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Aerodynamic impact of going topless?

Discussion in 'Roadster' started by gregd, May 27, 2017.

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  1. gregd

    gregd Active Member

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    I was scanning the Model 3 thread More efficient to open a window or use the A/C ? about whether it's better to use the AC or open the windows, and started wondering what the impact is of driving a Roadster with the top off.

    I understand that around town, there is no aerodynamic effect. But what about at freeway speeds? I presume there will be an impact, but how much? Would it matter if the windows were up or down, with the top off?
     
  2. dhrivnak

    dhrivnak Active Member

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    It is minor. On the track I normally get an ending 1/4 mile right at 100 with the top on. And 99 with it off so I translated to a 1% hit. Not enough to put it back on.
     
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  3. PhatCat

    PhatCat Kisco Kid

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    i expected so much more given the title of this thread
     
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  4. BluestarE3

    BluestarE3 Active Member

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    Why, were you hoping for something like this?

    [​IMG]
     
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  5. PhatCat

    PhatCat Kisco Kid

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    Actually i figured some aerospace engineer would say the effect is less if she faces forward given the dual nosecone configuration
     
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  6. bonnie

    bonnie Oil is for sissies.

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    I can see your posts.
     
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  7. dpeilow

    dpeilow Moderator

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    I swear I get better range with the top off. I thought this might be drag effect of the canvas roof acting like a streamer, but it may also be due to it more likely to be a warmer day and hence better performance from the cells.
     
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  8. ChadS

    ChadS Petroleum is for sissies

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    In a thread a few years back, I wrote the following. The speeds weren't very high and there were several variables so this is not definitive, but the difference is likely small as we couldn't see any difference at all:

    A couple of years ago 6 Roadster owners took a ~55mph trip from Bellevue WA to Mt Rainier. Tomsax kept track of statistics (they are in a thread here somewhere). As far as we could tell, there was no difference in energy usage between the cars with a hard top, with a soft top, with a mesh top, and with no top. No real difference between 1.5 and 2.0 cars either. The cars with more and/or heavier passengers did use a small amount more energy. The only big difference was that one car had a bike on a rack - that car used about 15% more energy.
     
  9. smac

    smac Active Member

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    #9 smac, May 27, 2017
    Last edited: May 27, 2017
    Not a direct comparison, but my Lotus variants over the years have been noticeably more thirsty when driving at 70 mph+ without the roof off. Given the windshield roll hoop and top of the window glass are common I'd expect the same relative differences.

    Again in my experience, lower speeds, it makes little difference. Once you start getting uncomfortable buffeting in the cabin (and any thing paper gets sucked out the roof) that's what corresponds to lousy gas mileage. My thinking is there is a point where the aerodynamics breaks down.

    I haven't done enough mileage using the Visium hardtop to do a comparison between the soft top / no top but I'd be surprised if the Visium wasn't the winner of the three options.

    (The ironic thing is my current one probably has less range than the Tesla, so I do sometimes get range anxiety in an ICE, hence taking more of an interest in MPG than I should for such a car ;) )
     
  10. gregd

    gregd Active Member

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    Thanks, all, both for the info and the smiles :)

    I kind of guessed the effect might be small, given the urban legend (or was it Myth Busters?) that one can drive in the rain above about 40 mph without getting wet. And I am betting that taking the top off would win out over the A/C on a moderately hot day, given what the A/C does to my range.
     
  11. smac

    smac Active Member

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    I've tried that exact experiment.. In a Lotus you still get wet ;)

    (I've also tried driving at >150 mph on the autobahn in a Lotus with the roof off, you get deaf ;) )
     
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  12. markwj

    markwj Moderator, Asia Pacific

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    Not a myth. I've tried it in a Tesla Roadster (not intentionally). For the amount of rain that time, the speed threshold seemed to be about 50kph; Above that I stayed dry. You can also experiment, top off, by pushing your hand up near the rear roll bar. I get an inch or so before I hit full force air resistance (vs immediate at the front near the windshield).

    So, I reckon there is not much resistance from that part of the car. At speed, the air is being pushed up and over by the windshield.
     
  13. Pando

    Pando Member

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    Sorry, wrong thread...
     
  14. Doug_G

    Doug_G Lead Moderator

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    In my experience, there is a modest negative impact on range at highway speeds when you go topless.
     
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  15. thefortunes

    thefortunes Member

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    Interesting, in my experience I have a positive impact on range running topless.

    But that may be a result of better weather vs. true aero effect as @dpeilow mentioned above.
     
  16. daniel

    daniel Active Member

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    The Roadster gets so much range that I don't care about small differences. I'd rather be comfortable. That means cloth top on in cold or rainy weather, mesh top in warm or hot sunny weather. No top is fine in town, when the weather is nice, but on the freeway it's too much wind for comfort.
     
  17. gregd

    gregd Active Member

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    Sure, and most of the time, comfort and enjoyment is the first priority. But I've got some trips coming up within the next month, and if, for example, taking the top off cost me 20%, I probably wouldn't do it even if it would be more pleasant, because it would impact my ability to make the trip.
     
  18. malcolm

    malcolm Active Member

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    Topless?

    It depends.

    Are you planning to go the "full Maurice"?

    Big hair, chest wig, medallions.

    It makes quite a difference, aerodynamically speaking.

    It gives the car a Hee Bee Bee Gee Cd.
     
  19. smac

    smac Active Member

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    I'd highly recommend the Visium top if you can get one. The thing I hate most about having the top on is the cabin feels dark/claustrophobic.

    The Visium makes it feel _much_ airier, almost like going topless, but on freeway driving trips it's a godsend for wind/noise.

    I haven't got a mesh top, but I'd love to know from those that have both the mesh and Visium how they compare.
     
  20. daniel

    daniel Active Member

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    I had never heard of the Visium top before reading the above and doing a Google search. The Visium appears to be a solid tinted glass hard top. So it would be suitable when you need a fully-enclosed top. The mesh top is for when you want the free flow of air, but not the full-on hurricane of topless on the freeway.

    I love the sun when I'm hiking or kayaking, but I don't like full sun when sitting. I like the cloth top in chilly or rainy weather, and the mesh top in warm, sunny weather. In fact, I love the mesh top. I suppose the glass top could be good in cloudy weather when it might rain but you want a bit more light, but it also must be heavy. Both the cloth top and the mesh top are lightweight and easy to put on and take off. I can imagine any hard top being heavy and awkward.

    The mesh top is also great when you are parked in the sun for a couple of hours. With any enclosed top the car would be an oven. With the mesh top, no problem.

    I hear ya.
     

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