When driving, the tires on your car come in contact with different types of terrain including asphalt, pot holes, speed bumps, gravel, and even bare earth. Depending on the quality of the terrain, a car could enjoy a smooth ride or a bumpy one. Fortunately, your car comes equipped with a system that not only absorbs the jarring impact of an uneven road, but also helps keep your tires touching the surface, allowing for better control of the vehicle. This is your springing system.
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Modern springing systems are based on four basic designs. Get to know these four spring classes: leaf springs, coil springs, torsion bars, and air springs.
Leaf springs are made up of a cascading set of blades held together by a U-bolt. The different lengths of the blades allow for the steady support for the entire vehicle the next while remaining flexible. First used on horse carriages, leaf springs were widely used in the rear suspension of cars until the late '80s, but has since been abandoned for newer systems.
Pros: Leaf springs can take on the heaviest of loads, which is why they are still pretty common in heavy-duty trucks and commercial vehicles.
Cons: They are very noisy, a lot bulkier, and provide a more rigid ride than today' systems.
If you know what a Slinky is, then you know what a coil spring looks like. This is a helical type of spring, which is basically a heavy-duty torsion bar wound around an axis.
Pros: These springs can take on heavy loads just as well as leaf springs, without the accompanying noise problems.
Cons: They can be extremely bouncy especially when poor shocks are used, and that's why they're rarely used in smaller vehicles. Their inability to take torque reaction and side thrust is why they are often paired with shock absorbers to enhance their reliability and efficiency.
This type of spring relies on the twisting properties of a straight steel rod to mimic the performance of coil springs. Attached to the vehicle's control arm, the rod gets twisted along its length as the arm moves up and down in reaction to the road surface.
Pros: Since torsion bars are just metal rods, their biggest advantage is that they save space while allowing easy adjustment of ride height. They are also very affordable, while being easy to maintain and replace.
Cons: A torsion bar's simplicity is both its biggest advantage most glaring drawback. If a suspension system lacks complexity, it won't be able to handle bumps very well.
This spring type relies on the compressing quality of air to absorb impact. Air springs are made up of a cylindrical air chamber strategically placed between the undercarriage and wheel.
Pros: Air springs are great for achieving extremely smooth performance, helping a vehicle glide over bumps. With these springs, a car is literally floating on air.
Cons: Air springs are very expensive. They also tend to fail more frequently than other suspension parts, but as long as you pay more attention to maintenance, problems can be avoided.
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.
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.
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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 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.
The yellow shims are anti-friction devices designed to allow the leaves of the springs to slide more easily between each other, offering better ride and handling. © Terrain Tamer.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.
Coil spring with bumpstop for when the coil is fully compressed, which means it runs out of 'travel'. The bump stop is a thick piece of rubber for that last-resort shock absorption. © Ultimate Suspension.Some people even convert their leaf-sprung vehicles to coil using kits such as this one from Superior Engineering:
LC76 leaf-to-coil conversion kit © 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 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.
Torsion bar springs at the top, four shocks below for front and rear axles © Ultimate Suspension.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 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.
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.
Airbag helper spring within a coil © Ultimate SuspensionMany 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, 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.
Leaf spring manufacturing © Ultimate SuspensionNext up,we'll cover dampers, and in the last article in this series will bring it all together with buying advice.
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