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50 m.p.h. Export Models
ONE of the new Thornycroft chassis to be exhibited at the Commercial Motor Show at Earls Court will be the 7-ton sixwheeled S.A.R. model, which, together with a larger version, a 10-tonner, is at present being supplied on contract to the South African Railways. An initial order of 96 of the six-wheelers, together with 60 four-wheeled models, is at present being executed. The smaller model, if it may be so termed, will operate at a laden weight of 19 tons and draw a 12-ton trailer The 10-tonner will have a laden weight of 20 tons, and is designed to pull a 25-ton trailer.
Specified to have a maximum speed of 50 m.p.h. and to operate in extremely hilly areas, it has been necessary to design a special engine to meet these requirements. To fulfil the purpose, the maker has produced a six-cylindered, 11.3-litre petrol engine, which develops 155 b.h.p. at 2,000 r.p.m., and has a maximum torque of 535 lb./ft. at 1,200 r.p.m. To be known as the GRN 6/1 engine, it has a bore of 41 ins, and a stroke of 6i ins. It is fitted with twin Solee down-draught carburetters, and for steady idling the manifolding incorporates a hot-spot and balancing pipe between the carburetters.
Special attention has been paid to cooling arrangements, some idea of which may be gained from the largediameter fan which absorbs 8 h.p. at 2,000 r.p.m. engine speed. The ignition coil is attached to the front of the engine to be coolea by the air from the fan stream. Air is supplied to the engine through an A.C. combined oil-bath air cleaner and silencer, and is positioned in the cab. The crankcase i breather pipe is connected into the air cleaner.
The cylinder block is bolted to the crankcase, and houses the camshaft which operates the overheadlvalve gear through push rods and rockers. Dry-pattern cylinder liners are fitted to the block. Engine, clutch and gearbox, all iron castings, are combined as a unit which is retained in the eltassis by trunnion bearings on both sides of the flywheel housing, and a single trunnion mounting from the front of the crankcase.
A large-diameter single dry plate clutch, of Thornycroft design, carries the transmission to the main and auxiliary gearboxes, which provide eight forward and two reverse ratios. The auxiliary box, which has a ratio of 2.26 to 1, when used in conjunction with the main gearbox, affords ratios of 11.59, 6.22, 5.13, 3.53, 2.75, 2.26, 1.56 and 1 to 1 forward, and 17.38 and 7.69 to 1 reverse. Propeller shafts, with a flexibly mounted intermediate bearing and Hardy Spicer couplings, convey the power to the bogie axles.
Both axles, of fully floating pattern, have overhead worm gear. There is no intermediate differential between the two axles because the maximum traction will be required when these models are traversing soft ground.
To stop a vehicle of 45 tons laden weight from high speed, demands good braking, which in both models is fulfilled by a Clayton Dewandre air-pressure system, acting on all wheels. Common to Thornycroft practice, main and auxiliary air reservoirs are used to obtain consistent retardation under all conditions. Brake drums of 161k ins, diameter and shoes 4 ins, wide on the front axle, and 6 ins, wide on the rear axles, give a total brake frictional area of 1,659 sq. ins.
With the exception of the rear end of the frames, and tyre equipment, the chassis are almost identical. Both have an 18-ft. 4-in, wheelbase, and their maximum frame section is
12 ins. deep, 31 ins_ flange and T5E in thickness
There being one ton difference only between the two models in the fully laden condition, it follows that the tyre equipment is more or less the same The 7-tonner is fitted with 13.00 by 20 tyres on all six wheels, and the larger model—retaining the 13 00 by 20 for the front axle, is equipped with 14.00 by 20 tyres on the bogie axles.