What is suspension and how does it work?

Welcome to the first article in our Beginners’ Guide to Suspension series, where we discuss everything you need to know about how suspension works for your car and caravan from a practical point of view. No hard sell here.  This is what we’ll cover:

In this article, we’ll cover what suspension is

Before we get into twin-tubes vs monotube we need to explore why suspension exists and the job it does for the towcar and caravan.  Would we still need it if every road was perfectly flat and smooth?  Here we’ll look at vehicle dynamics, body control, traction, handling, primary and secondary ride, and unsprung mass with and without a trailer.  We’ll also cover the suspension terms and components such as springs, damper, swaybars, links, arms, bumpstops and more.

In future articles, we’ll cover…

How suspension works – springs, shocks and more

Springs are a fundamental part of any suspension, so we’ll go over all the common types; leaf, coil, torsion, air and variants such as progressive vs constant load.  How they work, and the pros and cons.

The other fundamental of suspension is shock absorbers.  There are many to choose from, and you get more choices than springs.  So we’ll cover the pros and cons of the main types. We’ll also cover how shocks work, including valving, shims, tuning, rod speeds gasses, aeration, fade, position-sensitivity, adjustability and heat management designs.

Caravan suspension

Your caravan has a need for suspension which is similar to, but different to the towcar. We’ll explore what you need in trailer suspension and how it works. This will include single axle, tandem, loadshare, springs, shocks and more.

Buying advice

We’ll finish up this beginners’ guide to how suspension works with some buying advice, which won’t be “hand over your coin for the most expensive kit there is”.

Before we get into the nitty gritty of how suspension works, let’s talk about what suspension is.

What is suspension?

Suspension never gets the glory, but it is perhaps a more important factor in vehicle performance than the engine. Suspension is the component of a car that connects the body to the wheels and allows the car to safely and effectively traverse different terrains and driving scenarios.

Your suspension has two main jobs…

Despite the importance, the suspension really only has two jobs: ride and handling. For ride, it has to absorb and dampen shocks from uneven terrain for a comfortable journey (‘ride’), and secondly, keep all four wheels solidly on the ground to provide traction, control and braking while reducing body roll and inspiring the driver confidence –which can be summarised as ‘handling’.  The suspension has to work even on bitumen roads, which are never completely flat and level, and even if they were, the vehicle would still be accelerating, braking and turning with associated body roll, which needs to be controlled by the suspension.

The basics of suspension design are simple too.  There’s a spring, which absorbs the bump in the road by compressing, allowing the wheel to move up relative to the rest of the vehicle so your spine remains as you’d like it. That’s great, but then the spring rebounds, compresses, rebounds…and the vehicle behaves like a kangaroo, compromising both ride and handling. 

So to damp this rebound/compression cycle, there are dampers, inaccurately known in Australia as shock absorber, as it’s really the springs which absorb the shock. The dampers also help control the compression of the spring, and carries the weight of the vehicle. So if you removed the dampers you wouldn’t notice much difference until you went over a bump! Actually, you’d notice even on a billiard table-smooth surface because of the weight shift when a vehicle accelerates, brakes or corners – it’d feel something like a cross between a seesaw and a swing. 

Suspension design is complex…

That brings us to the complexity of suspension design. Even though it has just two jobs to do, there is a wide range of situations and terrains in which it must do those jobs. Of course, optimising suspension for one situation will lead to a compromise for another. Let’s take towing as an example.  When you’re hooked up to say a 2,500kg trailer you might have 200kg of extra weight on your towbar, which translates to around 300kg on the rear axle.  That’s a lot, and you’d want stiff suspension to handle that load and control the trailer.  On the other hand, as soon as you unhitch, you want softer suspension more suited to the lighter weight. 

Your new towcar has a compromise suspension. The carmaker doesn’t know if you’ll drive the vehicle around the suburbs unloaded, or fully loaded across a desert. So, they fit a compromise suspension which tries to do everything, and therefore, does nothing well. This is why I recommend changing suspension to a setup appropriate to the use of the vehicle. Every vehicle on the market will sag in the back once you attach a heavy caravan, except those with height-adjustable air suspension.

Now as ever, there is jargon. So let’s cover that off before we talk about what you may or may not want to do with your towcar’s suspension:

Suspension jargon

Ride

This refers to how comfortable the car is over any given terrain and how well it insulates the occupants.  Primary ride is large bumps, secondary ride is smaller ones.

Handling

This is how well the car responds to driver input.  Generally, the smoother the ride, the less inclined the car is to tell the driver what’s going on, and respond to inputs.  This is why sports cars have a rough ride, it’s a tradeoff for better handling.

Spring

This is the bit that supports the car on its axles or wheels and is compressible. Spring can be coil, leaf, torsion bar or even a bag filled with air, which is not to be confused with the airbags, which are there to help take the load from metal springs such as coils and leaves. The title photo shows a caravan’s coil spring with two shocks next to it.

Shock absorber

When a spring is compressed it bounces back, similar to a football dropped on concrete.  You don’t want that bouncing, you want one compression to absorb the shock, then back to normal. That’s where the shock absorber comes in – it damps the bouncing.  There are many types of shocks, such as twin-tube, monotube, foam, remote reservoir and more, which we’ll cover later.

Damper

See above!

Flex/travel

This is the movement of the suspension up and down relative to the body. Typically, lots of flex is good for offroading.

Bumpstop

If the suspension runs out of travel components, it will hit the bumpstop, which is a little rubber pyramid designed to provide some shock absorption as a last resort.

Sprung and unsprung mass

Sprung mass is any weight that is controlled by the suspension, such as the vehicle’s chassis and body. Unsprung mass is not controlled by the suspension and would be components such as the wheels and perhaps the axles in the case of beam-axled vehicles.

Rollbar/swaybar

When a vehicle corners the body will roll or tilt.  This is undesirable for both ride and handling, and so a rod linking the left and right wheels is used to reduce the roll. That rod is called an anti-rollbar or swaybar.

Links and control arms

These are various components that connect the wheel to the chassis. Most often discussed when large suspension changes such as a significant ‘lift’ (increasing the ride height) is in order. Towcars typically don’t need such lifts, which are more for hardcore 4×4 people.

Okay, that’s the terms out of the way. In the next articles in this series, we’ll take a look how how the suspension works when you’re towing. Stay tuned!

Read next in this series:

• Understanding caravan suspension – A guide for beginners
• Types of suspension springs and how they work
• Types of shock absorbers and how they work
• Suspension buying advice for towing