Read Post 562 on page 19 first –
Polar Moment explained.
In Physics M
oment is the turning effect or rotational tendency produced by a force acting on an object around a pivot point or axis.
Pitch Roll and Yaw
Every race car driver knows about Roll. It is the left to right turning motion the car moves in a turn. The second motion is Pitch. Think of your softball days where you had to pitch the softball underhand. Your hand pitched the ball from a low position to a high position from back to front. Your car pitches up under acceleration and pitches down under braking.
Yaw is the side to side motion during cornering and the least understood motion. Why is understanding Yaw important?
Race Car rotation in cornering is the deliberate, controlled turning or yaw of the car's body around its vertical center of gravity when in left hand turn.
A race car rotates around its vertical (z-axis) through the center of gravity during cornering, and the
Polar Moment of inertia measures how its mass is distributed relative to that axis.
You have to re-read this several times.
Ideally, we want to build the race car with the desired percent of left side weight and best front to rear weight. Only when we achieve this do we want to add lead weight to meet the minimum weight requirement when we scale. We want to add this Mass or weight at the Polar Moment as this is the point where it will move “ the least” when cornering.
Think of the old teeter totter at the local school. The tetter totter board is your Wheelbase. If two people of equal weight sit on it at the same position from the middle pivot, we have Minimum teetering or movement. The center pivot is the Polar Moment. Now bounce up and down and the teeter totter moves (Pitches) but notice the center pivot has the “ least movement” relative to the rest of the board. We have front to rear weight bias.
So if I go get the neighbors fat kid and put him on the right side of the teeter totter opposite me, the teeter totter will tilt in my favor. I have to scoot toward the center pivot point to “ equal up” the weight distribution.
In fact, an observer looking at this mess from the side would see my weight was shifted 10% bias toward the center pivot point. This is why we mount the heavy Battery and additional weigh bars as close to Polar Moment as possible. We do not want a bunch of “sling weight” at the rear bumper! If you have to add weight, place it where it moves the least during Roll, Pitch and Yaw.
Our race car really has Two tetter totters. A front to rear (wheel base) ( Pitch) Moment and a side to side (track width) Yaw Moment.
If we were to measure Left to Right side weights over the Track width distance and find the position to the center pivot point we have 60% left side weight bias. Our second tetter totter (Yaw Moment). This is why early Spots cars started moving brake rotors from the wheel mount calipers to the inboard brake caliper mounts. Lower polar moment.
Polar Moment is the intersection of these two points AT CENTER OF GRAVITY HEIGHT.
This is how your race car "Rotates".
How Polar Moment of Inertia Affects Cornering
- Low Polar Moment of Inertia (Mass Concentrated in the Middle):
- Achieved in layouts like mid-engine cars (e.g., placing heavy components, engines, and brakes near the center of the wheelbase).
- Requires less turning force (torque) to change direction.
- Allows the car to rotate (yaw) quicker, transition through chicanes faster, and feel more agile.
- Risk: If it is too low, the car can become "twitchy" or over-responsive at the limit.
By the way, most door slammers run 18 inch CG Height and super late model cars with dry sump oil pans and aluminum heads get as low as 16” CG Height. There is a method you can use to measure exact CG on your car with wheel scales and a floor jack. Read Post 562 on page 19.
