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Shock Absorber: What is it and How it Works?

Our car ALWAYS shake when we drive, it just depends how much it shakes. Shock absorber stops that! Image courtesy of Katowice / CC BY-SA 4.0.
Our car ALWAYS shake when we drive, it just depends how much it shakes.

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Image courtesy of Katowice / CC BY-SA 4.0

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Shock absorber makes our ride comfortable by dampening the car vibrations we get from uneven road.

But, here’s a fact.

Roads are never perfectly smooth. (I guess Malaysians would know it best…)

Bumps, road surface markings, potholes or even imperfection of the road would turn our car into an over-excited rabbit, jumping up and down, up and down, up and down…

You get the point.

Without shock absorbers to dampen out these vibrations, we would still feel the bounciness long after we drove past the road bumps.

Your kids would probably find it fun, but it’s definitely not comfortable.

What is a shock absorber?

Shock absorber is just an oil pump, really.

We call it the “shock absorber” or “shock” or “damper” because it absorbs and dampens the shock.

All you really need is a piston, some oil and voila, you have a shock absorber.

Mono-tube and twin-tube are different versions of shock absorber.
Mono-tube and twin-tube are different versions of shock absorber.

Okay, it’s not as simple as that, but you get the general idea.

It works hand in hand with our car suspension system to…

  • Softens vertical car movement (up and down). It takes the kinetic energy of the unwanted movement and converts them into heat energy which then gets dissipated into the surrounding.
  • Stabilizes side-to-side movement. Shock absorber dampens side movements too. It stabilizes vehicle by preventing excess body lean and body roll during cornering or sharp turns.
  • Maintain tyre contact with the road. Shock absorber pushes your tyre against the road surface to make sure that you tyre sticks to the road at all times. If your tyre is mid-air, you can’t steer or brake!
  • Slows down suspension wear. Our car suspensions are made of other components like springs, linkages. Having a shock absorber that softens the impact helps reduces the load on the other components.

How shock absorber works?

The story starts with a road bump.

When our car encounters an uneven surface, it bounces.

Based on Newton’s first law, a moving object will keep moving unless an external force act on it.

What he’s trying to say is that if nothing interferes, our car will just keep bouncing. Like this graph here.

A familiar graph to an engineering student! Car bounces are like sine waves. It oscillates back and forth between positive, zero and negative amplitude. Original image by Omegatron / CC BY-SA 3.0, edited by D S AUTO.
A familiar graph to an engineering student! Car bounces are like sine waves. It oscillates back and forth between positive, zero and negative amplitude.

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Original image by Omegatron / CC BY-SA 3.0, edited by D S AUTO.

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Ideally, we don’t want any vibration in our car at all. In another words, we don’t want the red line to go up and down, we only want it to be flat at zero.

This is where shock absorbers come in.

Every time the car bounces, the shock gets transferred through our tyres and onto our car suspension system.

As the car suspension bounces up and down, shock absorbers dampens out the vibrations bit by bit and we eventually don’t feel it anymore.

Like this.

Shock absorber takes away the kinetic energy, which evens out the amplitude. We call this damping. Image courtesy of Steelpillow / CC BY-SA 3.0.
Shock absorber takes away the kinetic energy, which evens out the amplitude. We call this damping.

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Image courtesy of Steelpillow / CC BY-SA 3.0.

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It achieves this by repeating the (i) compression and (ii) extension cycle.

Each compression-extension cycle removes some of the kinetic energy from the vibration.

Buckle up, because things are about to get technical!

Phase #1: Compression

Compression cycle of a shock absorber.
Compression cycle is when the piston presses down.

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Image originally by A7N8X under CC BY-SA 3.0, edited by D S AUTO.

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  • As the car suspension moves down, the energy of the spring gets transferred to the upper mount of the shock absorber.
  • The upper mount moves downward, pushing the piston down as well.
  • The piston have holes (a.k.a orifices) on the piston, which lets fluid seep through as the piston moves down the pressure tube.
  • But there is a catch! The orifices are very tiny and it only allow small amount of fluid to pass through. This resists the downward motion. It is at this point that the kinetic energy gets converted into heat energy, which reduces the car vibrations.
  • The oil in the shock absorber can heat up to 100 to 120 degree Celsius but the heat quickly gets dissipated into the surrounding.

Phase #2: Extension

Extension cycle of a shock absorber.
Extension cycle is when the piston retracts upward.

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Image originally by A7N8X under CC BY-SA 3.0, edited by D S AUTO.

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  • As the car rebounces (moving upward), the piston retracts into the starting position.
  • As it moves up, the same principle applies. Oil flows through the tiny orifices, which again resists the upward stroke.
  • Now, it is ready for the next compression cycle.

It’s worth pointing out that these two phases occur in very quick succession.

By repeating the cycles, the bounciness eventually becomes unnoticeable.

Like this.

To wrap it all up, this amazing video by jordi3736 sums up perfectly how shock absorber dampens vibrations.

Now, you may have noticed that the first cycle removes a lot more amplitude than the next cycle that follows.

Why?

Because shock absorber automatically adjusts to road conditions. The stronger the vibration, the more effective it is.

If that sounds like gibberish to you, here’s a great example that you can relate to.

Remember when you are in the swimming pool? The harder you punch, the harder the water resists your punch.

Our punches are like the piston, while the swimming pool is like the oil within the piston.

So, when the movement is great, the oil will resist the piston downward movement harder, which means it’s converting more kinetic energy into heat energy.

This slows down the suspension more.

Cool, right?

Different types of shock absorbers

The sheer amount of shock absorbers will shock you. (Please excuse the pun :D)

Yes, it is true that all shock absorbers dampens vibrations.

But, different vehicles types and suspension may require a different design of shock absorber which may look different and work slightly differently.

There are just oh so many variations out there.

If I have to list out all the types and variations of shock absorbers, this would probably be a 10,000 words article.

Nobody likes a 10,000 words article. So, let’s only cover just the main bits for now.

All shock absorbers can be categorized into two major types.

Type #1: Traditional Hydraulic Shock Absorbers

As you can see, hydraulic shock absorbers have very simple structure.
As you can see, hydraulic shock absorbers have very simple structure.

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Image courtesy of Michael Robinson (left image) and Stephan Ridgway (right image) under CC BY-SA 3.0 and CC BY-SA 2.0 respectively.

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A traditional hydraulic shock absorber has a very basic design. It features a piston in a tube filled with oil, very similar to mono-tube shock absorbers (discussed below) but without the gas chamber.

As of 2019, these type of traditional shock absorbers are rarely found in passenger cars because of cavitation issues, which essentially means that the oil bubbles.

Here’s why.

Oil contains approximately 10% air. When the shock absorber’s piston goes up and down rapidly, the air and oil molecule will start to separate, thus forming bubbles. This causes the damping force to reduce significantly – up to 35%.

Unfortunately, that makes our ride less comfortable.

Type #2: Gas Pressure Shock Absorbers

This is a coil-over gas shock absorber, which appears very similar with strut, visually. But unlike strut, it is not a structural part and does not support the weight of your vehicle.
This is a coil-over gas shock absorber, which appears very similar with strut, visually. But unlike strut, it is not a structural part and does not support the weight of your vehicle.

Gas pressure shock absorbers are usually found on the rear of the vehicle.

It has to be used together with a spring system. We can use a coil spring, leaf or air springs or torsion bars with this type of shock absorber.

Its design are usually very lean and simplistic, featuring a piston oil-pump body. The two ends are joints that attaches to the car’s knuckle.

That’s about it. That’s the whole shock absorber.

Because of its simplicity, the manufacturing cost are very cheap. When you have a worn shock absorber, it makes more economical sense to just replace this instead of repairing it.

The most common type of gas pressure shock absorbers are:

  • Mono-tube shock absorbers
  • Twin-tube shock absorbers

Mono-tube shock absorbers

This awesome short video by Monroe showcases the inner components of a mono-tube shock absorber.

Mono- is a prefix that means “one”. So, a mono-tube shock absorber quite literally means one tube.

The tube is divided into two chambers, separated with a free-floating piston.

  • On one side, is the pressure tube that is filled with oil.
  • On the other side, is the gas chamber that is filled with pressurized nitrogen gas.

As you can see, when compared to the more traditional hydraulic shock absorber, this type of shock absorber have an additional component – the pressurized nitrogen gas.

The nitrogen gas chamber is separated by a floating piston (shown as a red line in this picture.)
The nitrogen gas chamber is separated by a floating piston (shown as a red line in this picture.)

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Image originally by A7N8X under CC BY-SA 3.0, edited by D S AUTO.

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So why do we need the pressurized nitrogen gas?

The answer, is to reduce likelihood of cavitation.

The pressurized nitrogen gas acts as a compression force on the oil to keep it “tight and compact” so it is less likely to bubble.

Now, because the shock absorber only have one tube instead of two, we have more space to fit in a larger piston.

With a larger piston surface area, the oil can resist the piston better, which creates a greater damping force.

But precisely because of this, mono-tube shock absorbers tend to feel firmer and harsher. Mono-tube are usually used on SUV or four wheel drive that drive on harsh road conditions.

Twin-tube Shock Absorbers

Another great video by Monroe. They even compared the difference between mono- and twin-tube shock absorber.

Twin means “two”, so a twin-tube shock absorber simply means that it has two tubes.

  • The inner tube, is the pressure tube that is filled with oil.
  • The outer tube, contains nitrogen gas.

So instead of having a floating piston that separates the gas, nitrogen gas are found in the secondary tube of a twin-tube shock absorber.

The nitrogen gas chamber is found in the outer tube.
The nitrogen gas chamber is found in the outer tube.

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Image originally by A7N8X under CC BY-SA 3.0, edited by D S AUTO.

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Twin-tube boast a superior manufacturing process, which means it is slightly cheaper to produce than mono-tube shock absorbers.

It also has a smaller piston. With smaller surface area, the piston resists the oil lesser so that the dampening effect feels softer, which makes it more comfortable to ride in.

Also, the gas is not separated, so aeration is possible. This causes the oil to overheat and foam. You can lose up to 35% loss in efficiency when the oil foams.

They can only be mounted in an upright position. If you mount it horizontally, you will get oil at the bottom, and gas at the top, which doesn’t work.

Here’s a quick summary for mono- and twin-tube shock absorbers.

Mono-tubeTwin-tube
No. of Tubes12
Price$$$
DirectionalityCan be mounted in any direction.Can be mounted upright only.
AdvantagePerformance based.
Good for harsh terrain.
Comfort based.
Good for city drive.

Type #2: Struts

Struts are usually found on the front of a front-wheel drive vehicle. It does the same thing as conventional shock absorbers, but it doesn’t just dampens shocks, they also support the weight of the vehicle.

Struts are like a two-in-one deal. You get the suspension part AND the shock absorber.

By combining the coil spring, shock absorbers, upper control arm and upper ball joint into a single unit, they become a structural part of the suspension system.

If you remove struts from your car, your front bumper would be dragging on the ground.

The most common type of strut is a MacPherson strut.

MacPherson Strut

MacPherson strut have a coil spring on top too, which looks the same as a coilover shock absorber. But pay attention to the bottom, that's how you differentiate it.
MacPherson strut have a coil spring on top too, which looks the same as a coilover shock absorber. But pay attention to the bottom, that’s how you differentiate it.

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Image originally by Stahlkocher under CC BY-SA 3.0.

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MacPherson strut is usually found on the front of the car. It is the most popular strut because it is structurally simpler and space efficient. It replaces the need for an upper control arm, ball joints and rubber mountings.

Plus, just like a coilover shock absorber, you can mount the spring directly on top of MacPherson struts, saving even more space.

As a structural part of the suspension, it is designed to be studier to absorb shock while supporting side loads.

Fun fact.

MacPherson strut is a part of your suspension system so it directly affects your alignment angles (camber and casters). So if you replace the strut, make sure you get an alignment afterwards.

The Bottom Line

Well, that’s the gentle introduction into shock absorber.

If you have any questions, feel free to leave it in the comment section below. We’ll be more than happy to answer them.

Until then, drive safe and drive smart!

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