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Thread: What is more aerodynamic: Wide flared fenders or a wider body?

  1. #1
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    What is more aerodynamic: Wide flared fenders or a wider body?

    Let's suppose you are designing a supercar with a narrow cabin and a wide track.

    What would be more aerodynamic?
    A. To have a narrow body with the wheels being covered by wide fender flares. Maybe somewhat streamlined fender flares.
    B. To have a wider body covering the wheels.

  2. #2
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    Just to let you know, instead of starting multiple threads, there is a thread where you can ask all these questions.

    The Technical Questions Thread - Page 81 - Ultimatecarpage.com forums

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    Quote Originally Posted by Eggnog
    A string is exactly 9 long
    A very well designed set of flares has the potential to be more aero (less change in cross sectional area at the cabin), but most are not well designed.
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    check out the box fish, you dont need a tear drop shape to have low drag

    http://en.wikipedia.org/wiki/Mercedes-Benz_Bionic

    a good modern example of shape optimization is the 2012 Nissan GTR. basically looks exactly the same as the 2011 model

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    coefficient of drag = cross sectional area
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  6. #6
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    Quote Originally Posted by Cyco View Post
    A very well designed set of flares has the potential to be more aero (less change in cross sectional area at the cabin), but most are not well designed.
    Please elaborate.

    I don't understand how flares can provide LESS change in cross section area. If you get a narrow body and wide flares the difference in area would be larger than a wide body which has more or less a constant area.

  7. #7
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    Quote Originally Posted by Big time View Post
    Please elaborate.

    I don't understand how flares can provide LESS change in cross section area. If you get a narrow body and wide flares the difference in area would be larger than a wide body which has more or less a constant area.
    You asked which is more aerodynamic; simply put, the lowest CD should be most efficient... but isn't determined by frontal area alone.
    It may be achieved both fendered or by complete coverage of the wheels (flares or not) but there are many other calculations required.

    To wit:

    CD = Coefficient of Drag is a reflection of the aerodynamic efficiency of the shape

    CL = Coefficient of Lift

    CS = Coefficient of Side Force

    q = Dynamic Pressure in the Test Section

    r = Mass Density of Air

    A = Frontal Area of Vehicle

    V = Velocity

    Which brings us to an example of the required energy to propel an object to designed terminal velocity:

    HP = (DV)/550 = (CDqAV)/550 = (CDA 1/2 r V2)V / 550 = (CDA 1/2 rV3)/ 550

    Wherein

    D = Drag Force

    CD = Coefficient of Drag is a reflection of the aerodynamic efficiency of the shape

    V = Velocity

    q = Dynamic Pressure in the Test Section

    A = Frontal Area of Vehicle

    r = Mass Density of Air


    Aerodynamic Drag:

    CD: Coefficient of Drag

    Invariant with velocity unless there is Reynolds Number sensitivity. The lower the CD the more efficient the shape is to the air.
    With a reference frontal area of 21 ft2, a production car will have a CD around the 0.340 range


    CDA: Coefficient of Drag Area

    Coefficient of Drag x Area: CD x A

    Reynolds Number: the ratio of inertial forces (vsρ) to viscous forces (μ/L) and consequently it quantifies the relative importance of these two types of forces for given flow conditions.

    Dynamic Pressure: (PSF) A2 measures the test section speed in q which is the dynamic pressure.

    q = r V2

    Flow Rate: (ft3/minute)

    CFM = Area (ft2) x Speed (ft/min)

    Source: APPENDIX
    Last edited by csl177; 12-26-2011 at 09:40 PM. Reason: added source
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  8. #8
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    Quote Originally Posted by Big time View Post
    Please elaborate.

    I don't understand how flares can provide LESS change in cross section area. If you get a narrow body and wide flares the difference in area would be larger than a wide body which has more or less a constant area.
    For a closed cockpit, i.e. Sedan/Coupe type car if the flares taper in at the front as the windscreen slopes up, and taper out at the back as the rear window tapers down then the cross section or the car has the potential to remain constant.

    If, however, you have an open cockpit car eg Lotus 7, Sprite, etc running no windscreen, or only a tiny air deflector then the cockpit has no additional cross sectional area and you would be generally better off going for the wider body.

    General rules of thumb only, you must measure to make sure! That is why I quoted Eggnog.
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