STUTTGART-CLITHEROE NGAPORE
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Mercedes-Benz 5-tonner Equipped with Primrose Third-axle Conversion for Use in Malaya at 12 tons Gross Weight: 16.5 m.p.g. Obtained at 29.5 m.p.h. Average Speed By JOHN F. MOON
A.M.I.R.T.E.
MALAYA is a beautiful country, its wealth including vast rubber plantations and a year-round sunny climate. Its natural resources, however, arc not backed up by an equally rich road system and, although most of the main network of roads is metalled, the roads are narrow and the majority of bridges are not particularly strong. For this reason two-axled vehicles are limited to a gross weight of eight tons, and the legal maximum for threeaxle chassis is 12 tons.
It is because of these low legal maxima that an unusual application of a third-axle conversion has recently been carried out in this country. The Daimler-Benz agents in Singapore, The Cycle and Carriage Co. (1926), Ltd., were anxious to supply goods vehicles with legal payload capacities of between seven and eight tons, but they appreciated that the only way this could be done would be to import third-axle conversions of reasonably light four-wheeled chassis, factory-built six-wheelers being far too big and expensive for operation at 12-tons gross. Thus it was arranged that Primrose Group Sales should carry out a conversion on a Mercedes-Benz L328 5-ton semi-forwardcontrol goods chassis and supply conversion kits to enable The Cycle and Carriage Co. themselves to convert further chassis shipped in directly from Germany. A contract was placed with Primrose through William Duff and Sons (1930), Ltd., 32 Lime Street, London, E.C.3, who arranged for an L328 chassis to be sent direct to Clitheroe from Germany so that the initial conversion could be carried out, following which the chassis was to be shipped to Singapore. As soon as the chassis was finished, n 1 6 however, I was able to carry out a road test of this interesting vehicle, the test showing particularly good fuel economy and good driver comfort. However, because of the low torque output of the engine, acceleration and hill-climbing performances were rather poor compared with those of British four-wheeled chassis running at about the same gross weight.
As parts of Malaya can be very hilly indeed, this might be found a disadvantage, but the whole object of this conversion has been to provide the cheapest possible vehicle which could legally operate at 12 tons gross in that country, and to have used a higher-powered Mercedes-Benz chassis, such as the L322 6.5-tonner, would have meant. increasing the cost of the converted chassis beyond reasonable limits. Even so, I cannot help feeling that there are several British-built four-wheelers which would have been. equally suitable for this application.
The Mercedes-Benz 328 range of forwardand semiforward-control goods vehicles went into production about 12 months ago and replaced the 312 range, of which nearly 100,000 were produced. For solo operation the 328 models are rated at 8.9 tons gross, whilst the train-weight rating is 16.4 tons, this allowing it to operate in Germany at the requisite legal power-to-weight ratio of 6 b.h.p. per ton gross weight.
A Daimler-Benz OM 312, 4.58-litre, six-cylinder directinjection diesel engine powers the 328 modals, this being by no means a new design, but one which has been fully proved in service. Its net (DIN) power output is 100 b.h.p. at the comparatively high speed of 3,000 r.p.m., and the
net torque output peaks at 195 lb.-ft. at an engine speed of 1,600 r.p.m. Interesting features of the engine include a water-nil heat exchanger and an air intake, the ducting of which actually passes through the radiator header tank. The air does not. get heated, as it is drawn through at high velocity, but because the inlet funnel is ahead of the radiator cleaner air flows. into, the engine.
In unit with the engine is a Daimler-Benz five-speed synchromesh gearbox, top gear of which is direct. The gear-change action of this box was found to be perfectly satisfactory on test, but I could not help feeling that there is too wide a gap between fourth and top gears, the respective maximum speeds in these ratios being 28 and 47 m.p.h.
A two-piece propeller shaft takes the drive into the rear axle, which has hypoid-bevel final gearing giving a reduction ratio of (.857 to 1.
Long semi-elliptic springs are a feature of the suspension at front and rear, each of them being almost 5 ft. long and controlled by telescopic dampers in the case of the front axle and supplemented by helper leaves at the rear axle. The chassis frame, which has seven cross-members riveted in position, has 0.3125-in.-thick side members, the upper flanges of which are flat rearwards from the back of the cab. The maximum side-member depth is 8.57 in. and the flange width is 2.37 in.
Daimler-Benz recirculating-ball steering is employed and, in common with current Continental practice, the steering column is universally jointed so that the plane of the steering wheel is only 37' off horizontal. The A.T.E. (Teves) hydraulic braking system has leading-and-trailing-shoe units at both axles and is boosted by a Westinghouse airpressure servo which works at a maximum air pressure of 75 psi.
The semi-forward-control cab is a standard DaimlerBenz product as used also on the L323, L322 and L327 chassis. It has a four-point mounting to the chassis frame and, as usual with Daimler-Benz cabs, the finish is of a particularly high order. A two-man passenger seat is a standard fitting, although the intrusion of the rear engine cowl into the cab space restricts the foot-room available to the centre passenger. The cab is , generously glazed, although the screen line is on the high side and a driver of average height cannot see much of the bonnet from the driving seat. Nor can either of the front wings be seen, but sprung width indicators are fitted as standard equipment.
So much for the standard chassis. The right-hand-drive chassis converted by Primrose originally had a wheelbase of 13 ft. 9t in., this model being supplied in Germany with a 14-ft. 9-in.-long body. For the conversion, Primrose used their familiar hydraulically compensated, coil-sprung third axle. The location of the driving axle was left undisturbed, the effective wheelbase of the vehicle as a six-wheeler being 1 ft. 11+ in. •greater than standard because of the bogiecentre dimension of 3 ft. 10+ The original side members were not altered drastically, and none of the cross-members was discarded. The chassis length was increased by adding 51 in. X 4 in. X + in. channels which were fitted inside the side members and extended as far forward as the rear-spring front hanger brackets, these being bolted in place. A non-standard crossmember was added in the vicinity of the rear hanger brackets, immediately to the rear of which another crossmember was added, this one carrying the trailing-axle coil . springs. The standard Mercedes-Benz rear cross-member was retained and bolted to the rear ends of the added channels. Running the length of the converted frame as far forward as the front-spring rear hanger brackets, a 3 in. X 2 in. X 1 in. channel was added to form a runner. The object of this was partly to strengthen the frame and partly to ensure a high enough body height to give adequate clearance above the coil-spring reaction cross-member.
The trailing axle itself was based on a standard Primrose 4i-in.-diameter tube but, contrary to normal Primrose practice, it did not carry the same hubs and brake assemblies as the driving axle, these not being available. Instead, therefore, Girling brakes with a diameter of 151 in. and 41-in.-wide linings were employed, adding 255 sq. in. frictional area to the 341 sq. in. of the basic chassis.
Details of this Primrose third-axle conversion are wellknown to British operators, hut, for the benefit of overseas readers, let me just say briefly that the trailing axle is carried on quarter-elliptic leaf springs riveted at their forward ends and free to slide against curved brackets which locate the ends of the springs laterally. The main suspension is provided by the two coil springs, which are supplemented by Aeon rubber helper springs and controlled by telescopic dampers. Two hydraulic cylinders interposed between the chassis frame and the driving axle Are piped to larger hydraulic cylinders against which the i 8
upper ends -of the coil springs butt. This system ensures that a correct balance is maintained between the individual axle loadings of the bogie and, as has been proved on previous tests conducted by The Commercial Motor, good traction characteristics over rough ground and up adverse gradients are given by the single driving axle, both when moving forward and in reverse.
As received from Germany the L328 chassis was equipped with 8.25-20 (10-ply) tyres. Because there was no need at all for twin tyres on the trailing axle, single 8.25=-20 (10-ply) tyres were fitted on centre-disc wheels, the effective track being the same as that of the driving axle. Thus equipped, the six-wheeled chassis and cab had a kerb weight of 3 tons 18 cwt. For the purposes of my test a temporary platform body, carrying long concrete slabs and a few iron weights, was mounted on the chassis, the combined weight of this imposed load totalling 8 tons 61. cwt. This meant that the vehicle was tested at a gross weight of 12 tons 8+ cwt.-8+ cwt. above the legal maximum at which it will be able to operate in Malaya.
I collected the vehicle from the Clitheroe depot of Primrose Group Sales and was soon to regret that this particular example was destined for Malaya. The temperature hardly rose above zero the whole time-and no heater had been fitted in the cab! However, well wrapped up, and accompanied by Primrose's Kenneth Pimm, we headed for the Preston By-pass, the M6 motorway, where a 17-mile fuelconsumption test was conducted.
The amount of traffic and the severity of the gradients on this motorway made the conditions of the test almost identical to those that could be expected on a normal Malay main road, although I was driving more slowly than most Malay drivers would, keeping the speed down to about 33 m.p.h. except on down-grades. The run was completed at an average speed of 29.5 m.p.h., and the resulting fuel-consumption rate was extremely satisfactory —16.5 m.p.g., giving a time-load-mileage factor of 6,018. This fuel figure compares very favourably with what could be expected from a British vehicle running at about the same gross weight and possibly explains why this DaimlerBenz OM 312 engine has remained so popular over the years.
Gradient performance tests were carried out next, Parbold Hill being used for these. This incline is 0.75 mile long, with an average gradient of 1 in 12, and the ambient temperature at the time of these tests was —12°C. (28°F.). The radiator was unblanked, and the temperature of the water in it was measured at 72°C. (162°F.) before making the climb.
The ascent was more rapid than I had expected, being completed in 4 minutes 10 seconds. The road speed never dropped below 8 m.p.h., and this occurred while in second gear—the lowest ratio used, which was engaged for a total time of 1 minute 13 seconds. Because of the low ambient temperature, the ascent caused the temperature to rise to only 77°C. (170°F.). The cooling system is pressurized, however, so, even allowing for the fact that the vehicle will be operating in ambient temperatures of 32°C. (90°F.) or above, engine cooling should be adequate.
My usual fade test was carried out down Parbold Hill, the vehicle being allowed to coast in neutral, the footbrake being relied on to restrict the speed to about 20 m.p.h. The descent lasted 2 minutes 22 seconds and at the bottom of the hill a full-pressure stop was made from 20 m.p.h„ a Tapley retardation meter reading of 50 per cent. resulting. This figure compares very favourably indeed with the average figure of 67.5 per cent, obtained later on in the test when the drums were cold and shows that the generous drum and lining area of this light six-wheeler should be proof against any fade occurring even though, unlike most German vehicles, this particular chassis did not have an • exhaust brake. While descending the hill, only very light pedal pressure was needed, the line pressure never exceeding 15 psi. When making the crash stop at the bottom both the trailing-axle wheels and the near-side front wheel locked.
Following this fade test I drove the vehicle back up the hill and stopped it on the steepest section, the gradient here being 1 in 6.5. Here the handbrake, which is of the umbrella type and pulls out from beneath the facia panel, held the vehicle with ease, even though the drums of both rear axles would still be quite hot from the fade test. Then I attempted a restart in second gear. not really expecting this to be successful. However, by dint of slipping the clutch deliberately a little, I managed to get the six-wheeler moving, and then once the clutch was fully engaged the engine pulled away at such low revs that the dynamo was not charging—a remarkable indication of low-speed torque output.
Fortunately, and despite the low air temperatures, managed to find a at piece of road on which to conduct braking tests. The surface, however, was wet. In view of the conditions the retardation figures obtained were extremely good. When stopping from both the test speeds it was obvious that the trailing-axle wheels were locking and hopping slightly, and the near-side front wheel was locking also. Despite this, the Mercedes-Benz remained quite stable while coming to a standstill. There was no evidence of detrimental lag in the air-pressure system and, although a larger servo had not been fitted to cope with the extra effort required by the trailing-axle brakes, it was obvious that the braking system was entirely adequate.
Handbrake efficiency was high, its application at 20 m.p.h. producing consistent Tapley-meter readings of 29 per cent. The power of the handbrake was sufficient to lock the wheels of the trailing axle.
The same stretch of road was used for the acceleration tests, and quite satisfactory figures were obtained between a standstill and 30 m.p.h., second, third, fourth and top gears being used. Unfortunately, it was obvious that there was a slight gradient on the road, because it proved impossible to exceed 20 m.p.h. in one direction when making the direct-drive acceleration tests from 10 m.p.h. upwards. No figures can be quoted for this test, therefore, which serves to emphasize that this is a vehicle with a gearbox which has to be used. When making these direct-drive attempts, incidentally, I noticed that there was a certain amount of transmission roughness at about 14 m.p.h.
Taken generally, the Mercedes-Benz six-wheeler was quite a pleasant vehicle to handle, engine-noise level as heard in the cab being particularly low. The engine seems to be fairly quiet at most speeds until approaching the governed speed, and the front of the cab is obviously very well insulated, because when cruising at between 30 and 40 m.p.h. there is little more noise in the cab than can be expected from a petrol-engined vehicle.
The front suspension was a bit harsh, but this would presumably be because the front-axle loading of the test vehicle was 121 cwt. less than the intended loading when the [328 is operated as a conventional four-wheeler. Not unassociated with this would be the slight vagueness of the steering, making it necessary to "saw" at the wheel when travelling at normal cruising speeds. Conversely, however, the steering was found to be a little on the heavy side when manoeuvring at walking pace, and this could obviously not be anything to do with the axle loading.
As already remarked, the gear-change action was quite pleasant and the clutch was light to use. On the debit side, though, I noticed that the gearbox tended to whine when in the indirect ratios. Also light to use was the brake pedal.
The driving seat is fully adjustable, the complete seat being movable in a fore and aft direction, whilst the cushion-height can be altered and the rake of the squab adjusted. Despite the absence of a heater, it was not as cold in the cab as might have been expected, so it was obvious that the cab insulation was pretty effective. So far as hot-weather operation is concerned, there are several freshair outlets into each side of the cab (a fan is incorporated in the system for extra effect), whilst additional cooling air can be obtained through two vents above the windscreen which give a screen of cool air across the ceiling of the cab, and through the large swivelling quarter lights in the cab doors which can be turned to act as scoops. The engine cowl in the cab is a fixed component, so this is fully draught-proofed to avoid the chance of hot air from the engine compartment being forced into the cab.
All in all, this was a most interesting vehicle to test. In any case, 12-ton-gross six-wheelers must be something of a rarity, whilst there can be few examples—as yet—of British-German co-operation on a commercial vehicle. There would obviously be no call for a conversion of this sort on this chassis in this country—at present the L328 is not even being imported by the British concessionnaires —but it obviously is a fairly satisfactory way of filling the particular need in this case. Certainly the conversion is a very workmanlike job and should, if anything, enhance the life of the vehicle. It certainly will not shorten it, and if there had ever been any fear of this, Daimler-Benz A.G. would never have approved the Primrose system.