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Shock Absorbers for Dummies

Many of us sheepishly avoid eye contact when the subject of shock absorbers comes up. It seems that everyone we speak to is an authority on the subject, often with strong views on the best shockers for our Criusers.

This page is an attempt to explain in simple terms what a shock absorber is, what it does and the different types available. Hopefully it'll arm us with some self respect to at least maintain eye contact with the experts.
What's a shock absorber?

It's a device that offers resistance when the suspension moves up and down. The neat thing about shock absorbers is that they only offer resistance when there is movement and the amount of resistance is related to the speed at which the suspension moves. Typically, the faster the movement, the higher the resistance.

Why do we need shock absorbers?

Our cruisers use springs to keep the vehicle up. Just like any spring, if we compress or stretch it and then let go, it will keep bouncing up and down for a long time.

If we follow the movement of the vehicle (magenta curve) after hitting a bump, without shock absorbers, the vehicle would continue to bounce up and down well after hitting the bump. Shock absorbers offer resistance to this uncontrolled movement so that the vehicle quickly returns back to a level ride on level ground.

In the case above, by offering resistance to (or more correctly damping) the movement, it takes energy away and dissipates it as heat (that's why shock absorbers get hot).

How does a shock absorber work?

Most shock absorbers for our Cruisers are based on a piston pushing (as the suspension compresses - compression) and pulling (as the suspension extends - rebound) inside a cylinder containing hydraulic fluid.

 The piston contains holes, slots, valves, shims etc which controls the amount of fluid which can pass through it. A basic interpretation shown to the left displays 4 large holes in the piston. As the piston moves in the cylinder, fluid is forced through the holes to the other side of the piston and whilst doing so, creates a resistance to the movement. The amount of resistance is often referred to as the damping rate. 

There's a lot more in there of course. There are more valves, etc at the base, gas chambers etc which we won't go into right now.

As you may have guessed, by altering the size and number of holes, you can alter the amount of restriction to flow and hence the amount of damping force the shock absorber exerts. We alter the damping rate.

In this example, with 2 smaller holes instead of the previous 4 large holes, the damping rate will be much higher. 

Where things become clever here is the amount of restriction can vary with speed with which the piston moves inside the cylinder. You may have come across the term "multi stage valving" used to describe a shock absorber, or advertising claims of "5 stage valving". This describes the fact that there are a number of distinct stages in the amount of restriction to flow within the shock absorber.

Another important aspect of damper performance is the thickness (or viscosity) of the hydraulic fluid we use. The thicker it is, the higher the damping rate. Conversely, the thinner the oil, the lighter the damping. This is important to remember as we learn about shock absorber fade.

So why do we need different damping rates?

The general design requirements of shock absorbers for our Cruisers is to have "light damping" or minimal restriction for slow speed movement (to keep things comfortable and to enable the wheels to track over small undulations). For high speed piston movement, we'd be looking for high damping or high restriction to keep the suspension from bottoming out or extending back too quickly.

Yes, you did read it correctly. The bit about extending back. You see a shock absorber works both ways and interestingly, the amount of restriction or damping rate as the suspension compresses (damper compression rate) is different to that as the suspension rises back (damper rebound rate).

This of course is a very simple way of looking at it and shock absorber design is a real science which we're almost insulting by telling it in such a simple way.... But it is Shock Absorbers for Dummies after all.

What's all this stuff about fade?

A shock absorber's worst enemy is heat. High heat can alter the viscosity of the hydraulic oil and it becomes thinner. If we remember, the thinner the oil, the smaller the resistance and hence a lowering of the damping rate. The car feels loose in the suspension and tends to bounce around much more, with higher chances of bottoming out.

Another problem with some designs can be aeration.  There's gas or air inside the shock absorber as well as oil. The gas or air may mix with the hydraulic oil which again offers less resistance and lowers the damping rate. It's a bit like shaking up a Coke bottle. Looks like liquid until you release the top and all the gas comes bubbling out of the liquid.  As we'll soon see, there are a number of designs which address the problem.
People keep talking about different types of shock absorbers. What do they mean?

Ah, we're into the salesman's territory here. Ever been talking to the suspension sales person and (s)he throws you a line like "They're foam cell shockers mate, ya can't go wrong". Naturally you agree and subsequently avoid any further eye contact.

Let's see if we can explain some of it.

Twin Tube Gas 

This type of shock absorber has two tubes. An outer reservoir tube and inner pressure tube. The piston moves up and down inside the pressure tube and oil is forced through the compression valve or the rebound valve. 

As the oil displaces, an inert gas such as nitrogen at relatively low pressure (around 5 atmospheres) is maintained on the oil in the reservoir tube. 

The reason why an inert gas at pressure is used, is to reduce the chances of aeration. This type of shock absorber eventually will fade if used hard and simply requires time to cool for full damping to return. 

Foam Cell 

Again a twin tube but an interesting design where instead of allowing any gas to come in contact with the hydraulic oil,  nitrogen impregnated foam cells are used. 

These shock absorbers virtually eliminate aeration and will generally hang on a bit longer than the twin tube gas shock absorbers before fading. Unfortunately if this type of shock absorber is overheated, it will not regain its damping qualities when cooled.

Mono Tube 

As the name implies, this type of shock absorber uses only one tube and the piston valving controls both rebound and compression damping. A floating piston separates the nitrogen gas from the oil and the gas is at a high pressure generally above 20 atmospheres. 

This type of shock absorber is more prone to stone damage than the twin tube types. Once the tube is hit, the rod piston and/or floating piston will no longer seal properly against the cylinder wall. 

With the gas at the bottom of the shock absorber, it's difficult to design one with as much travel as the twin tube designs above. As such it's a popular shock absorber on road cars and as it heats up, the gas pressure increases and exerts more force on the oil. The end result is that the damping rates increase with heat. 

Twin Tube Hydraulic 

This type of shock absorber is similar to the twin tube gas type except instead of using an inert gas at pressure, it simply uses air at atmospheric pressure. 

Generally, it's prone to aeration and will fade quickly. Allowing the shock absorber to cool will see a return of its damping qualities.