>From 100 MPH down to 50 MPH
the predominate retarding force is drag not rolling friction..
F=MA
where F is drag plus rolling friction and M is the mass of the car and A is
the difference
in speed divided by the time it took to slow down..
Try a coast down test from say 35 to 10 MPH to estimate rolling friction.
Subtract low speed results from the high speed results to estimate drag.
Drag is the aero coef times frontal area. Divide drag by the frontal area
of your car to get the aero coeef. Repeat above 20 times and take average.
To be exact take it to a wind tunnel.
Now is this technical enough of a preface? I was just have fun noticing that
modern cars
are more slippery than older ones. I also observed that the same techniques
MG used at Sebring are the same GM used to increase its fleet milage.
My Buick is proportionately wider, taller
and heavier than my MGBGT. So the differences in mass and frontal area
tend to cancel each other out, at least to the first decimal place...but I
had to
first assume they were tear drops...
> 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.
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