Lowest CD?? (Thanks Zukiru)

Discussion in 'Other Manufacturers' started by WhatIsChazaq, Jun 3, 2008.

  1. WhatIsChazaq

    WhatIsChazaq Active Member

    I saw Zukiru's sig listing his Subie XT and the .29 CD. My Forester is a brick by comparison at .35

    Isn't the XT C/D one of the lowest ever in a production vehicle? Any others close?
     
  2. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    The Honda Insight clocks in at 0.25 CD.
     
  3. TheForce

    TheForce He who posts articles

    And the Prius ( which is still in production ) clocks in at 0.26 CD.
     
  4. Zukibot

    Zukibot Member

    Really glad you posted this. I had been trying (unsuccessfully) today, to find his posted CD. I had thought the 0.29 was for the '08 SX4 Sport though.
     
  5. Zukiru

    Zukiru 0.29 coefficient of drag

    nope the .29 is for the XT which was the lowest CD for a Japanese car in 1985 when the vehicle was designed. it's still pretty low.

    it impresses me because back in 85 they had to really wind tunnel test a cr to get these low numbers.

    today it can be done with computers, while that's also awesome!, there's just something cool, and old school about being a first for Japan!

    the suzuki's CD must suck cause I have yet to find it listed.
     
  6. msantos

    msantos Eco Accelerometrist

    Not to be totally outdone, the HCH-2 comes in at 0.27 CoD

    Cheers;

    MSantos
     
  7. Vooch

    Vooch Well-Known Member

  8. AbACUZ

    AbACUZ Juniorest member

    0.34 here

    Can not believe the 04 WRX wagon is lower 0.33

    have to research this ....
     
  9. Ophbalance

    Ophbalance Administrator Staff Member

    The 99 Elantra is a .33 as well.
     
  10. Zukiru

    Zukiru 0.29 coefficient of drag

    I'd imagine that's just because of the paste on saab nose. I doubt they bothered to make sure it was as aerodynamic as the original.
     
  11. Jaral

    Jaral Well-Known Member

    Isnt the total drag something like Cd x frontal surface area? So a small/narrow/short car has less drag than a large car with the same Cd?
     
  12. seftonm

    seftonm Veteran Staff Member

    Correct. Cd is how "slippery" the object is to the air. Multiplying the Cd by the frontal area of the object will give a figure which more closely represents how much resistance to movement the air produces. A Boeing 747 has a Cd about 10x lower than that of an Insight, but I doubt that it has less total drag after the Cd is multiplied by its frontal area.

    Back to the original question, the GM EV1 had an excellent Cd at 0.19.
     
  13. Ophbalance

    Ophbalance Administrator Staff Member

    So how much affect would moving from a Cd of 0.33 to 0.30 have on FE? Is there any easy to find equation for this?
     
  14. lightfoot

    lightfoot Reformed speeder

    The square of the speed and the density of the medium (air) factor in as well. From Wikipedia:

    "The overall drag of an object is characterized by a dimensionless number called the drag coefficient, and is calculated using the drag equation. Assuming a constant drag coefficient, drag will vary as the square of velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity. The standard equation for drag is one half the coefficient of drag multiplied by the fluid mass density, the cross sectional area of the specified item, and the square of the velocity."

    http://en.wikipedia.org/wiki/Drag_(physics)
     
  15. AbACUZ

    AbACUZ Juniorest member

    But the Saab nose is more aerodynamic, and the scoop on the hood is more aerodynamic, this has me really puzzled, , im going to have to get the 92x crew on this to find out why.

    I also second the question as to how much more efficient will the 0.30 would be over a 0.33






    oh and the first one that says.. 0.03 gets a pie on the face :D hehehhe
     
  16. Vooch

    Vooch Well-Known Member

    the wiki article has a fair amount of background - underbody, wheel width, and so forth have a great effect
     
  17. lightfoot

    lightfoot Reformed speeder

    Seems to me that one of the main reasons to slow down is to minimize aero drag and remove it from the equation as much as possible. That makes the drag coefficient much less important than it would be at higher speeds.
     
  18. Ophbalance

    Ophbalance Administrator Staff Member

    Yes. But, driving 45 in a 70 just isn't practical ;).
     
  19. lightfoot

    lightfoot Reformed speeder

    Before we know it that may be very practical!

    45-50 in a 65 works quite nicely around here, with a bit of "traffic management" astern. It did take a while to get comfortable with doing that.
     
  20. trackermpg

    trackermpg Well-Known Member

    When it comes to Aero/drag, etc. nothing is ever quite as simple or intuitive as it may look to us. There is an infinite amount of interactions and effects such as ram air (cooling) flow and subsequently how efficiently that flow is recovered back into the free stream, interference drag between body parts that join at angles, airflow spillage/direction changes into lower pressure areas, flow separation, wetted area (total surface area), etc. No matter what is calculated, nothing is true until it is proven. Cd has to be demonstrated after the fact, although modern CFD software and wind tunnels go a long way towards accurate predictions.

    Sorry, but since the aero portion of drag in a car does = 1/2 rho (density) x Velocity squared x Area (frontal) x Cd, so unfortunately the answer really is by a factor of .03, or just under 10%. A car with a Cd of .30 would have ~.91 of the drag of a car with a Cd of .33 at the same speed. That part is simple! Apple would be good.:eek:

    When calculating total drag for an aircraft, the drag of the fuselage moving through the air is only one part. The reason for low Cd numbers for the fuselage is typically because the air has a very long way to be able to make its direction changes and rejoin at the back end, whereas in a car those changes are sometimes forced to be made in a more severe fashion. The back end of a car is relatively horrible aerodynamically compared to aircraft, hence the higher Cds of most cars compared to aircraft, and conversely the success of Basjoos' boat tail. Notice the rooflines of most efficient cars today, and you'll notice the most gentle curving practical to delaw flow separation and it's subsequent reduction in Cd. While the Cd of 747 is much lower than most modern cars, however, there absolutely is considerably more frontal area and surface (wetted) area of the body - a LOT more! The drag caused by skin friction is proportionate to the amount of surface area (skin) and can be reduced by using the "area rule" concept of using a coke bottle shape in places where other protruding devices begin, i.e. wings, engines, etc. What cars don't have to contend with in the total drag equation is the drag created by creating lift. The induced drag (by-product of lift) is highest at low speeds and decreases exponentially as V decreases. When you have to create lift equal to or greater than several hundred thousand pounds, the induced drag amount is staggering when V is low. The faster you go, the lower the induced lift/drag becomes because of the effect that V squared has on lift production, but the parasitic drag (frontal area x Cd, skin friction drag, interference drag, etc.) increases exponentially exactly as it does for cars.

    Again, nothing in aerodynamics is likely to be as simple as we may think, and nothing is real till proven (measured) in reality! Never underestimate the amount of engineering that goes into a car these days, and never assume we can automatically make something "cleaner" than they did without having subsequent consequences either aerodynamically or mechanically.
     
    Last edited: Jun 6, 2008

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