here is outside the box idea. How about you build a ply wood enclosure for
the car of a known shape that has a charcteristic Cd for the shape a
rectangle perhaps. You will also know the area. At that point you do a coast
down from 50 to 25. Remove the box and do it again. If you know the area of
both shapes and all the other vaibles can't you sort it out from there.
dave
> -----Original Message-----
> From: owner-land-speed@autox.team.net
> [mailto:owner-land-speed@autox.team.net]On Behalf Of drmayf
> Sent: Sunday, November 13, 2005 11:33 AM
> To: Elon
> Cc: land-speed submit
> Subject: Re: Area, as in CdA
>
>
> Elon, a series of coast down tests are good. The real issues begin when
> trying to separate out the various parameters. Things like gear drag,
> rotating inertia, brake drag, rolling resistance. You might be able to
> quantify mechanical gear drag using a orque wrench and measuring the
> torque it takes to rotate everything fron the rear axle tot he gear box
> and then using the formula for finding lost horsepower. You would ned
> to measure wheel rpm for finding that parameter. Then you need to try
> and figure out rolling resistance. That requires that you weigh the car
> in the as run condition and then getting the tires warm, measuring the
> tire pressure. Brake drag wight also be measures similarly to the gear
> drag technique. The for the aero drag, you will need actual air density,
> wind speeds wind angle, roadway grade, the frontal area, and of course
> the starting and stopping speeds. This is not a trivial task!
>
> OR
>
> You could just do the coast down and figure out the combined drag
> coefficient for the car under the given conditions. Means that the Cd
> would be a composite of all things above for this car and this car only.
> Which woould work for determining enhancement changes to the car.
>
> OR
>
> Take it to a wind tunnel with a rolling road surface.
>
> Things like this become fun into themselves.
>
> Keep us oinformed of how and what you are doing with the dynamics of the
> car.
>
> mayf
>
> Elon wrote:
> <snip>
>
> > But seriously everyone,
> >Doesn't a coast-down tell you the product of the CdA and all the other
> >variables? It doesn't tell you just the Cd or A, no?
> >
> >
> ><snip>
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