg, 29%
    more details below

    Drafting a Big Rig
    Myth: Drafting a big rig saves fuel

    They emphasized again and again how dangerous drafting a big rig is: ~3/4 of truck/car accidents are caused by person driving the car and you're driving in the blind spot of the truck. 150ft is minimum recommended following distance at 55mph, so even the 100ft test is considered dangerous.

    Small-scale test
    NASA let them do a small scale test to study the aerodynamics of a big rig. They captured video of smoke travelling over a minature big rig and verified that there is a low pressure area behind. They then stuck a minature car to a force gauge to study the difference with and without drafting.

    • 7 car lengths: 21% drag reduction
    • 10ft: 60%
    • 6ft: 80%
    • 2ft: 93%
    Full-scale test
    Freightliner lent the MythBusters one of their new Cascadia big rigs, which they are billing as the most aerodynamic big rig on the market.

    Mike Ryan, Hollywood stunt driver, was there to educate Grant on the ins and outs of drafting. Also on-hand was Andrew Smith, test engineer, who helped them hook up a computer to the fuel injection system to accurately measure the fuel consumption.

    • 55mph control: 32mpg
    • 100ft: 35.5mpg, 11% improvement
    • 50ft: 38.5mpg, 20%
    • 20ft: 40.5mpg, 27%
    • 10ft: 44.5mpg, 39%
    • 2ft: 41mpg, 29%
    The fuel economy actually dropped at 2ft. Andrew Smith's theory was that at 2ft, Grant got nervous with the throttle as it was difficult to maintain that 2ft gap.

    Annotated Mythbusters: Episode 80: Big Rig Myths
     
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  13. Steve3008

    Steve3008 Member

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  14. Steve3008

    Steve3008 Member

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    Hey so how far can the radar see when it's tracking what's happening in front of the car you are following? Seems this would make drafting a lot safer if it can "see" what's going on. If it does not feel confident it could increase the trailing distance.
     
  15. jareade

    jareade Supporting Member

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    This was my immediate thought as well.
     
  16. artsci

    artsci Sponsor

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    I've drafted many times and it's saved my rear more than once. There's way too much angst about it on this board. If you draft a reasonable distance behind it's perfectly safe and there's a substantial range benefit. I only draft trucks with large mud flaps -- thus small stones etc are not a problem. Lots of folks follow other cars way too close at speed -- a car length or two. Now that's the defiinition of danger.
     
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  17. Boatguy

    Boatguy Member

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    Generally speaking I think you're right.

    However, if unforeseen conditions (cold, wind, etc.) cause your 10% reserve to evaporate, it could very well get you to the next SC. I think it's a technique to have in your tool bag, particularly because TACC is very good at maintaining a set distance and far more vigilant than most drivers.
     
  18. scottm

    scottm Active Member

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    Dodge the draft! I want a hard hook-up. Like an auto-extending bar with a magnetic clamp.
     
  19. supratachophobia

    supratachophobia Active Member

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    I thought about this with the electro magnets from office doors. They are 24v can easily be operated outside of their designed enclosure. I'm actually working on a Thor hammer idea for Halloween.....
     
  20. BerTX

    BerTX Active Member

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    I only draft in a high headwind, and then with a TACC setting of 7. Unless it's a gravel truck, in which case TACC is set to 200. ;)

    I did draft a bit the time I got to Price, UT and all power was out, including the Supercharger. Skipping Superchargers is not something the MS60D is very good at, but I made it to the next one!
     
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