AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

DESIGNING 32-TOIN ;ROSS A TICS

2nd December 1966
Page 90
Page 91
Page 92
Page 95
Page 90, 2nd December 1966 — DESIGNING 32-TOIN ;ROSS A TICS
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

rTHERE are many ways of killing a cat, and also it would seem

there are many ways of obtaining the new gross vehicle weight of 32 tons, now available for articulated vehicles within the Ministry of Transport Regulations. These, allowing the upgrading of vehicles above 24 tons, apply to vehicles built after February 1963, and give rise to an unnecessary urgency which has given designers and manufacturers considerable problems, not always appreciated by the people who use the vehicles.

Firstly there is the problem of converting existing vehicles. The most significant design feature for the uprated gross tonnage is the stringent requirements for braking performance. Two completely independent brake systems are now obligatory—thus ensuring safety in the event of failure of either system.

The brake efficiencies laid down in these regulations are minimum of 50 per cent for the main system and 25 per cent ft the auxiliary system. An efficiency of 16 per cent is required ft the hand or parking brake, this being the equivalent of holdin on a 1 in 6 gradient. In upgrading, the regulations also allow ft the axle loadings to be increased, the maximum allowable on single axle with tyres in twin formation or low profiles is no 10 tons. In order to obtain this 10 tons at the driving axle, tk SMMT recommended imposed fifth-wheel loading has been it creased to 9 tons for two-axle tractive units.

Thus it will be seen that conversion of existing vehicles mear considerable modification to the brake system and must also ensure that the driving axle is loaded to the capacity in order to obtain something like reasonably acceptable traction. We have many two-axle tractive units capable of hauling 32 tons already in service, and having been given the opportunity to carry the new tonnage, hauliers are only too anxious to do so without any delay.

Alternative trailer arrangements which will be used with these two-axle tractive units can now be examined, although it is my opinion that using two-axle units for 32 tons gross can only be a temporary expediency, tractability being only just acceptable with the limit of 10 tons at the driving axle.

The Construction and Use Regulations permit a gross vehicle weight of 32 tons on four-axle articulated vehicles provided that the centres of extreme axles are not less than 38 feet and the overall length of vehicle does not exceed 42ft. 7.75in. These requirements allow a mere 4ft. 7.75in. for the total dimensions of tractive unit front overhang and trailer rear overhang, which gives the designer no latitude whatsoever in positioning his bogie. It is normal when designing a carrier to position the platform, fifth-wheel mounting and bogie so that an equally distributed load gives the required loadings at the fifth wheel and at the bogie.

It can be seen from Illustration 1 that this vehicle is not a balanced machine as the centre of the body does not coincide with the theoretical centre of payload. Furthermore, due to the extreme length of this combination, interchange between tractive units must be very carefully checked: if a shorter wheelbase unit is coupled the extreme axle centres will be too close for the gross tonnage, and if a longer wheelbase unit is coupled the overall length will be excessive. Both conditions would make the vehicle illegal.

The second combination (Illustration 2) must utilize a trailer with three axles. This configuration has several favourable aspects worthy of commendation. By introducing a third axle the distance between the centres of extreme axles can be reduced to 32 ft., a reduction of 6 ft., and this does lend itself to a more balanced machine as the centres of body and payload now coincide. Furthermore, by adding an extra axle to the trailer, the increase in payload achieved between 30 tons and 32 tons g.v.w. is likely to be in the region of 30 cwt. This increase is as good as can be achieved with any sort of arrangement. The cost of such a set-up is also likely to be favourable since axles and suspension are simple and all three axles are identical, and the number of tyres can be reduced from eight to six, each axle carrying only 6 tons each.

On the other hand there are disadvantages attached to this type of trailer, because the wide spread of the three axles is most certain to set up tyre scrub which will prove to be unacceptable. The leading axle tyres wear more rapidly than those on the other two axles; this is indicative of the fact that a three-axle bogie will rotate not about the centre axle but about a line somewhere between the last two axles. This being so it is obvious that the effective wheelbase of the whole combination will be increased, and cornering characteristics—and in particular cut-in—will be adversely affected.

Such shortcomings in the behaviour of three-axle trailers fitted with rigid axles has led to the development of trailers incorporating various designs of non-rigid axles.

The most simple example utilizes normal trailing axles with leaf-spring suspension, the ends of which have lateral clearance in the hanger brackets on one or more axles. This clearance is controlled by side-loaded springs; when cornering the side loading engendered allows the spring and axle to I\ivander—thereby tracking within certain confines.

A more refined type of self-tracking axle incorporates the Ackerman principle of steerage, and is provided with king pin assemblies similar in every way to a tractive unit front axle, except that they are swept forward in order to obtain the necessary caster action. It must be noted that this forward offsetting of the king pin has an undesirable effect when the vehicle is in reverse motion, and because of this a locking device is normally provided which will lock the axle in a central position. This locking device is attached to the track rod which joins the two stub axles, and can be remotely controlled from the driving cab.

An axle such as this could, in the extreme, be fitted to all three positions of a three-axle trailer, but in fact this is not practicable due to probable weight limitations. The lack of lateral stability when cornering at speed can also be a serious drawback to a bogie set-up which does not utilize at least one fixed axle.

Two self-tracking axles, mounted one fore and one aft of a normal fixed axle, have the beneficial effect of practically eliminating tyre scrub. But due to the fact that the turning centre of the assembly will be on the fixed axle centre line, the wheelbase will not effectively be reduced and no reduction of cut-in will be achieved. It must also be noted that to achieve reasonable side stability, and because of the limited loading of these trailing axles, the centre axle would need to be loaded considerably higher than the outer two axles. And to avoid uneven or uneconomical tyre equipment, small size tyres would be fitted in twin formation on the centre axle and in single formation on the two outer axles. A similar set-up using two tracking axles, both fitted to the rear of a fixed axle, would have the effect of shortening the effective wheelbase considerably, thus eliminating tyre scrub and reducing cut-in. I know of no experiments with this configuration but oversteer would need to be checked very carefully.

One more arrangement, using two fixed axles and with the rearmost axle self-tracking, presents no problems, tyre scrub is reduced to acceptable limits and cut-in on cornering is also improved to a very satisfactory degree. This I regard as the best method to adopt if three-axle trailers are essential to obtain the gross loading with two-axle tractive units. But it must be remembered that the distribution between axles of the 18-ton ground load must be considered very carefully and critically. Well-balanced braking must be designed and heights of fifth-wheel mountings be consistent, as any excessive angular difference in carrier frame could affect the individual loadings to a surprising degree and unbalanced braking would result.

It must be quite obvious that it is impossible to obtain an ideal geometry of steering with a long-wheelbase articulated vehicle; that is to say, it is impossible to eliminate both cut-in of the inside edge of the trailer and outswing of the rear corner. A normal fixed threeaxle trailer is shown in Illustration 2. If this trailer were fitted with three steerable axles (Illustration 3) so that the trailer could prescribe a path exactly related to the path of the fifth-wheel kingpin, it would be necessary for the trailer wheels to alter steering lock progressively from the straight ahead at the outset of the turn to something like 45 deg. maximum lock at the worst condition. If this geometry could be arranged, the cut-in of the trailer would be at a minimum but at the expense of considerable swing out of the tail end. This could only be achieved by the use of turntable mounted axles, and to go to this extreme would not be practicable.

Steering gears have, however, been perfected which give a high degree of steerability, with axles mounted on turntables and interconnected through linkages. In some cases these are controlled by an A-frame, the apex of which is pin-mounted to the trailer chassis, and in other cases are controlled by direct linkage to the rear end of the tractor. A further steering gear takes its controlling motion from the angular movement of the tractor and trailer through the medium of pinion and crown wheel mounted between the fifth-wheel plates. These systems are both more expensive and heavier than the simple tracking type and I would think that the weight disadvantage and also their complexity will preclude their use in this country in any quantity, unless legislation is implemented stating maximum allowable turning circles to be achieved. Steering axles are most sensitive to wheelbase to obtain the correct angular movements of wheels, so it would seem that each instalment must be suitably tailored.

There are several types of three-axle tractive units, but basically they can be divided into two groups: the traditional six-wheeler units having a double-drive bogie, and the twin-steer units having two steering axles and a single driving axle.

Both types have advantages and disadvantages common only to their particular breed. The double-drive is probably the most used three-axle unit in this country. Whatever the suspension, whether there be fitted a non-reactive system—which entails considerable linkage—or a single centre-mounted laminated leaf spring suspension—which requires very heavy side-members to obviate high stresses due to the considerable rear overhang from the spring bracket—or whether the more elaborate beam and spring system is used, it is inevitable that this type of machine must carry a considerable weight penalty when compared with the twin-steer units.

The double-drive outfit does however have the great attribute of retaining maximum traction under conditions of poor adhesion, such as gravel, soft earth or mud. But except for operators who work continually off the road I think we can discount this type of vehicle for normal tninking service at 32 tons in this country. For it must be remembered that the five-axle 32 gross outfit must enable the operator to increase his payload by well over 1 ton in comparison with a 30 ton g.v.w. four-axle outfit if the extra expense is to be justified. Weight is not the only disadvantage. The higher cost of this Thiele is certainly a consideration, as is also the fact that the louble-drive unit, due to its fifth-wheel position being very much 'urther in front of the rearmost drive axle, will not have the nterchangeability with carriers used with the rest of a mixed leet. The fifth-wheel coupling will also be considerably higher han is necessary on any other type of motive unit, and this eature not only affects interchange between carriers but also nevitably raises the centre of gravity of the outfit as a whole. Reeling control is also adversely affected: the double-drive rear 'ogle has to be steered by a single front axle and even though )ower-assisted steering will reduce driver fatigue, the wear of this ixle and its tyre equipment will be greatly increased—the shorter he wheelbase the more this will apply.

So we come to the three-axle unit having twin-steered axles, and here, I think, we have the genus of vehicle which is at this :ime being more rapidly advanced in design than any other road vehicle. The more traditional twin-steer with first and second axles close together, the fifth wheel mounting well ahead of the drive axle in order to load the two front axles to their capacity, will consequently need a frame of considerable section and weight to reduce stress. This type of twin-steer is but a derivative of the rigid eight-wheeler—shortened in wheelbase and having one of the rear axles removed. The second twin-steer configuration has the additional steering axle close coupled to the driving axle, the fifth wheel can be in a more normal position and will allow certain interchange with various carriers in the fleet as well as being lighter in weight.

Both of these two twin-steer chassis (having a single-drive axle) can suffer from lack of traction under certain conditions of terrain, and dubious tractibility even under good road conditions, a maximum driving axle ground load of 10 tons and 32 tons gross vehicle loading gives only 31 per cent adhesive weight. This is the major fault with any single-drive unit and the need arises to accommodate some design of load transfer system which will satisfactorily obtain a minimum of 37 per cent adhesive weight for traction.

The only British example incorporating this feature at present is the Scarnmell Trunker (Illustration 4), an extremely short wheelbase, compact unit, with exceptional manoeuvrability which utilizes an air bellows to control the loading in the second axle suspension—so that under normal laden conditions the front and second axle ground loads are both 3.5 tons and the driving axle 10 tons.

By operating an air valve in the driver's cab the axle loadings can be instantly altered to 4.5 tons front, 0.6 tons second and 11.85 tons driving axle, this giving ample tractibility.

Handling, manoeuvrability and safety are the essential requirements for heavy haulage machines, with extremely congested roads, tight roundabouts and corners and in particular works and factory entrances which were designed and built long before the vehicles we now must accept as commonplace were even vaguely visualized. If for no other reason, it is my opinion that the ideal articulated vehicle must consist of a three-axle, shortwheelbase, twin-steer motive unit, with provision for obtaining at least 37 per cent adhesive driving axle weight, a horse power to tons weight ratio of at least 6, and having preferably a close ratio semi-automatic gear box, coupled to a two-axle semi-trailer, the front trailer axle to be a conventional fixed axle and the 2nd trailing axle to be of the self-steering type. The braking systems for both driving axle and trailing axle must be controlled by loadsensitive valve gear to ensure absolutely safe braking under all conditions of loading and road adhesion.

It now seems certain that it will only be a matter of time before the maximum vehicle loads and lengths will be still further increased to enable the carriage of 40 ft.-long containers, but it is at this moment a matter of conjecture as to whether the allowable maximum will be 36 or 38 tons; the overall length must be increased by at least 6 feet to something like 48.5 feet. It is also unknown at present whether axle loadings are likely to be increased or if axle spacings will be affected. With so many unknown quantities it is impossible to make any straightforward gaze into the future. The future is indeed hazy. There is little doubt that we shall all need the luck from a shamrock if we are to be inflicted in this country with 48.5 ft.-long 38-ton g.v.w. artics as a common and every-mile occurrence, in order that we may keep up with Continental trends where the roads and bridges were designed to accept such machines.

Even if given special dispensation, these vehicles—as container transport grows more common—will present problems for all concerned: to traffic control and to the drivers in particular and also to the general public—because although these vehicles will be primarily motorway users, they will still need to get from the docks, warehouses and factories to the motorway beginning. The problems for the designer are also likely to be considerable. But unlike everyone else concerned the designer would not be happy if he hadn't any problems.

But the vehicle I visualize for the future, whatever the tonnage, will need to be as comfortable to drive as the modern private car. This will necessitate considerable thought being given to the design of suspension units—and to transmission assemblies and the crew cabin.

Independent suspensions for the road wheels could help solve the problem—and a twin-steer, double-drive motive unit with fully automatic transmission could be the possible answer.

Design improvements in heavy transport vehicles have in the last decade taken giant strides and no doubt this will continue until the acme is reached, not only in comfort but also in safety.

Tags

People: ROSS A TICS

comments powered by Disqus