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A spoiler (or wing) provides a downward force to keep the end of the car on
the ground (front or back). This is particularly needed for taking corners as
the added downforce increases the friction on the tires allowing the car to move
faster without breaking free. The trouble is this wing produces drag so it's a
battle to keep the end of the car down effectively while producing the least
amount of drag. Porsche Racing found out that if you have a reverse slope at
the tail this will reduce turbulent drag (see below) as well as
having a flat, closed underbody.
<-- direction of travel
___
|________
__ / <-- like this but much steeper
__//\\___/
\/
By placing an air dam in the front of the car and the sides it forces air over
the hood (which produces downward force, good) as well as keeping air from going
underneath the car. This creates a vacuum which pulls the vehicle closer to the
ground (good). Remember, the basic shape of a car is very similar to an
airplane wing so the faster a car goes the more it wants to lift off the ground
and take flight. That's why the Porsche 930 (the 911 turbo) has a whale tail as
standard equipment -- it would move so fast that the car would literally lift
off the ground enough that steering became useless, as if the car was
hydroplaning.
As a side note, the extreme majority of "spoilers" you see on cars nowadays are
strictly for looks. Only at very high speeds do some of them become effective
(+100mph). The primary reason is because the "spoiler" isn't in the main air
stream and it's too close to the car's body. The spoilers that actually work on
street cars are the ones that are sticking up into the air stream away from the
body of the car, like the basket handle you see on the new Supra, or have
a duck tail (see below) stretching across the entire rearend, like what you
would see in truck racing.
<-- direction of travel
___
|________/ <-- duck tail
__ |
__//\\____|
\/
The reason why pickup trucks get better mileage with a shell is because the
tailgate is no longer acting as an air brake! If these people put a tonneau
cover over the bed or removed the tailgate they would get better mileage and
acceleration because they wouldn't be hauling around the weight of the shell. A
Fastback has a better coefficient of drag than a Squareback as the sloping rear
roof line doesn't create as sudden pressure drop, thus reducing the amount of
suction (drag) applied to the rear. The squared butt is NOT an improvement in
aerodynamics.
That ends today's lesson on automotive aerodynamics. I hope you took notes
because tomorrow we'll talk about installing instant 20,000hp into your Type III
and completely solving you heating problems with our engine of the week: the
DC-10 gas turbine!
Toby "hmmm, I guess those physics classes can be useful" Erkson
air_cooled_nut@pobox.com
'72 VW Squareback 1.6L bored and stroked to 2.0L
'75 Porsche 914 1.8L for sale
Portland, Oregon
http://www.geocities.com/MotorCity/8501/
______________________________ Reply Separator _________________________________
Subject: Re: Fastback marketing
Author: type-3-errors@umich.edu at SMTPGATE
Date: 7/30/97 6:02 PM
I just figured the Squareback would have a lower coefficient due to the wagon
tail end - it's the same as the people who get better gas mileage with a shell
on their pickup trucks than without it.
> For what it's worth, I HAVE seen published explainations that the
> spoilers on the rear of race cars are there to force a high pressure
> region over the tapering rear of the car where, without the spoiler,
> a low pressure area would develop and lead to large amounts of vortex
> shedding and consequent energy loss (drag.)
Hmmm, I thought the spoilers were there to keep the rear end from lifting off
the ground, due to the same principles you describe above.
> but the only way you could confuse them with sports cars is if
> you've never seen a Lotus Super Seven.
Never trust any car that was not mass-produced. :-p
Bye,
Everett and his 1965 1500 S
Transporter site: http://ebus.simplenet.com/Volkswagen