Here is some research by Mitsubishi Motors on them
http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf
As cheap as they are I am buying a set for my bug!
Here is a different "metal" type too
http://www.hallwindmeter.com/vortex.php
http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf
As cheap as they are I am buying a set for my bug!
Here is a different "metal" type too
http://www.hallwindmeter.com/vortex.php