A factor in choosing the right aftermarket shock absorber for your vehicle is understanding the measurements of the shock absorber. There are three basic measurements to a shock absorber, which will influence the purchasing of an aftermarket shock. Shock absorbers come in a variety of styles and types, depending on factors such as daily driving habits, vehicle type and vehicle weight, to name a few. Measuring for new shocks should take no longer than half an hour, even if you have never performed this type of research before.
Lift the vehicle in the front or rear, based upon which shocks you wish to replace. Place a two-ton or greater capacity jack underneath the lower control arm, or axle housing, depending on which is available. Manipulate the jack so that it lifts the wheel off of the ground. Use a wheel lug wrench to remove the wheel completely from the vehicle.
Measure the compressed length of the shock absorber, which is the length of the shock absorber with the weight of the vehicle upon it. By lifting the vehicle by the lower control arm or axle housing, you are compressing the shock to the same length it would be at if the wheel were on the ground. Measure from the center of the top shock mounting bolt to the center of the bottom mounting bolt. Write the measurement down as "compressed length."
Reinstall the wheel onto the vehicle. Tighten the lug nuts so that the wheel is pushed flush against the hub of the vehicle. Lower the vehicle. Use a wheel lug wrench to tighten the wheel nuts to the manufacturer's specifications for foot-pounds of torque.
Place the tape measure against the side of the top cylinder on the shock, or the fat part of the shock if the shock is inverted. Measure the distance across the cylinder---this will become your diameter.
Write down the measurement as "diameter."
Perform a mathematical equation to find the circumference of the cylinder. The circumference of a cylinder is a measurement of the cylinder as a whole circle. The formula for circumference is "Diameter times Pi (3.14)." For example, if the diameter is 3.5 inches, the circumference will be 3.5 x 3.14 (10.99). Round up to the nearest ½-inch, which would make your circumference 11 inches. Write your answer down as "circumference."
Use a two-ton or greater capacity jack to lift the vehicle in the front or rear, depending on which shocks you wish to replace. Lift the vehicle by the body lift points, which are located underneath the vehicle (generally a few inches underneath the body) and the same distance from the front or rear of the wheel well. The purpose of this process is to lift the body of the vehicle off of the ground while allowing the shock absorber to extend to its maximum length on the vehicle. Leave the wheel assembly on for this section because the weight of the wheel and tire assembly will give the true extended length.
Lie underneath the vehicle, just on the inside of the tire you are going to perform your measurement on. Measure the distance between the center of the top mounting bolt and the bottom mounting bolt of the shock.
Write your measurement down as "extended length" of the shock absorber. This will allow you to gain knowledge of how far your shock absorber travels between compressed length and extended length. The difference between the compressed and extended length is the travel distance. You can figure this out by subtracting the compressed length from the extended length.
