Various Makes & Functions of Shock

The following are extracts from the articles published in the American and Australian 4x4 magazines.

Your 4WD sways into corners, plowing and rocking, then stumbling with oversteer, Those OEM shocks barely manageable when new, now seem lifeless, leaving your 4WD oscillating and rebounding violently. Speed bumps jar your jaw so hard that you need to replace cracked fillings.

The traditional OEM (Original Equipment) shock for most 4WDs, however, has been a twin-tube hydraulic type. Functional and less expensive to produce, hydraulic shocks can serve a wide range of uses. The drawback is the quality and tuning level of most OEM standard shocks which have a shorter life span than upgrade shocks, especially when put to hard work. The primary purpose of shock absorber is to tame spring oscillations, the kinetic force that grows to extremes on rough washboard roads at speed. In the taming of this kinetic energy, the dampening action of shock absorber generates tremendous heat. Heat and rapid piston movement create foaming. Low-cost OEM standard shocks seldom resist the fast cycling, heat build-up and fluid foaming that lead to shock absorber "fade".

Fade does occur when a hydraulic shock absorber extends and collapses at a high rate of speed. Engineers note that as shock absorber moves beyond 150 cycles per minute, hydraulic oil begins to mix with air within the shock absorber. This aerates the oil. Foamy aerated oil, with a much lower viscosity, moves through the shock absorber’s valving system at a higher rate of speed Under these conditions, the shocks absorber’s load rating deteriorates, and at the extreme, the shocks simply fail to dampen the spring oscillations. Common symptoms of fade are a harsh ride, poor stability on corners and wheelhop.

Spring rates aim at a reasonable ride, smooth suspension and proper load carrying capacity. The shock absorbers’ job is to sense chassis loads and spring travel, then regulate fluid damping to control jounce and rebound.

Twin-tube hydraulic shock absorbers, whether gas cell or not, operate similarly. Some shock valving is as basic as a rod moving a sealed piston, with a series of sized holes to control fluid movement. More sophisticated shocks use precise, spring-balanced metering systems to adjust pressure and fluid flow.

When wheels hit bumps and rise, the shock absorber allows fluid to move in a precise volume through the metering orifice(s). Engineers can create specific damping effects for each rate of spring travel. Valves also allow fluid to move rapidly upon suspension impact in situations like the bone-jarring holes that emerge from nowhere.

These same valves close during rebound, forcing large amounts of fluid to squeeze through metering orifices. This establishes a greater damping action on rebound than bounce.

Valving ratios and fluid flow is a science. We depend on the research, shock bench studies and skid pad tests of manufacturers to create the right damping rates for each and individual 4WDs. As a rule, shocks with a 40 percent bounce to 60 percent rebound ratio work well in most multipurpose light 4WD environments.

Before getting hung up on ratios, however, keep in mind that spring rate varies and other factors may alter your 4WD’s needs. Shock absorbers are not all the same, and there’s nothing "universal" when matching shocks to your 4WD’s chassis but getting a complete set of shocks and springs produced by a same reputable suspension company with matching rates for a specific usage is the best bet.

Before they design a shock absorber, engineers consider the spring or torsion bar design, wheel travel, intended terrain, vehicle weight distribution, load capacity, sprung and unsprung mass, plus the velocity that shock fluid must travel in inches per second. Even then, testing in the field will serve as the real world stress test.

Oversize wheels and tires, stiffer lift kit springs and other chassis requirements increase the load on shock absorbers. Factory engineers and many aftermarket multiple shock kit builders spend hours working with CAD drawings and skid pad tests to confirm optimal shock absorber design and mounting angles. Although many show trucks give the impression that more is better, the real incentive for multiple gangs of shock focuses on proper valving rates and useful mounting angles.

An offshoot of the standard twintube hydraulic shock absorber is the twin-tube low-pressure gas-charged shock. These shocks counter the air pocket weaknesses of standard hydraulic shocks.

Instead of handling fluid displacement with an air pocket, low-pressure gas shock designs replace that air with an inertgas, typically nitrogen. Charge pressures range from 60-200 psi. The main goal with gas-charged shocks is to eliminate aeration of fluid, the common cause of fade.

The down side with gas-charged shocks is "pump up". Like any other shock absorber design, heat builds up with fast cycling, and the gas pressure increases. Foaming, however, is far less likely with these shocks than with standard hydraulic types. Undesirable stiffness and erratic damping can occur when heat raises gas pressures. These shocks are more suitable for 4WDs with leaf springs suspension as due to the friction in between the many leaves they do not work the shocks as hard than the coil sprung 4WDs.

An example of low-pressure gas-charged shocks is : Tough Dog Nitrogen Gas Shock.

Other improvements over standard hydraulic shocks are the foam cell and gas cell designs. These shocks counter the troublesome air pocket built into standard shocks.

Is an easy way to prevent air foaming. The nylon foam cell material which has tiny closed cells that contain bubbles of gas cannot mix with the oil. As a result, air that might otherwise cause foaming is no longer a threat.

Because the air pocket is now isolated from the oil, odd mounting angles are not a problem. These shocks work well in all positions, and air-free, non-foaming fluid action can provide precise tuning.

Foam cell shock absorbers offer better valve tuning for a wider range of operating conditions. Fade is far less likely to occur. Unlike gas-charged shocks, pump up is non-existent. Although fluid viscosity does change under severe conditions, these shocks, stand a better chance of delivering long and consistent performance. These shocks are especially good for 4WDs with coil springs suspension which demand very heavy duty shocks.

An example of foam cell shock is : Tough Dog Heavy Duty Foam Cell Shock.

Often called "nitrogen cell" designs, offer another improvement over the standard hydraulic shock. Compressible gas-filled plastic cell occupy the space normally allotted to air pockets. The cell prevents air from mixing with oil, which reduces the risk of fluid aeration (foaming) that causes shock fade.

Unlike foam cell material, the non-permeable gas bag must stay sealed and intact. When buying cellular shocks, always consider quality. We have cut out a versions of these shocks absorbers and found the plastic bag split open and completely out of place.

Once on your truck, the only other test for a plastic cell shock is whether they still perform effectively. When shock fade occurs over washboard roads, chances are good that the nitrogen or gas-filled plastic bag has ruptured.

Examples of plastic cellular gas shocks are 1) Rancho Shock 2) Explorer Pro-Comp Cellular Gas Shock.

Shock is not only contributing to the comfort level but also the handling, control and safety of your 4WD and NOT ALL SHOCKS are made equal in quality !!

The thrill of the rattle, bump, knock and roll of a roller-coaster is loved by many but not when they happen on your 4WD. These are inherent in most 4WDs with cheap and inefficient suspension system especially the shock absorbers.

The shock absorber is very often ignored whilst we emphasized a lot on a vehicle external and internal looks and the engine performance and etc when we are looking to buy a new 4WD or when we are upgrading our 4WD.

Although often overlooked, the humble shock absorber plays a very important role in keeping the wheel on the ground for best possible road holding, to control the bounce, roll and dip of the vehicle body and at the same time to absorb whatever shocks (as the name says) generated from the surface the wheel is moving over.

The purpose of the springs fitted to a suspension system is to dissipate the sudden shock transmitted to the vehicle and finally to its passengers when a wheel hits a bump. If it is a small bump it will create a small shock but if it is a big bump it will create a spine thumping force that can throw you off your seat. But the after effect of fitting the spring is that the vehicle will tends to bounce and bounce and also sway and dip rather than one bounce for each bump when the vehicle moves over an undulating surface, it is just like a loose ball rolling down a staircase. This is when the shock absorbers or dampers as you may call them come into play to control its enthusiasm.

The switch to coil spring suspension on many 4WDs has greatly increased the importance of the shock absorber in the overall scale of things. The reason being that a leaf spring has a certain amount of self damping effect thanks to the friction created between the many leaves of the leaf spring stack. As a leaf spring flexes, the many leaves stacked together rub against each other and restrict the number of oscillation of the entire spring. Thus, it imposes less burden on the shock to control the oscillation. Whereas, the coil spring does not have the same friction. So, it would continue to bounce, sway and oscillate up and down long after the vehicle has gone over the bump that causes its first bounce. The newer 4WD now with air bag suspension has even demanded a new breed of super heavy duty shock to control the bounce and sway. A typical air bag suspension oscillates many times more than a conventional leaf spring suspension. So unless you don’t mind your vehicle body bouncing up and down swaying from side to side or dipping to the front or to the rear whenever you do a braking or acceleration you are going to need really good and efficient shock damping.

If you don’t bother to fit efficient shocks you are going to suffer from a whole lot of problems. Every time the vehicle body bounces, sway or dip it affects the downward pressure on the tyre (sometimes it may even lift the wheel off the ground), the tyre tread efficiency and gripping with the road. That mean you get inferior grip and poor road holding that will affect the safety of your life and excessive flexing of the tyre will cause premature wear and tear. And because of the ever bouncing of the tyre off the ground while it is still turning you will also get irregular tyre wear pattern, such as scalloping of the edge of the tyre tread.

Not only will road holding suffers, your vehicle braking efficiency will be reduced very much as the wheels being braked will lift off the ground and hop instead of in contact with the ground for good braking. That is why vehicles with inferior shock absorbers take a longer distance to stop in an emergency. Apart from the above, the wheel’s bearings, steering components and bushes of all the linkage will also suffer premature wear and tear not to mention the body fatigue and discomfort of the person who ride in the vehicle.

With about 15000 valve port variations for a shock absorber it is important that you get the right ones to suit your 4WD and your specific usage. If you stick to well know, popular and reliable brands and explain to a suspension specialist just what you want to do with your 4WD and the load you often carry, you will stand a good chance of getting the right valves for your 4WD. If you shop for cheap units you will probably end up with a set of shock absorbers that look the same on the outside but inside is made from inferior components, with simple and incorrect valving which usually offer a much shorter working life. In other words, you get what you pay for ! Our advice is to go for the best you can afford but again not necessary the most expensive is the best. So, consider value for money and check out whether they carry the manufacturer’s warranty, for how long and the reliability of getting replacement if anything goes wrong later.

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