What magical forces combine to give you a smooth ride? The main components are the springs and the dampers, and the major factor that creates a spring is the shape. If the material is long and relatively thin, the natural elasticity of the steel will ensure that a force will cause a visible movement. That is why a leaf spring or a torsion bar is long. A coil spring can be regarded as a spiral torsion bar.
Springs have many other uses on a vehicle – they close engine valves, regulate oil pressure, apply clutch pressure and perform numerous other duties. They also take on many different shapes, but this article will mainly concentrate on suspension springs.
Steel is surprisingly elastic, ie it behaves like rubber, but the forces needed to cause stretching or shrinking are vastly greater than those in the case of rubber, and the movement is very small. Steel obeys Hooke’s law, which states that when a material is loaded the extension is proportional to the load, but only up to a point, known as the elastic limit, and if the load is increased past this point the material takes on a permanent set. Furthermore, any load that causes an extension or compression to a point below the elastic limit will not induce a permanent set, ie if the load is removed the material will return to its original shape.
Here, a suspension spring is just a piece of steel shaped to extend or compress in unison with the wheel’s up-and-down movement. The main design secret is to choose the spring dimensions and the steel alloy correctly, so that the steel remains elastic within its range of movement. A permanent set would cause the spring to sag, leading to the car settling closer to the road surface and a softening of the suspension’s compliance. After many years, the material becomes fatigued, and on older cars the springs have to be removed and retempered.
A road spring has two primary duties to perform. It has to keep the wheels in contact with the road as much as possible, to improve roadholding, and at the same time, flex to absorb energy and improve comfort levels for the vehicle’s occupants. These are obviously conflicting requirements, which is why suspension design is still to some extent more of an art than a science.
Imagine what would happen if a spring-less car hit a 50 mm step. In theory, the wheels, and everything else, including passengers, would move upwards by 50 mm, causing an instantaneous upwards acceleration of close to infinite metres per second squared. But in practice, all the components flex, including the seat pads,
so that the acceleration would be reduced. However, it would still be enough to give everybody a nasty jolt.
Imagine if the same car was fitted with springs, but no dampers (incorrectly named shock absorbers). The springs would first compress and then extend, as they started to vibrate. The vibration will normally last for four or five cycles before the friction in the suspension links absorbs the energy. Meanwhile, the car will bounce and the wheels will hop, seriously reducing roadholding. When I was an apprentice, we often drove cars without dampers, just for fun, and it was easy to get the car sideways just by opening the throttle. It was amazing to see how much bounce even the slightest bump induced.
Bouncing can be greatly reduced by fitting dampers to the springs, and this is where the art of good suspension design comes in. If the damping is too severe, or the springs too hard, the upward acceleration becomes uncomfortable, whereas if the damping is too soft, the bouncing will be uncomfortable. If the springs are too soft, the suspension will simply bottom out on the bump stops, again causing discomfort.
The fact that springs vibrate means that a knowledge of vibration terminology is useful, and the various terms are explained in more detail in the accompanying panel.
The frequency is the number of complete vibration cycles per second, measured in hertz (Hz) so that one hertz is one cycle per second, while the amplitude is the size of the up or down movement. Whether a spring is hard or soft depends on the spring rate, which is the force compressing or extending it divided by the amount of movement in millimetres. This, in turn, depends to a small extent on the particular steel alloy, but is mainly a factor of the dimensions and shape of the spring.
With leaf springs, the rate depends on the width, thickness, length and the number of leaves. If another leaf is added, the spring will get harder, ie the rate will increase.
With coil springs, the rate depends on the material diameter and the length, and the latter, in turn, is determined by the coil diameter and the number of coils. This means, for example, that if you want to lower your car’s suspension by making the coils shorter, the rate will increase, ie the springs will get harder, because you’re effectively reducing the amount of steel available to absorb the movement. An easy way to see this is to clamp a length of small diameter steel rod in a vice, attach a vice grip to one end, and try to twist the rod. (Although, the rod is straight, the principle still applies to coiled steel.) You’ll discover that the longer the portion that sticks out from the vice, the easier it is to twist the rod. The correct way to lower a car and keep the rate the same is to buy springs that are shorter but have the same rate as the original set. The new springs would probably be coiled from thinner material.
A torsion bar is just a straight coil, but most of them are adjustable at one end, and this leads to the temptation to apply more or less pre-load. This cannot change the spring’s rate, and affects only the size of the force that will cause initial movement. In fact, any spring has a pre-load, which depends on the way it is mounted, how much it is compressed during installation, and to what extent the static forces due to the car’s mass deform it. Increasing the pre-load will cause discomfort, because the spring will only flex when it meets bumps of more than a certain size.
The overall rate of a suspension system is a combination of the spring rate and the rate of the tyre, which behaves like a spring. The combined rate of the two is not difficult to calculate, but the answer is surprising, because one would expect that a soft and a hard spring combined in series would have a rate somewhere between the two rates, but this is not so. The combined spring rate is softer than either of the component springs, as shown in the panel.
Spring types:
Leaf springs have been used for well over 400 years in something close to their present form, and for thousands of years before that in the form of a bow to propel arrows. In fact, it is one the most successful designs of all time, and is still in use on most trucks. In earlier days, complete sets of leaves were often doubled-up, one inverted above the other, to form a flat ellipse. Such springs were known as full elliptic springs. Most modern applications use only the bottom half of the ellipse, which is why such springs are called semi-elliptic. In the ’20s and ’30s, some cars were fitted with quarter elliptic springs, consisting of half a semi-elliptic, with the thick end clamped to the chassis and the eye fixed to the axle.
Leaf springs have a number of advantages. They are robust, and in an emergency a broken leaf can be replaced by the average blacksmith or machine shop, making such a spring ideal for a vehicle that has to travel into areas where modern repair facilities are not available. The inter-leaf friction has a certain damping effect, and the shape helps to locate the axle.
When leaf springs are manufactured, the individual leaves are given different curvatures, with the shortest leaf given the greatest curvature. This equalises the stress distribution, but causes the upper side of each leaf to rub against the underside of the leaf above it. This causes friction and wear, and on older designs the whole spring was often enclosed in a gaiter containing thick oil. Some modern springs have anti-friction material fitted between the leaves, while the edges are often rounded to reduce peak stress values.
Coil springs are lighter than leaf springs, and can absorb almost twice the energy for the same volume of material, so that a coil spring performing the same duty as a leaf need only weigh half as much. But this advantage is offset by the requirement that the coil and axle be located by separate arms. Some coil springs are close-coiled at one end, so that as the coils bottom, the effective length changes and the rate increases, ie the coil becomes harder.
Torsion bars are essentially straightened coils, and the small amount of space taken up by torsion bars also means that a number of four-wheel drive vehicles, especially ones with a separate chassis, use torsion bars for the front suspension, because the location of the coil would get in the way of the driveshaft.
Rubber springs have been used to a limited extent, notably on some Minis, and on the rear suspension of some trucks. The rubber is usually stressed in torsion or shear, and has the advantage that it can store a greater amount of energy than steel for the same mass. It also transmits very little noise. Air springs have been tried at various times, are often fitted to buses, and are again in fashion on a number of the latest luxury cars. The units, consisting of air under pressure housed in a bag that fits in the space where a coil spring would normally be, are now computer controlled. The advantages include the fact that the bounce frequency between laden and unladen conditions varies less than with a steel spring, and the ride height can be maintained, whatever the load, by increasing the pressure.
However, the best example of an air spring is the car tyre, which absorbs enough energy to qualify as a suspension spring.
A modern tyre has a spring rate that is typically 20 times higher than the rate of the suspension springs, and it’s interesting to note that the load carried has very little influence on the tyre rate.
Comfort is a very vague term, and the modern car is a result of many studies into the effect of the amplitude (size of the bounce) and frequency of a vibration on the human body. This was first studied scientifically by Maurice Olley, a great suspension pioneer, who worked under Henry Royce at Rolls-Royce, but later migrated to Cadillac. In 1932 he built a “bouncing chair” at one of the General Motors proving grounds in the US. It was built in such a way that a vertical movement of adjustable amplitude and frequency could be imparted to it.
Volunteers were asked to sit in this chair, and the amplitude was set at a given value while the frequency was gradually increased until the subject signalled that he or she felt uncomfortable. This experiment was repeated at many different amplitudes, and the results were tabulated. This enabled a relationship between the amplitude and the frequency to be established in such a way that values outside a threshold limit could be considered uncomfortable, while most people would regard values inside the limit to be comfortable. The results show that if the frequency is increased from three to six hertz, then the acceleration must be decreased from 1,5 to 0,24 m/s2.
Until the early ’30s, when most cars still had a beam front axle, it was usual for the front springs to be stiffer than the rear springs because they had to control the motion of the axle and wheels relative to the chassis, and any softening would have made the steering erratic. The rear springs were softer to improve comfort.
Maurice Olley’s 1932 investigations also included building a special car with masses on outriggers front and rear, to make it easy to adjust the moment of inertia (the pendulum effect), so that its effect on handling and comfort could be determined. He discovered that if the front suspension was made softer than the rear, the level of comfort would be vastly improved. This could not be done with a beam front axle without introducing a rapid side-to-side movement of the front wheels, called shimmy, as well as making the steering very sloppy. This result led to the general introduction of independent front suspension a few years later.
