Frame Rigidity and Suspension Flexibility in the Six-wheeler.
Page 43
Page 44
Page 45
If you've noticed an error in this article please click here to report it so we can fix it.
DURING the past year or two Immense progress has been made in the development and design of the rigid frame six-wheeled chassis, and the efforts made have culmiriated in the W.D. subsidy model, which many people are inclined to look upon as "the last word" in this class of vehicle, and it certainly does put up an excellent performance . and possesses the great advantage that it is almost as good for,cross-country running as a vehicle designed solely with this purpose in view, whilst as a means for transport on good or bad roads, it is actually better in many respects than the ordinary four-wheeled type. There is, however, no such condition as finality in design, and if is a mistake to consider anything as being sufficiently near to perfection, for that way lies stagnation and the arrest of healthy growth.
Progress demands continual modification, adaptation, and even supersession. For instance, the so-called rigid-frame six-wheeler of the normal pattern has anything but a really rigid frame, and it is this lack of rigidity which sets chassis designers and body-builders some problems which are often extremely difficult to elucidate. The prime cause appears to be that whilst great care is usually taken to provide three-point suspension for the engine and gearbox, that most important member, the frame, which is supposed to form a satisfactory foundation for the body, is four-point mounted on two sets of wheels which, when travelling over rough country, are more often than not at widely different levels and angles. The springs may do much to alleviate the torsional stresses on the frame which result, but that they are not altogether effective in this can easily be confirmed by anyone who has seen a vehicle with a frame supported at four points traversing rough country.
A simple and apparently effective solution to this problem has been found by the designers of a new six-wheeled chassis which we describe elsewhere in this issue. In this model the frame is given effective three-point mounting by providing a centrally pivoted cross-spring for the front axle, and the relative position of the frame at any time thus becomes a mean of the positions of the four driving wheels of the bogie, with the result that there is practically no torsion on it.
It might be thought that the effect of this would B17 be to reduce the vehicle stability, but in practice this does not appear to be the case, although in the particular vehicle to which we allude the rear springs support each frame side member at two points, which may exercise a beneficial effect in this direction. It has already been shown by drastic tests carried out on the six-wheeler that the one weakness appears to lie in the arrangement of the -front axle and springing, for when directly crossing narrow trenches with practically opposing faces the wheels stubbed against them and sometimes forced back the front axle. With the type of crossspringing on the latest six-wheeler robust radius arms convey such stresses direct to the chassis frame, but the front axle must necessarily be of great strength.
An Enterprising and Prosperous Industry.
ONE of the pleasant duties of the staff of a technical journal, such as The Commercial Motor, is to visit the manufacturers of commercial vehicles to discuss technical matters and recent advances with the designers and production staff, to join the directors and heads of departments at lunch and to exchange views upon general and particular policy, often to argue such matters from opposite sides and to inspect the works and the products which are going through. The information arising out of these visits is, of course, afterwards placed at the disposal of our readers, either in the columns of the journal, or in the voluminous correspondence which flows through the editor's desk.
., It is more than pleasant to be able to place on record our sincere appreciation of the ability, the fixity of purpose and the pluck which have enabled the commercial motor industry to survive the post tvar upheaval, which has ruined other industries nd which at one time threatened the manufacure of industrial vehicles with extinction. The ooding of the world's markets with cheap exService lorries was a severe infliction and we remember prophesying seven lean years for the Industry as a consequence. Whilst some concerns found the road to recovery before the seven years had elapsed, taking the trade as a whole the transitional period has been a fair average, and the industry seems not only to be well on the road to recovery, but to be bounding forward along it. The principal concerns are full up with crders ; there is evidence of progress and of advanced thought at every factory and in every concern. Our pages, in fact, testify to the continuous work in producing new and better models, so much so that the character and range of commercial motor vehicles available seven years ago would look paltry when compared with those which to-day appeal to transport users. If the commercial motor industry is once again, and so quickly, prosperous and busy, it is the outcome of enterprise, the reward of endurance and determination to surmount all difficulties, and the result of giving good value and good service.
'The Merits of the Unit System.
EITER since the first example of the combined engine, clutch and gearbox was put into the chassis of a motorcar (and we happened to be present, many years ago, when the unit was being assembled on the bench), discussion upon the merits and demerits of the system has never ceased. In the course of the years which have elapsed we have seen the idea growing in favour, albeit somewhat slowly in the commercial vehicle world, and we have never met with any experience that has shaken or tended to shake our confidence in it.
',It seems so sound to take the whole of the parts of the power unit and to make a bench job of their marriage, to secure the alignment of shafts which the system offers, and to know that this alignment cannot be lost through distortion of the vehicle frame or in misinformed or ill-judged action on the part of workmen during a subsequent overhaul. The saving of weight alone should be a matter of some moment to the commercial vehicle user ; it ranges from 14 cwt. to 2i cwt., according to the power of the engine, and if consideration be given to the cost in fuel, lubricants, tyres and wear and tear of moving this avoidable weight throughout the whole length of the life of a chassis (which for accountancy purposes may be put at 150,000 miles, but is invariably longer), it will be seen that a substantial saving can be effected by the adoption of the unit system. The chief argument against combining the three principal components of a chassis is the possibility of loss of accessibility. This argumenthas lost all its force now that the leather-faced cone dutch has given place to the dryplate clutch, which calls for practically no attention between the normal overhaul periods for the whole unit, provided it be safeguarded against The inroads of dust and grit. Adjustments of
the clutch-operating mechanism and of the clutchbrake pad and the checking of the engine timing gear are all rendered easy by the provision of a large inspection door to the clutch pit. We have found that the gearbox of a combined unit is rendered more accessible for inspection by thus being brought forward away from the cab or body, whilst the gearbox can always be detached and removed from the bell housing if it be not desired to remove the whole unit to the bench for attention. These and other minor arguments all go to show that the unit system has well proved its usefulness and justified the good opinion held for it in many quarters.