The most important principal behind panhad rods
is to make them as long as possible - short ones can cause
more problems than they solve.
The "best" technique for horizontal location of solid rear axles
is the Watts linkage. It works without changing the roll center
height as a result of body roll.
A panhard rod does cause a change in roll center height with
body roll but the effect is dependent on the actual geometry.
The goal is to minimise the change in this point:
roll center = intersection of the longitudinal axis of the panhard
rod and the centerline of the car (i.e. along the
driveshaft)
projected to the vertical plane passing through
the axle housing.
A short panhard rod does a poor job of mimising the variation! The
longest rod you can make is the best because it minimises the change.
Not only does the change in roll center adversely affect handling and
predictibility (a) it is considered a bad thing to move the roll center
above
the axle b) the tendency of a car to understeer or oversteer is affected
by the relative heights of the front and rear roll centers (among other
things).
In a stock spridget, the rear roll center stays at a constant height.
c) jacking forces are introduced by panhard rods.
With panhard rods the change in roll center is a non-linear function
(sine) of the angle of the panhard rod (as distinct from the body).
A short rod changes angle at a much faster rate (v body roll)
than a long one. Therefore the roll center height changes further and
more rapidly with a short panhard rod then with a long panhard rod.
Finally, the angle of the panhard rod also results in a jacking force - the
force along the panhard rod projected onto the vertical plane. This is also
a function of the sine of the panhard rod angle. This jacking force can also
lead to evil handling traits. Note that the horizontal projection of that
force
is what actuall keep the axle located horizontally! The horizontal
projection
is constant for a given corner speed, therefore the compressive load along
the rod and the vertical projection (jacking force) increases as the rod
shortens.
It should also be clear now why the combo anti-tramp rods/panhard rods
are not a good design and why you should strive to make the panhard rod
horizontal. If you are really fixed on handling, you also fit the panhard
rod
with an appropriate number of bags of cement sitting in the drivers seat.
For some of thus, this makes a big difference (and is tiring work lifting
all
those bags in and out!!!!)
OK, so how much difference does this all make? Not much if you don't
push the car (but then why bother with the panhard rod in the first place).
If you are driving hard enough to *really* require a rod, then you will be
seeing the adverse effects of a poor design.
Mike
----- Original Message -----
From <NewNGsInfo at cs.com>
To: <spridgets@autox.team.net>
Sent: Tuesday, February 06, 2001 1:28 AM
Subject: More Tech Session Pics
> All,
>
> I have also posted some Tech. session pics at:>
>
> http://ourworld-top.cs.com/JCareyPage/news.html
>
> G'day
> John Carey
> http://ourworld.cs.com/JCareyPage/JCareyPage.htm
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