[Shotimes] rear spoiler

[Zach Leahy]

Depends on the surface, but the average calculation is

F=1/2*a*rho*Cd*V^2

A is a constant to make up for unit differences, Cd is the coefficient
of drag, for a perfectly flat object of infinite size, I believe it
will be "1" (think cliff face), but may be higher.  Cd is generally a
backwards calculated value based on testing, not on design analysis. 
What is behind the surface (like the back of a car, not so much the
front) has a huge impact of the Cd. IIRC, a rear glass at 30 degrees
is the worst for cd, I'd have to drag out my vehicle dynamics book to
see.

rho is the air density, and V is obviously velocity.  It's been
several years since I have used this formula, but it does show that as
your Speed increaced linearly, the drag force increaces exponentially.
 That's why it takes a lot more horsepower to go from 130 to 140mph vs
20 to 30 mph.  So when a ricebox claims they can pull 160mph in their
civic, pull this out and show them some errors in their calculation. 
It soons turns into, well my friend told me that, and his a has a tubo
and a blower, and NoS.  It's kinda funny.  Use the 220hp of the SHO as
an example, if 220hp can get 140mph, how much more does it take to get
150mph.  Answer, is a lot more than 225hp.

Now, this gives you the actual DRAG based on the air flowing past the
car, the amount of downforce, or actually in nearly any passenger car,
the amount of lift, generated by the air is based on a different
formula, and different calculations.  Ine has to know the location of
the center of gravity of th car as well (I balanced it once, iirc it's
about 6-8 inches behind the end of the SHO lettering on the driver's
door, I needed it for a simulation in that course.  I proved tha a SHO
could outrun a Mustang cobra R, and a Porche GTR given that the road
was a sheet of ice covered in water (nearly zero CoF), turns out the
other cars don't have enough weight, and it's not centered close
enough to the drive wheels, to get enough traction to put the power to
the ground)  But to give you a run through of the calculations for
lift/downforce (the difference is merely a negative sign) I would have
to go dig my textbook out of the barn, and I don't really want to do
that as it is packed away with a lot of other stuff.

Z

On 6/22/05, Co-Op <Co-Op@schramminc.com> wrote:
> Is that a fast corner?  I doubt that a spoiler would help an SHO at speeds
> under 75 or 80.
> 
> You could hammer form a piece of aluminum sheet to attach to a stock
> spoiler.  It shouldn't be too hard, and would be removable for the ride home
> from the track.
> 
> Does anyone know how much force wind has per square foot at, say 75, 100,
> and 125 mph?
> 
> -----Original Message-----
> From: van Oss [mailto:vanOss@centurytel.net]
> Sent: Wednesday, June 22, 2005 1:22 PM
> To: shotimes@autox.team.net
> Subject: [Shotimes] rear spoiler
> 
> 
> My 92 came without a rear spoiler.  I have one on hand in the correct color
> and am debating whether to install it.
> 
> There is only one place I've wanted some rear downforce, at the entry to
> turn 1 at Brainerd.  At the end of that mile-long straight, the rear end
> feels awfully light to me.
> 
> On the other hand, I have no aesthetic interest in a spoiler and I only go
> to Brainerd about once a year.
> 
> I know this is partly subjective, but I'm soliciting your opinions.  Thanks.
> 
> VO
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