Reminds me of a joke. Three scientists were taken to a room with a
chicken.
The biologist comes out and stated "its of such and such species". The
chemist emerges and states "it is of this and such complex hydrocarbons
compounds".
The physicist returns and states "first assume it is a sphere".
Aerodynamic force doesn't become significant until around 45-50 MPH.
So if you drive around town at 45 or less aerodynamics wouldn't matter.
One way to see the aero effect is to run up to 100 MPH on a level road
on a still
day.
Put the car in neutral and note how long it takes to slow down to 50
MPH.
My '93 Buick Roadmaster station wagon takes a long time, much much
longer
than my MGBGT. With an SUV it seams like you're putting on the brakes!
My Buick has no rain gutters and the windshield has a steep slope. It
gets
24-26 MPG running 80 MPH all day. It has a 5.7 Litter engine.
My MGBGT has a 1.8 litter engine and gets about the same mileage.
My MGBGT has lots of sharp edges such as rain gutters and
headkight recesses. Compare a stock MGBGT to the Sebring
one. They cut off the gutters and put cowls over the headlamps
to lower the CD and go faster. Buick did the same thing to
meet its Federally mandated fleet mileage goals.
Richard Lindsay wrote:
> Bill writes,
>
> > By the way, you techies out there, does aerodynamic design
> > actually save on gas and propulsive energy to the point
> > that it matters in a car not driven faster than 87mph
> > (139.2kph)?
>
> A couple of interesting points before a discussion:
>
> (1) At idle, 100% of the engine's <available> power is being
> used just to overcome the internal friction of the engine.
> I have always thought that that was cool.
>
> (2) At maximum speed, 100% of the engine's <available> power
> is being used to overcome internal friction, rolling
> resistance and to overcome aerodynamic drag.
>
> Aerodynamics is a VERY complex subject involving so many
> variables that it is often studied as an empirical science,
> hence the continued need for wind tunnels. Yet, good math
> exists for realistic calculations.
> Coefficient of drag varies with the frontal area of the body
> but 3-D design shape enters into actual drag. A tear drop is
> about the most aerodynamic shape and a flat plate perpendicular
> to the direction of fluid flow, is about the worst. Some
> math:
>
> Cd = D / (0.5 * R * V^2 * A)
>
> where,
>
> Cd is the coefficient of drag
> D is drag, as measured
> R is fluid (air) density or about 1.22 kg/m^3
> V is velocity in m/s
> A is frontal area
>
> Rearranging, one can solve for drag. Notice that drag
> increases with the SQUARE of velocity (e.g. a doubling in
> velocity causes a four fold increase in drag, a tripling in
> velocity causes an eight fold increase, etc.!)
> Here are a few Cd values for a few cars. Sorry, the MGB
> is not in my reference material but given a drawing of the
> frontal area, an estimate is included below;
>
> Circular plate: Cd = 1.17
> Tear drop: Cd = 0.04
> Porsche 911: Cd = 0.38
> VW Beetle: Cd = 0.48
> 3-series BMW: Cd = 0.38
> 7-series BMW: Cd = 0.43
> MGB: Cd ~ 0.42
> March INDY race car: Cd = 1.06
>
> The INDY car is a high Cd car! Why? Because increased
> drag, caused by the wings, is traded for increased downforce.
> Slower on the straights, faster in the turns.
> So, to answer the question, finally; AurAerodynamicape
> matters at slow (sub 80mph) speed ONLY if the aerodynamic
> design is VERY poor or vehicle is very big (read: moving
> van or perhaps a 4x8 sheet of plywood handled up on a roof!).
> Review the Cd numbers above. They are not very different
> for all practical designs. HOWEVER, fuel economy is a hot
> topic because it is right in our faces all the time. As I
> posted earlier, I know people who have spent $20,000+ to
> achieve 8mpg improvement in mileage! In reality, they
> just wanted a new car but couldn't live with the honesty of
> admitting that they "just wanted it."
>
> Best
>
> Rick Geek Lindsay
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