IS THE ARTICULATED SIX-W ELER WRONGLY LAID OUT?
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More Weight Should be Carried by the Front Wheels. Bigger Load Capacity and. Better Road-hol4ing Would Result It Would Involve Moving Forwards the Turntable and Lengthening the Tractive Unit, but Not the Overall Dimensions
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pROPER weight' distribution affects performance, maintenance, cost and safety, so it pay§ to get it right," says Mr. F. B. Lautzenhiser, chief transportation engineer of the International Harvester Co., in an article based on an S.A.E. paper in a recent number of our eminent American contemporary, " The Commercial Car Journal." A irt aspect of the subject which is emphasized, and which can hardly fail to be of great interest, in this country, relates -to the layout of articulated vehicles. Attention is drawn to the desirability of mounting the
turntable, or fifth wheel, as they call it over there, well in front of the driving axle, instead of almost directly above it, as is the usual practice.
Our diagrams are based on the arguments put forward, which certainly seem sound and are fairly selfexplanatory.
In the case of a four-wheeled lorry, no one would normally select a layout which resulted in the whole of the weight of the body and pay-load being carried by the rear wheels, but one seems to be acting equally illogically in placing the semi-trailer king-pin in line with the driving-axle centre, whch, so far as the tractive unit is concerned, comes to virtually the same thing.
The two four-wheelers illustrated are both carrying 8 tons. The short-wheelbase vehicle has a rear-axle loading of 10 tons and an undesirably long overhang. By shifting the rear axle backwards, 2 tons are transferred from the rear to the front axle, with a big improvement in weight distribution and a shortening of the awkward overhang.
Before proceeding, let us remember that, in calculating weight distribution, the rule is this : If the centre of gravity is x ft. from the front axle and y ft. from the
rear axle, then the former carries y x weight -h (x y), whilst the latter carries x x weight ÷ (x y). In other words, the percentage is in inverse ratio to the distance of the weight centre from the axle.
Now turn to the drawings of the two articulated platform lorries. Note, first, that they are both of the same overall length. The upper diagram shows that the percentages of the gross weight on the front, driving and trailing axles, respectively, are 8, 48 and 44. It is generally agreed, and obviously the best for tyre-carrying capacity, that the ideal distribution is 20, 40 and 40 per cent.
By moving the second axle rearwards, not only do we succeed in adding another ton to the pay-load, but also we bring the weight distribution much nearer to the ideal. Roughly, 16 per cent. of the gross weight is then supported by the front wheels, whilst the driving wheels carry 40 per cent, and the trailing wheels 44.
The alteration in the layout has reduced the driving axle loading by half a ton, despite the extra ton of load, and doubled the weight on the front wheels, thereby greatly improving the road-holding qualities of the outfit
We have based the above weight distributions on the assumption that the tare weight of the complete vehicle, in both cases, is 4 tons, made up of lf tons for the tractive unit and 21 tons for the semi-trailer. It will be seen that the lighter outfit has a gross weight of 12 tons, of which 8 tons is pay-load. On the turntable is imposed half the pay-load and about 1 ton of the trailer tare weight. The driving wheels, therefore, carry this amount plus, say, I ton of the tractor tare weight. On the trailing wheels we have 4 tons of payload and about 11 tons of trailer tare weight—more than at the turntable end, because the axle and wheels are• included.
On the larger vehicle of the two, 50 per Cent. of the pay-load now becomes 4j tons, so that on the turntable there are 4i tons plus 1 ton—the same proportion as
before of the unladen weight. Unlike the earlier case, however, the turntable weight is not directly applied to the back axle. As the king-pin is 30 ins, in front of the drivingwheel axis and 120 ins, behind the front wheel centre line, one-fifth of the turntable weight will be carried by the front axle and four-fifths by the driving axle. To the nearest quarter of a ton, these proportions of 51 tons are 1 ton and 41 tons. Accordingly, with the tare weight of' the tractive unit split up as before, there are altogether 2 tons on the front axle and 51 on the driving axle. The latter figure is actually less than before, in spite of the greater load. (Lengths 120 and 30 are purely arbitrary.) Still more nearly could the ideal be approached if the, position of the trailing axle were altered. It would need to go back only a little way to transfer the 4 per cent. to the turntable. Then an extra 3 per cent. would be added to the driving wheels and about 1 per cent. to the front axle.
Thus, one direct result of the forward turntable is a bigger pay-load capacity, because the arrangement permits all tyres to be laden to the maximum.
Another, is greater safety when hill-climbing. One of our diagrams shows an exaggerated case. The weight of the front part of the trailer is represented by a vertical downward force through the turntable centre, When the king-pin axis intersects 'that of the driving axle, this force is well behind the wheel centres. There fore, it actually creates an upward force through the front hubs, an effect that may well be positively dangerous. With the forward turntable, however, the vertical weight line remains well in front of the driving axle as the gradient steepens, so that some of the trailer weight is still . materially helping to keep the steering wheels down to their job.
Yet another good feature is illustrated in the sketch . of the two types rounding a bend. The tendency of the trailer to continue going straight ahead results in an outward force through the turntable and • parallel with the axle of the tractive unit. On the one hand, the whole of this force has to be resisted by the driving wheels; on the other, it is shared by the driving and steering wheels, the proportions being inversely related to the distances, as in the case of the weight distribution explained earlier. Thus, roadholding characteristics are improved and .safety promoted.
Increasing the wheelbase of the tractive unit may be regarded by some as a drawback, but it does not involve any addition to the overall length of the outfit. Moreover, it actually allows a longer trailer, for if this be of the square-fronted type the distance between the body front and the cab can be lessened as the forward overhang of the trailer is increased. This is shown graphically in one set of the accompanying diagrams. There seems to be only one real criticism of the scheme, namely, that the jockey wheels of a detachable semitrailer would need to be rather far back. In consequence, the stability of the unit when uncoupled will be impaired.
As is indicated by our final pair of sketches, the question of space between trailer front and cab does not arise if the end be rounded or if the turntable be of the retractor type, as incorporated in certain Carrimore semitrailers. Nevertheless,, the farther back from the cab that the king-pin is placed, the longer the radius of curvature of the body front and the less the reduction in loading space, or the smaller is the distance of movement the retractor mechanism 'has to accommodate.
That, roughly, is a summary of the case for the forward turntable, and there seems to be general agreement among its advocates that the extent to which it , should be advanced is 10-13 per cent. of the tractor wheelbase. We locik forward to hearing the views of those who are opposed to the scheme.