T3: RE: Rear skirt aerodynamics

["Erkson, Toby" <toby.erkson@intel.com>]

Short answer: Quite possibly.
Long answer:  That depends, do you have a wind tunnel?  :)

I cannot give a concrete answer, but from what I've read (and John, PLEASE 
step in anytime!) a squared-off vehicle, like a Squareback or Bus, would 
benefit with a longer rear valence.  From what I remember, adding a tail to 
the top helps as well.  The tail would be located at the very rear edge, 
like where the rear rain gutter is, and it would need to be short in height 
and somewhat triangular (no basket-handles, okay?!).
                                  
              tail -->  /^\       
     top of car        /  |       
       ______________/_ _ \       
       <-- front          |       
                          |       
                                  
Making the rear valance longer is beneficial to a point.  Too long and it
will 
interfere with driving, like scraping on ramped surfaces or acting like a 
snowplow in the winter (don't laugh, this happened to the Porsche 914 and
the 
engineers to design a different one!).  Like I said, I know from personal 
experience in my own lab (Squareback zipping along at 80mph with a small oil

leak) that the stock skirt (valance) works as intended!

To gain more aerodynamic efficiency you would want to include side skirts
and an
air dam (also known as a front spoiler).

We are lucky that the surfaces of the car are rounded and this helps
out...funny
how manufacturers are going back to the rounded style after many years of
making
sharp, angular bodies (with higher drag factors).
    Toby Erkson
    air_cooled_nut@pobox.com  <-- Please use this address for email
responses
    '72 VW Squareback 1.6L bored and stroked to 2.0L
    '75 Porsche 914 1.8L, ORPCA member
    Portland, Oregon, http://www.geocities.com/MotorCity/8501/


______________________________ Reply Separator
_________________________________
Subject: RE: Rear skirt aerodynamics
Author:  Heads Together Systems - Steve Farmer [SMTP:hts@hts.com.au]  at
MSXGATE
Date:    4/1/98 7:17 PM

Toby Erkson wrote...

[snip]
>;).  But, as John J. and I know, and by my personal experience, you should 
>have that apron there.  It helps reduce the lower vortices that occur back 
>there and this lowers the vehicles coefficient of drag.

Does this mean that if we have a custom removable rear skirt, made of 
fibreglass i assume, that we could make it even deeper and gain more 
aerodynamic efficiency?

Regards,
Steve Farmer