To Kill a steering vaguary

["Graydon D. Stuckey" <graydon@apollo.kettering.edu>]

On Tue, 2 Feb 1999, Janet Scruggs wrote:

> Velgia brings up a good point : Eibach springs have lowered the car ~1.5"
> and as such some have suggested that bump steer is more likely and, I am

I'm not convinced that bump steer is that big of an issue on this car 
when lowered.

> is the correct alignment spec for non-OE parameters like tire size and
> suspension height... any theories out there?  This renews my earlier
> question of the list regarding tire pressures and recovering suspension
> response characteristics when changing to lower profiles.

My first response would be with respect to castor.  Typically, castor 
provides for straight tracking down the road. I think the Quattro uses 
3-4 deg in stock spec.  However, if you have lower profile tires, you 
will get stronger camber thrust since the sidewalls won't "give" quite as 
easily.  (Camber thrust, BTW is a lateral force produced by the tire when 
it is at an angle to the road - ie camber)  So, if you have a 
significant amount of castor, and an increased amount of camber thrust, 
then that camber thrust acts as an input to the steering system and 
attemps to change the steering angle of the steering wheels.  You would 
have to counteract this input by steering in the opposite direction at the 
steering wheel.

	So, I might try a little less castor with lower profile tires.  
BTW, when you lowered the car, you got more castor also, so that's even 
more reason to slot the strut towers a little to get less castor IMHO.

	As a point of reference, most modern race cars with significant 
downforce use little to no castor.  Castor also provides increased camber 
during turn-in, but it also raises the car when you turn the wheels, and 
that screws up the ground effect, so they use no castor.  I drove a race 
car a while back which had zero castor, and it still self centered the 
steering wheel due a couple other natural forces which accomplish the 
same goal.  It was very easy to steer.  I liked it, and decided to run my 
RX7 with about 1-2 deg of castor (factory spec was 4 deg).   It worked great!

> PS... I wonder if vagueness in steering is affected by the ritual three
> shots of tequila that I have before each test drive.  Just a thought.

Hmmmm...  I guess it depends on what you do with those three shots!  You 
mix them with the Pentosin, right?  No?  Oh.

Later,
Graydon D. Stuckey
No Audis!  :-(
Yet!  :-)