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The natural oscillation frequency of the suspension, expressed in Hertz (cycles per second). It combines the wheel rate and sprung mass into a single number that describes how "stiff" the suspension feels, independent of the vehicle's weight.
Staniforth's Competition Car Suspension explains: "Knowing the frequency of a vehicle suspension is the best way to understand its stiffness. The natural frequency of the suspension is the rate at which it will move up and down in cycles per second (hertz). When we are looking at suspension frequency, it is the sprung weight of the vehicle and the wheel rate that are at play." He adds the context: "In much the same way as a loudspeaker or induction system is tuned to take advantage of resonant frequencies, suspension frequency is used to optimize the vehicle for its purpose." Gillespie's Fundamentals of Vehicle Dynamics provides the engineering range: typical values run from "1 to 2 Hz" for passenger cars to 3-5 Hz for race cars. The formula is straightforward: frequency = (1/2pi) * sqrt(wheel rate / sprung mass). This makes ride frequency the universal comparison metric — a 2.0 Hz front end feels the same whether achieved by heavy springs on a heavy car or light springs on a light car.