I think one of the reasons that the software can't tell you much about
bump steer is the use of a trunnion for the lower mount. Bump steer occurs
when the dimensions stray from these specifications:
--Inboard tie rod end located along a line drawn from the upper and lower
inboard suspension pivots.
--Outboard tie rod end located along a line drawn between the upper and
lower outboard ends
--angle of the tie rod such that a line extended through it intersects the
instant center of the upper and lower arms (a line drawn through their
axis that extends until it intersects.
You can pretty much visualize why this is so--the steering arm mimics the
combined motion of the upper and lower arms, and so there is no steering
input caused by suspension travel.
With a trunnion, the steering input axis (vertical rotation of the
trunnion) and the suspension pivot (outboard horizontal rotation) do not
occur around the same point, as they would if the lower mount was a ball
joint. You can see this easily by looking at the first drawing (the one
from the manual).
I haven't been able to reconcile this difference--even using Susprog3D,
which permits trunnion front ends to be modeled. I suspect that trunnions
like the TR3/4/6 ones inherently introduce some bump steer and the only
way to eliminate it is to directly measure the bump steer and try changes
that eliminate it within the useful range of the suspension travel--and
then be sure the suspension can't exceed this limit. It doesn't have to be
so--I suspect you could build a trunnion that had both axis pivoting
around the same point. But the question is "why". It seems like a lower
ball joint makes far more sense to begin with. I wonder why they monkeyed
around with these strange devices?
-----Original Message-----
From: Larry Young [mailto:cartravel@pobox.com]
Sent: Tuesday, May 27, 2003 8:10 AM
To: fot@autox.team.net
Subject: TR3/4 Suspension Geometry
I've written a small report on the calculation and measurements I've
made on TR3/4 suspension (see
http://home.swbell.net/cartrip/TRsuspension.PDF). No guarantees, but I
believe I finally got all the basic measurements right. I hope some of
you find it to be useful.
Larry Young
|