Resource Information for Auto Repair Projects:
Though gas shocks and struts have been installed on many cars and trucks since the 1970s and are standard equipment on most new vehicles today, the idea of using pressurized gas to prevent foaming or aeration of the hydraulic fluid inside a shock absorber actually dates back to 1953. A French physicist by the name of DeCarbon invented the worldís first pressurized monotube shock absorber.
The reason for using gas pressurization is to fight cavitation. When a vehicle is traveling down the road and hits a bump or a pothole, the compression and extension of the suspension telescopes the shock or strut. This forces the piston inside to push its way through a column of oil (hydraulic fluid), creating resistance as the oil is forced through small orifices in the piston. The motion of the piston also creates an area of high pressure ahead of it and an area of low pressure behind it.
Itís the low pressure area directly behind the piston that causes trouble because it allows tiny air bubbles to form in the oil (cavitation). The faster the piston pumps up and down, the more rapidly cavitation aerates the oil on both sides of the piston and churns it into foam. The result is a "foam zone" around the piston that offers little resistance and causes the shockís dampening ability to fade.
Thatís where gas charging comes in. Pressurizing the oil inside the shock with nitrogen gas prevents the formation of bubbles in the low pressure zone behind the piston.
How much pressure does it take? In a twin-tube shock, most manufacturers use about 100 to 150 psi, though some go up to as much as 250 psi depending on the application. The gas charge is located in the top of the outside chamber. With monotube shocks and struts, a floating piston separates the gas from the oil. Because of the larger surface area, a much higher gas charge is normally used: typically 360 to 400 psi. As a result, gas pressurized shocks and struts have more consistent ride control characteristics than nonpressurized shocks and wonít fade on rough roads or under hard use. Gas pressure also makes the shock more responsive for a firmer, more stable ride.
IMPORTANCE OF GOOD RIDE CONTROL
Maintaining good ride control across a broad range of driving conditions is important because not only does it improve ride quality, but also handling stability and driving safety. By dampening the movements of the suspension, the tires remain in contact with the road surface. This prevents the tires from bouncing and skipping with every bump and dip in the road. Tires that do not stay in contact with the road canít provide good traction, steering stability or braking friction.
The issue of vehicle stability is especially critical with SUVs because of their high center of gravity. When making sudden steering maneuvers or turning sharply, the body experiences a lot more roll than a typical passenger car. If the shocks canít keep the body under control, it may increase the potential for a rollover as the body and suspension react to the sudden shift in g-forces.
One aftermarket shock manufacturer talks about the importance of a vehicleís "safety triangle," which refers to the relationship between the tires, brakes and shocks or struts. These three components work together to maintain traction, vehicle stability and control. If any of these components are worn or not working normally, it reduces the margin of safety designed into the vehicle and may create a potential hazard.
The best shocks in the world wonít be much help if the tires are bald or the brake pedal is all the way to the floor. Likewise, a brand new set of set of all-season tires and freshly relined brakes may not be able to provide the shortest possible stopping distance if the shocks are weak. Even the most sophisticated ABS system or traction control system cannot function at peak efficiency if the tires are hopping and skipping over every bump in the road. So itís absolutely essential to keep the tires in firm contact with the road under all driving conditions ó and that requires good shocks.
Unfortunately, worn shocks are not as obvious to most motorists as worn tires or brakes. Worn tires they can see. Worn brakes they can often hear or feel through the brake pedal. But worn shocks are not as conspicuous.
Many motorists donít really understand the role of the shock absorbers or struts in a vehicleís suspension. They know it has something to do with bumps, but donít know itís actually the springs that take the bumps. The shocks are there to dampen spring rebound and suspension motion. Therefore, few motorists associate shocks with steering and handling stability, body control, traction or braking.
A motorist who is experiencing a shudder or shimmy after hitting a bump may think something is wrong with the steering rather than suspect worn shocks. If their vehicle handles poorly, leans like a wounded ship when cornering, or sways or veers in strong crosswinds, they may not realize the shocks play a major role in countering such things.
Because shocks wear slowly over time, the loss of ride control is usually gradual and occurs over many miles. Consequently, people get used to how their vehicle rides and handles, and often donít realize how loose and sloppy their suspension has become compared to when it was new. Thatís why technicians need to pay more attention not only to visually inspecting shocks and struts for leaks and damage, but to also check how the shocks perform any time a vehicle is road tested or driven (that includes before or after other repairs have been made, too).
When the time comes to replace a set of shocks on a vehicle, therefore, itís important to make sure the replacement shocks equal or exceed the level of performance engineered into the OEM shocks. Many aftermarket shocks do, but some may not.
According to one shock supplier, technicians have to be more careful these days about what kind of shocks they install. Many OEM shocks are fairly sophisticated and tuned to a specific vehicle application. If a cheap set of replacement shocks are installed, it may actually downgrade a vehicleís ride control stability. The replacement shocks may feel better than the old worn ones, but if theyíre not up to the same level of performance that the original shocks had when they were new, the customer may not get the best value for his money.
At the very least, you should recommend an OEM-equivalent type of replacement gas shock or strut. But the best approach is to evaluate your customerís driving needs and then recommend a specialty product that provides even better performance.
Most people want "like-new" or even "better-than-new" performance from a set of replacement shocks, so itís important to make sure the product you recommend delivers the level of ride control that meets your customerís expectations.
Donít make the mistake of thinking all gas shocks and struts are the same. Theyíre not. Besides the basic differences in construction (twin-tube vs. monotube) and gas pressure (low pressure and high pressure), youíll find significant differences in valving strategies.
The trend today is to design shocks that provide firm, crisp handling and a boulevard smooth ride. A performance shock with relatively stiff valving can provide the kind of firm control thatís needed to keep a vehicleís body flat on a winding road. But stiff valving creates ride harshness and increases feedback through the suspension. Conversely, a shock with relatively soft valving smooths out the bumps, roughness and vibration, but typically lacks the firmness required for quick, responsive handling.
The piston assembly inside a shock or strut has a spring loaded metering valve or a deflection disk that controls how much oil bypasses the piston as it pumps up and down. In the bottom of a twin-tube shock (but not monotube shocks) is a second metering valve that allows some of the oil to circulate back and forth between the pressure cavity and the outer reservoir. By changing the pressure at which these valves open and close, a shockís dampening characteristics can be tuned as needed. The higher the opening pressure, the firmer the shock. The lower the opening pressure, the softer the shock.
Gas pressure really isnít a factor here because the valves are reacting to hydraulic pressure created by the movement of the piston. Even so, the use of gas pressurization means the valving can be made somewhat firmer than in a nonpressurized shock because cavitation isnít a factor. This improves ride control without increasing harshness.
Shock manufacturers also use "staged" valving to modify the resistance of the piston assembly as velocity and/or travel increase. For many years, one leading shock manufacturer has put several small grooves into the side of the piston pressure tube to create a "comfort zone." The grooves create a leak path for the oil to bypass the piston when the shock is operating in its normal midrange of travel. But when the piston travels more than a few inches in either direction, it goes past the grooves and encounters more resistance.
There are also manually adjustable shocks and struts that allow the driver to fine-tune the dampening rate to suit the occasion. Manually adjustable shocks have been around for many years and are popular with people who drive a dual-purpose vehicle (vehicles that are grocery getters during weekdays and raced or driven off-road on the weekends). But these kinds of products require a physical adjustment when a change in valving is desired ó which is not a very practical approach for everyday driving.
Electronic shocks that provide remote adjustment or that can respond to changing road conditions have also been around for a number of years. The main advantage of the electronic units is that their damping characteristics can be changed in milliseconds to react to changing road conditions: firm when extra control is needed, and soft for a boulevard-smooth ride when less control is needed. The damping characteristics can also be modified according to steering inputs, braking and vehicle dynamics (yaw and pitch).
Electronically adjustable shocks and struts use one of two operating principles. A small electric actuator motor mounted either atop the unit or inside the shock/strut itself rotates a control rod or selector valve that opens or closes metering orifices in the piston valve. Others use a solenoid to change the relative stiffness of the shock as it travels through compression and rebound.
Many people thought electronic suspensions would make conventional double-acting hydraulic shock absorbers and struts obsolete, but the high cost of high tech suspensions has limited their use to luxury sedans and expensive sports cars. In fact, on some cars the vehicle makers have gone back to the more reliable and less costly conventional gas shocks and struts.
Another drawback with electronic shocks and struts is that the control electronics is usually integrated into a body control computer or ride control system. Consequently, there may be little or no flexibility to upgrade the system if replacement is needed. And in some cases, replacement units may not even be available from aftermarket suppliers (though that situation has improved in recent years).
Aftermarket "add-on" electronic ride control systems have also been introduced by various manufacturers, but only appeal to a small segment of the market because of the high cost of such systems.
In recent years, several aftermarket shock suppliers have introduced new products that have "adaptive" valving so shocks can adjust to changing road conditions. The result is improved ride control and better handling performance without increasing ride harshness.
One manufacturer uses a "velocity sensitive technology" (VST) that provides relatively soft valving for normal driving. When the tire hits a bump, the VST valve automatically closes and increases the stiffness of the shock or strut. The same thing happens when the tire rebounds from the bump.
Reacting to the sudden change in speed or velocity of the piston, the VST valve again automatically closes to instantly provide greater damping resistance and control spring recoil. The result is reduced tire bounce, better handling, traction and stability.
Another supplier has developed what it calls an "inertia active system" that can instantly switch from soft to firm as conditions dictate. The inertia valve continuously meters the rebound circuit and adjusts according to what is needed. When high resistance is needed, as when cornering or braking hard on a smooth surface, the inertia valve closes producing high dampening force. This keeps the vehicleís chassis stable and level, reducing body roll and nose dive for a safer, more controlled ride.
When low resistance is needed, as when a tire hits a bump or pothole, the inertia valve opens to reduce the dampening force. This allows the suspension to absorb jolts without transmitting them to the chassis for a smoother ride.
A similar type of adaptive valving is also used by another manufacturer in one of their gas shock/strut product lines. Their impact sensing "Reflex" valve allows oil to momentarily bypass the piston when a hard bump is encountered. The valve can react in 15 milliseconds to change the shockís damping characteristics. This occurs anytime an impact or acceleration of 1.5 gís is experienced. As soon as the impact is overcome, the valve closes and returns to the normal setting. This ability to instantly switch from firm to soft provides a noticeably smoother ride as well as better handling.
The manufacturer says their tests show a decrease in vehicle body roll of 12 percent during a J-turn maneuver, and 18 percent less pitch (nose dive) during hard braking compared to brand new original equipment shocks. Vehicles with a high center of gravity, including light trucks, minivans and sport utility vehicles (SUVs), typically need all the ride control help they can get when it comes to tight turns, evasive maneuvers and emergency braking. So for these vehicles, the new adaptive valving offered by various manufacturers offers a significant improvement over the OEM shocks in most cases, and adds an extra margin of driving safety.
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