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By the Technical Editors IT took eight months to produce the Thornycroft Mighty Antar prototype, the 100-ton-gross tractor with machinery-carrying semi-trailer, which was described in "The Commercial Motor" last week. The order was first placed in April, 1949, and the prototype was on the road by the end of December.
Apart from a clogged injector nozzle, no other trouble has been experienced in almost 1,000 miles of operation, most of it across country at Bagshot Heath. The engine " calendar " indicates that it has completed 10,000,000 revolutions since it was fitted in the chassis.
It was a notable moment when the Antar was put at our disposal and we climbed into the cab, starting the engine from cold without preliminary use of the heater. The Ki-gass equipment works on one bank of cylinders and is required only below 8 degrees C.
The engine roared into life within a second, and we drove a short distance along the road to pick up the Crane 50-ton machinery-carrying semi-trailer. A spot brake test made on the way indicated a stopping distance of less than 30 ft. from 20 m.p.h. This is equal to 15 ft. per sec. per sec
After attaching the semi-trailer, we drove across Bagshot Heath, finding the engine power sufficient to pull away from rest in second gear in the main gearbox and overdrive in the auxiliary. The trailer is unsprung, hut the suspension of the tractorcan reasonably be described as "too good."
Driving across furrowed ground at 15 m.p.h. would be uncomfortable in any normal cross-country vehicle, but although grab rails are provided
B12 in the Antar cab for the use of the crew, they were not required diiring this severe trial. Drivers will be strongly tempted to travel fast across country without realizing the punishment meted out to the vehicle.
Although there was ample power to spare from the Rover Meteorite 250 b.h.p, oil engine, we purposely used high gear ratios to observe the torque at low speeds, and found the power unit to be as happy at 1,000 r.p.m. as at its maximum governed point.
There are 12 ratios available between the main and auxiliary gearboxes, all gears being in constant mesh and having dog-clutch engagement. The only occasion on which the lowest ratio was employed was when spin developed on one wheel at a particularly soft section and the unit was in danger of becoming bogged.
A third differential has been fitted to the Kirkstall bogie at the request of the Iraq Petroleum Co., which has bought the vehicle. ' Except when climbing inclines, second and third gears, with overdrive auxiliary, were suitable for all sections of the course.
Clutch engagement, with assistance from the Clayton Dewandre power equipment, was remarkably
light. Incidentally, the clutch unit has its own cowled fan attached to the end of the clutch shaft. The hydraulic steering boost took all the hard work out of manoeuvring, both on loose ground and on the road.
The hydraulic servo system works at a pressure of 750 lb. per sq. in., the skew-gear pump being driven from the engine.
When attempting to make a tight turn we noticed that the wheels of the semi-trailer were suffering damage, being dragged sideways against the line of travel. Although the tractor has a turning circle of under 70 ft., it is not advisable to turn the complete unit in less than 100 ft.
. We had previously weighed up the prospects of fuel consumption being measured in gallons per mile, but ultimately found it to be at the rate of approximately 2-1 m.p.g. The gross train weight was a mere 70 tons. It is understood that road operation will afford 4 in.p,g.
• Maximum temperatures recorded at Bagshot were as follows: Radiators, 135 degrees F.; cylinder heads, 140 degrees F.; engine oil, 110 degrees F.. and ambient, 48 degrees F. The radiators were covered for approximately half their surface area. Clayton Dewandre Still-tube radiators are used to cool the oil and water. Oil pressure remained constant at .55 lb. per sq. in. when starting from cold and in normal operation The Comjoints double-skinned pop-riveted cab has withstood the rigorous treatment at Bagshot without sign of movement or tear. Both windscreens are fixed, but the quarter lights can be opened and afford extractor ventilation. An electric fan extractor is also provided in the cab roof.
We drove along a near-by road to make acceleration and braking trials. As no speedometer is fitted, we measured the road speed by
engine revolutions and gear ratios. Starting from rest in second overdrive gears and engaging subsequent ratios at peak engine revolutions, we reached 20 m.p.h. in 46 secs., 25 m.p.h. in 76 secs. and 28 m.p.h., the governed speed, in 85 secs. These are average figures computed from runs made in opposite directions.
The brake stopping distances of 45 ft. from 20 m.p.h. and 75 ft. from 25 m.p.h are reasonable for a 70-tonner, but the Thornycroft engineers are trying to improve on them. There is a brief time delay because the system is arranged for the semi-trailer brakes to be applied before those on the tractor, and with the prototype it takes between one and two seconds for the full pressure to be reached. The results we obtained afford a deceleration rate of 9 ft. per sec. per sec., but the ultimate aim is to improve the overall brake efficiency to 15 ft. per sec. per sec