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Roadster drag coefficient

Discussion in 'Roadster' started by asgard, Jul 11, 2016.

  1. asgard

    asgard Member

    Aug 17, 2013
    Does anyone on this forum happen to know if the reported drag coefficient is measured with the hardtop on or without a top? The only known report of the drag coefficient seems to be from this article.
    The quoted number of 0.35 for the Roadster seems suspiciously high.
    • Helpful x 1
  2. Andrew Wolfe

    Andrew Wolfe Roadster 472 - S 440

    Jun 20, 2011
    Los Gatos, CA
    I think that is correct.

    1) remember that the drag coefficient is the drag divided by the frontal area. If the frontal area is small, then the coefficient tends to be higher.

    2) Many cars with a Cd less than 0.35 are bean shaped. Interesting styling tends to increase Cd.

    3) Turbulence kills Cd. That would include a vertical rear window at the midpoint of the car, cooling ducts, louvers, etc.
  3. MileHighMotoring

    May 27, 2015
    Parker, CO
    I'm sure it was best case scenario, meaning hardtop on? The Lotus Elise is 0.41 so they did improve it quite a bit.
  4. ecarfan

    ecarfan Well-Known Member

    Sep 21, 2013
    San Mateo, CA
    Yes, it is unfortunate that Tesla did not reshape the rear deck lid to make it a smoothly sloping shape. The Roadster CD is poor. I am sure that the next generation Roadster will have a far lower CD.
  5. gregd

    gregd Active Member

    Dec 31, 2014
    Very little hard data, but based on remaining range after a trip it seems that the additional drag when running with the top off costs about as much as running the A/C. Noticed this a couple of times driving home (mostly freeway, uphill). What I don't know is how much of those trips did the car use the A/C for cooling the battery... that might have been significant factor.
  6. Timothy

    Timothy Driving on Sunshine

    May 4, 2011
    Drag coefficient. I was always told that the Alfa Romeo 1969 Spider--boat tail (smooth rounded tail end)--had a better drag coefficient than the 1974 Spider which had a Kamm tail (square cut off) but otherwise was essentially the same car. Certainly not intuitive.

    My 1967 Alfa Giulia Super (below) is a very square looking car. It was marketed as 'the car designed by the wind' purportedly one of the first cars tested in a wind tunnel. It was supposed to be more aerodynamic than an XKE in wind tunnel testing. Again, not intuitive. (This is all oral Alfa tradition. Who know if it is supported by fact!)

    Anyhow, I wonder if the vertical window on the Roadster adversely affects the car's aerodynamics if the Kamm tail concept improves the situation on these Alfas?

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