From: Roland Zuk <roland@digiweb.com>
>It is true that a high roll center may create a
>propensity for roll jacking, but semi-trailing arm
>suspensions do *not* have a high roll center.
>With a trailing or semi-trailing arm arrangement,
>the designer can place the roll center anywhere
>between ground level and the elevation of the arm
>pivots, depending on the angle of the arms
>relative to the axle.
Close; actually, the roll center winds up being a
function of the angle of the pivot axis vs. the axle
centerline. Draw the pivot axis of each arm out to
where it intersects the vertical plane of the axle
centerline (the "instantaneous center" or "I.C.");
then draw a line from this point to the tire contact
patch for the wheel connected to that arm. Do this
for both sides, and where the two tire-to-I.C. lines
intersect is the roll center. The roll center can
actually be above the arm pivots in certain cases,
though this is rare in practice. It can also be
*below* ground level!
There are different explanations of the layout in the
literature; I'm following the one in Milliken and
Milliken (pp 636-8 if anyone's *really* bored).
I see now that I should have written "can be" rather
than "is" in my original post. I'll stick by the
statement that roll center for a semi-trailing arm
"can be" high in comparison to a strut or double
wishbone suspension.
>*Swing axles* are NOT semi-trailing arm...
They kind-of are. :) Full trailing, semi-trailing,
and swing axle suspensions are all on a continuum
(ooh, there's a word) as you vary the pivot axis angle
from 0 to 90 degrees. (But if we argue topology now
we'll take time away from the "Whiners Pick the
Winners" thread. ;)
Cheers,
Craig Blome
Bid and sell for free at http://auctions.yahoo.com
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