One of the most fundamental parts of a vehicle's suspension is the shock absorber. Depending on what's under a rig, there are plenty of drivers that may not be able to identify the control arms, or the D-bushes, or perhaps even the springs, but everyone can point to a shockie.

Typically held on by only one nut on each end, they are usually the easiest suspension component to remove and replace, and often the first thing a budding enthusiast looks at doing themselves. And yet most drivers do not truly know what they do.

A shock absorber looks pretty similar to a bicycle pump, both inside and out. They both have a shaft that telescopically moves in and out of a chamber. But while a bike with a flat tyre and no pump will not make it home, a 4WD without a shock usually will. We know we need them because every vehicle has them, but why are there so many types? Why do some cost a lot more than others? What makes them different? And which one is best for me? To understand the reasons for the answers to these questions, you need to understand how they work and why we need them.

THE BASICS

The aim of a vehicle's suspension is to keep the body and occupants as level and comfortable as possible. If you didn't have suspension, the chassis would rise and fall on every bump, so instead arms move up and down to negate highs and lows in the surface.

The springs carry the weight of the vehicle and force the wheels to follow the contours of the surface. When you hit a bump or a hole, the spring is what absorbs the impact. It is calculated to be of sufficient strength to support one corner of the vehicle at around the middle of its compression, which allows it to compress and extend as much as possible and keep the arms around the centre of their travel.

An input or release of energy as the wheel hits a high or low upsets the equilibrium, causing the spring to expand and contract. To counter this, the shock puts resistance on suspension movement. If you didn't have shock absorbers, your springs would continue to bounce after every undulation, you would lose traction and steering, and struggle to keep it in a straight line.

How does the shock provide resistance? Thinking back to the bicycle pump, when you push in the plunger to force air out the end, there is resistance. This is because you are forcing a large volume of air through a tiny hole. Shocks use the same principle, only with oil.

The shaft you can sometimes see coming out of the top of shocks has a piston at the bottom, with seals holding it tight in a chamber. There are holes in the pistons of assorted sizes with spring-loaded valves (dashpots) that open or close depending on the pressure and the direction of the piston. The valves make it harder for oil to flow past the piston, and can provide different amounts of resistance going down (bump) versus up (rebound).

Because of the different spring ratings of the dashpots, resistance can also be varied with different oil pressures. Most are set such that the harder or more sudden the suspension movement, the greater the shocks will resist and slow it down.

The volume of oil required to fill the pressure chamber is different above the piston than below, so at the bottom of the chamber there is another orifice that leads to a second chamber (reservoir) around the inner chamber, for storing surplus oil. There are more dashpots between them to further inhibit oil flow.