1012 Variable rate leaf suspension springs

The purpose of the suspension is to protect the body from the shocks caused by the vehicle moving over an uneven road surface. If the axle were bolted directly to the chassis instead of through the media of the springs, the vehicle chassis and body would try to follow a similar road roughness contour and would therefore lift and fall accordingly. With increased speed the wheel passing over a bump would bounce up and leave the road so that the grip between the tyre and ground would be lost. Effectively no tractive effort, braking retardation or steering control could take place under these conditions.

A suspension system is necessary to separate the axle and wheels from the chassis so that when the wheels contact bumps in the road the vertical deflection is absorbed by the elasticity of the spring material, the strain energy absorbed by the springs on impact being given out on rebound but under damped and controlled conditions. The deflection of the springs enables the tyres to remain in contact with the contour of the road under most operating conditions. Consequently the spring insulates the body from shocks, protects the goods being transported and prevents excessively high stresses being imposed on the chassis which would lead to fatigue failure. It also ensures that the driver is cushioned from road vibrations transmitted through the wheels and axle, thereby improving the quality of the ride. The use of springs permits the wheels to follow the road contour and the chassis and body to maintain a steady mean height as the vehicle is driven along the road. This is achieved by the springs continuously extending and contracting between the axle and chassis, thereby dissipating the energy imparted to the wheels and suspension assembly.

A vehicle suspension is designed to permit the springs to deflect from an unladen to laden condition and also to allow further deflection caused by a wheel rapidly rolling over some obstacle or pot hole in the road so that the impact of the unsprung axle and wheel responds to bump and rebound movement. How easily the suspension deflects when loaded statically or dynamically will depend upon the stiffness of the springs (spring rate) which is defined as the load per unit deflection.

Applied load i.e. Spring stiffness or rate S = ———,—:-

Deflection

A low spring stiffness (low spring rate) implies that the spring will gently bounce up and down in its free state which has a low natural frequency of vibration and therefore provides a soft ride. Conversely a high spring stiffness (high spring rate) refers to a spring which has a high natural frequency of vibration which produces a hard uncomfortable ride if it supports only a relatively light load. Front and rear suspensions have natural frequencies of vibration roughly between 60 and 90 cycles per minute. The front suspension usually has a slightly lower frequency than the rear. Typical suspension natural frequencies would be 75/85 cycles per minute for the front and rear respectively. Spring frequencies below 60 cycles per minute promote car sickness whereas frequencies above 90 cycles per minute tend to produce harsh bumpy rides. Increasing the vehicle load or static deflection for a given set of front and rear spring stiffness reduces the ride frequency and softens the ride. Reducing the laden vehicle weight raises the frequency of vibration and the ride hardness.

Vehicle laden weight, static suspension deflection, spring stiffness and ride comfort are all inter-related and produce conflicting characteristics.

For a car there is not a great deal of difference between its unladen and fully laden weight; the main difference being the driver, three passengers, luggage and full fuel tank as opposed to maybe a half full fuel tank and the driver only. Thus if the car weighs 1000 kg and the three passengers, luggage and full fuel tank weigh a further 300 kg, the ratio of laden and unladen weight will be 1300/1000 = 1.3:1. Under these varying conditions, the static suspension deflection can be easily accommodated by soft low spring rates which can limit the static suspension deflection to a maximum of about 50 mm with very little variation in the natural frequency of vibration of the suspension system. For a heavy goods vehicle, if the unladen weight on one of the rear axles is 2000 kg and its fully laden capacity is 10 000 kg, then the ratio of laden to unladen weight would be 10000/2000 = 5:1. It therefore follows that if the spring stiffness for the axle suspension is designed to give the best ride with the unladen axle, a soft low spring rate would be required. Unfortunately, as the axle becomes fully laden, the suspension would deflect maybe five times the unladen static deflection of, say, 50 mm which would amount to 250 mm. This large change from unladen to fully laden chassis height would cause considerable practical complications and therefore could not be acceptable.

If the suspension spring stiffnesses were to be designed to give the best ride when fully laden, the change in suspension deflection could be reduced to something between 50 and 75 mm when fully laden. The major disadvantage of utilizing high spring rates which give near optimum ride conditions when fully laden would be that when the axle is unladen, the stiffness of the springs would be far too high so that a very hard uncomfortable ride would result, followed by mechanical damage to the various chassis and body structures.

It is obvious that a single spring rate is unsuitable and that a dual or progressive spring rate is essential to cope with large variations in vehicle payload and to restrict the suspension's vertical lift or fall to a manageable amount.

10.12.1 Dual rate helper springs (Fig. 10.84(a)) This arrangement is basically a main semi-elliptic leaf spring with a similar but smaller auxiliary spring located above the main spring. This spring is anchored to the chassis at the front via a shackle pin to the spring hanger so that the driving thrust can be transmitted from the axle and wheel to the chassis. The rear end of the spring only supports

Fig. 10.84(a-g) Variable rate leaf spring suspension

the downward load and does not constrain the fore and after movement of the spring.

In the unladen state only the main spring supports the vehicle weight and any payload carried (Fig. 10.84(a)) is subjected to a relatively soft ride. Above approximately one third load, the ends of the auxiliary helper spring contact the abutments mounted on the chassis. The vertical downward deflection is now opposed by both sets of springs which considerably increase the total spring rate and also restrict the axle to chassis movement. The method ofproviding two spring rates, one for lightly laden and a second for near fully laden condition, is adopted by many heavy goods vehicles.

10.12.2 Dual rate extended leaf springs (Fig. 10.84(b))

With this semi-elliptic leaf spring layout the axle is clamped slightly offset to the mid-position of the spring. The front end of the spring is shackled to the fixed hanger, whereas the rear end when unloaded bears against the outer slipper block. The full span of the spring is effective when operating the vehicle partially loaded. A slight progressive stiffening of the spring occurs with small increases in load, due to the main spring blade rolling on the curved slipper pad from the outermost position towards its innermost position because of the effective spring span shortening. Hence the first deflection stage of the spring provides a very small increase in spring stiffness which is desirable to maintain a soft ride.

Once the vehicle is approximately one third laden, the deflection of the spring brings the main blade into contact with the inner slipper block. This considerably shortens the spring length and the corresponding stiffening of the spring prevents excessive vertical deflection. Further loading of the axle will make the main blade roll round the second slipper block, thereby providing the second stage with a small amount of progressive stiffening. Suspension springing of this type has been successful on heavy on/off road vehicles.

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