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The Frunk: A decision of style or functionality?

Discussion in 'Tesla, Inc.' started by KarenRei, Jul 27, 2017.

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  1. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    I've been thinking a bit about Tesla's frunks... and I'm having trouble convincing myself that they're a good idea.

    The main reason that this trend began of having a long hood area in cars began is because in an ICE vehicle you normally have a big engine located there. This later became extended to the utility of having a longer deceleration distance during impacts.

    However, this runs afoul of aerodynamics. The optimal aerodynamic shape is not to have a long taper at the front, but at the back. That is, an ideal shape for enclosing volume is an airfoil (2d) / "teardrop shape" (3d), and thus, an egg-shaped front-end. Indeed, other designers, such as Aptera, took this path to lower their drag coefficient significantly:

    An EV has no need for the large engine in the front. Indeed, Tesla just leaves an empty space where an engine would otherwise have been there. So that reason is gone entirely. The crash deceleration length argument still stands.... sort of. Because part of the reason ICE vehicles need such a long space for deceleration is that a lot of the engine doesn't compact well, and you don't want to push the engine into the passenger safety cell. Which is clearly not applicable in this situation. And even if you wanted length, there's no reason that the front end should approach its maximum height so quickly, then relatively level off, then resume a sharper rake at the windshield. Nothing about that is inaccordance with aerodynamics; it's just a style that people expect from their cars.

    [​IMG]

    Above: Up, rapidly reaches max height, mostly levels off for most of the length of the hood, then resumes a shaper rake again. Not a particularly optimal shape for aerodynamics - but what people expect from a "normal" car.

    Having the extra storage space in the front is nice, but you'd get better aerodynamics having the same amount of storage space elsewhere in the vehicle by allowing for a more gradual taper in the back**. So I can't see that as a justification.

    It's no insignificant issue... we're talking something like a 25% sacrifice to aero, aka something like 40-50 miles range, and slower charging from a given power supply (lower Wh/mi = more miles added to the pack per hour of charging at a given number of watts).

    So.... is this really just a style thing? Keeping the storage at the front rather than going to a more airfoil/teardrop front end, because that's what people expect the shape of a car to be? If so.... Tesla's already removed the use of fake or needless grilles with the Model 3, breaking with one aspect of what people expect cars to look like. Hopefully they'll continue with this trend in the future and let physics dictate the basic shape, with style adding in its "sleek lines" and so forth atop it.

    What do you think?


    ** Re, taper: there's lots of different ways you can do it. In general, though, you don't want to taper too much before the rear passengers, if you want meaningful rear passenger space - but then you want to taper as much as you can in the rear storage. The rate of taper cannot exceed too strong of an angle (which depends on your speed); otherwise you encourage flow separation, which means you drag more of a low pressure wake behind you. If you can't prevent flow separation any more due to length constraints, it's better to have a sharp cutoff, sometimes with vortex generators, to create a recirculating zone behind the vehicle. The same principles also apply (to a lesser extent) to the sides, due to crosswinds; the net flow across a vehicle is the sum of the vectors of the wind and the vehicle's motion, and is not always perfectly aligned with with the direction of travel.
     
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  2. Tiger

    Tiger Member

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    The long frontal part is also a safety feature, safer for pedestrians and safer in head2head collisions.

    However, I would advocate for a power liftgate also for the frunk (both open and close via click of keyfob). Easy solution for avoiding automatical opening while driving is simply to reverse the opening hood (open up from windshield edge).
     
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  3. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    #3 KarenRei, Jul 27, 2017
    Last edited: Jul 27, 2017
    Except for paragraph 3 of what I wrote above. To reiterate it:

    1) Since there is no large, poorly-compactable ICE up front, an EV doesn't need as long of a deceleration length up front for equivalent Gs in an accident)

    2) Even if you did want as long of a deceleration length regardless, a traditional hood shape (rapidly to full height, relatively level, then resume the steeper rake with the windshield) is not good aerodynamics. Doing the entire front end with a continuously flowing rake is better aerodynamics. And while that increases the size the front windshield relative to the amount of viewing height out of it, it also pushes the A-pillars further away from you, giving you a smaller blind spot.

    It just seems to me that Tesla is carrying over ICE styling which exists in ICEs for a reason that doesn't apply to EVs (aka, "need to cram a big engine in there"). Sort of like putting fake grilles on EVs because ICEs have them. Or how early "horseless carriages" had the driver at horse height (and otherwise quite resembled a horse-drawn carriage), because that's what people were used to.
     
  4. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    I think the aspects of a frunk's function - crumple zone and storage - plus its form - it suggests a cushion of safety to the cabin crew and it does seem familiar - outweigh the aerodynamic benefits you lay out.

    I can't decide if your perspective is European or American: your other posts demonstrate perfect familiarity with your stated location, but your writing style and, if I may, mannerisms suggest not even North American (i.e., possibly Canadian) but rather straightouta the USofA. Now, that's really neither here nor there, but Class-8 owner-operators, when given the opportunity*, choose long-nose tractors rather than cabovers 10 times out of 10....despite the greater visibility, superior maneuverability and benefits regarding wheelbase restrictions that the latter provide.

    Utterly critical to the success of any EV revolution is the mass adoption of such vehicles. Around here, much deviation from a reasonably standardized shape garners the sobriquet "weirdmobile". They don't sell. The mathematics is easy: 25% better aerodynamics times 0 sales...the revolution fails.

    *: I am aware that cabovers are extremely prevalent in much of Europe. Mandated cab-lengths and overall length restrictions determine same. In NoAm and I think in Australia this externality is, for the most part, not present to skew preferences.
     
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  5. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Well read. Grew up in the US, but am a permanent resident of Iceland. :)

    And for the record, I'm not saying it was the wrong decision because - as you stated, 25% better aerodynamics times 0 sales.... That said, style trends to change with the style, and form slowly catches up to function. And even today there's a market for better streamlining. Aptera didn't have anywhere near what Tesla has going for it, was an almost unknown, anything but having a market capitalization similar to GM, was launching at a time where EVs weren't nearly as popular, public charging stations almost nonexistent, and had half the range of a Model 3... but it still had something like 4k reservations. Given its situation, that was no slouch.

    I just hope we'll see the better streamlining that EVs allow for in the future. The hardest thing for me pulling the trigger for a Model 3 was how "conventional" the styling is, "style over function" details like this.... but the thing that got me to pull the trigger for it was how they're nevertheless pushing much harder for function than all of their major competitors ;) I just hope that by the time my Model 3 gives up the ghost, there will be a true streamliner by a serious manufacturer on the market.
     
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  6. jaguar36

    jaguar36 Active Member

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    I'm not gonna argue too much with you, as you've shown in other threads you are unwilling to accept any beliefs beside your own, but I will say, if you look at any other vehicle designed primary for aero (aka top speed record setters) none of them are tear drop shaped. They all have a nose of some sort.

    I also find the large frunk to be super useful and am saddened that they've shrunk it so much.
     
  7. Boourns

    Boourns Member

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    Perhaps the overriding factor is Elon's directive of no weirdmobiles. Tesla was started to bring EVs into the mainstream. If the S looked like the Aperta you linked, we'd all be on the Chevy Bolt board right now, and Tesla would be a footnote in history. "Hey remember the time the PayPal guy lost all of his money?"
     
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  8. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    #8 KarenRei, Jul 27, 2017
    Last edited: Jul 27, 2017
    Did you just drop in to throw out an insult?

    You provided two links, both to the same thing, a bike from Aerovelo. Due to the need to contain an semi-upright human, aka taller than wide, they've oriented its taper primarily laterally. If you trace out a horizontal cross section of the bike, you'll see that it follows the classic zero-lift airfoil shape. An airfoil extended greatly along one axis forms a wing, while an airfoil tapering on two axes resembles a teardrop**. Aerovelo is roughly halfway between these.

    Do you think it's style that makes airplane wings shaped the way they are?

    It's also worth warning that the "fastest vehicles" is not a good way to judge drag coefficients. Racing vehicles often have terrible drag coefficients, because they're focused on creating downforce. F1 cars can have drag coefficients in excess of 1.0.

    Space is space, regardless of where it's located in a vehicle.

    ** - To be fair, an airfoil rotated around an axis is a bit sharper than a stereotypical teardrop - it's not a hemisphere on the frontend, but, as mentioned in my first post, more of an egg-shaped front end. For example, an airplane nose. The ideal degree of curve also depends on the velocity, just as with any airfoil. A second caveat is that this is only applicable to subsonic regimes; transonic and supersonic velocities generally favour pointier leading edges, creating conical shocks around the surface. Supersonic airfoils are generally either "doubly-pointy teardrops" (circular arc) or have a diamond-shaped cross section (double wedge)
     
  9. Camera-Cruiser

    Camera-Cruiser Fully Charged

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    My kid wanted a 60's era VW micro bus because it was so retro, cool, and could haul lots of stuff, etc. I said nope, because YOU are crumple zone for all of your stuff. Now when you graduate college, pay back your student loans etc., buy what you want. She'll be 30 and have 2 kids before that happens. No worries here.

    Realistically, I love the visability provided by cab forward and cab over design, but until our cars have magnetic repulsion to keep our fleshy bits from getting squished, I'll live longer with a descently sized crumple zone.
     
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  10. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    Without looking anything up, just going by memory -

    wasn't Buckminster Fuller's thingymajingy (¿Maxion?) the most aerodynamic vehicle ever? Big and round in front and tapering just as you wrote?
     
  11. BluestarE3

    BluestarE3 Active Member

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    Dymaxion car - Wikipedia
     
  12. mal_tsla

    mal_tsla Member

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    I think the reason Model S is shaped that way is because "it looks good" and it is "efficient enough" .. that is, the car achieves usable, practical EV range without needing to look "strange."

    Model X is shaped a little more "tear drop" and more "strange" (indeed, many call the car Fugly in photographs, although it does look better in person). I think that's because Model X, being larger, taller, and heavier, isn't "efficient enough" if it looks like a traditional SUV. Thus, it needs to look a little more radical to be usable.

    It's a decision of style with a limit on the negative impact to practicality. As it should be, IMO.

    modelX-vs-modelS.png
     
  13. KarenRei

    KarenRei KarenRei KarenRei KarenRei KarenRei KarenRei

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    Indeed - it was one of a number of vehicles over the years like that (Dymaxion had a key flaw in that its rear steering was a horrible idea). Note that there's no single "right" shape because you can taper horizontally, vertically, or both; you can decide to sacrifice / narrow the backseat or interior height for aero or keep it; you can have different length requirements; you can keep the front and rear wheels inline or narrow the rear wheelbase (even down to one wheel); and a whole host of other things. It depends on your needs and how far you'd want to go. Some changes in pursuit of aero are definitely not painless. But some are relatively painless, and are only not done out of style reasons, of "that's not the look we're used to".

    From a less radical perspective, the Chrysler Airflow was a great example of changes to style for the benefit of aerodynamics that at the time were considered radical but now look positively outdated. By all accounts, it was a superb car, but didn't get enough sales to justify keeping it alive for more than a few years. Yet its laid the groundwork for future trends that were increasingly adopted over the coming years and decades; it's easy to see its influence on, say, the original Volkswagen Beetle.
     
  14. ItsNotAboutTheMoney

    ItsNotAboutTheMoney Well-Known Member

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    Efficiency pushes the cabin forwards
    Safety pushes it back.
    I'm sure that autonomy will change the shape of cars.
     
  15. SucreTease

    SucreTease Member

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    I am reminded of how many complaints I have read about the "egg shape" of the Model X. Its aerodynamic design is already challenging the SUV market, and it is already testing the limits of how much aerodynamics the current market is willing to accept.
     
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  16. Quick2Judge

    Quick2Judge Member

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    @KarenRei. As I think of the most aerodynamic vehicles... Jets come to mind.... And yeah those all have a tear drop shape. In fact the bigger they get there more they look like tear drops. So you definitely have a point on the efficiency point of view. Sales/adaptability cannot be ignored. Thanks for giving me something to think about.
     
  17. Uncle Paul

    Uncle Paul Member

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    Aero is only one component of EV design.

    Very few people will purchase an extreme aero design that comprimises other factors.
     
  18. AnxietyRanger

    AnxietyRanger Well-Known Member

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    Just a note adding to your message: Both Model X and Model 3 havat the strange, bulging look, while Model S is indeed very sleek. So I don't think it is just about the SUV part, it is part of Tesla's design evolution, headroom and liking large-windowed greenhouses...

    Tesla is actually more and more going Fiat Multipla (the facelift) these days, with Model X and Model 3 design...
     
  19. David29

    David29 Active Member

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    This is very informative, so thank you for that.
    It has bothered me for some time that the Tesla Model S rear seat space is so cramped, both side to side and front to back. In thinking about it, I had decided that the reason was that Tesla put a premium on locating the driver's seat roughly midway between the axles, and then the second priority was to have ample trunk space with room for a third row of seats. This was a guess, of course. Your post and the discussion that follows does suggest that the main reason for the cramped second row may well be the appearance of the front end, hence the (unnecessarily?) long hood.
    For most of my purposes, I would prefer a more spacious rear seat area over the frunk as it is (even if people complain that it has gotten smaller). While I appreciate the energy absorption benefit of the long hood, I would expect that the engineers could achieve sufficient energy absorption with a shorter hood and the right structure beneath.
    Although we have not yet seen dimensions of Model 3, it looks to me as if Model 3 will shift the space balance such that there is proportionally more space for the second row and less for the hood (otherwise the shorter car would have too little space for the second row). I hope that Model 3 will have good rear seat space, anyway. And it will still need both decent aerodynamics and to meet crash tests.
     
  20. mal_tsla

    mal_tsla Member

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    Good point. But, to my eye, model 3 has a more flat hood to raked windshield transition like the model S, and has the long hood/short rear deck proportions typical of a RWD ICE car.

    Meanwhile, the model X is often compared to a minivan style due to its short, continuously sloping hood and less dramatic transition to the windshield rake angle.

    I haven't mocked them up side by side, though, because I don't have an easy source for the profile photo at similar scale like for the S and X.


     
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