AEC FIRS WITH A 44-TON TRACTOR
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THE SCOTTISH SHOW will see a number of new tractive units designed for operation on normal haulage at 44 tons, the figure that is being talked about as a possible future maximum gross weight limit in Britain. The first of these to be announced is the AEC Mammoth Major 2VTG6R4, a conventional 6 x 4 chassis based on the export tractive unit for 38 tons gross shown at Earls Court in 1968.
Except for having right-hand steering instead of left-hand and a 12ft wheelbase instead of 11ft 6in., the latest AEC model is almost identical to the 38-ton chassis. It has the AEC 800 Series diesel but with the maximum rating of 272 bhp gross (BS) at 2,600 rpm which was introduced shortly after the 13.1 litre V8 was first shown at the 1968 Commercial Motor Show with its original output rating of 247 bhp gross (BS); maximum gross torque (BS) of the higher-power version of the V8 is 638 lb. ft. at 1,400 rpm.
Like the 38-ton Mammoth Major tractive unit, the latest design is available with either a six-speed or 10-speed-splitter gearbox or a 10-speed semi-automatic (this will be in the chassis at Glasgow) and power steering is standard. The two-spring rear suspension is also the same but while the spiral-bevel-andhub-reduction driving axles of the earlier 38-ton model which give a 17-ton bogie rating are standard, there is an option cif spiral-bevel /double-helical double-reduction axles which give a bogie weight rating of 20 tons. To get to the designed maximum weight of 44 tons, the latter arrangement will be needed unless a three-axle trailer is used with the Mammoth Major as the front axle limit is 6.5 tons and even with a wide-spread bogie on a tandem trailer, the total of the acle weights would be only 43.5 tons. With he 20-ton bogie in this case, the figure of 44 ons is exceeded by 2.5 tons which will allow i useful margin for uneven loading.
Four axle ratios are available—from 5.65 o 7.83 to I—in the case of the hub-reducion axles and there are five listed for the iption-5.67 to 9.04 to 1. In both cases a ockable interaxle differential is standard ind in the lighter axles lockable differentials n each driving head are offered as an )ption.
Front-axle brake-shoe width has been ncreased from 61n. to 7in. to take care of he extra gross weight. This is now the same s at the rear and brake sizes are still 5.5in. all round. Front tyres are 11.00-20 adial steel cord with 10.00-20 tyres of the ame type at the rear.
'Ian revised
When the 38-ton tractive unit was anlounced a year ago, it was said that UK ,ersions would have a secondary brake ystem utilizing only the front axle of the ractive unit (with the trailer). But this plan las been revised and dual-diaphragm lockype actuators are fitted to the front and earmost axles to provide for this function ind for parking. The service brakes have ndependent circuits for the tractor and semirailer operating systems and the secondary ;ystem is controlled by a hand valve in front )f the driver. The secondary diaphragms of he tractive unit actuators and the locking 1evices inside them are controlled for parkng by a second small valve close to the iriver's seat. AEC recommends that the ;econdary system be used for short stop—
at traffic lights and so on—with the parking system used only when the vehicle is to be stationary for some time or when left unattended.
The usual Ergomatic Thru-way cab employed in conjunction with the V8 is fitted on the latest model; the engine located below the seat pressing and this gives across-cab access and allows a double passenger seat. There is the normal good standard of interior fittings and finish. The cab is mounted 5in. higher than on the original design with V8 engine.
Estimated chassis /cab herb weight of the model with the standard rear bogie is 7 tons 4.5cwt, and 7 tons 7.75cwt with the 20-ton bogie. And while the chassis is limited to 44 tons for normal operation, maximum gross train weight when not restricted by regulations is 56 tons with the 17-ton bogie and 65 tons with the 20-ton design.