Two Desert Giants on Trial
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THE second and third prototypes of the Atkinson Omega have been undergoing acceptance trials by Aramco, to whose specifications they have been constructed. The tests have taken place at the Ministry of Supply's proving grounds .at the F.V.R.D.E. and Bagshot
Heath. • .
The two large tractors are outwardly
sitnilar .-to the original prototype previously described attd reported in The commercial AdotOr on November 15 and Noven-tber 29 last year. However, engines and transmission systems have been altered.
One vehicle is powered by a RollsRoyce C6.SFL six-cylindered super
charged oil engine, as employed before,. but the -power rating has been increased to 275. b.h.p. at 2,100 r.p,m. Whereas a, Wilson eight-speed ,preselective gearbox. was coupled to this engine on the original Model, a fully automatic four-Speed box is now used in combination with a torque converter. The auxiliary gearbox and axle ratios remain the same.
The only change in the chassis specification is the use of larger front tyres which are now 18.00 by 24-in 10-ply Atlas Flotation. This alteration is intended to lessen the tendency for the front wheels to dig into soft sand.
A Cummins NRT6 six-cylindered turbo-charged oil engine powers the other vehicle. With an output of 335 b.b.p. at 2,100 Lp.m. and 900 lb.-ft. torque at 1,500 r.p.m., this engine is coupled to a Wilson eight-speed preselective gearbox, as used on the original prototype. To make full use of the greater engine power the axle ratios have been raised to 8.7 to 1, permitting a maximum speed of about 43 m.p.h.
At the request of Aramco, both new models have been. lightened considerably. The massive chassis structure of the original prototype has been redesigned and a total weight saving of about 2 tons -achieved, whilst retaining an adequate strength safety margin.
As tested, the two tractors were coupled to Tank transporter trailers lent by the Ministry of Supply.. One was loaded with a Tank and the other With ballast to give an all-up weight of about 62 tons in each case. Impressive performances were put up by both vehicles, although their characteristics are dissimilar.
The Rolls-Royce engine delivers its power smoothly :and with an unusual degree of silence. The Self-Changing Gears automatic gearbox performs admirably. Upward gear_ changes are noticeable only through the alteration in engine speed.Downward changes are more apparent, but Swift and smooth,
No manual gear-change override is provided on the Atkinson, as Arameo think the vehicle will have to contend with inexperienced drivers and that the greater the simplification of controls, the better.
Tractive-effort tests on the Rolls-Royceengined Omega gave a figure of 17 tons with low ratio engaged on the auxiliary gearbox.
Hill-climbing was rather disappointing. An attempt at a restart on the 1-in-4 test slope at the F.V.R.D.E. failed when the torque converter stalled. However, such gradients are unlikely to be encountered in the type of desert operation for which this vehicle is intended, but should it be necessary, a larger converter with a greater torque capacity can be supplied by Self-Changing Gears, Ltd.
On the rough track, at Bagshot Heath the transmission coped admirably with changing conditions, always providing a
smooth flow of power and traction on all surfaces.
Less smooth and much noisier, the Cummins-engined Omega makes no secret of its power. With finger-tip control of the air-operated RV30 eight-forwardspeed Wilson gearbox, this model requires little more effort to drive than the vehicle with fully automatic transmission.
The 1-in-4 test slope was ascended, a restart, being made in second gear. First gear and low ratio of the auxiliary gearbox gave an impressively steady ascent and restart on the 1-in-3 gradient. An attempt to restart on this slope when the concrete slabs were wet produced the awesome spectacle of all 10 wheels spinning at once.
Tractive-effort tests showed again the capabilities of this vehicle. Though the designed tractive effort is 67,000 lb., a reading of 18 tons was the highest recorded, solely because of a lack of adhesion between the smooth-treaded flotation tyres fitted and the tarmacadam surface of the test area. Even with the tyres deflated to 20 p.s.i. from the 25 p.s.i. normal running pressure, the front and rear wheels spun, producing alarming distortion of the tyre casings.