Unsprung weight is the vehicle weight that is not supported by the springs.

Examples include: Tire/wheel assembly; brake rotors and calipers (or drums and components); wheel bearings; steering knuckle; hanging weight of the control arm (or trailing arms on rear axles); differential and axle weight; 1/2 of the spring and shock absorber weights.

Unsprung corner weight is usually around 70-120 lbs.

Dimension A – Measure the distance from the control arm pivot point on the subframe (centerline of the bushing) to the point on the control arm directly under the center of the spring or coil-over assembly.

Dimension B – Measure the distance from the control arm pivot point on the subframe to the centerline of the ball joint.

Note: If you are running reverse offset wheels, then measure to the center of the wheel.

Using a protractor or similar measuring device, find the angle of the centerline of the spring or coil-over assembly from the horizontal of the control arm.

In most cases, this will be somewhere between 75 and 90 degrees, and 90 degrees can be used for the angle.

This measurement helps determine the “force angle” and resultant spring force applied to the control arm.

Shock Ride Height from Extended Height

Determine the total travel of the shock absorber using the shock manufacturer’s catalog; or by pulling the shock shaft to the full extension position and measuring the length of the chrome shaft. Generally, the shock should be compressed 40% – 50% of its travel at ride height.

For example, if a shock has 4.8″ of travel and you want the sprung weight of the vehicle to compress the shock 45% at ride height, you would enter 2.16″ (45% x 4.8″) of shock compression to ride height.

Sprung Weight = Corner Weight – Unsprung Weight.

It is the weight of the vehicle that is supported by the spring and is the only weight used when calculating spring rates.

Motion Ratio = (Dimension A / Dimension B) * sin(Spring Angle)

The motion ratio is the mechanical advantage (lever ratio) that the wheel has over the spring in compressing it.

Static Load = Sprung Weight / Motion Ratio.

The static load is the load that the spring sees from the sprung weight acting through the motion ratio.

Spring Rate = Static Load / Shock Ride Height.

You should always find the closest spring rate available for your application. When in doubt, choose a lower spring rate.

It is easier to achieve handling and performance with a lower spring rate and a “stiff” stabilizer bar or shock.

Effective Wheel Rate = Spring Rate * (Motion Ratio)2

Wheel Rate is the effective spring rate at the wheel, due to the leverage advantage the wheel has with respect to the spring on the control arm.