Spotlight o uspens ion
Page 52
Page 53
If you've noticed an error in this article please click here to report it so we can fix it.
ASYMPOSIUM on vehicle ride was held on Monday and Tuesday at the Advanced School of Automobile Engineering, Cranfield, Bedfordshire. Some 110 delegates heard papers on various aspects of the subject, including the characteristics of various forms of suspension and some of the problems arising from evaluation and development. Two papers, summarized on these pages, are relevant to commercial vehicles.
" THE accelerations of military vehicles during cross " country operation" was the title of a paper by D. Cardwell, assistant director of basic research, Fighting Vehicles Research and Development Establishment. This paper described a series of tests made by the F.V.R.D.E. on various types of tracked and wheeled vehicles to investigate the performance of their suspension systems under crosscountry conditions.
The objects of the survey were to collect data on the subject, which were required for several purposes. The design of equipment likely to be temporarily or permanently carried in vehicles making cross-country journeys has hitherto been based on inadequate data relating to the accelerations actually experienced. Information of this kind was required to assist designers of future vehicles, whilst the effects, from the crew point a view were also of importance. No attempt was made to make a close study of individual types of vehicle, since general data were required, but studies were carried out on vehicles with various .types of suspension.
As well as tracked vehicles of between 9.5 and 65 tons in weight, wheeled vehicles with capacities of between ton and 10 tons were tested. These were grouped into those having independent suspension for all wheels, those with beam. axles front and rear, a six-wheeler with beam axle at the front and rocking beams at the rear, and a trailer.
Each vehicle was driven over a series of test courses. These included a stretch of simulated Belgian pave with irregular potholes, divided into two graded sections. One had steps of between 1 in. and 1.5 in. between individual stones and the other had 1-in. to 2.5-in. steps. A more completely artificial course comprised rectangular-section beams laid at irregular intervals on a level concrete surface. The height of the beams was either 1 in. or 2 in. Trials were also conducted on natural courses of various kinds and degrees of severity on Bagshot Heath or, in the case of tracked vehicles, Long Valley, Farnborough, Hants.
Electronic equipment was used to record the accelerations experienced at various points in each vehicle. This was of the type which recorded results by means of a pen on a spool of paper, and it was found valuable to be able to check the results as the tests took place. Og was taken as indicating the condition with the vehicle at rest on the ground.
Thetests Thetests were carried out at the maximum speeds possible, subject only to the discretion of the driver as to what degree of discomfort was tolerable. It was found, in practice, that the driver formed the first line of defence against damage to the vehicle's suspension because he would slow down for any apparent severe irregularities. Generally he would try to limit the speed so as to avoid losing contact with his seat, since this implied an inability to respond to lateral forces.
It was found, in this connection, that a firm padded driver's seat was more satisfactory than a soft resilient seat for cross-country work, because of the avoidance of resonant effects. On the artificial courses, displacement of the driver rarely exceeded 1 in. but, on the natural courses, displacements of 4 in. occurred occasionally. It was considered that a displacement of between 4 and 5 in. represented the maximum tolerable.
c22
A seat belt was found to be beneficial to the drivt under these conditions; but it was considered that th vehicle suspension was liable to require strengthening if h took advantage of the ability to achieve higher speec without undue discomfort.
Under cross-country conditions it was found that the rid quality depended mainly on the natural frequency of di suspension system. This, in itself, confirmed the expects tion that a soft spring would be the most comfortabh although it was qualified by the effects of resonance an of bump and rebound stops.
It was found, somewhat surprisingly, that independer suspension systems differed little from beam axle system in terms of ride quality in itself under these condition; The principal merit of independent suspension in the ch cumstances of the tests was in improved steering contro Some investigations were carried out into the problem of loose loads. It was found to be not uncommon for loose load to " jump " to a height of 5 ft. during the test5 Tentative runs with a 0.5-ton loose load gave rise to sorn anxiety for the safety of vehicle and driver. It was con sidered highly desirable that, wherever possible, all equip ment should be firmly attached to the vehicle for cross country running.
So far as the vertical accelerations experienced weri concerned it was found that somewhere between 2g an 2.5g was the general maximum; occasional bumps of uj to 4g were experienced on the vehicles. It was consideret that these were probably caused by minor misjudgment on the part of the driver, due to the terrain proving tt undulate more than it appeared to do.
The maximum amplitude of oscillations was found to bl at either the front or rear of the vehicle and measured uj to 10 in.
Trailers were found to undergo more severe accelerations reaching more than 6g at times. It was considered that thi was because they had no crew, the driver of the track)] tending to accelerate as soon as his own vehicle had negoti ated each awkward part of the course.
In the dicussion which followed, Mr. V. E. Gough, o the Dunlop Rubber Co., Ltd., suggested that a suspensiot system designed so that motion can be expected to dit away after the first impact would give the driver greate confidence under cross-country conditions. An analogy o this was in the inherent stability associated with an under steering characteristic as opposed to the instability of ; vehicle which oversteers. Mr. Cardwell said that althougl this aspect of the subject had not been specifically studied he felt intuitively that the idea was sound.
In answer to other queries Mr. Bradfield, of F.V.R.D.E. stated that experience showed that a vehicle which conk withstand 250 miles of the 2-in beam course and 50 mile of the pave course without damage was unlikely to givt appreciable trouble in normal service. Damage to tht vehicle superstructure was often as great a problem a4 damage to the suspension.
THE paper given by Mr. A. J. Hirst, chief technica engineer of Metalastik, Ltd., dealt with the applicatiot of rubber springing to vehicles. Rubber had the charac teristic of being able to store large amounts of strait nergy in relation to its unit weight and the space it ,ceupied, he said. It was, however, easy to throw away is advantage in weight and space due to the metal parts Iso required to form the complete suspension system. he greatest advantage in this respect was obtainable if le rubber could be used to guide the axle or its equivalent s well as carry the load.
The simplest form of rubber spring was that in which le rubber was loaded entirely in compression and this ad proved very suitable for heavy railway applications in Ihich a 30-ton load was supported on a 13.5-in.-dia. unit pith a height of 16.5 in. and a deflection of 2.125 in.
Although it was possible to produce a somewhat softer pring by altering the rubber mix or using a hollow unit f this form, the development of springs suitable for direct aading on road vehicles had proved impracticable. This as because the spring became relatively too tall and .arrow to resist buckling. It was necessary to use the ubber in combined shear and compression, and pairs of .nits of this form had been successfully used in wee ormations for direct loads of five tons.
The independent front suspension used on current buses nd coaches built for their own services by the Binningam and Midland Motor Omnibus Co., Ltd., incorporated ubber spring units employing this principle. In these an inequal-length double-wishbone layout was used, with the pring mounted in a semi-diagonal position. A stabilizing date incorporated at the mid-point of the spring was finked o the upper wishbone in such a way as to equalize deflecions. This system was suitable for sprung weights of oetween 1.5 and 3 tons per wheel; it could be adapted for ise with weights down to 1 ton per wheel.
The rear suspension used on 13.M.M.O. vehicles was of he link type in which pairs of rubber bushes are conlected by links at front and rear of each side of the axle, Inc bush of each pair being attached to the frame and one to the axle. This was suitable for sprung loads of .5 to 2 tons per wheel, although on a lightweight singlelecker it could fall to 1 ton per wheel. The high ratio If unsprung to sprung weight was a problem on.this type if vehicle and Mr. Hirst favoured the use of single rear yres for this reason although he acknowledged that they ad drawbacks in other directions.
The link type of suspension had an inherent advantage n that the characteristics could be adjusted to give variable ate effects. With the links in compression, for example, here was a useful "tumbler-switching " effect.
Neither of these systems was suitable for vehicles of he private car size. Reference was made to the suspenion of the B.M.C. Mini models, in which direct loading if rubber springs was achieved by use of a system of everage, and Mr. Hirst remarked that the resulting arrangenent, although causing high loadings, gave a low unsprung weight—even by independent suspension standards.
The designer's initial attitude to rubber suspension had oeen one of caution, particular worries being the effect of a.tigue and the reliability of bonding. Neither of these lad given appreciable trouble in practice.
The phenomenon of creep did, however, remain, although ts magnitude was less than sometimes realized. Some reduction could be achieved by choice of the rubber mix. Natural rubber had proved best so far, but some synthetic materials were giving experimental results which gave grounds for optimism in this respect.
The measurement of creep effects on actual vehicles was complex, for, in addition to static and dynamic creep, there was a negative creep effect which depended on the temperature and a work-softening effect, all of which had an effect on the small variations of vehicle height. The effect of wide variations• of temperature did not, incidentally, appear to have any ill effect on rubber suspension. Support for this impression was given by its use on railway rolling stock in Russia.
The inherent damping effect of rubber sometimes caused designers to wonder whether separate hydraulic dampers could be eliminated. Although it was possible to aim at this in the choice of material and in the design stage, Mr. Hirst felt that more could be gained from aiming at increasing the stress range of the rubber.
Mr. Hirst gave calculated figures for the vertical accelerations of the body and axle of a lorry with suspension having various rates. Account was taken of the resilience of the tyre and the variation in this due to load. Alteration in the spring rate from 4,000 lb./in. to 500 lb./in. reduced the vertical acceleration of the body in unladen condition when the wheel rolled over a 0.1-in, step from .212g to .038g with sprung weight and axle weight both of 2,500 lb. and a tyre deflection rate of 16,000 lb./in,
Other figures showed that a reduction in unsprung weight caused an increase in axle acceleration, but it must be remembered that the reduction in the mass to be held down counterbalanced the effect of this. It was felt that unsprung weight did have a considerable influence on the ride because of the uncomfortable nature of the motion caused by axle tramp. Moreover, the higher natural frequency of a lighter axle unit enabled road irregularities to be followed more precisely.
Details were given of an analysis of ride characteristics on a B.M.M.O. D.9 front-engined double-decker bus from which it was concluded that the independent front suspension and relatively large front overhang of this model were beneficial to upper-deck ride comfort.
During the discussion, Mr. A. E. Moulton, of Moulton Developments, Ltd., was able to confirm the reliability of rubber suspension. Some two million rubber spring units of the type used in the B.M.C. Mini models were now in service and no actual failures of the springs had been reported. With regard to creep, he felt that although it was a nuisance its effect was minor and could be counteracted quite simply when necessary, for example by adding washers. On the question of global temperatures, 20,000 vehicles were now in service in Canada without giving rise to complaint, the stiffening effect due to low temperature evidently being insufficient to become noticeable to any serious degree.
Mr. D. Flodkin, of the Rootes group, struck rather a pessimistic note in connection with rubber suspension. He felt that its disadvantages, in terms of creep and the comparatively wide tolerances which had to be accepted, were not counterbalanced by sufficient advantages to justify its use in place of steel.
Mr. Hirst, in reply. said that although each material must be judged on its merits, rubber offered advantages particularly in weight and running costs. Rubber springs used on buses had been kept in service without any attention after mileages of 250,000. A.A.T.