Types of suspension springs and how they work

In this second article of our Beginners’ Guide to Suspension series, Robert Pepper discusses the different types of suspension springs and how they work.

The two main parts of suspension are the spring and the shock absorber, terms we defined in the previous article.  This time, we’ll look at the different types of suspension springs. When we get to the buying advice part of this series, you’ll need to know the difference between a leaf and a coil, and also what airbags are. So it’s all useful background information.

Before we discuss the types of suspension springs – let’s talk about its purpose

The spring is there to support the vehicle’s weight and absorb bumps. When the vehicle travels over a bump, the wheel will move up over the bump. You don’t want that force transmitted to the rest of the vehicle, so the spring compresses and absorbs the bump. Then, when you’re over the bump, the spring extends once more. In a perfect world the wheels move up and down, and the car body stays precisely level and calm. You can see this in desert racer videos.

Unless you want to get into major vehicular surgery, you don’t get any choice in the type of suspension spring – coil, leaf, torsion bar, etc – as that’s set by the vehicle or caravan manufacturer. However, you can change the nature of how that spring works with different versions of that type of suspension spring and choose one more appropriate to towing rather than the generalised version fitted by the carmaker. For example, coil springs come in different heights, compression rates and so on for different purposes. In the same way, you can also choose a variety of different dampers. As the differences are internal, they all fit the same mounts, and we’ll cover that in the next article.

Types of suspension springs and how they work

Leaf springs

The oldest type of suspension spring is a leaf spring, dating back from the horse and cart days, which is why it is sometimes called a cart spring.  They are simple packs of multiple long rectangles of thin steel, which combine to form a spring. 

Leaves are simple and robust, and are often fitted to trucks and the back of utes. This has given rise to a myth about leaf springs being good for heavy-duty but that’s a correlation-causation confusion. Leaves are no better than, say coils for load bearing. It just so happens they’re fitted to heavy-duty vehicles because they are simple and strong, hence people thing they’re fitted because they can handle a load.

Leaf springs are heavy and provide both poor handling and ride. When the wheel moves up and down so too do the leaves move between each other, providing a measure of damping. This is why some trailers don’t have dampers, relying on the leaf spring’s natural damping effect. However, you really want the spring to act purely as a spring and leave the damping to the purpose-designed component, which is precisely valved for the job, namely the shock absorber. 

Another reason this type of suspension spring handles poorly is that it doesn’t locate the axle well, allowing it to twist under torque from the driveshaft when the vehicle goes over bumps.  Essentially, leaf-sprung vehicles are likely to handle and ride worse than any other type of spring, but a well-designed leaf-sprung vehicle can outperform a poorly designed coil-sprung vehicle.

You can replace leaf springs with heavier or lighter-duty versions, which have more or fewer leaves, or thicknesses of leaves, or raise the car’s ride height, known as a suspension lift.

Parabolic leaf springs

Parabolic suspension springs are a type of leaf spring that shape the leaves like a parabola or elongated oval. They offer the same carrying capacity as conventional leaves but at a lighter weight, as there are fewer leaves in a pack, typically two. As there’s less inter-leaf friction, they ride and handle better too. So if you swap to them, the damper will need to do more work. As a result, you should always consider springs and dampers as a complete, matched set. However, they are more expensive.

Coil springs

Now we come to coils. The most common type of suspension spring, fitted to almost all cars and 4x4s except for the back of most utes, which tend to be leaves. The coil spring can’t locate the axle by itself, so it needs control arms and links, but with that in place, it is robust, light and offers great handling as the jobs of spring and dampers are separated, and the wheel/axle movement can be precisely controlled.

As with leaves, you can replace the coils typically with versions that are stiffer and/or taller, giving a slightly higher ride height and less compression under load, ideal for the heavy tow ball mass of a big trailer.  You can also choose progressive springs as a type of coil suspension spring, which are soft for the first part of the compression and then stiffen up. The idea is that unloaded, the car had a soft ride, yet when loaded there’s the stiffness needed to deal with the weight.

Some people even convert their leaf-sprung vehicles to coil using kits such as this one from Superior Engineering:

The upgrade is done to improve ride and handling beyond what could be achieved with a simple replacement of the leaf springs.

Torsion bar springs

Torsion bar springs are a rare type of suspension these days. They are essentially a long bar firmly gripped at one end, and the other end can twist, which is where the spring comes from. Ride height is easy to adjust without replacing the bar, and stiffer ones can be made for heavy-duty work.

Air springs

Air springs are the one type of suspension spring that is versatile enough to deal with any load as they simply inflate more to deal with extra weight and that is why you now see them on many trucks. This type of spring suspension can vary the ride height at the touch of a button, which means the car can be low for on-road, very low for car parks, or high for off-road. In addition, it makes hitching up a trailer easier if the hitch isn’t at precisely the right height. 

Air springs work really well for towing, so there is no need to change them for aftermarket versions. It is possible to swap coils for airbags, but that is an expensive engineering exercise.  The most common vehicles with air suspension are Land Rover and Range Rover vehicles, pretty much all of them from the Discovery 3 of 2005 to the present day.

The disadvantage of air springs is complexity and expense; they are reliable but cannot match a plain steel spring and are more expensive to replace.

Not to be confused with ‘airbags’ …

There are also ‘airbags’, which are a type of supplementary air springs designed as load-bearing helpers to suspension springs like coils or leaves.

The idea here is that when the vehicle is not loaded the airbag has no or little pressure, and the metal spring takes all the load. When the vehicle is loaded, you’d inflate the airbag, and it would become a load-bearing spring, helping the metal spring. The idea is that you can run less heavy-duty, more comfortable springs when unloaded or not towing, but when you are loaded, you use the airbag to avoid the suspension sagging.

The concept does work but you need to be careful not to over-rely on the airbag and use it as a supplement, not a primary spring.

Base weight considerations

Many touring vehicles will have a heavier base weight than standard, so owners might fit a constant-load spring designed for the new base load. For example, say your vehicle, stock standard, weighs 2,000kg and has a GVM of 3,000kg, so the car marker would have designed the suspension for that range of weights.  You then add a bunch of accessories to it, so the new base weight is 2,600kg, and when you’re camping or towing, you’ll go to 3,000kg.  You should then consider new springs designed for a 2,600kg load, at least, as the car will never again weigh only 2,000kg but 2,600-3,000kg.

Regardless of the type of suspension springs you have – quality matters

Regardless of the type of suspension springs, quality matters. You don’t ever want to waste money on cheap no-name parts, and that’s true of springs too. The quality of the material, design and manufacturing process all play a part in the quality of the spring and you get what you pay for.  

Next up,we’ll cover dampers, and in the last article in this series will bring it all together with buying advice.