Nice try Martin. If you had prefaced your comments, given the same weight
and the same rolling friction, you might have invented a great empirical
test. What ever happened to inertia? Bill
On Sat, 2 Jun 2001, Martin wrote:
> 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
Bill Saidel (856) 225-6336
Dept. of Biology (856) 225-6312 FAX
Science Building E-Mail: saidel@crab.rutgers.edu
Rutgers University
315 Penn Street
Camden, NJ 08102-1411
USA
For more information about my science, check
http://crab.rutgers.edu/~saidel/saidel.html
"Sacred cows make divine hamburgers."
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