Okay, John's analysis makes some sense, and it's the first explanation I've
heard for why having the Panhard bar one way vs. the other should make a
difference.
To answer Mike's question, a properly configured torque arm should only
resist axle torque, and a properly set up Panhard rod should only resist
lateral deflection of the axle. These two should be able to be used together
to get rid of a whole lot of undesirable behavior. The wild card is how you
think the leaf springs get into the picture, and cause binding or other
secondary effects (as an example, running hard urethane front spring
bushings will make the leaf springs act as anti-sway bars because on any
turn you're twisting them as well as compressing and extending them).
My opinion is that for lateral location, a properly set up Panhard rod
(locate it horizontally, as low as you can without hitting the rear cover,
the tire well, or running into the exhaust system) is the way to go because
it's simple and it will work. If you are a stickler for proper rear axle
motion, then build a Mumford link and you'll be able to set the roll center
below the pavement if that's what you want. A Watt's linkage is a waste of
time because there are three moving/rotating parts, two frame brackets, and
a big rear axle bracket required, and even with all that you end up with a
roll center that's higher than what you can get with a Panhard rod.
Actually, to have things more correct (move the roll center with the chassis
rather than the axle) you should put the center pivot on the frame, and to
do that you have to build a big structure onto where the tire well used to
be.
Regards,
Theo
-----Original Message-----
From: Carmods@aol.com [mailto:Carmods@aol.com]
Sent: Wednesday, December 20, 2000 8:40 AM
To: owner-tigers@autox.team.net; tigers@autox.team.net
Subject: Re: RE; Panhard Rod
To those interested in Panhard locations, here is my analysis to answer some
of the recent questions.
The stock Mark I rod runs from below the wheel centerline of the axle on the
left side to high on the body on the right side. The Mark II runs the
opposite direction. The instant roll center of the rear suspension is
defined
as the point at which the Rod intersects the centerline of the car.
When accelerating out of a RIGHT turn, the body rolls to the left, raising
the right pivot. When this happens the instant roll center of the rear rises
increasing the roll stiffness of the rear suspension. Two things now happen.
The engine torque into the rear axle as well as the increased rear roll
stiffness cause vertical loads to be shifted from the right rear and the
left
front tires to the left rear tire. As the left front and right rear tires
lose vertical loads they lose cornering power. These cornering forces are
additive and transferred to the left rear tire. As the force exceeds the
tire
capability there will be an extreme oversteer in right turns and lifting and
spinning of the right tire.
When accelerating out of a LEFT turn, the body rolls to the right, lowering
the right pivot. When this happens the instant roll center of the rear
suspension lowers, decreasing the roll stiffness. Again two things happen.
The engine torque into the rear axle as well as the decreased rear roll
stiffness causes cornering force capability to be shifted to the right front
tire. As the force exceeds the right tire capability the car will have
understeer. Having a car that handles different when turning left or right
is
not fun.
With the Tiger Mark II, which has the Panhard Rod running from the right
side
of the axle to the left side of the body, the opposite dynamics happen and
these loads subtract from each other. The left and right turn differences
are
improved and it is easier to balance front and rear roll stiffness.
So, to improve the Tiger rear geometry in order of complexity, you can
change
the Panhard Rod to the Mark II design, make the Panhard Rod horizontal by
lowering the frame attachment pivot or install a Watt's linkage. Good luck
in
finding room and feel free to add more ideas.
John Logan
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