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Slipstreaming/Drafting

Discussion in 'Model X: Battery & Charging' started by Tozla, Feb 18, 2017.

  1. Tozla

    Tozla Member

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    Does slipstreaming/drafting work with EVs? I assume it does. If you are driving behind say a large semi you won't be cutting through the air as you would if you were in front of the semi. Would your range be extended doing this or is it so minuscule that it wouldn't matter?
     
  2. daniel

    daniel Active Member

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    Drafting will definitely improve your range. It has nothing to do with what kind of motor the car has. It can also alter your destination, as in, instead of ending up at Grandma's house for dinner, you end up in the morgue.
     
  3. Tozla

    Tozla Member

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    My grandma was such a poor cook we'd likely end up there anyway.
     
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  4. vgrinshpun

    vgrinshpun Supporting Member

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    There is no need to have unsafe following distance to realize energy consumption benefits of driving behind trucks. Following one or better caravan of two or more trucks at 2 second interval will save 10-15% on energy consumption. This is based on real life personal experience.
     
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  5. OCRyan

    OCRyan Member

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  6. vandacca

    vandacca ReActive Member

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    I concur. Even with a travel distance of '7', I found a 10-15% range increase. AutoPilot comes in handy in this scenario.

    However, you run a greater risk of getting hit by flying rocks/road-debris, so there is that to consider.
     
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  7. SDRick

    SDRick Member

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    I stay as far away from trucks as possible. Saves the paint and glass from chips and the air tends to be better.
     
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  8. vgrinshpun

    vgrinshpun Supporting Member

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    Once again, although lectures are appropriate for some instances and generally are appreciated, one DOES NOT have to keep unsafe distance from a truck to realize substantial benefits in energy consumption. The article referenced in your post notes that safe following distance is 150 feet at 55mph, which is equivalent to following interval of time of 1.86 seconds. As I posted above, 10-15% savings in energy consumption is easily achieved just by following truck with a safe interval of time, 2 seconds as I posted above. Note that this is more that what article claims the safe following distance is.

    Just to clarify, I do not follow trucks as a rule, but knowing that you can do this safely, in a pinch, and how much of energy one can actually save, is something worth to keep in the back pocket. Beats reading lectures all the time.

    Front of my car is covered with Xpell Ultimate, which does excellent job of protecting the paint.
     
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  9. SDRick

    SDRick Member

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    Does the Xpel work on the windshield?
     
  10. Xolt

    Xolt Member

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    I would imagine my model X windshield may be one of the most expensive windshields available and probably the most expensive in a normal production car. With that in mind drafting behind a truck will cost way more that any possible savings in economy after a stone chips my windshield. Another thought is that with the incredibly low drag coefficient of the model S the savings of drafting while possible are not near those savings of a less aerodynamic vehicle. I'm guessing the X is the most aerodynamic SUV so that's enough for me and I'm not taking the chance. The aerodynamic advantage of drafting is more significant at higher speeds. Fast driving in a draft will be very similar to the range of slower driving not in a draft as the main difference is the rolling resistance of the tires, that is also the advantage of the 20" vs 22" X wheels lower rolling resistance.
     
  11. vgrinshpun

    vgrinshpun Supporting Member

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    Have no idea - did not try it.

    As somebody living in major metropolitan area and driving a lot (71k miles on my MS in 3 yrs 8 months), following any vehicle at safe following distance is a challenge, so there is no 100% guarantee of keeping ones car in pristine condition anyways, unless it is always kept in a garage and just marveled upon couple of times a day.

    There is no argument that following trucks, even at safe following distance has some cost to it in slightly increased risk of accident/damage. It should be avoided if possible, that's all. However, in a pinch, when one needs to extract few more miles from a charge, it is OK to follow trucks, especially caravan of them, and it is useful to know how much energy one can actually save. I learned this on a long distance trip back in 2014, when charging facilities and supercharges were not as prevalent as they are now, when I did a wrong turn in the middle of the trip leg and would not have made it to my destination if I did not follow trucks. It was a white knuckle experience for a while, since I did not know what to expect in terms of energy savings.

    So I feel that sharing experience and actual energy savings is much more useful that cliché lectures.
     
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  12. Xolt

    Xolt Member

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    I think planning ahead is by far the best option and even with range issues and the ability to tailgate to pickup a few extra miles it is still better today to find a plug share point if needed.
     
  13. vgrinshpun

    vgrinshpun Supporting Member

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    Agreed. However, knowing what kind of energy savings one could expect by using safe following distance would not hurt, and that was the reason for my original post. The potential savings are not just few extra miles, but tens of miles (10-15%).

    I have no problem with emphasizing caution. My problem is when information is substituted with meaningless lectures.
     
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  14. Xolt

    Xolt Member

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    Are implying that by saying the drag coefficient of the S is so low that the benefit of drafting is minimal is meaningless lecturing? And the far elevated risk of the potential of a stone breaking a windshield is also the same? I remember in "Back to the future" Micheal J Fox tagging a ride behind a pickup on his skateboard, it works but it's hardly a good idea, but sorry for the meaningless lecture. Just kidding it is just factual data and common sense.
     
  15. vgrinshpun

    vgrinshpun Supporting Member

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    10-15% is not "so low". On a 230 mile leg it is 23 - 35 miles.

    Meaningless is when lecturing on the obvious (increased risk of damage/accident when following trucks) replaces actual real life data on potential savings of energy. Providing both (providing real life data while mentioning the slight increase in risk), however, is not meaningless. That's the point.
     
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  16. MasterT

    MasterT Member

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    ⬆⬆⬆⬆⬆⬆ that was a meaningless lecture

    Lighten up. We get it - drafting is not an acceptable course of actions for you, even if you dont have any other options, except towing.
    But others do it and do it quite successfully achieving measurable range improvement and safely unlike @daniel who instead of
    get successfully to their intended destinations
     
  17. Xolt

    Xolt Member

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    23--35 mile gain is substantial but that would be figuring that you are drafting a truck from charging station to empty and that is not a real world scenario I'm sure you would agree. Not arguing it reduces drag but exaggerated numbers that would not play out in the real world is my point. That is why I was repeating the low drag coefficient because in a large SUV the benefit is on on the high side and a S would be much lower. I also have a new BMW X5 and it has hp and torque gauges and the benefits can be readily observed as you close in on a trailer. The max benefit depending on the type and aerodynamics of the trailer is around one car length at highway speeds, by 4 car lengths it is really starting to drop off. I have not observed the power consumption in my XP90D drafting but I'll check it out.
     
  18. vgrinshpun

    vgrinshpun Supporting Member

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    #18 vgrinshpun, Feb 18, 2017
    Last edited: Feb 18, 2017
    We are getting a bit circular here, so this would be my last post on the subject as I consider it beaten beyond death at this point.

    I am not sure what is the significance of BMW X5 and it's hp and torque gauges. My real life data, as was mentioned, was based on one of my trips in Model S and it's energy consumption readout. Most of the way I was following trucks with the safe time interval of approximately 2 seconds.

    I actually looked at my pictures as I remembered snapping the dashboard picture upon arrival during the specific trip I was mentioning above. I did found the snapshot, discovering in the process that I have poor memory on dates (trip actually happen in September of 2013, rather than in 2014 as I mentioned), but also that actual savings in energy consumption over the whole length of the trip were 17.8% (300 / 365). Here is actual snap shot from the trip showing energy consumption of 300Wh/mile as well as EVTripPlanner.com output for the same trip, including matching car load, temperatures, speed, etc.

    As seen from the info below, the difference was between not making to the destination (EV Trip Planner 81.8kWh is more than useable capacity of the 85kWh battery) and arriving with 27 miles to spare.

    upload_2017-2-18_12-48-33.jpg snap1.png
     
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  19. daniel

    daniel Active Member

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    This web site

    Maintain a Safe Following Distance (The 3 Second Rule)

    and this one

    Teaching a Teen to Drive | Being a Role Model | Following Distance | State Farm

    both recommend a three-second rule for following distance.
     
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  20. snellenr

    snellenr Member

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    Real-world data: Heading west from Amarillo on I-40 at 65 mph into a 20 mph headwind (per Tesla Waze), I was using 410 kWh/mi and running well below the trip power curve. After tucking in 2-3 secs behind a truck (a cab towing a pair of cabs), consumption dropped to around 350 kWh/mi and I gradually recovered to the curve (well, close enough for comfort).

    Traffic wasn't too heavy, visibility was excellent, and I never had the feeling that I couldn't react to traffic. I'm driving a classic P85, so autopilot didn't come into play.
     

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