[s-cars] Caster talk (long), was 2B plates installed with a twist

[Djdawson2 at aol.com]

In a message dated 7/16/2004 4:37:13 PM Mountain Daylight Time, 
ricoletsinger at yahoo.com writes:

> Reading Neil's description it sounds great.  Less
> tramlining, more steering weight, increased negative
> camber on turn-in, this is all awesome.

And all true...

  What are the > downsides to adding castor?  It has to be a
> trade-off...
> 

Not really, if kept within reason.  The more caster you have, the more the 
tire's center of drag is placed behind its steering pivot.  The result is a self 
aligning property at speed.  BMW uses a bunch of it (it's visible, just turn 
the wheels either way and observe the camber change), Audi doesn't on the urS 
cars.

In race car set up, caster angle is considered the least important, as long 
as it falls between 2 and 6 degrees.  More than that, and the car is working 
against the driver under cornering, and has an unresponsive feel... even though 
in reality it isn't unresponsive.  It just doesn't have that "eager to turn 
in" feeling.  Also, in a race car, playing with the caster angle doesn't result 
in any significant gains or losses in overall performance.

Bottom line, some additional caster is a great thing for the urS cars, as 
they are relatively twitchy at speed (always called tramlining on the list).  

The only other angle that one can change to add straight line stability is 
kingpin inclination.  On a car equipped with upper and lower control arms, this 
is the angle (from vertical) that would result from drawing a line between the 
upper and lower ball joints.  On strut equipped vehicles, this would be the 
angle from vertical of the line drawn between the lower ball joint, and the 
upper strut mount.  This angle provides self alignment even when sitting still, 
as the weight of the vehicle itself is working to center the steering.  The 
reason why... just as increasing caster adds camber as you turn the wheel, a 
larger kingpin inclination actually raises the car up as you turn in.  So the 
stabilizing force to center the steering is the vehicle weight.  Changing caster 
is the same as changing kingpin inclination, in that you are changing the 
relationship between the top and bottom steering pivot points.  The only difference 
is that caster is changing the front to rear relationship, and kingpin 
inclination is changing the side to side relationship.
That's all I've got....
HTH,
Dave in CO