DeTomaso Mailing List: January 2001, Message #242

On Mon, 8 Jan 2001, John Bentley wrote:
> If the lever arm was 1 foot or ten from the car, the height of the car is 
> not going to change.  The car weighs the same and the spring rate isn't 
> changing.  As long as you are grabbing onto the car (suspension) at the hub 
> at the same location for both arms, the distance you extend the hub will 
> not effect the static height of the car.
> 
> Where would the force come from to compress the spring?

I've had a bit of fun watching people struggle with this, and I have to
admit I smiled at the mental picture of folks scratching their heads as they
wrote.

If the A-arms were exactly equal length, all of this speculation would be
easier.  :-) If they were, there would be no change in lever arm with a
change in offset, as the A-arm bushings and ball (or heim) joints would
define the pivot points.

I could swear we went through this a couple of years ago.  Here's my
thinking...

Since the A-arms are not (to my knowledge) equal-length, there is a (for lack
of a better term) meta-pivot - a second virtual pivot point - formed by the
rotation of the hub relative to vertical as it moves through it's travel.
I.e., the hub (and thus the wheel) develops negative camber as it moves
upward, and it develops positive camber as it moves downward.  This is true
for nearly every car, since it helps to counter body roll.

There is a lever arm there (defined by this virtual pivot point, which is
defined by the inner and outer A-arm attachments), but I don't know the
measurements (or probably the math anymore) to define it and calculate it.
I can tell you this.  This virtual pivot point is way, way "inside" of the
hub... I figure it's at least several feet inboard.  

Therefore, a change in length of 0.5" (due to a change in wheel offset)
would be lost in the noise.

End of problem.  :-)

-Andy (the geek)
#3822 (not pivoting anything right now)