Polar Moment

Polar moment of inertia in a race vehicle is the center of all forces.  It is the place that the race vehicle pivots about.  The polar moment is also the center of weight in the vehicle or actually the center of gravity.  The farther away from the polar moment weight is added to the vehicle, the more of a pendulum effect it will have when the vehicle changes directions.

Let's not complicate this with some scientific explanation.  Here's an example that will hopefully illustrate how it works.  Think of your vehicle as a set of barbells with 50 pounds of weight at each end.  Imagine trying to quickly turn and stop the bar bell while holding it.  You will notice that itís hard to turn and hard to stop turning once it starts because of momentum.  Now lets move the weights in close to the center of the bar and try it again.  Notice now that the bar turns, and stops turning, easier because the weights have less distance to travel.  Reduced polar moment of inertia makes a vehicle feel like it "wants" to change direction.  The vehicle feels better and the suspension is easier to tune.

The idea is to build your vehicle as light as possible, get the weight percentages correct, and then add weight at the polar moment to get the vehicle up to the minimum weight requirement to satisfy the rules.  The more of the vehicles weight you can put at polar moment, the easier the vehicle will react to direction changes.  Keep in mind however that the polar moment is also usually about where the driver sits.  This makes the vehicle more difficult to drive because the rotational feeling from the vehicle to the driver is minimized.

Notice that we get the proper weight percentages before we add weight to meet minimum rule standards.  The correct weight percentages in the vehicle are MUCH more important than polar moment.  There have been vehicles that had over 100 pounds of weight in the rear bumper, but because the weight percentages were correct, it ran in the top 5 all the time.

Building a chassis low in weight allows you to use polar moment to your advantage.  Do NOT compromise driver safety however for this.

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Last modified: January 01, 2016

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