THORNYCROFT'S NEW I GHT I 2-TONNER TESTED
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The Stag, Introduced at the Commercial Show, Manifests Desirable Qualities for Fast Main-road Heavy Haulage. It Weighs Under 7 Tons, and Has An Auxiliary Gearbox and a 100 b.h.p. Petrol Engine
FOR the rapid transport of big loads over main roads, the new Thornycroft Stag would seem to be a particularly suitable vehicle when due regard is paid to existing taxation regulations and modern road conditions. It is light, powerfully engined and appropriately geared. rro save weight, much ingenuity in
design has been exercised, and full advantage has been taken of the steels and alloys with maximum strength and minimum weight that are now available. The strength of the frame and suspension systems is undoubtedly adequate for the purpose of the vehicle.
B26 The high average speeds which the vehicle has been built to maintain are made possible by the installation of an engine developing approximately 100 b.h.p. and a gearbox having four fairly close ratios. A fifth emergency ratio is provided by an auxiliary gearbox. Thus, under ordinary conditions, the driver has only the one gear lever to manipulate. For the major part of the distance the machine covers it will be employing a direct drive, and the double reduction afforded by the two gearboxes used together will be utilized only for ascending exceptionally steep hills, or for starting from rest in difficult circumstances.
Alternatively, an over-speed auxiliary gearbox is offered, in conjunction• with appropriate ' ratios,, elsewhere, but this arrangemen? seems the less desirable because an indirect drive is then employed at high road speeds. • The low first ratio of 88.5 to 1 should rarely be needed. More often probably the auxiliary box will be employed with second gear, which combination affords the usefully low ratio of 52.5 to 1. Thus, theoretically, the arrangement in the vehicle we tested seems to be almost ideal.
An idea of its efficacy in practice can be obtained from the results of the acceleration and hill-climbing tests. With regard to the former, the manner in which the main curve in the accompanying graph rises steadily and with only brief interruptions as the four main ratios are progressively engaged, affords unquestionable evidence that they are well chosen. Furthermore, an ascent of Fairleigh Hill, near Basingstoke, which is about 1-} mile long and has a maximum gradient of 1 in 8, satisfied us that the machine possessed good hill-climbing powers. We easily negotiated the steepest portion at about 6 m.p.h. in "low second," and accomplished a restarttwithout having to employ the lowest ratio.
There are, however, many circumstances, encountered under ordinary working conditions, where it is desirable to advance or to reverse slowly up'a short steep incline, in which the emergency gear would prove invaluable.
. One such we experienced during our day's test. It was the approach from a parking ground to the main road at a level higher by some feet; and its surface was soft. Having been compelled by passing traffic to wait on the slope, we were enabled by reason of the 98.5-to-1 gear to start away again with ease and without wheelspin. The combination of circumstances afforded also a good
demonstration of the facility with which the hand brake could be operated.
• Consumption was measured over a circular course 284miles in length, which included nine gradients of 1 in 20 or steeper, of which four are nearer (and, in one case, severer than) 1 in 10. On the return journey, where the road traverses high ground, we drove into the teeth of a high-velocity wind. A two-gallon test tank was used for this trial, and after each refilling with a measured quantity we carried on until the supply was exhausted. The distances covered on each two gallons from the start were 9.5 miles, 8.2 miles and 10.3 miles, an average of 4.66 m,p.g., whilst the rate for the complete journey, for which six gallons and 1-3pt, were consumed, was 4.60 m.p.g. This would almost certainly be improved upon on mainroad haulage, for the course was a stiff one. Furthermore, it must be borne in mind that the vehicle had covered less than 100 miles.
Frictional area of braking surface per ton laden weight is rather low on this Thornycroft, but, nevertheless, the retardation is adequate—even for .such a speedy machine—as the accompanying braking graph manifests. The diameter of the drums and width and thickness of faCings should ensure longevity. Employing the Dewandre triple vacuum servo system, the foot brake acts on all six wheels, whilst the hand brake applies only the brakes on the four bogie wheels.
Between the main vacuum cylinder and the main tension rod there is a short cross-shaft carried in bearings in a large aluminium casting, bolted and braced to the frame longitudinal. This might appear to be an unnecessarily elaborate arrangement, but it permits light construction without sacrifice of rigidity. Nearly every brake arm is of builtup construction, two welded-on plates replacing the more orthodox solid arm and simplifying linkage.
Examination of the chassis reveals many other noteworthy features. The auxiliary gearbox is bolted at the front to a horseshoe-shaped mounting, the arms of which, as it were, embrace it, and, in turn, are bolted to brackets on the frame at points in transverse line with the centre of gravity of the box. One reason for this arrangement is to en-. able a single-type of gearbox casting to be employed for a number of systems of mounting ; for example, it can be bolted to the back of the main gearbox. The bearer fixings permit frame distortion, and a universal joint at the front end of the intermediate propeller shaft allows misalignment of the two gearboxes.
Where the frame is heavily stressed above the bogie, the members are reinforced by lattice pieces within the channel. A box cross-member lends great strength at the anchorage of the main rear springs, which are of the duplicated inverted semi-elliptic type. The method of supporting the springchair pivots warrants investigation, as does the attachment of the axles to the spring ends.
Lubrication of all bogie-suspension bearings and much of the brake gear is effected by a Tecalemit inertia lubricator mounted between the near 828 'side bogie wheels. This is a commendable feature, simplifying maintenance and diminishing the chance of wear through neglect at important points.
On the road we found the machine pleasant to drive. The steering is light for so heavy a vehicle, and it is easy to hold the machine to the desired course. The six-cylindered engine is not new, but has been improved in respect of certain details, • and reduced in weight. It possesses the flexibility and smoothness characteristic of its • type to a marked degree, is responsive, mechanically quiet, and an easy starter by hand. The clutch is first-class, and the brakes call for little exertion in operation.
To change gear a knack has to be acquired. For the sake of rendering a quick change obtainable, a closely adjusted clutch brake is incorporated. Whilst enabling noiseless changes to be accomplished by the adept, it is baffling to the inexpert.
We imagine the designer, in producing this whole clutch and gear layout to have aimed at the minimizing of step losses, with the ideal represented by the infinitely variable gear in mind. Nor would we attempt to belittle his success in achieving this conjectured ambition, for our brief experience at the wheel was enough to create an impression that, with little more familiarity, we could put up some high averages over long distances of trunk routes, and it should be remembered that the efficiency of the ordinary gear drive is hard to improve upon.