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AVERAGES
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ONE of the biggest attractions at Earls Court last year was the Big Bedford 7-tonner, a cab-overengine model weighing 3 tons unladen in any of its roles as a load carrier. Its price and features are undoubtedly attractive, and operators have since been keen to learn more of its capabilities and performance.
"The Commercial Motor" was given the first opportunity of testing one of the first long-wheelbase chassis from the production line, which is now housed in the new factory at Luton. There is nothing austere about the 7-tonner, despite its low price, and its performance is well up to the Bedford standard in respect of fuel economy, speed and smooth riding. On a give-and-take course, the fuel-consumption rate worked out to 9.3 m.p.g. at an average speed of 29 m.p.h.
In many ways the 7-tonner is a grown-up version of other Bedford vehicles, its specification including a hypoid axle and synchromesh gears, features such as are found on other models in the range, The engine, too, has some points in common with the " Extra Duty " engine fitted to the smaller Bedfords. The " tone," at all speeds, is unmistakeably Bedford, and with its smooth torque at low speeds it has the same good characteristics.
Engine Details The 4.92-litre 94 b.h.p. overhead-valve petrol engine, which produces 220 lb.-ft. net torque, has full-length dry cylinder liners, seven-bearing crankshaft, inductionhardened crankshaft journals, and copper-lined bearing shells, which are normally associated with compressionignition engines. To add to the potential life of the power unit, it has a controlled breathing system in which all air, for combustion or ventilation, passes through oil bath cleaners before entering the induction pipe or crank chamber.
A synchromesh gearbox, with constant-mesh singlehelical-form gears for the third and second ratios, was first introduced in the Big Bedford, and has since been extended to the 5-tonners, and others of the range. This refinement is not often found in low-priced commercial_ vehicles; it is a great advance in the direction of noise reduction, No high-pitched whine comes from the gearbox when the intermediate ratios are in use. A further feature of the 7-tonner is the employment of a hypoiddrive rear axle, in which the differential unit is carried centrally in the chassis, with half-shafts of equal length.
In the hydro-mechanical braking system the front brake shoes are operated hydraulically and those at the rear by mechanical means. The Lockheed brake master cylinder, operated by a Clayton-Dewandre vacuum servo unit, incorporates two pistons in tandem; one part of the system being connected direct to the front axle, whilst the other actuates the rear-axle shoes through a relay of rods and levers. This tandem piston arrangement ensures that the rear brakes remain in operation should there be a failure in the front hydraulic system, and vice versa.
The same size servo unit, 110 mm. bore, and of 6-in. stroke, is common to the Bedford 5and 7-tonners. Four years of prototype testing in the Chilterns has, according to the manufacturer, proved that this servo unit, in conjunction with an increase of nearly 500 sq. ins. in
facing area, is entirely adequate for operation of the larger models in hilly areas.
The test vehicle, of 13-ft. wheelbase, had the 16-ft. lorry body fitted, and was equipped with dual-marked 8.25 by 20-in. and 35 by 7-1-in. tyres, and in this condition the unladen weight was 2 tons 19 cwt. The addition of concrete blocks firmly secured against movement, test equipment, tools and a large quantity of rain water, brought the running weight up to 10 tons 81 cwt.
In bringing the cab fully forward over the engine, the weight distribution closely approximates to the ideal balance, the split weights taken at the time of the test showing 341 per cent, of the load to be carried by the front axle and 651 per cent. on the rear axle.
My test course for the Bedford was planned over a route on which I have formerly tested others of its el:is% and after a short run from Luton, bringing oil and water temperatures to a normal working level, an assault was made on Bison Hill. At the start of this climb, the ambient reading was 46 degrees F., radiator top-tank temperature 158 degrees F., and the 'double-skinned " engine cover chilly to the touch..
Smooth Transmission
The Bedford started quite well, and second and third gears were engaged successively until reaching the first steep bend. Here there were hurried changes down to the lowest ratio as the Tapley gradient meter registered 1 in 61, and a practically instantaneous stop-start test made at this point to prove the smoothness of the transmission and 'the ability of the vehicle to start off from a difficult position. Assuming-a transmission efficiency of 90 per cent. and rolling resistance 2 per cent., the Bedford can negotiate a l-in-4 incline when operating at the maximum gross weight.
After making this getaway, the remainder of the climb was made at 5 m.p.h., in the lowest ratio, on part throttle. This caused a slight warming of the engine cover, but at its hottest, this did not reach tepid level, and • would have caused no discomfort' to a third occupant in the cab, who would have his legs resting on the cover.
• Incidentally, a three-seater cab can be fitted if required. The radiator temperature rose by 8 degrees F. during this climb, proving that this is an adequate margin for overseas operation. The radiator core has a frontal area of 4i sq. ft. and the system is pressurized to raise the boiling point of the water to 224 degrees F. at sea level.
In normal use the brakes are well up to Bedford standard, the vehicle being brought to rest in 28.5 ft. from 20 m.p.h. and 58 ft. from 30 m.p.h. It is an interesting comment on current commercial vehicle design, however, that brakes that are efficient for normal everyday use do not always show up well in the type of brake-fade test to which I subjected this truck. The Bedford engineers have always insisted that the brakes shall be efficient with the servo inoperative—their specification of 0.4g is, in fact, particularly high. This means that the brake pedal cannot have such a large reserve travel available to take care of drum expansion resulting from prolonged application of the brakes.
I was not altogether surprised, therefore, at the results obtained from my brake-fade test. Following the hillclimb, I engaged third gear and drove at full throttle against the brakes. An initial Tapley reading of 63 per cent. was obtained, and as the drums heated, so the efficiency figure decreased to 40 per cent. at the end of a mile and to 26 per cent, at two miles. This rate of retardation was maintained for a further mile, after
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which the pedal had reached floorboard level and the Tapley reading fell to 5 per cent.
Undoubtedly this was a severe test and represents conditions which would not normally arise in this country. In some export territories, however, where long gradients have to be negotiated, it is relatively easy to get into trouble, particularly with inexperienced drivers. Incidentally, on this type of test the better the power-weight ratio of the vehicle, the worse it will show up, and so the high road performance of the Bedford handicaps it in this respect. Nevertheless, for a 7-ton vehicle in the 30 m.p.h. class, reserve pedal travel is important because the operation of high-speed schedules means more frequent application of the brakes, often from higher speeds than with a vehicle in the 20 m.p.h. class.
The brake efficiency recuperation was fairly constant and was complete in four miles when driving at a steady 20 m.p.h. Incidentally, the maximum water temperature recorded when driving against the brakes was no greater than when climbing Bison Hill.
After this I drove at high speed across to Hatfield for the short-performance trials, the speedometer needle lovering over the 45 m.p.h. mark on level ground, withut using full throttle. The Bedford handles like a car, )e 20-in.-diameter steering wheel and 26-to-l-ratio reering box lightening the effort required. Moreover, le easy-change gearbox is proof against noisy ngagement.
At the lower engine revolutions, the Bedford mainlined a steady pull in top gear at 15 m.p.h. during ormal driving, and indeed there was no trace of engine inking or snatch during the 10-30 m.p.h. acceleration sts using direct drive. The 7-tonner requires 39 secs. ) reach the legal speed limit using -fourth gear and 6 secs. passing through all ratios.
Economy in Fuel Consumption Fuel consumption trials were conducted over the taker's standard 9-mile route, beginning on the Hatfield y-pass, and which embodies a climb over Welwyn Hill uring the outward and return runs. I was-using a pressvitch stop watch to time the period during which direct rive was employed. This worked out to 93.5 per cent. [the total time on the outward leg and 95 per cent. uring the return. The return climb over Welwyn Hill rather shorter than on the outward run, and this is ;ually reflected in the fuel consumption rate. The ures were 9 m.p.g. on the outward run and 9.6 m.p.g.
the return, the average thus being 9.3 m.p.g.
Having completed the tests, I passed the vehicle over the works representative, and as a passenger I sat tck to take stock of the Bedford. When fully laden.
e suspension is good; the occasional bump is felt as e wheels ride over undulations, such as sunken man)1e covers, but apart from this there is little to criticize the Bedford springing. Shock absorbers would not
required for road operation in this country, but it ight be advisable to install them for use on unmade ads overseas.
Although close visibility immediately in front of the.
7-tonner is not as good as in the earlier semi-forward control chassis, the angle of vision is improved by extensive glazing, which-is extended to the corner panels in the back of the cab.
The attention paid to driving comfort will be well appreciated by the crew, because in addition to the ample room and moderate temperature in the cab, the driving seat is adjustable and the interior well trimmed for easy cleaning. Driving fatigue is lessened by the general light handling, and this feature led me to measure the brake pedal effort in relation to Tapley readings.
An emergency stop, which according to the previous tests brought the vehicle to rest in less than 30 ft. from 20 m.p.h., produced a maximum Tapley reading of 78 per cent. A less energetic thrust on the pedal (280 lb. according to the Pressometer) produced a figure of 65 per cent, and at lower pressures of 150 lb. and 140 lb. the Tapley showed readings of 51 per cent. and 50 per cent. respectively. By suitably adjusting the driving seat, full thrust can be applied on the pedal without undue effort by the driver.