Re: roll centers

[Peter Kemos <peterk@domain.elided>]

thanks to all who replied.... i have added some comments/questions to one
response in particular attached below:


> %Hey, don't let this thread die yet, lots of good discussion and
> %information.
> %Here's a question which Fred Puhn's book does not specifically address:
> %How does one calculate front roll center on a double A arm suspension
> %where the lower A arm is NOT parallel to the ground, but rather angles
> %upward from the chassis to the hub/spindle/wheel assembly?  All of
> %Fred's examples have the lower A arm parallel to the ground.
>
> The analysis is the same, it is just that the A arm is not at what some
> consider to be optimal (Parallel to ground).
>
> %Extrapolating from his examples, the upward angled lower A arm would
> %locate the instantaneous roll center quite high up on the vehicle, at
> %approximately a height of the top of the tire.
>
> Are you confusing instant center and (instantaneous) roll center here?

probably, i was trying to use Fred's verbage, going from memory.

>
> Extending the lines of the A arms gives the instantaneous center, about
> which the tire is moving.  Then connecting that intersection with the
> (opposite) tire contact patch gives the (instantaneous) roll center.  Now
> the roll center will migrate as the suspension moves, so that is why I call
> that instant too.

this was the key answer i was seeking (that the intersecting point is connected
with the contact patch of the OPPOSITE tire), thereby resulting in an above
ground instantaneous roll center, relatively higher than if the the lower A arm
were parallel to the ground.



>
> %If one were then to
> %connect this point with the tire contact patch on the same side of the
> %vehicle, it would result in a roll center well below ground level.
> %If on the other hand one is supposed to connect the instantaneous point
> %with the contact patch of the tire on the opposite side of the car, it
> %would result in a relatively higher roll center (well above ground
> %level) than if the lower arm were parallel.
>
> This is why we want parallel to the ground lower A arms.
>

yes, but with our cars (115 series Alfas) wouldn't the opposite be true?  it is
accepted that the rear roll center on these cars is very high (top of the
diff), while the front r/c is quite low (in stock suspension configuration).
as such, the roll axis is tilted sharply upward from front to rear.  to
alleviate this and level the roll axis out, one can lower the rear roll center
by means of a panhard rod or sliding block, but could one not also RAISE the
front r/c to further contribute towards leveling the roll axis?

if this is correct so far, then wouldn't the tilted lower A arms raise the
front r/c and thereby achieve better suspension geometry by correcting, to some
degree, the disparity in front to rear r/c heights?

>
> %
> %Does this make sense to anybody?
> %Could anyone please comment?  The car specifically is a 115 series GTV.
> %
> %Thanks,
> %Peter Kemos
>
> Steve Moresi
> SLO, CA
> 74 Spider
> 64 Spider

Thanks again,
Peter Kemos

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