Useable Wheel Travel is the total vertical distance that the suspension allows your wheels to move without binding. If the lower coilover mount is welded to the axle, then this measurement is that same as the shock stroke.
Coilover Angle is the angle at which the coilover is mounted, measured in degrees from absolute vertical in any direction.
Lower Arm Length is the length of the lower A-arm, trailing arm, or 4-link bar measured from the frame pivot to the wheel center.
Shock Mount Distance is the measurement from the frame side pivot of the lower link to the lower coilover mounting point. If the lower coilover mount is welded to the axle, then this measurement is equal to the lower arm length.
Sitting Droop Ratio is the percentage of the shock shaft positioned inside the shock body at ride height. (For example, if you want 6" of shaft showing at ride height on a 14" coilover, that's 8" of shaft inside the shock: 8/14 = 57%)
Corner Sprung Weight is the total weight supported by the coilover. Sprung weight does not include axle or wheel weights.
Dual Rate Spring Calculation Guide
Performance off-road coilover shocks are designed to have two different springs installed on them such that the vehicle transitions from a softer combined spring rate to the stiffer lower spring at some point during the compression stroke. For 8" and smaller coilovers, there might only be space for a single spring while 16" and 18" coilovers may require a third "tender" spring to prevent the springs from going loose at full extension.
In a dual rate coilover setup, the upper primary spring is placed on top of the secondary spring which is either the same rate or heavier than the primary spring. During the initial stroke, the two springs will compress together to give you a lighter combined rate (see table below) for the initial up stroke until the slider hits the transition nuts (secondary nuts) at which pint the upper spring is taken out of the equation and all remaining up travel is transferred to the lower secondary spring.
For dual rate coilovers you will need to multiply the spring rate calculated above by two to get the required spring rates. While running the same spring top and bottom will still give you a transition, the secondary spring should be about 25% to 50% stiffer than the primary spring to make that transition more significant.
Effective Primary Spring Rate In A Dual Rate Configuration
Combined effective coilover spring rate listed in lbs./in.
Triple Rate / Tender Springs / Helper Springs
Tender springs, also known as helper springs, are used in applications were the primary springs don't allow the coilover to settle down low enough under the weight of the vehicle to reach the desired ride height. This happens most often with long travel coilovers in applications with a relatively low ride height. Tender springs are low rate (50 to 90 lbs./in.), short (3" to 5"), and often made with flat wire. They remain compressed under load until the other springs fully extend at which point the helper spring expands keeping a load on the springs to prevent them from going loose.
Video: Coilover Springs, Spring Rates, and Calculations