IS A REVC SUSPENSION 'ION IN ERDUE?
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Part 1 -overnents Needed in the Suspension of Goods 'cies to Avoid Damage to Bodywork When ling Light, say Bodybuilders and Operators
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1-: practical information were required by a market
research worker on the springing of a private car, he would obtain a cross-section of driver and passenger opinion on the vehicle's road holding and on the comfort provided. A different approach would be necessary if the object were to obtain realistic views on the suspension characteristics of a heavy goods vehicle. It would then be more appropriate to seek the opinions of maintenance engineers and of bodybuilders.
Comfort for the driver may become of greater importance later on, but as yet it is secondary to overall running costs; the road holding provided by the conventional system may be regarded as adequate for most operating conditions. Maintenance is, therefore, the first consideration, and although some of the problems that apply to a bus or coach suspension correspond to those inherent in the design of private-car systems, the maintenance question has priority.
Conventional Springing
To show the wide interpretation of the term " maintenance " that is necessary in the present context, it is appropriate to quote the most outspoken comment on conventional springing heard during my investigation on the subject. This was made by a well-known specialist bodybuilder, Mr. Leslie Wilsdon, of Wilsdon and Co., Ltd., Solihull. .
In Mr. Wilsdon's view, a revolution in suspension design is long overdue. One of the most difficult problems that confront van builders, he states, is "the rough ride the body gets when the vehicle is returning light," which may lead to loosening of the joints and sometimes fracture of the members, unless the structure is unnecessarily robust—and heavy—for normal running in the laden condition.
This opinion is shared by many other bodybuilders and by a large number of those operators, notably C licensees, whose vehicles regularly make return runs without a load. As well as-mentioning damage to cabs and bodywork, C-licence operators complain that light running increases tyre wear and is often responsible for half-shaft fracture and other transmission troubles.
The extent to which hard springing can increase main c.16 tenance costs may have escaped recognition in some instances because the vehicle is grossly overloaded for at least 50 per cent, of its running time. A disproportionately heavy expenditure on repairs is then incurred on account of overloading, and any troubles resulting from the former cause are more easily overlooked.
Providing an unvarying ride under all conditions of loading is normally represented as the ideal at which suspension designers should aim, and vehicle manufacturers would welcome an increase in the effective spring base to improve stability. The "ideal "—in whatever form it takes—will be far more difficult to achieve in the commercial vehicle field than in the design of a private-car system with a constant-periodicity or other type of advanced characteristic. The most serious factor is the very high ratio of gross weight to unladen weight, which, taking into consideration the accepted need to design lorry chassis for an overload, may amount in an
extreme case to as much as to 1. In comparison, if a baby car is loaded with a full complement of passengers and luggage, the ratio, is unlikely to exceed 1-I to 1.
Great improvements have been made in the design of conventional leaf springs since the early days of the motor industry, and they have the outstanding advantage that no auxiliary linkage is required to cater for torque reactions, which reduces first cost and avoids complication. Moreover, the advances that have improved the reliability of leaf springs have also enabled their weight to be reduced. This is of particular importance when a soft suspension is required because weight is approximately proportional to flexibility. This weight factor is, however, a major obstacle to the use of leaf springs for systems giving a variable rate increase. Springs with auxiliary leaves which are brought into action at various stages of deflection, normally enable a two-rate or three-rate suspension to be employed, and are a feature of a number of existing vehicles. • They cannot, however, give a fully cushioned unladen ride without sacrificing laden stability.
Constant periodicity can be achieved with a leaf-spring system by varying the thickness of the leaves proportionate to their length (the shorter leaves are thicker than the longer ones) and by varying the gap dimensions. Such springs are relatively costly to produce and have not, as yet, been fully exploited. The rear springs of the Leyland Atlantean double-decker are of this type and provide a periodicity of 92 cycles per minute.
Apart from increasing the overall weight of the vehicle, a heavy spring has the serious disadvantage, particularly on a lightweight chassis, that the force required to accelerate its mass when hitting a bump is proportionately great, and even if the suspension has a low periodicity, vibration may be imparted to the chassis.
Coil springs and torsion-bars have a lower weight than
leaf springs for a given loading but cannot act as torque members or be employed to give a variable rate unless used in conjunction with a complicated linkage, or a separate " wind-up " device. The leaf type is, of the three, the more likely choice of designers as a spring element for a constantperiodicity suspension system.
One of the most important practical merits of the leaf spring is that its maintenance and replacement are thoroughly understood by .the average mechanic and parts. are readily obtainable. Moreover, if the operator decides to increase the stiffness of the suspension, he can adapt the simple method of adding an extra leaf.
Every goods-vehicle operator whom I questioned on the subject of. suspension said that no system would be acceptable unless repairs coUld be easily undertaken at a low cost. A. new system would obviously be suspect, and this assertion is supported by one of the foremost designers of commercial vehicles in the country. He emphasizes that he would not initially introduce an "entirely different" system• because sales resistance would be high however good the proven advantages of the springing.
. This designer believes that the conventional type of suspension will eventually be superseded by the rubber or air type, but envisages a rubber auxiliary being employed in conjunction with a leaf spring as an interim measure to satisfy the "conservative operator's mind."
The cushioning properties of air have long been regarded by some leading experts as offering a form of springing that could revolutionize suspensions for all types of vehicle, and the use by the Citroen company of an hydro-pneumatic system on the DS19 car and the successful application of air-sack units by General Motors to coaches and load-carrying vehicles, give practical evidence that air can be an excellent suspension medium. Moreover, reports from America show that the latter type is highly reliable and that its use reduces maintenance.
Suspension Matches Loading One of the outstanding advantages of both systems is that the springs are automatically adjusted to match the load, so that changes in the weight carried make no difference to the suspension characteristics or to the height of the vehicle. In the Citroen car the nitrogen contained in relatively small suspension units of spherical form is displaced by hydraulic fluid, the pressure of which is varied automatically to compensate load changes.
In the General Motors system the amount of air in the bellows is automatically increased to match the load, and the effective volume remains constant. Pressure release is also automatic in both systems, any change "up or down" being subject to a lag of several seconds to obviate variations as a result of normal spring action. The air bellows is supplied from a reservoir.
As applied to a G.M.C. tractor Unit twin bellows are employed at the front and larger single units at the rear. The axles are of a rigid type, and torque reaction is controlled by, transverse links and radius arms. An anti-roll bar is fitted at the rear.
Operating on a similar principle the air cells of the system produced by the American General Tire and Rubber Company, which are now made in this country by the Silentbloc group of companies, comprise a twocompartment sandwich-type rubber sack connected by a restricted orifice giving shock-absorber action. Solid rubber pads on the inside of the cells provide emergency springing in the event of air failure.
Constant Gas Quantity It is pertinent that the Citroen DSO car is generally credited with more advanced suspension characteristics than any vehicle with conventional springs, and it is significant, therefore., that the periodicity of the units
increases with higher deflections. From information available it would appear that the characteristic of an air bellows or cell is ysiirtuallr of the constantperiodicity type, and an investigation of the fundamental difference between the two systems may give a pointer. to future trends that will affect commercial-vehicle design.
Whereas extra air is introduced to, or released from, an air bellows or cell to compensate load changes, the amount of gas in the Citroen pneumatic unit is, it should be emphasized, constant. The wheel load of the latter is applied through a system giving a mechanical advantage, and it can be assumed that the gas operates_ at a very much greater pressure.
Under conditions of high instantaneous loading, it would seem probable, therefore, that Compression is more nearly adiabatic (which is the characteristic of piston-engine compression) than isothermal, in that much of the heat generated is retained for a sufficient time to increase the cylinder pressure and consequently the periodicity of the suspension with large deflections.
In the case of a relatively slow movement of the wheels, less heat would be retained and the periodicity would be lower. In operation, this should result in the relatively soft springing when the deflections are small and when the cycle time is proportionately of long duration, and much harder springing when larger deflections result from a rapidly applied load. Is this the ideal?
Citroen System Criticized
It has been suggested that Citroen engineers developed hydro-pneumatic suspension for the car in preference to adopting an air-sack type despite the added complications involved, because the system occupies a comparatively small space. Critics of the car suggest that the first cost of the suspension will preclude its application to other models and that maintenance problems will present insuperable difficulties after a few years in service. The first cost of the car is, however, relatively low, and it is intended to appeal to motorists who require a business vehicle that is both reliable and easy to maintain.
Mounted between the axle assembly and the chassis, the units of the air bellows and cell systems are normally employed without mechanical advantage in the. same way that rubber compression springs are used. The volume of air and the heat dissipation area are proportionately large, and heat generation would not, it may be concluded, have any material effect on the cushioning provided. If an air-pressure system is part of the stan
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dard equipment of the vehicle, the cost of the suspension, when the units are produced in quantity, should not be so much greater than that of conventional springs as to prove a deterrent to operators.
Although the protagonists of air springing claim that rubber suspension is already obsolete and say that British manufacturers, if they are wise, will concentrate on the development of air systems, rubber units may well provide the type of springing, at an acceptable cost, that would be suitable for vehicles in quantity production.
It should be pointed out that rubber can be compared with steel in .so far that its properties and performance characteristics Vary widely with treatment during production processes and according to the constituents of the compound. Notable advances have recently been made in evolving compounds and treatments for new applications, and developments are in progress which may greatly extend the scope of the material, This may particularly apply to the introduction of rubber-based suspension systems for both private cars and commercial vehicles.
Stressing rubber in compression is considered by some authorities to offer the most satisfactory way of employing the material to its best advantage as, a spring element. They point out that for the rubber to be stressed in shear it must be bonded chemically to some other material at a pressure which may be as much as 20 times the working pressure, and they state that this may cause deterioration of its properties.
If rubber be stressed in tension it is liable to tear and rupture and is, therefore, unreliable. A compound with the minimum of carbon black has improved antitear and anti-rupture properties, but its cost is very high compared with compounds of larger carbon-black 'content.
Rubber in Compression
When acting in compression, rubber may be deflected up to 45 per cent., and by employing a suitable compound it is possible to reduce hysteresis losses to a minimum. Reduction of hysteresis impairs the self-damping property of the material, which must he compensated for by auxiliary damping, but it also eliminates overheating and deterioration from this cause, and the characteristics of the rubber can be more exactly matched to suspension requirements.
Rubber stressed in shear has the advantage that it occupies less space for a given load rating than rubber in compression. The Metalastik concern have shown that the material can be employed in toggle-link bushes in such a way that it is initially stressed in shear, and subsequently in both shear and compression, to give a constant-periodicity suspension. This also applies to the Metalastik multi-disc type of spring employed for independent front suspension.
Midland Red maintenance engineers state that the toggle-link unit operates for more than 100,000 miles without replacement becoming necessary, and this tribute to rubber is typical of those heard in connection with older types of rubber suspension, notably the compression springs employed by the Scammell company. There can be no doubt, therefore, of the maintenance advantages offered by the material if .it is developed scientifically for the particular application. One of the few criticisms heard of rubber systems is that some forms are "almost impossible to damp effectively."
As an alternative to air being used as the only suspension medium, it is considered by some authorities that it could act as an auxiliary to rubber.