How does a shock absorber work?

One way to think of how a shock absorber, or damper, works is to think about how it doesn’t work.

A car without dampers is like a pogo stick. ‘Boing-ga-boing’ is the less technical way of describing it; ‘underdamped’ is the technical term.

But how do you stop a car from ‘boinging’ all over the road when it hits a bump? The short answer is with a liquid-filled cylinder and a piston with holes in it.

But shock absorbers do more than just absorb shock or prevent ‘boinging’. They are one of the key contributors to a car’s ride comfort and handling.

So, how do they work?

Imagine a syringe with water on both sides of the rubber piston. Put a cap on the top and bottom, then drill a small hole through the piston. Now, push on the syringe plunger… it pushes back: a little if you push lightly, a lot if you push hard.

When you don’t push, it does nothing. Attach the syringe to your pogo stick, or your car’s suspension, and, hey presto, no more ‘boinging’.

Physics would tell us that damper tuning is as simple as working out the right ratio based on the stiffness of the spring, the weight of the car and the weight of the wheel and suspension.

Then, Bob’s your uncle, you have the perfect compromise between smashing into a bump or ‘boinging’ off it.

Well, it isn’t quite that simple. Like most things that come in syringes, many years of development and painstaking finetuning are needed to find the perfect harmony.

A great deal of trial and error is done by car-makers when a new model is in development.

Prototype cars are driven hundreds of times with different damper iterations, trying to find the perfect balance between ride and handling.

Never ones to keep things simple, engineers have, over many decades, come up with more and more intricate and incredible ways of controlling oil flow through pistons.

First they tried changing the shape of the holes; then they blocked the holes with metal discs that would flex away with enough flow; then they cut slits in the discs. Their inventiveness knew no bounds.

Engineers, being engineers, couldn’t stop at that. They tried to figure out ways of changing the damping force actively. They tried zapping magnetic particles in the hydraulic fluid with electricity to thicken it – like adding aeroplane jelly crystals to the water in our syringe.

Then they played with fancy electronic valves. That might sound incredible, having two-amp solenoids take on two-tonne forces of a motor vehicle and win, but it’s amazing what a small solenoid can do with a little help.

This is where some very clever hydraulic tricks are used to close the hole in our syringe rubber. The solenoid only needs to ‘hold its finger’ over a much smaller hole that holds the main hole shut. Oil pressure is redirected behind our little solenoid to help it keep its finger over the hole.

Once you have these intricately manufactured valves plumbed into your damper, the world is your oyster. Well again, not quite…

You have to work out a computer algorithm, which takes live data from a myriad of sensors, at 500 times a second, processes it, and sends a current to our friendly solenoid.

Let’s get back to basics. Why do shocks need gas? One advantage of liquids, like the oil in your shock absorbers or water in our syringe, is that, unlike gases, they can’t be compressed.

This poses a problem. Remembering that there is water on both sides of the syringe rubber, as the plunger rod goes in it will displace some of the water, like a tea bag in a cup of tea, but our syringe is capped at both ends…

Hydraulic lock will be a term familiar to anyone who has ever tried to drive their uncle’s Valiant through a dam. How does the damper rod fit into the damper without hydraulically locking it?

Basically, you need some air, or preferably nitrogen, inside the damper. Getting air inside a device you don’t want any air inside of poses further challenges. Twin-tube, monotube (single-tube) and external reservoir or accumulator designs are all varied ways of skinning that cat, in order of effectiveness.

Then there is the problem of cavitation. It acts like adding bubble bath to the syringe water. All that energy has to go somewhere. As the shock absorber piston moves up and down on a bumpy road the liquid flow heats up the fluid.

This, coupled with the big pressure variations inside the damper, can create instantaneous air bubbles – like a boat’s propeller or a kettle on the boil. A syringe full of compressible bubbles can’t do a lot of damping.

So, a lot of work goes into keeping the flow inside the damper as smooth or non-turbulent as possible. The gas pressure from the aforementioned nitrogen helps too.

There you have it: the workings of a damper in a few easy steps. Spare a thought for your trusty dampers, or shock absorbers as they are colloquially known, the next time you drive over a bump.