Elon;
I think the main point of having a high polar moment of inertia in an LSR
vehicle is to keep the car controllable. A low PM vehicle is great for
autocross and road racing as it can change direction fast. Once you lose it
in a mid-engine car it spins very quickly. This makes it difficult for the
driver to gather it up again
Regards, Neil Tucson, AZ
-----Original Message-----
From: land-speed-bounces+neil=dbelltech.com@autox.team.net
[mailto:land-speed-bounces+neil=dbelltech.com@autox.team.net] On Behalf Of
Elon
Sent: Tuesday, January 15, 2008 7:42 PM
To: land-speed submit
Subject: [Land-speed] Polar Moment
A few days ago Ed mentioned how important Polar Moment (PM) was and felt a
car should be set up with a high polar moment. Jack also confirmed such a
configuration. With Jack's outstanding experience, a contrarian view of LOW
PM would be difficult to support.
However, I have been weighing this conflict of Low vs. High PM for a while
and would appreciate your thoughts. There is universal agreement, and no
question that the CG must be in front of the Center of Pressure (CP). That
provides aerodynamic stability. Any aero disturbance or road disturbance
will always have a tendency to correct back to the path the CG is following.
This is an inherently stable AERO configuration.
I wonder about High PM. As long as the car is traveling in a straight line,
neither Low nor High PM should make significant difference because the
controlling, stabilizing force is AERODYNAMIC stability. I know current
thinking is a high PM will resist an upsetting force. It is the Barbell
effect in that it takes a greater force to start a weight ROTATING the
farther out it is from the CG (or the heavier it is). However, the down-side
is that it takes just as great a force to stop the rotation and RETURN the
barbell back in-line. And that is the root problem. If a disturbance is
GREAT ENOUGH TO OVERCOME the built-in AERO stability, and start the barbell
revolving, then it may continue rotation, (i.e. a spin).
Even though a light barbell will react more easily to a yawing disturbance,
any correction will return it to a center-line position far easier than a
heavy barbell. And the important Aero Balance works far MORE EFFECTIVELY on
a light barbell. I can understand how a high PM car will feel less twitchy,
but is it really safer? Looking back at the banning of the rear-engine
modified roadsters, these were extremely high PM vehicles. They were also
widow-makers. -Elon
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