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The Iron Duke Resource Site


 

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Welcome to my newly redesigned Fiero 2M4 resource site.  The original site recorded over 12000 visits,  hopefully this one will also prove as useful.
Ira Crummey

Polar Moment of Inertia:

Inertia is the tendency for a mass at rest to remain at rest, and a mass in motion to remain in motion, in a straight line, unless acted upon by an outside force. In an automobile the outside force is the force generated by the tires against the road surface, the engine turns the wheels to accelerate the car, the brakes slow the wheels to decelerate the car and the front wheels turn to change direction of the car. Everything comes down to the interaction of those four small contact patches and the road surface.

The concept of Polar Moment of Inertia is an extension of Newton’s law of inertia. In simple terms it refers to how difficult it is to get an object to rotate on an axis. The farther away from the axis of rotation the mass is, the harder it is to make it turn. Conversely, if the mass is concentrated near the centre, it is easier to make it turn. Mid engine cars have a low polar moment of inertia which makes it easier to make them rotate, and therefore easier for them to turn. This is a double edged sword however, mid engine cars will react more quickly to changes in tire angle (they turn faster) but this also means that they are inherently easier to spin as well.

 (all diagrams by Ira Crummey)

High Polar Moment Designs:

Note how in each of these designs the major masses such as the drivetrain (in red) and the fuel tank (in green) are placed at the extreme ends of the chassis.  This results in a high Polar Moment of Inertia.  Designs such as these are very stable in straight line travel, making them "safe" handling designs well suited for the "family hauler".

Low Polar Moment Design:
(Pontiac Fiero)

Note that in the Fiero most of the drivetrain mass is within the wheelbase, as is the centrally located fuel tank.