From horse-drawn carts to cars, trucks, and trains: The leaf spring has been there, done that, and is still going strong.

What Are Leaf Springs?

  These are the kind of springs used on those horse-drawn carts: Metal strips that bend. And that's basically it. A spring can be made from a single leaf (strip), or several. The more leaves used, the stiffer the spring will be; you can't simply make the leaves thicker, as they are then not individually flexible enough. When a strip bends, it effectively does so about it's centreline: One side is slightly stretched, and the other slightly contracted; the thicker the strip is, the more amplified this effect is, so a set of thin leaves together is more pliable and suitable for use in a suspension system than a single thick one.

  You may have already spotted that, with two or more leaves stacked together, the upper surface of one leaf will be touching the lower surface of the next - one of which will be stretching, and the other contracting, as the spring flexes. To cope with this, the leaves have to be able to slide over each other, which is why leaves are held together with clamps around their outside rather than being bolted through.

  A further complication of the way springs bend is that the overall length of the spring changes as it flexes, so one end has to be hinged with a shackle between the spring and it's mount to cope with this.

Leaf-Sprung Suspension Designs

  This picture shows the front suspension on a Ford Model T, with the front axle mounted to a single spring running across the width of the car. You may notice that the spring has a shackle at both ends to take up changes in the spring length. This is because the change in length would try to move the axle to one side if only a single shackle was used, so in order to maintain the central positioning as the spring compresses, a shackle has to be at the other end as well. Normally this would allow the axle to move from side to side at random, as neither end of the spring is solidly attached to it, which is why a bracing frame is used to hold the lateral position of the axle assembly, without impeding it's vertical movement. Having the spring running across the width has the advantage that you only need one (so it's cheap), and also means that the suspension can be quite soft and responsive to vertical movements of one wheel, while still being stiff enough to allow for the normal loading of both wheels on a flat surface.

  The most common method of building a leaf-sprung suspension system, and the one you are most likely to see in general use, is to mount two leaf springs lengthwise on the chassis, and then affix the axle across them. This picture of the rear suspension on a Ford pick-up on the right is a good example of this. When designing a system in this way, we are less concerned about the change in length of the spring as it flexes, and make do with just the one shackle. Yes, the change in length does tend towards moving the axle in the direction of the shackle, but this is not overly critical. What is critical, however, is which end of the spring you have the shackle fitted to.

  As we said, the compression of the suspension causes an increase in the length of the spring between it's mounting eyes, taken up by the shackle. The axle, mounted roughly in the middle of the spring, also gets moved slightly, in the direction of the shackle. Now, if both the wheels, left and right, are compressing the suspension the same amount, they will both move the same amount towards the ends with the shackles. However, if one wheel is compressing it's suspension more than the other, that wheel will move further towards it's shackle than the one on the other side - effectively trying to turn the axle. This understandably has the effect of trying to steer the vehicle in one direction or the other - a trait known as bump-steer, because it's normally most evident when one wheel hits a bump and another doesn't (bump-steer rears it's head for other reasons, too, which we'll look at in later sections).

  The reason why it matters which end of a spring the shackle is mounted on (and, therefore, which way it will try and steer under compression) is because of the weight transfer during cornering. As a car travels round a corner, it's weight (acting on the car's centre of gravity) causes it to lean towards the outside of the corner - known as body roll - as the weight transfer compresses the suspension on that side. If the shackles are mounted towards the inside of the car (i.e. at the rear end of the front springs, and/or the front end of the rear springs), the effect will be to shorten the wheelbase (the distance between the centres of the wheels along the length of the vehicle) on the side that is being compressed - the outside of the corner. This induces a steering effect in the opposite direction to the way the car is being steered by the driver, which makes the handling of the car...interesting.

  If, instead, the shackles are mounted towards the outside of the car (i.e. at the front end of the front springs, and/or the rear end of the rear springs), the turning effect due to body roll will be in the same direction as the driver is trying to steer the car in. While this may make the vehicle steer a little bit more than intended, it's better than having the vehicle try to turn in the opposite direction to the driver's input. It's not to say you can't have the shackles facing inward - you may not notice any bad side effects, especially if it's only on one end of the vehicle, or there isn't much suspension travel (movement) - but you don't want to end up in a hedge because of it. Most of the time, a shackle will only be mounted in this way on a modified off-roader (having the shackles away from the ends of the vehicle helps with ground clearance and stops them getting damaged), in which case a slight change in on-road handling is probably acceptable.

Variations in Leaf Springs

  The leaf springs found in most applications are described as semi-elliptic, in reference to their shape. When first used on horse-drawn vehicles, it was not unusual for the spring to be an oval (elliptical) shape (right). So because what we would look on as being a normal leaf spring only uses half of that design, it's called semi-elliptical. Logical, huh? In a similar vein, if you only use half of one of those, you call it quarter-elliptical. Quarter-ellipticals usually work by having one end (which would've been the centre on a semi-elliptical) bolted to the chassis, and the axle etc bolted to the other. Semi-ellipticals are by far the most common, and are likely to be the only type of leaf spring you encounter.

  A lot of the time, it would be preferable to have soft suspension to improve both comfort, and how well the suspension can deal with smaller bumps - but a softer suspension will be too easily compressed fully by a larger impact. So, what is required is a system whereby the suspension is normally soft, but gets stiffer the more it is compressed - this is often described as a rising-rate system. With a leaf spring, an easy way to achieve this is to have extra helper leaves, which are straighter than the others. If, for example, you have a leaf spring with four standard leaves and two helper leaves, most of the time it will behave as a four-leaf spring. Under large loads, however, the standard leaves will deform under load until they are parallel with the helper leaves. Once the spring is in this state, any extra loading also has to flex the two helper leaves as well - meaning that the spring now behaves as if it was a six-leaf spring. By changing how many regular leaves and how many helper leaves a spring has, as well as their curvatures, a leaf spring can be tuned to give a desired response under different loadings.

  Most leaf springs use leaves that have the same thickness along their length. However, a parabolic leaf spring uses leaves which are thicker in the middle and thinner at the ends - the way it tapers being given by a parabolic formula, hence the name. This gives a much more flexible, supple spring, which reacts better to small loads. This is because of better distribution of the load across the whole length of the spring, helped by the fact that less of the individual leaves' surfaces are touching each other (less friction between them as they slide over each other). This allows for a spring which is soft and responsive for small bumps, but which can still be stiff enough to take heavier loads without using up all of it's travel - particularly if helper leaves are fitted. Parabolic leaves are now used in most applications of leaf springs, and are also a popular retro-fit addition to vehicles with normal, flat-leaf, semi-elliptic leaf springs, such as older Land Rovers, where their additional flexibility compared to normal leaf springs (which are generally stiffer overall in order to cope with heavy loading, at the expense of light-load performance) improves their off-road performance.

  Although almost all leaf springs are made from steel, it is not uncommon for fibreglass to be used, although normally only for single-leaf springs, for applications where light weight is a priority, and heavy loads aren't going to be carried.

Advantages & Disadvantages Leaf Springs

  The leaf spring has a major card up it's sleeve when it comes to designing suspension systems: It's an all-in-one package. Once you've got your axle etc bolted to the spring, and the spring bolted to the chassis, the spring itself will hold everything in place, so you don't need to worry about the extra expense and complication of linkages etc. Furthermore, the way the leaves of the spring move against each other as it flexes gives a frictional resistance to movement, helping to stop the spring bouncing out of control.

  Also, leaf springs can be built to take huge loads without needing to be overly large or expensive - this makes them ideal for the lorries and trains they are so often used on.

  Unfortunately, there are some serious downsides, too. As well as the way a leaf spring streches and contracts under flexing that we discussed above, moving the axle etc that it's bolted to around with it, the friction between the leaves can work against you. As the spring gets older, and begins to rust, the leaves won't slide over each other as easily, making the spring stiffer than it's meant to be - leaf springs have to be well looked-after in order to keep their performance. Also, that friction means that there's a minimum force required to overcome it, and get the spring working, so the suspension may not respond well to small changes in the road surface.

  While parabolic-leafed springs alleviate many of the problems with leaf-sprung suspension, especially relating to small-bump response and suppleness, they can only do so much. All in all, for passenger car and performance applications, the leaf spring has been eclipsed by alternative methods of springing. It lives on in heavy-duty applications - trucks, trains, military vehicles - where small-bump response can be sacrificed for brute strength and simplicity.

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