Road Test of Dennil
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axim 16-ton-gross
four-wheeler By...,. P.
feature is the brake design. Wet weather prevented the best possible stopping distance figures being obtained but as it was the figures were very good for the conditions. The general feel of the brakes on normal running was nearperfect, with no appreciable lag, excellent response and progression and good retardation.
In other respects too the Maxim design can be praised. The standard of the cab from the comfort and quality angles is very high, suspension is excellent and the steering and gear change action are light. However, there will never be a perfect vehicle and there are two points on which the Maxim can be criticized. One is that the clutch is heavy and the other is that the level of noise in the cab is fairly high. This latter point is receiving attention and a "noise-reducing kit" was to have been fitted to the vehicle I tested, but this had not arrived in time. When a Maxim is fitted with the extra equipment I hope to get a run in the vehicle and if so will report the result.
When the Maxim was introduced by Dennis at the last Commercial Motor Show it was the first standard model to include the Cummins VALE lightweight diesel in its specification. This engine is, at the moment, the sole fitting, but after seeing the Maxim six-wheeler at the Scottish Show fitted with the new Perkins V8 (the fitting was done by Perkins) it would not surprise me to see that power unit offered as optional in the Maxim before long: Range make-up The Maxim range consists of the four-wheeler tested (available in wheelbases of 10ft 5 in., 12 ft. 6.5 in., 14 ft. 8 in. and 17 ft. 5 in.), six-wheelers in three wheelbases for up to 22 tons gross and a tractive unit with a wheelbase of 7 ft. 11 in. or 9 ft. 7 in. for operation at up to 26 tons gross train weight. It is considered by Dennis that 26 tons is the maximum practicable with the Cummins V8 employed, which produces a maximum of 178 b.h.p. net at 2,300 r.p.m. and 293 lb.ft. maximum net torque at 1,750 r.p.m. The standard gearbox is the Dennis UM, five-speed, constant-mesh unit as in the test vehicle. On the fourwheelers the rear axle is made by Centrax Ltd. under licence to Rockwell Standard of America; on the rigids it has a single-speed driving head, whilst on the tractive unit there is a two-speed head. On the six-wheeler models a double-drive two-speed bogie made by Moss Gears Ltd. is employed.
As already pointed out, one of the most commendable results of the road test was braking performance. Secondary systems are incorporated on all the Maxim models, even though some of them cannot be used at the increased weights because axle spacings are below the specified figures —for example, the 10 ft. 5 in. four-wheeler. The secondary system is a complete duplication of the main system, there being triple diaphragm chambers at all wheels. The main and emergency sections of these are fed by separate circuits (and reservoirs) through a biased dual foot-valve. With this type of valve, air is fed to each system simultaneously with that in the secondary circuit 25 psi. behind the main.
If there is a fault in the service system the secondary is applied automatically with only a minimal delay and, of course, no extra action by the driver. This, in my opinion, is the ideal type of mainand emergency-brake system and far better than those requiring the driver to operate a hand valve or something similar for his emergency brakes.
Brake tests were delayed until the afternoon of the test day in the hope that the weather would clear up, but this did not happen and they had to be carried out on a wet road. On none of the tests was there any locking of the wheels, even though maximum retardations as recorded by a Tapley meter were 75 per cent from both 20 and 30 m.p.h.
The surface was a good one for brake testing, being rough asphalt giving good drainage for the rain, but the stopping distance must have been extended because of the wet. Even so the stopping distances were very good, particularly from 30 m.p.h. where 17 ftls2 represented an actual efficiency of 52 per cent. Overall decelerations on dry roads of up to 20 ft./s2 were said to have been obtained on tests carried out by Dennis and I can well believe this.
Separate tests of the secondary system were not carried out because of it being a duplicate of the service system, even to the extent of layout of pipe runs. The Neate multipull handbrake produced a very good efficiency of 27 per cent, and although a number of pulls on the lever were necessary before the rear-wheel brakes were fully applied there was no more delay than there would be in the secondary air-pressure systems of some large artics where the semi-trailer brakes are fed from the tractive unit.
More than expected
Fuel-consumption tests were carried out on an 11-1 mile out-and-return circuit over a section of the Farnham by-pass on A31. This is an undulating road, mainly dualcarriageway and carrying only light traffic. The fuelconsumption return when running at up to 40 m.p.h. was exactly 12 m.p.g., which was very good indeed and, I would guess, more than many people would expect. On a high-speed test with an average speed of 44.6 m.p.h., 9-6 m.p.g. was returned. This was also very good and the interesting point is that the figures for the time-loadmileage factors (the product of gross weight, fuel consumption and average speed) differed by only 1-5 per cent.
Before very long we will probably be hearing again about vehicles having troubles with fuel freezing, and an incidental advantage with the Cummins engine should overcome this. It was noted when checking the fuel left in the test tank at the end of the runs that it was warm; this is because that with the Cummins PT fuel system, the fuel recirculates between the fuel tank and engine. It is fed to the pump/injector units by way of passages in the heads and in the process gets heated up and in turn warms the contents of the fuel tank. It was noticeable that the fuel tank was being dried of the rain on its exterior by the warmth inside.
Acceleration tests confirmed the feeling of lively performance that was obvious as soon as the Maxim was first driven. The times returned both through-the-gears and in direct-drive compared well with those obtained on tests with medium-weight vehicles having capacities of from 5 to 7 tons. Even though peak torque is not reached with the Cummins V8 until 1,750 r.p.m., there was a good pull-away from 500 r.p.m. or so on the directdrive runs, the engine showing no distress at all.
Maximum speeds in the gears of the Maxim tested were 6, 15, 28, 45 and 59 m.p.h. and it is interesting that when changing up into third, fourth and top from the maximum engine speed in the lower ratio the engine speed in the new gear is above the point of maximum torque. This means that the gearbox ratios are well matched to the engine characteristics and that the fitting of a two-speed axle on this particular model would be superfluous.
The manufacturer does not offer a two-speed axle on the four-wheeler, although this is standard on the 6 x 4 and tractive unit where the higher weight makes it more desirable to keep the engine running closer to the point of maximum torque; grossing 16 tons, the torque from 1,700 r.p.m. to the maximum engine, speed is quite adequate. A fairly short hill was used for the hill-climbing performance tests, this being Woolmer Hill, near Hindhead. Although only 0-3 miles long, Woolmer is very steep, the average gradient being 1 in 7-5 and the maximum 1 in 5. The total time for a maximum-power ascent was 2 min. 3 sec., of which 34-2 sec. was spent in first gear; the minimum speed on the climb was 5 m.p.h. Brake fade was checked by coasting the Maxim down the hill with the brakes applied to keep the speed at 20 After a run which took 1 min. 5 sec., a maximumpressure brake stop from 20 m.p.h. produced a Tapleymeter reading of 70 per cent. As this was only 5 per cent less than the best figure obtained with cold drums, fade should not be a problem with the Maxim. In fact, when the' linings were hot the efficiency appeared to have improved because the rear wheels locked for a short distance and the resulting skid could have been the reason for the reduction in efficiency. Stop-and-restart tests were carried out on the steepest section. Easy restarts in bottom and reverse gears were made and the handbrake held the vehicle comfortably when facing up and down the hill respectively.
There was a strong smell from the clutch after the stopand-start tests on Woolmer Hill, caused by the need to have a fairly high engine speed when starting off. This was not only the case on steep inclines such as Woolmer Hill, but also on general driving. I found that the engine stalled easily when starting off using the normal technique, although—as found on acceleration tests—once roiling, the engine pulled away happily from very slow speeds.
The need to use an entirely different driving pattern was obvious as soon as [took over the wheel of the Maxim. I never really mastered upward gear changes despite the Dennis UM gearbox having a very light change and well-defined gate. The most difficult thing to get accustomed to is that instead of the engine speed in the next higher gear selected being relatively low, it is generally over 1,500 r.p.m. if the engine had been run near to the maximum in the lower gear; 1.500 r.p.rn. is often the engine speed when an upward change is made on commercial vehicles. An engine tachometer is a standard fitting on the Maxim, which is a worthwhile item for a driver taking over the vehicle for the first time.
It certainly helped me in making clean changes.
Apart from the embarrassment at making imperfect gear changes in front of Lou Jackson of Dennis, who drove the vehicle without a blemish, I ,thoroughly enjoyed driving the test vehicle. The only real criticism from this point would make is that the clutch is unduly heavy. A twinplate clutch or some form of power-assistance would be an advantage in reducing the driver's effort, as the clutch was quite out of keeping with the rest of the controls and the general character of the Maxim. The brakes had a particularly good feel. There was no feeling of lag in the system and the retardation was progressive and proportionate to the travel of the brake treadle so that it was easy to get the right degree of braking for any particular circumstance. The powerassisted steering had a light action and there was no vagueness.
I have already said that the gear-change action was very good and, added to this, the lever is conveniently placed to the left hand. Locating the handbrake lever on the right-hand side of the driver's seat makes this convenient also. A bad point about the latter unit, though, is that part of the mechanism is exposed and this was covered in grease which could easily come in contact with a driver's clothes.
The interior of the Maxim cab is fitted and finished to private-car standards and this applies to the seating, instruments and the amount of padding provided. Switches for lights and so on are mounted above the windscreen on the driver's side and the headlights and side lights on the offside and nearside can be operated independently. With the switches there are two sockets, for an electric razor and a " wander " light, and other facilities not normally provided in a commercial vehicle include a clock and a cigarette lighter. The standard heater/demister unit was found to be very effective and the air-operated wipers cleared a large area of the large windscreen. At the beginning I referred to the Maxim being the first vehicle tested with Dunlop tubeless RB6 radial-steel cord tyres. This is a fairly new development by Dunlop and only 10.00-20 are at present available. Whilst the tyres are readily available the special wheels needed to go with them are not. This fact is holding up the wider use of tubeless tyres, says Dunlop. The wheels have a rubber sealing ring in the detachable rim, and improved supplies should be available by next spring; 9.00 and 11.00 sizes of the same tyre will also be obtainable.