Finding the correct length for aftermarket shocks takes more than an axle at full droop and a tape measure. Many factors need to be considered, so we headed down to Bilstein in Poway, California, to see what tips these shock experts have for measuring for shocks.
The first order of business is to know your extended length. Extended length is the overall length at the shock’s longest point, measured from the mounting center to mounting center, whether your application uses an eye or a bar pin. The only real exception is on stem-mount shocks, which should be measured from the stem base to the center of the mounting hole.
When choosing an extended length, you must be careful not to overextend the brake lines, exceed safe driveshaft angles, or on coil-sprung vehicles, allow the coil spring to unseat. It is also essential to note the motion ratio of the shock. The motion ratio is the amount of shock shaft travel for a given amount of wheel travel. For example, if a shock is mounted vertically on a solid axle, the motion ratio is 1:1. If a shock is mounted at an angle, the motion ratio will change as the shock shaft goes through its range of motion. This has an impact on extended length because a shock mounted at an angle, but measured only in the vertical plane (the lower shock mount to the bumpstop mount, for example), can result in a shock that is too short and suspension droop that is not optimized for the suspension travel available.
On some applications where lots of unsprung mass and high speeds are present together, it is important to pair shocks with limiting straps. Most shocks are not designed for the high tensile loads that may be experienced at full droop when heavy axles and high-velocity shock cycling are mixed.
Next, you’ll want to measure for collapsed length, or the length of the shock at its shortest point. Things to look out for when measuring for collapsed length are chassis interference issues, overwrapping of the leaf-spring pack, and coil bind on a coil suspension. It is also important to know that as a general rule, installing longer shocks will almost always require a corresponding change to the height of the bumpstop. The reason for this is that the longer a shock’s travel is, the longer the collapsed length is. This is why suspension manufacturers often include bumpstop extensions with lift kits. When measuring for collapsed length, it is safest to include the entire height of the bumpstop, as the typical OE-style bumpstops will compress at least 1⁄3-½ of their height.
In addition to understanding collapsed length, measuring for ride height is equally as important. Knowing the length of the shock body and the amount of shock shaft compression remaining will ensure you don’t over compress the shock and bottom out on the shock body, causing permanent damage. Certain shocks, such as the Bilstein 5160, move the dividing piston to the reservoir, thereby shortening the body and increasing the amount of uptravel the shock shaft has. These shocks are a great upgrade where suspension uptravel is at a premium.
Finally, if you are in doubt about how much your shock shaft is cycling, run travel indicators, such as O-rings or zip ties on the shaft to reveal how far the shock shaft is traveling during compression. This is a great visual illustration as to what is happening under your rig in the real world.