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A customer contacted our support desk this week. Having browsed our website for replacement shock absorbers for his vehicle, he found that we list two different compatible shock absorbers. The first is a regular KYB Excel-G Oil-filled shock, while the second is a gas-filled KYB Gas-Adjust. He wanted to know what the difference is, and which one is ‘better’ for his vehicle.
Before answering, let us quickly familiarize with the basic elements of a vehicle’s suspension system.
What is Suspension?
The suspension can be defined as the link between the road and the car body. The purpose of the suspension is to minimise – as far as possible in a controlled fashion – the vibrations from the wheels and the car body caused by uneven road surfaces or changes of direction.
The job of the suspension system is exactly that, namely to prevent these vibrations from being transmitted to the car body in order to reduce rocking, pitching and diving, avoid swaying, and ensure optimum road contact and traction with minimum slip.
The suspension system comprises a range of components, including strut support bearings, springs, shock absorbers, connecting rods, stabilisers, axle supports/wheel supports, wheel bearings, suspension arms (control arms and pull rods), wheel brakes, rims, tyres, final drives and steering.
In this article we will talk about the springs and shock absorbers, and how they work in tandem.
What the springs do?
Imagine jumping from a decent height down to the ground. You would probably feel nervous and brace yourself so a significant impact as you hit the rigid ground. Imagine jumping from the same height onto a giant trampoline. The impact would be far less.
Similarly, the spring in the suspension system cushions the effects of road unevenness and impacts from the road, turning these into (up and down) vibrations.
You may have heard terms “sprung mass” and “unsprung mass”. Sprung mass is mass that is suspended by a car’s suspension. Its everything that would move down and up if a giant hand came out of the sky and pressed the car’s roof down. That includes the body, chassis, engine, gearbox, luggage, passengers and everything else around them. Even your pet Boerbul on the way to the dog park.
Unsprung mass is the opposite. Unsprung mass is everything which does not enjoy the benefit of suspension beneath it. This category includes the suspension itself, the wheels and wheel hubs, the tyres, elements of the braking system such as the brake discs, calipers, pads and brake lines. The axles, suspension arms and linkages, depending on their design, could be unsprung or semi-sprung, -i.e. sprung at one end.
The spring forms an important link between the individual suspension components, connecting the sprung and unsprung masses in the vehicle. In terms of a car’s suspension comfort, the basic rule is that the smaller the ratio of unsprung to sprung mass, the greater the comfort. In simple terms, you want more sprung mass and less unsprung mass. (Just another reason to take the Boerbul road tripping with you).
We have all noticed that bigger and heavier cars are more comfortable to drive. Having a ton or two of sprung mass does wonders in smoothing bumps out. The springs must work harder due to all the weight above them, and thus they absorb more of the impact before it reaches the cabin. If you drive on a road with bumps, a heavier car will suffer far less of a kick-back than a lighter car as it rides over the same imperfections. No matter how well tuned the suspension is, a one-ton car will never feel as smooth and relaxing as a two-ton one.
On the flip side, a lightweight car is more efficient, more responsive, feels more alive through corners and will stop faster, among several other advantages. Let’s bear this mind and move on.
What the shock absorber does?
Let us continue with our trampoline example. You land on the trampoline and the springs cushion your fall. They extend and contract, and you bob up and down several times before coming to a rest. Having only a spring on your vehicle would have a similar effect. Your car would go over a bump and vibrate up and down several times like a bouncing ball, until all the kinetic energy has dissipated.
The job of the shock absorbers is to reduce and slow down these vibrations from the springs, which is why the technical, and most correct term for them, are vibration dampers. Vibration dampers convert the kinetic energy into thermal energy through fluid friction.
A shock absorber is basically an oil pump. A piston is attached to the end of a piston rod and works against hydraulic fluid in the pressure tube. As the suspension travels up and down, the hydraulic fluid is forced through tiny holes known as ‘orifices’ inside the piston. As the orifices only allow a small amount of fluid through the piston, the piston is slowed which in turn slows down spring and suspension movement.
The friction in turn generates heat. The shock absorbers can heat up to between 100 and 120oC in the process.
The Interaction between the spring and the damper
The shock absorber and the spring work in tandem. When you drive over a bump, for example, the obstacle forces the wheel up into the wheel housing. In the process, the spring is compressed. The shock absorber is now in its compression stage. Once the spring has levelled out the obstacle, the shock absorber must slow down the movement of the spring as it releases its tension with great force. The shock absorber is now in its extension or rebound stage.
Now, let us look at the difference between a hydraulic shock and a gas shock.
Hydraulic Shock Absorbers
This type of shock absorber comprises a working cylinder (tube), a compression chamber and a piston rod that directs the working fluid (hydraulic oil).
Oil shock absorbers are only two-pipe, their working fluid is only hydraulic oil and can only work in one direction (only under compression).
A disadvantage of this type of shock absorber is the presence of an air mixture in the compression chamber. This is called “Aeration” here air leaked into the system, and become mixed with or dissolved in the hydraulic fluid. In shock absorbers, the presence of up to 10% air in the hydraulic fluid is quite normal. The subsequent problem is known as “Cavitation”. Under load, the air and oil molecules separate resulting in a noticeable drop in the damping force. On longer journeys on country roads and motorways, a loss of up to 35% damping force can be measured. In practical terms, this translates into poorer road holding, and subsequently worse cornering and braking; only after a break does the damper regain its full force and the vehicle its original driving performance.
Cavitation also occurs when driving on poor road surfaces. Due to the poor heat dissipation of oil shock absorbers, the continued imperfections of the road surface cause the oil in the shock absorbers begins to boil and bubbles to form. The bubbles very quickly through the piston valve which significantly degrades the performance and effectiveness of the shock absorbers. Additionally, cavitation alters the viscosity of the hydraulic oil and makes it flow more easily. As we said above, we prefer the oil to move slower through the orifices, as that is what provides maximal damping.
Despite these shortcomings, hydraulic shock absorbers also have several positive characteristics that are worth noting.
A huge advantage of this type of shock absorbers is their affordable price and the huge selection of brands and models that can be easily found at most good motor spares shops.
In addition, oil shock absorbers remain “soft” and comfortable and provide a very comfortable ride, and if you drive quietly, without heavy loads and high speeds, or if you drive more often on city streets and short distances, oil shock absorbers are very effective.
Gas Pressure Shock Absorbers
Unlike oil shock absorbers, the gas chamber is not filled with air, but with gaseous nitrogen, which is pumped under high pressure up to 28 ATM. Gas shock absorber contains both oil and gas.
The two working substances are located in one chamber but are separated from each other by a special membrane. Nitrogen gas is used to compress the oil, preventing cavitation and bubble formation.
Since nitrogen is pumped under high pressure, this causes the piston to always be in a compressed state, so when overcoming bumps on the road, the stability of the shock absorber changes nonlinearly, which ensures good and stable handling of the car.
The use of nitrogen ensures that the shock absorber oil is permanently under pressure, thereby preventing cavitation, even under load. Gas pressure shock absorbers therefore do not suffer from the loss of damping force, even under load.
Consequently, the driving performance of a car fitted with gas pressure shock absorbers can be precisely defined and remains reliably stable, whatever the situation; this constitutes a substantial bonus in terms of safety.
Because of their more complex design, gas shock absorbers are more expensive than their (entirely) oil counterparts. they are more rigid than hydraulic ones, which affects both driving comfort and other suspension elements that wear out faster and require more frequent replacement.
Its boils down to…. your driving needs and “restore” or “more?”
Before buying a shock absorber, you should ask yourself, how do I want my vehicle to perform and how will I use my vehicle? Choosing one shock over the other is a matter of choosing performance. Also ask yourself if you want to “restore” your vehicle’s normal capabilities, or do you want something “more”.
The choice of shock absorbers for your car is dependent only on you, your driving skills, the conditions in which you most often ride, and on whether you prefer a softer or stiffer suspension.
As we mentioned earlier regarding the sprung and unsprung masses, there is always a trade-off balance between comfort and handling. The same is true with shock absorbers. Oil shocks provide better comfort while gas shocks offer better road holding, cornering and braking.
There are no good or bad shock absorbers, there are only shock absorbers that can be useful for your driving style. If you have a choice of two compatible shock absorbers your vehicle, one oil and one gas, evaluate which is better suited to your needs.
If your car originally came with oil shock absorbers and you were happy with the ride quality and handling, then when the time comes to replace your shock you may as well fit a new pair of oil shock absorbers, which will ‘restore’ the car’s ride to it’s original characteristics.
If you weren’t happy with the original handling, say there was excessive body roll or you would like to corner at higher speeds, i.e. you want “more’ from your car, than you may want to upgrade to gas shocks.
If you have a sports car model or you like a tougher ride or you frequently drive on bad roads, then you might consider buying gas shock absorbers, knowing that this type can provide you with better grip and more stability in long journey.
However, if you drive a regular vehicle in a measured fashion, oil shock absorbers are a great (and cheap) solution for your car.