In this second article of our Beginners’ Guide to Suspension series, Robert Pepper discusses the different types of shock absorbers (dampers) and how they work.
So far we’ve covered the basics of suspension, and taken a detailed look at springs. Now it’s time to go into detail about the different types of the other big suspension component, the ‘shocks’ or shock absorbers as they’re often called in Australia. Just to make life confusing, overseas, they’re more accurately known as a damper.
Your vehicle’s body and chassis are connected to the axles by springs. – and springs are bouncy. Think of dropping a ball onto concrete; it’ll bounce, bounce, bounce. Now, imagine if that’s what your car did every time you hit the slightest bump. And it’ll do the same every time you accelerate, turn or brake.
What you need is some way of damping out that bounce movement, and that’s why we have different types of dampers (or shock absorbers). The spring supports the weight of the car, and when a wheel hits say a small hole in the road the spring compresses, absorbing the shock. The spring then extends and should extend back to its original dimensions. That’s where the damper comes in, to damp out what would otherwise be a kangaroo hop. Yes, the damper does a bit of shock absorption too, but that’s really the job of the spring.
How the different types of shock absorbers (dampers) work
The basic operation of shock absorbers (dampers) is to restrict the movement of a piston by forcing fluid through small gaps. This is similar to operating a coffee plunger or a bicycle pump. If you’ve used the latter you’ll recall pumps become hot during use, and that’s exactly what happens with a damper; the energy absorbed has to go somewhere, and it becomes heat. Dissipating that heat is a major task of damper designers, as is tuning the damper for optimum operation in a variety of conditions and loads.
Here’s a simple diagram of a damper. There are a few different types of shock absorbers and this one is a monotube, as there’s one tube (mono). However, it illustrates the concept for all dampers.
The tuning is done by determining the number, size and type of valves for the oil to flow through. Larger valves let more oil through, as does using a greater number of valves. There are mechanisms to prevent return flow and open or close extra valves depending on piston rod speed. The shock absorber (damper) also needs to work in both directions for compression and rebound. In short, it’s a skilled job and something of a black art to work out the best combination of valves for any given vehicle setup.
The different types of shock absorbers (dampers)
There are a number of different types of shock absorbers (dampers) including monotube, twin-tube, single, double, foam-cell and all sorts of other flavours, but they all dampen out the effects of a spring compressed by the vehicle moving over uneven terrain. Nitrogen gas is used as it is inert (does not significantly expand) when hot, and is even in many dampers that don’t feature a variant of the word ‘nitrogen’ in their name. “Gas dampers” generally refer to nitrogen dampers, even though the cheaper alternative, air, is also a gas.
The two major types of shock absorbers are monotube and twin-tube, and each design has its advantages.
Types of shock absorbers – Monotube
The monotube has more of the oil touching the outer tube, which means it is better at heat dissipation, something that kills damper performance and, by extension, vehicle performance. As the oil and gas are entirely separate, aeration (mixing of gas and oil) is impossible, so performance is maintained. Aeration means in effect a thinner oil, so there is less damping. This is known as damper fade, and is apparent to the occupants by a far bouncier (and more dangerous) ride, with much less handling control.
When a monotube overheats it hardens up rather than softens because the oil expands but cannot aerate. The relatively high gas pressure in the damper also helps prevent overheating in the first place. As a monotube has something like twice or three times the gas pressure (gas reaction force) of a twin-tube, it is constantly trying to extend, and that’s what you want off-road, when it forcibly pushes the wheel down. For this reason monos are also quicker to react than twins. Finally, a monotube can be mounted in any direction. Twin tubes can only be mounted at around a 45-degree angle from vertical.
Types of shock absorbers – Twin-tube
The main twin-tube advantage is damper travel length. For a given overall length, there will be more piston travel than with a twin as the gas doesn’t take up vertical space. Twin-tubes are also cheaper to manufacture, so for both of those reasons, original equipment dampers are almost always twins.
Which type of shock absorbers are the best?
One of the car world’s ongoing wars is about which type of shock absorbers are the best, but of more importance is the quality of the valve tuning and construction. Buy based on those criteria, and buy specific to how you want the vehicle to perform, accepting that you may need to compromise elsewhere. For example, heavy-duty suspension will make the vehicle feel much more controllable when loaded, but will bounce when unloaded. The various suspension components are part of an integrated whole, so changing one without considering the effects on another won’t lead to the best possible result, and there is always a compromise.
So where does this leave the tower? Your car’s original dampers are matched to its original springs, and that setup isn’t optimised for towing. It’s a general-purpose compromise across everything from unloaded suburban coffee runs to heavy-duty offroad. If you optimise your suspension for towing, then you’ll change both springs and shock absorbers to heavier-duty types. Stiffer or taller springs compress and extend differently from the originals, so they require different damping characteristics. Then there’s heat; corrugations are the damper killer, as the piston inside the damper moves up and down very quickly which generates heat and that can kill the shock.
So, if you tow in Australia, you’d want shock absorbers designed for heavier-duty work than standard and can handle the heat demands of corrugations, particularly as the rear axle has a lot of work to do when towing heavy loads.
The last article in this series will wrap up all the buying advice, but for now, that’s dampers (shocks) demystified!
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