Image courtesy of Discount Tire
Static Imbalance (left image)
Occurs when there is a heavy or light spot in the tire that prevents the tire from rolling evenly and causing the tire and wheel to undergo an up-and-down motion.
Dynamic Imbalance (right image)
Occurs when there is unequal weight on one or both sides of the tire/wheel assembly’s lateral centerline, thus creating a side-to-side wobble or wheel shimmy.
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This Matlab script calculates the vibration frequency of a tire due to imbalance at a given speed: tire.m
The forcing frequency equals vehicle speed divided by tire circumference for a simple case of one heavy spot.
A harmonic may occur at twice the frequency, or if there are two heavy spots.
– Tom Irvine