Designing for - Air Suspension D ISPLAYS of goods and passenger vehicles
Page 23
Page 24
If you've noticed an error in this article please click here to report it so we can fix it.
equipped with air suspension at recent Continental shows, notably at Frankfurt and Turin, have• aroused criticism of British manufacturers for their tardiness in exploiting this form of springing, despite its successful application to long-distance coaches in America over a number of years.
Practically every vehicle and trailer maker in this country is, however, experimenting with some form of air system or is closely considering its adoption after reviewing the advantages it offers. No manufacturers appear to doubt that the system would enable the suspension of their vehicles to be greatly ,improved, but reports are contradictory with regard to various technical aspects of the characteristics obtainable.
In his paper on the "Latest Progress in Motorbus Construction and Present Trends," read at the HamburgBerlin Congress of the International Union of Public Transport, Mr. L. Lagarrigue, of the Paris Undertaking, stated that the constant-volume air system was not acceptable for buses. More recently, the claim was made by Mr. H. Clements, of the British Belting and Asbestos Company, that it was necessary to preload an air system hydraulically to obtain satisfactory results. The two views are indicative of the complexity of the subject as seen by some vehicle and component designers. It is known that several manufacturers are evolving special chassis designs based on air-suspension units in preference to modifying existing chassis to accommodate the system. This policy should provide a means for reducing production costs as well as facilitating the introduction of advanced features. including those of potential benefit to the bodybuilder.
Big Future for Right Chassis The extent to which air suspension will aid the chassis designer cannot yet be accurately assessid. Its scope may be such that a rigid chassis could be provided in place of frame members that must, of necessity, act as part of the suspension system. Whether or not this will be the outcome is a moot point; there is little doubt that • those concerns who first produce a chassis which combines the structural advantages offered by the air spring with a form of chassis best suited to the service requirements of the vehicle will eventually command a sub stantial part of the market both in this country and overseas.
Future exports may well depend on the success of these projects, which will involve a large capital expenditure and a programme of intensified research; in some cases the opportunity may be taken to make other changes in design, including, for example, repositioning the power unit and transmission components. In total this may represent a major revolution in design, the import of which will not become apparent for a number of years.
In the meantime, it will be necessary to break down the inherent conservatism of the average operator in all the market areas and to establish beyond reasonable doubt the types of unit in every category which are the most promising in terms of development potential.
Prejudice to be Overcome A leading designer stated some years ago that operators would be prejudiced against any completely new form of suspension for a long period after it had been initially proved in service. Within the past few months another designer employed by the same company criticized existing designs of the air-bellows system as being too complex and too costly for vehicles in largescale production. He appealed for the elimination of the automatic levelling valve and its substitution by a simple form of valve control which could be reset with each change of load.
These comments show that, concomitant with the need to accelerate development work on all types of potentially suitable pneumatic and hydro-pneumatic system, vehicles should be marketed with substitution units so soon as it is known that they are reliable in service. Only in this way will it be possible to create favourable market conditions for the new models when they are ,produced at a later date. There is a sufficient number of operators who will accept air suspension immediately, if it is backed by the makers' guarantee, to justify adaptation of existing chassis. Moreover, these operators will, in the main, possess above-average technical knowledge, and their observations on the results obtained should be invaluable to manufacturers.
A fundamental difference between the constantvolume air-sack suspension and established hydropneumatic types is that the total weight of air is A211 increased to compensate for higher loading in the former system, whilst the latter incorporates a non-flexing air cylinder in which a given weight of air or gas is compressed in accordance with load fluctuations. With a heavily laden vehicle a high rate of spring deflection will cause the air to be compressed adiabatically in the hydraulically controlled system. This difference may have far-reaching implications. Although a hydro-pneumatic suspension is relatively complex the production of a low-cost type might be feasible, and it is of great importance that vehicle manufacturers and operators should have an early opportunity of comparing the merits of both systems, and their variants, based on accurate test results.