Shock and Strut Inspection
Shocks and struts are simple devices that dampen the movement of springs, suspension and vehicle pitch. For the most part, suspension works best with smooth weight transfers and the suspension is not compressed on the bump stops. As a vehicle brakes, accelerates, or corners, weight is transferred and the attitude changes. This can be helpful if done in an effective way. Weight transfer can also have momentum. If a driver makes a sudden lane change and a panic stop to the right, the weight will transfer to the left-front wheel. If that corner has a weak shock, the suspension could rapidly compress resulting in the vehicle going from an understeer condition to a “snap” oversteer condition which can roll the vehicle. The vehicle is better controlled with better shocks and struts. Good struts and shocks become even more important with ABS. Vehicles with ABS can turn while braking making vehicle pitch even more extreme.
Why Shocks Wear
Shocks wear because they work hard. On a smooth road, a shock strokes 1,750 times per mile. As a result, they wear out. Wear can occur between the piston and the tube allowing oil to pass through. Shocks are filled with oil that can have different weights. When the oil is col it flows like 15-weight, but once it heats up it flows like a 20-weight. The oil can become contaminated with debris, so it is important to replace the dust boots when replacing a shock. If they bottom out, the piston hits the bottom of the valve, and can cause damage to the piston and valving. Mileage is a poor indicator of poor shocks. Shocks can last between 40,000 and 70,000 miles. Constant inspection is the only way to determine the condition of the shock or strut.
There are three types of shock absorbers: mono-tube high-pressure gas, twin-tube low-pressure gas and twin-tube hydraulic (non-gas). Each design has certain characteristics that improve the performance of the vehicle. Gas is added to shocks and struts to keep the oil from foaming or aerating inside of the tube. If the gas escapes, the shocks can overheat.
Shocks use valves to control the oil flow to control the force necessary to move the shock. The valving in replacement shocks is usually vehicle specific. This is important to remember because a stiffer replacement could cause a ride complaint from the customer. Conventional shocks are a compromise between comfort and control. The newest shocks on the market can sense road conditions and determine the between road bumps, and vehicle pitch.
Struts are a suspension part that replaces the upper control arm and ball joint. They act like shock absorbers and act as a structural support for the suspension system. Because struts are a part of the suspension, they wear differently than shocks.
All struts and shocks should be replaced in sets of four to avoid balance problems when braking or accelerating. Struts should always be equipped with bump stops to prevent damage if the strut bottoms out. Springs should also be replaced to insure that the new shocks and struts last.
Adjustable shocks are great for vehicles with altered spring and ride heights. Most shocks are designed to fit standard height cars and can work with lowered cars as long as they don’t bottom out internally and become damaged. Some shocks are not position sensitive and will work properly anywhere in their stroke range. Different vehicle suspension designs have different stroke travels, but a good rule of thumb is that most vehicles can be lowered acceptably about 1-1/2 inches. Beyond that, the possibility of bottoming increases rapidly. When lowering a vehicle, be sure to use bump stops to protect the shocks and struts from bottoming out. Also remember that severely lowered vehicles typically have a negative effect on suspension geometry, ride quality and handling, as well as tire and suspension part wear.