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Re[2]: Fastback marketing -- nope, aerodynamics class!


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


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