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More aero: Roadster or Model S?

Discussion in 'Tesla Motors' started by S-2000 Roadster, Jul 9, 2011.

  1. S-2000 Roadster

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    I've been seeing some comments around the 'net that the Model S is more aerodynamic than the Roadster. While that's strictly true, I think it bears considering that the aerodynamic drag coefficient of a vehicle is a non-dimensional number. It does not directly represent the amount of force on a vehicle at any given speed.

    It seems like a more appropriate comparison would be the drag area of the Roadster versus the Model S. The Model S is larger than the Roadster, and surely it must overcome a greater force due to aerodynamic drag. If the Model S were scaled down to half size, then I suppose it would have one quarter the drag of the full size version, even though the drag coefficient would stay the same.

    Anyone have drag area numbers for the Roadster and/or Model S?

    As an example, the Prius has an impressively low drag coefficient, but the point is that it's quite a large vehicle. The Honda CRX and original Honda Insight don't have quite the same drag coefficient, but their drag area is significantly less than the Prius. That's how the original Honda Insight achieved an EPA mpg rating that's almost double that of the Prius (well, not literally double).

    I mostly bring this up because people need to be educated that larger cars will always suffer from larger energy losses no matter how low their drag coefficient. Driving a smaller car that fits your actual capacity needs, while applying similar efforts to reduce drag, will net a lower energy consumption due to lower drag area. Switching from ICE to electric is a good goal, but I hope we can encourage people to stop driving around 4-seater SUVs with only one person in them and no cargo.
     
  2. stopcrazypp

    stopcrazypp Well-Known Member

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    #2 stopcrazypp, Jul 10, 2011
    Last edited: Jul 10, 2011
    I'd be careful about assuming that a smaller vehicle is going to have less drag in general. In general, a long and low shape will do better than a short and tall shape (which seems to describe a lot of small cars today). The extra space you get from a longer car comes almost "free" in terms of aerodynamics (since you can make the frontal area roughly the same and still increase interior volume significantly).

    But you'll likely save on weight and manufacturing impact by buying a smaller car.

    As for SUVs, they tend to both have high drag coefficients AND large frontal areas, so definitely aren't very aerodynamic.
     
  3. JRP3

    JRP3 Hyperactive Member

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    The best number to use for comparison is cdA, coefficient of drag and area, if you can find it.
     
  4. Todd Burch

    Todd Burch Electron Pilot

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    Maybe you saw this?

    Some have questioned the dimensional estimates I used for the Model S, but they were the best I could find. From those numbers, the Model S has approximately a Cd 77% that of the Roadster, and a frontal area 25% larger than the Roadster...so the Model S has (1.25)*(0.77)= 96% of the Roadster's drag.
     
  5. S-2000 Roadster

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    Yes, I shouldn't be using a vague term like 'smaller' because it's not clear whether I'm talking about frontal area or cubic feet. The Smart is much 'bigger' than the Tesla Roadster in terms of height and therefore frontal area, but most people consider the Smart to be a 'small' car. I tend to think of it as huge because I naturally focus on the height, but I admit that it's tough to talk about 'small' when there are different ways to measure it.

    P.S. 'Short' is also a vague term because it's the opposite of both 'tall' and 'long.' Stupid English language... :cursing:
     
  6. JRP3

    JRP3 Hyperactive Member

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    Don't forget to consider width as well. The Smart may be tall but it's also narrow, area is height x width.
     
  7. richkae

    richkae VIN587

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    I just measured my Roadster:
    The width from outside of front tire to outside of front tire at the ground is about 66 inches.
    The width from outside of rear tire to outside of rear tire at the ground is about 68 inches.
    There is a lot negative camber, especially in the back so the bottom of the tires is actually slightly outside the fender line.
    The mirrors stick out about 5 inches wider than the car ( they are 11 inches from the mounting point, but that is on the A pillar which is well in from the side of the car )
    Its about 30 inches to the belt line ( where the side window starts )
    The width from the top of one side window to the other is about 42 inches, and from the bottom is about 54 inches.

    Calculating 68*30 ( body ) + 2*11*4 ( mirrors ) + 14*(54+42)/2 ( cabin ) - 5*48 ( underneath space between wheels ) = 2560 square inches

    My Prius is also 68 inches wide, about 60 inches tall ( and pretty darn square )
    The mirrors are 6x8 or so.
    So 68*60 + 6*8*2 - 5*48 = 3936 square inches

    The BMW 7 series is 74.9 inches wide, and 58.3 inches tall. With mirrors its 84 inches. I cant guess at the cabin shape, its probably closest to square like the Prius.
    74.9*58.3 ( body ) + 9.1*6 ( mirrors ) - 55*5 ( underneath guess ) = 4146 square inches


    Using .35 for the Roadster gives you .35*2560 = 896
    Using .26 for the Model S * BMW 7 series width = 1078

    20% more CdA than the Roadster. Considering how big the car is, thats pretty amazing.
     
  8. kgb

    kgb Member

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    #8 kgb, Jul 10, 2011
    Last edited: Jul 10, 2011
    from here: http://www.teslamotors.com/models/faq

    So, Model S...
    Height = 56 inches
    Width w/ mirrors = 86 inches

    Model S estimate (assuming same size mirrors) =
    76.9*56 (body) + 9.1*6 (mirrors) - 55*4.46 (under) = 4115.7 sq in

    so, CdA = 1070, 19.4% more than the roadster.

    Ok... so I wasted my time. This is well within the margin of error for these estimates. :redface:
     
  9. richkae

    richkae VIN587

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    I think that the Tesla Motors page that says: "Overall width with mirrors 7’2" (2189 mm)" is a misprint, and they mean the width of the body ignoring the mirrors.
     
  10. kgb

    kgb Member

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    #10 kgb, Jul 10, 2011
    Last edited: Jul 10, 2011
    7' 2" is 86". You think that doesn't include mirrors? When the BMW 7 series is 84" with mirrors? That would give the Model S a width of around 95" with mirrors, 11 inches more than the BMW 7 series. If you are right, that would make a significant difference in the frontal area and add a lot to the CdA.
     
  11. richkae

    richkae VIN587

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    Oops you are right. I miscalculated.
     
  12. Eberhard

    Eberhard #421 Model S #S32

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    I wonder why Model S should be wider then a BMW 7 series. Model S does not need a middle tunnel and can be slimmer while offering even more space. Slim but long reduce the air drag.
     
  13. Thumper

    Thumper Member

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    My guess is that the basic dimensions of the S were the result of thinking that went something like this. What range do we want this car to go? 300mi. OK. Now, about how many kwh do we need for that? X kwh. How large must that pack be if we want to keep it to a single layer inside the wheelbase and still be within the size of normal cars. This will yield basic footprint of the passenger area. Now tack on crush zone in front of driver and behind rear seats, wrap in in an attractive aero shell and bingo you've got an S. Keeping the pack to a single layer carried low has lots of advantages. It is protected in most crashes. Even side impacts will miss the battery most of the time because the offending other car will ride over the battery. In front and rear crashes, it is inside the safety zone. The weight is very low. Removal is relatively easy from the bottom. With current chemistry, it does make for a fairly large pack though which in turn requires a fairly large sedan.
     
  14. TEG

    TEG TMC Moderator

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    My guess would be that the Model S size was conceived more about passenger / cargo capacity, and not initially designed around the battery pack size / shape.
    I recall the early discussions were like "size like a BMW 7 series, or Mercedes E class."
    I bet they then started thinking about pack configuration, cell specifications, and range estimates.

    I know we like to say "it is all about the batteries", but in the case of the Model S, I think they wanted to make a large sedan that had unquestionable utility in terms of passenger space and cargo carrying capability. There is also the thought that a requirement was made that it must be able to carry 5 kids in back.
     
  15. Mycroft

    Mycroft Life happens

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    The fact that that's how many kids Elon has is sheer coincidence. :)
     
  16. Eberhard

    Eberhard #421 Model S #S32

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    The cells are 18mm wide with a total of 8000cells you have 80x100 = 144cm x 180cm this size should fit easily under the cars body
     
  17. Nik

    Nik Dreaming no more :-(

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    However, we've seen pictures that show that the battery is bigger than that. You can fit 8000 cylindrical cells into less than 144x180cm by arranging them in an ABABAB pattern, like a pack of cigarettes, so optimal spacing was not the only concern. Does coolant flow between the individual cells?
     
  18. strider

    strider Active Member

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    But there needs to be room for coolant, wiring, etc.
     
  19. Cobos

    Cobos S60 owner since 2013

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    As a side note I seriously hope that the measurements we got now will be shrunk slightly in width at least. A normal parking space here is 2,4m hence a Model S, means you either partly block the next spaces or you can't open the doors. :(
    With the mirrors the Model S is wider than any car I can find specs for here in Oslo, the regular MAN busses doing service as public transport busses are only 30cm wider. A slimmer version will also slightly decrease drag to keep this slightly on topic.

    Cobos
     

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