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J. COTTON, M.I.R.T.E. SHOD with tyre equipment for operation on sand in Sarawak, the Scamrnell four-wheel-drive tipper chassis lent to me for road test was handicapped in the cross-country trials at Farnborough. Nevertheless, although loaded to maximum capacity, it acquitted itself with honours in overcoming the hazards of the Tank-proving ground.
Scammell vehicles wererenowned for their ability during the war to go where others. would not. The new addition to the Scammell range has a larger engine, power assistance to steering, and a higher maximum speed, First details, with drawings of the new chassis, were published in "The Commercial Motor" on December 30, 1949, but to recapitulate, the power unit is the Scammell-Meadows 10.35-litre "square" oil engine, which develops 130 b h.p. at 1,900 r.p.m. Its maximum torque is 420 lb.-ft. at 1.000 r.p.m, It is retained in the chassis in conjunction with the clutch on a three-point rubber mounting, with a steel tie-rod at each side of the .clutch housing to provide positive
fore-and-aft location. . . .
A short intermediate shaft, with rubber. universal joints, carries the drive to, a six=speed constant-mesh
gearbox mounted on rubber. The transfer box is attached to the rear, of the gearbox and, in the case of the test chassis, had a .fixed reduction ratio of 1.31 to 1. with positive drive to the front wheels. Subsequent models will be available with an 'arrangement for.dis
engaging the drive to the front axle: . .
The front-axle . casing has a, standard centre. section and trumpets,..but...-the rear-axle housinghas bonstrengthened to ,iecommodate a 12-ton load. The bevel
c8 and epicyclic• gears of both axles are interchangeable. Triangulated radius arms, which are ball-mounted from a cross-member, locate the front-axle assembly and afford full articulation between limits determined by stops on the chassis. The front-axle suspension incorporates the Scammell rocking-beam arrangement in which the transversely mounted leaf spring is coupled to the frame through a central pivot.
A Clayton-Dewandre two-cylindered air-cooled cornpressor assists braking and steering. It is driven from the engine and draws air through the oil-bath air cleaner. In addition to the normal cam-and-double-roller steering box, the drop arm is linked to an external air-pressure arrangement. of Clayton-D.ewandre pattern. The steering column is divided by a differential, the cage of which operates a reaction valve and admits air to one or the other side of the servo system attached to the drop arm. The working pressure of the air equipment is 85-90 lb. per sq. in.
-frilade:alhazarpus cat journey over ice -covered roads to the Stakhiell 'Works on the day of the test, arriving in Watford at daybreak. The fuel tank of the teSt
chassis was filled, the fuel temperature being recorded, and an additional eight gallons carried in tins' for use when we reached Farnborough.' 1 had elected to make the fuel-consumption trial between Watford and Farnborough,. because this would be representative of normal operating conditions for vehicles of this size.
After checking the load and its distribution at the local Weighbridge, we started. While all other traffic was crawling and sliding along the road, the Scammell, with its front-wheel drive permanently _engaged, maintained a straight course at well over 20 m.p.h. As a precautionary measure, 'fifth speed was .engaged to descend, the hill into Rickmanss.tvorth. Shortlyafterwards the atmospheric temperature rose to above freez
ing point. . .
An early appointment at Farnborough left no alternative buit todrive fast for 35 rnileS So af to Maintain an average speed of over 20 m.p.h. Local traffic in Rickmansworth, Slough and Windsor made inroads into the
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scheduled time, and,' of course, driving through narrOw and congested streets with a.chassis measuring a fraction under 8 ft. 8 ins, at the front-wheel hubs did not.make for speedy travel. With such an overhang at the axle, my thoughts were of the Bpadicean chariot Heavily laden commercial vqhicles can usnally be found toiling up Bagshot Hill at IOW,-Speed, but on this occasion there was no 'other traffic with which to compare the pace Of the Scarnmell. Nevertheless, it made a good, clean climb in fifth_gear and without the:Apeed
falling below 19 ,Apart from this climb, all other main-road gradients appeared almost level, and there was no further occasion to use intermediate tears. .
We arrived at Pinehurst Corner, Farnborough, :eight minutes ahead of schedule, havisng driven 34.7 miles in 1 hr. 38 mins„ an average of, 21.3 mph., whichis decidedly fast for an I8.4on gross !oad 'As. the chassis had been stored OVernight -under cOver, the fuel was above day temperature when starting, and a slight compensation for its contraction should have been made when. ref uel I ing at Far n borough. . Although no adjustment had been ortade,-the fuel return of S:75 m.p.g. indicates that the Scammell is not a thirsty machine.
Initial braking tests were made inside the Ministry of Supply compound, and the results noted for comparison with those taken after the cross-country trials. We stopped at Chobham, which was on the way to the Tank-Proving ground, to test the effects of the staggered paving stones on the power operation of the steering. Before-Trioving off down the " straight "I made sure the air-pressure 5upp1y was at its maximum, and noticed that the gauge registered a steady decrease in propor
tion to the distance travelled. This test was much worse than could be found in normal service, because of the differential spacing of the paving stones, which are laised transversely across the path of the vehicle and are approximately 2-3 inS, above ground level.
300 Blows in 30 Seconds
From full pressure (according to the gauge) down to 60 lb. per sq. in., there was no indication at the steering wheel of the severity of the test, but when the air supply became more exhausted the oscillation of the .wheel War; violent. Assuming the stones to be, on •an average, 4 ft. apart, 300 heavy diagonal impacts at the -wheels in less than 30 secs. were required to reduce the pressure by 30 lb. per sq. in.
, Although the air system was almost at .zero pressure by the end of the trial, I turned the sharp corner without difficulty. I could not reduce the air pressure sufficiently on the road to test the servo effect on the steering, but my. expel-len-6e at Chobharn, was reasonably conclusive that the Scammell would not be _unduly cumbersomeif the air supply of the steering became defunct.
• The proving ground was drying after a :periodeof heavy' rain, and,' although there was less water in the gullies than usual, the ground was stickier than on other oceasions when I have conducted tests. The photographer, who was riding ahead of the test chassis on a six-wheel-drive recovery vehicle, gave warning of the state of the ground when he jumped out of the cab and sank above his knees in mud. Lifting tackle was almost required to rescue him.
Ploughing and Ditching!
Because of the narrow-section tyres, which were fitted to meet the operator's specification, I doubted whether the Scammell would complete the course with a clean sheet, but it ploughed through the five miles of ditches. furrows and other obstacles without becoming bogged. When testing the front-axle articulation and checking belly clearance, there was one occasion when the sparewheel carrier grounded, but the chassis was extricated without external assistance.
Seen from the accompanying vehicle, the Scammell rocking-beam axle was often. at a different angle from the rear axle and the remainder of the vehicle. This, as well as improving riding comfort, evens the load on the front wheels and improves traction, especially when crossing furrowed ground or climbing diagonally from ditches.
I noticed that the tread mark left by each tyre on the soft earth was slightly disrupted, suggesting that a wider tyre would be beneficial should worse conditions be encountered. The wisdom of fitting such small tyres ,was again in question when tackling the steep loosesurfaced gradients. The Scammell made a clean climb up the 1 in 3.8 and I in 3.18 sections, but developed wheeNpin on the 1 in 2.74 stone. It came to rest with all wheels churning up mud and stones. A second attempt was made without success, and the other steeper gradients could not be climbed To satisfy myself that the vehicle had not–failed through lack of power,-1 tackled the I in 2+. 'concretesurfaced slope and drove up without any noticeable fall
in engine revolutions. . The vehicle manufacturerwould, for normal piaposes, supply a 14-in.-section tyre, and with such equipment the Sceiminel should romp up. all the Farnborougli gradients.
I had planned to take the chassis through the local sand pits, but because of the packed surface, the test would have been without purpose It is dry and loose sand that provides .difficulties. . ,
The fuel tank had been refilled at the start of the cross-country trials, and after nine miles, of running in first and second gears slightly under three gallons were needed to restore the original level. This gives a fuel return of approximately 3 m.p.g. The radiator temperature, after 40 minutes' ploughing through mud in second geai, was 167 degrees F., with an ambient of 51 degrees F.
Brakes Well Shielded
• The brake drums and facings of the Scammell are well screened by the width of the wheels, and as no great depth of water had been encountered on the proving ground, the braking efficiency had remained unimpaired. Tests from 30 m.p.h. showed that the chassis could be brought to rest in 64 ft.
I took over the wheel for the acceleration tests and employed third gear when moving away from rest. The constant-mesh gears and sharp cut-off of the fuelinjection pump afford an easy and rapid change between 'ratios. The rate of acceleration, for a'heavy vehicle, is notably rapid. It took. 15.4 secs. to reach 20. m.p.h. and 30.2 sees. to 30 m.p.h. from a standing start. TOPgear runs showed that 27.8 secs. were needed to reach
30 m.p.h. from 10 m.p.h. •
When turning at the end of each test run, I tried to lower the nress-ure in the air system by repeated movements of the steering wheel, but with theengine running there was no decrease. After stalling the engine, it took 18 full turns of the steering wheel, in opposite directions, to reduce the pressure from 80 to 20 lb. per sq. in.
The run back to Watford was Made at a smart pace, the maximum speed allowed by the injection-pump governor being slightly over 30 m.p.h.
Comfort for Three
By moving the gear selector and gate forward in the cab, seating has been provided for a crew of two men and the driver. Although the cab has a screened roof and adequate ventilation for operation in hot countries. the crew can be comfortable in cooler elimates, because • with all windows closed the interior temperature was maintained at 60 degrees F. The steering and controls are well placed for comfortable long-distance driving, and I arrived back at the works not feeling noticeably tired after spending 100 miles in the cab.
Lubricant temperatures taken at the works were as .follows:—Front axle, 92 degrees F.egearbox., 114 degrees F.; transfer box, 147 degrees F.; rear axle, -46 degrees F. The radiator-water temperature was 155 degrees F. Because of the low ambient temperature two 'thirds of the radiator was covered for the day's tests.
The Scammell is well suited to any overseas condition. The high position of the frame, engine and transmission affords ample clearance for cross-country travel, and a sturdy bumper bar and a guard for the radiator and lamps give full protection at the front. The silencer tail pipe cannot become submerged, because it discharges the exhaust above the cab.
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