The Latest 7-ton Commer Goes Through the Hoop
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By L. J. COTTON, M.I.R.T.E.
IN addition to the underfloorengined 5-tonner and 7-tonner, a long wheelbase 7 ton model, which permits increased payload space, has been produced by Comrner Cars, Ltd., Luton. The former 7-tonner of 11-ft. 9-in, wheelbase was designed for a 16-ft. body, but to meet the requirements of operators, the manufacturer has developed a 13-ft. 6-in.-wheelbase chassis, which allows the body to be lengthened to 18 ft.
As well as producing this chassis, Commer Cars, Ltd., has designed a light-alloy platform body, the weight of the vehicle complete being under 3 tons. It is, therefore, entitled to travel at 30 m.p.h. in this country.
Apart from lengthening the wheelbase, there have been few modifications to the chassis. The brake servo motor has a 6-in.-diameter piston, whereas a 5-in. piston was used on the previous 7-tonner, tested in 1948, and wider brake shoes are employed on both axles. The total facing frictional area has been increased by 96 sq. ins. (26.8 per cent.).
A 24-gallon fuel tank replaces the 16-gallon tank.
Lengthening the wheelbase has altered the turning circle, which, 82 measured during my road test. was 56 ft. on both locks. Although the carburetter governor has now been removed, I could not at first appreciate the difference, because in fitting the small fuel-test tank there was a restriction in the supply. This was subsequently rectified before measuring the rates of acceleration and consumption.
A six'-cylindered overhead-valve petrol engine developing a net output of 109 b.h.p. at 3,000 r.p.m. is installed in me chassis at an angle of 66 degrees from the vertical and located below the driving seat.
Doubters Confounded When this unit was introduced, almost three years ago, there were many operators whodoubted the wisdom of treating the bores with a porous chromium deposit. -Experience has shown that cylinder-bore life is greatly lengthened by this treatment, and its success has prompted the manufacturer to extend the process to other commercial models of the Commer and Karrier ranges.
As the frame is level from the front axle to the rear, the 7-ton chassis provides no difficulties for the bodybuilder. The light-alloy
platform body, developed by Commer Cars, Ltd., has an underframe comprising longitudinals and cross-bearers of channel section with the top and.bottom flanges lipped for additional rigidity.
The crosrs-bearers are mounted on • ttie longitudinals through short bolsters which are bolted to the Crossbearers and to the longitudinals. The same type of material of 4-in, depth and 2-in, flange dimension is used for the longitudinals, cross-bearers and bolsters. Additional stiffening is provided by• angle reinforcements which are bolted to the cross-bearers, bolsters and longitudinals.
In mounting the underframe on the chassis, weight has been saved without sacrificing strength. Special bolts pass through the lower flanges of the longitudinals and the top flanges of the chassis side members. A i-in.-thick Tufnol packing piece is used to separate the chassis and body underframe.
The body floor comprises extruded hollow-section light;alloy boards and T-section clamp strips which overlap the boards and are fastened to the cross-bearers with coach bolts. The front panel, which stands 1 ft. 6 ins. above platform level, is constructed of hollow extruded boards which are tongued and grooved. The corner posts are of angle section and their attachments to the platform and underframe are reinforced.
In taxation condition, there was 1. cwt. to spare under the 3-ton weight limit when the vehicle was checked on the Luton weighbridge. The seven iron castings which were carried as payload were secured against movement by a sturdy wooden cradle, and the total running weight for the trials registered 10 tons 9 cwt., complete with crew, test equipment and a full fuel tank.
Although I have had a great deal of experience of Comrner vehicles in this country and in Persia, 1 neVer fail to enjoy the benefits of improved visibility, crew space, lightness of control and the comfortable cab temperature when driving an underfloor-engined vehicle.
These points were in my mind when driving from Luton to Dunstable during the warming-up period of the test. A. stretch of corrugated road surface also brought to my notice a further advantage of the longer wheelbase. There has been a modification to the springs since the underfloor-engined range was introduced, and the suspension is now softer.and has less rebound.
Bison Hill again formed the scene for climbing tests, and it was in the initial attempt that I found an
unofficial governor in the fuel line. This caused low gear to be employed at an early point on the hill before reaching the 1-in-6 i section, but, nevertheless, the climb was accomplished on part-throttle. The ease with which we completed the test was justification for not making a repeat run.
The temperature rise during the three-quarter-mile climb was negligible, but a greater difference was noticed in the next test, which comprised driving five miles at full throttle in third gear against the -brakes, at a speed of 15-18 m.p.h. This, I consider, is comparable to the work done in climbing a gradient. At the same time I was able to measure brake-fade characteristics over a decline of a similar length. I have met such hills and vales in my peregrinations.
With an atmospheric temperature of 62 degrees F., the radiator water was 167 degrees F.-at the start of the trial. The road selected for this test was undulating and quiet, and well suited to driving with one foot on the accelerator and the other hard down on the brake pedal.
Spot checks taken at the first, second and third mileposts proved retardation to be near optimum effi
ciency. After four miles, volumes of smoke poured from the drums, and the rate of retardation began to lessen, so I accelerated towards the latter part of the next section and measured the stopping distance from 20 m.p.h. at the fifth mile post.
This was 75 ft., and the braking effort had by this time almost completely faded. The stopping distance measured at the first, second and third mileposts varied between 18 ft. and 30 ft. Undoubtedly this test was severe, and the conditions are without parallel in this country.
I was surprised at the small rise in water temperature, the maximum
reading attained during the five mites of continuous gear work being 170 degrees F. This shows a mere 23degree rise.
Smoke pouring from the drums made the cab untenable at the conclusion of this trial, so I drove cautiously to a convenient parking site and stopped the engine, released the hand brake and left the lorry in low gear. I have previously noted the effect of parking a vehicle with overheated drums and the lever fully applied. This is a frequent cause of ovality in drums.
After a quarter of an hour's stop the test was resumed, and the works
representative drove to the Hatfield by-pass in readiness for a consumption trial. As before, when I tested the Commer 7-tonner in 1948, we drove nine miles out to Stevenage, . measured the quantity of fuel used, and returned over the same course with the same procedure at the end.
9 m.p.g. On Hilly Course The first leg of the course involves climbing the steeper side of Welwyn Hill, necessitating full use of third gear to maintain a good performance.. Slightly more fuel was, therefore, used on the outward run than on the return, but the total quantity used for 18 miles was within 0:01 pint of two gallons. This corresponds to a consumption rate of 9 m.p,g.
T endeavoured to maintain an average speed of 30 m.p.h. by keeping the speedometer needle between 33 m.p.h. and 35 m.p.h. when the road was clear. This resulted in an average speed for the course of 31 m.p.h., which confirms that the engine torque and overall gear ratio are right in proportion to the load.
A spot check of acceleration rate from rest and in direct drive produced results comparable with those s3 of my previous tests, the time factors being 27.4 seconds from 0 to 30 m.p.h., and 38 seconds from 1030 m.p.h. in top gear.
The increased frictional area of the braking system is a modification introduced chiefly to prolong the period between adjustment and replacement of the facings, and to give more consistent efficiency in relation to drum temperature. The high retardation rate has been main
tained, and the measured stopping distance of 49 ft. from 30 m.p h., obtained during my test, is an average figure for the medium range of load carriers. This is equivalent to 19.9 ft. per sec. per sec.