Suspension Definitions

Spring rate : Usually expressed in lbs/in, it’s the force required to compress a linear spring one inch. Linear springs have a constant spring rate. Progressive springs have progressive spring rate that rise has the spring load increase.

Corner weight: Is the weight of the car applied to ground through the tire. It’s made of the sprung weight plus the unsprung weight.

Unsprung weight: Weight of the part of the car that is not supported by the suspension. Includes the weight of the tire, the wheel hub, the brake and half the weight of the axle and suspension links.

Sprung weight: Weight of the car that loads the suspension. Equal to the overall weight minus the unsprung weight.

Spring mechanical advantage: It’s the ratio of the spring length over the wheel movement. Almost equal to one for McPherson struts.

Wheel rate: Spring rate times the square of the spring mechanical advantage.

Coil bind length: State of a spring were the coils touch each other. The spring then becomes solid with a nearly infinite rate.

Spring static deflection: The amount of deflection of a spring under the car sprung weight. For a linear spring, it’s equal to:

To design a suspension, 3 factors must be taken into account : the wheel frequency, the roll force balance and the fore and aft pitching period.

Wheel frequency : The wheel frequency of oscillation in cycle per second of a car without damper. It’s a function of the car sprung weight(W) and the wheel spring rate(K) :

Typical frequencies range from 1 cps for large soft sedan to 2.5 for a racecar without aerodynamic down force.

Roll force balance: It the front to rear distribution of the roll force of the spring and anti-roll bar effect. It affects the over/understeer property of a car. Weight distribution and wheel geometry also affect car steer property.

Pitching period: The pitching in the front and aft oscillation of a car on its suspension. It can be reduce to zero at a given speed by carefully using a slightly higher wheel frequency at the rear of the car. For a given wheel base and speed. Over a road bump, the rear suspension that it the bump after the front, will settle at the same moment than the front one. The speed V at which this phenomena occurs is:

Where L is the car wheel base in meters and f is the wheel frequency at the front and the rear. For a street car, this speed is usually set to the usual highway speed and to the mean track speed for race cars.