In a message dated 5/2/99 2:31:46 AM Central Daylight Time,
autox-owner@autox.team.net writes:
<< Date: Sat, 1 May 1999 16:47:53, -0500
From: HPKY59A@prodigy.com ( BENNY DEMENT)
Ok I know that ackerman( what I call "toe out on turns") is a
function of the angle of the steering arm.
What I don't know is how to measure the amount. would like to know
for both rear steering arm locations and for front steering arm
locations. Ya know ahead or behind the Axle.
Benny Dement 56 CSP >>
The geometry of the steering arms and center link determine the ackerman
angles. Measuring the effect can be tough. The basic rule is to take a line
through the steering axis ( line between upper and lower ball joint) and the
outer tie rod end and extend it to the rear of the car. Where the two lines
meet will give a rough idea. If the meet behind the rear axle, you won't have
enough to make the wheels remain toed out for average turns. In which case,
some static toe out may help. If the lines cross right at the rear axle (or a
line between the center of the rear tires for IRS folks) then with zero
static toe, the wheels should be very close to proper toe in the curves. And
if the lines cross in front of the rear axle, then the car will gain toe out
as the wheels are turned tighter. This would allow some toe in for stable
braking, and still get good wheel angles in the curves.
That is only an approximation though, as the location of the steering rack,
and the length of the tie rods will effect it too. The above assumes that
there is one solic rod joining the ends of the two steering arms.
A more precise way, (and possibly easier) is to use accurate turn plates
under the wheels to get the true turn angle of each tire. Then you can plot
where the spindle will point at a given turn angle. For visual types (Who
don't like trig) you can draw a scale picture of the car with a dot at the
center of each tire patch, and use a protractor to mark the angles of the
front wheels. Extand the lines til they hit, and see where it intersects the
rear wheel axis. If the two front tires cross ahead of the rear axle, the
inside tire will be trying to produce a greater slip angle and will add to
the turning force. Crossing behind the rear axle, the inside tire will be
producing no slip angle, possibly even fighting the outside tire. Ideally
they should intersect right on the axis of the rear tires, then the tire
scrub will be minimum, and slip angles should be equal. For any one radius,
the toe can be used to pput it where you want, but only perfect ackerman can
make it the same at any turn radius.
I set my toe to line them up at a 20 degree turn angle on the outside wheel.
This is about the max I see at local autocross events. With my wheelbase and
width, I needed just over 23 degrees at the inside wheel. The turn in was far
better then with zero toe.
JMHO of course, but I have tried it, and my times seem to back it up, too bad
I also did other changes to the car, so I can't say this alone made the big
difference.
Gary M.
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