MASTIFF 6x2 Laughs at 14 Tons
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New Thornycroft Six-wheeler Announced Today Shows Firstclass Fuel Economy at High Speed: General Performance and Handling Reaches High Standard By John F. Moon,
FIRST CLASS performance, above average driving comfort, sensible design and robust construction are regular features of current Thornycroft production. These points were made clear in the road test reports of the Trusty eightwheeler (October 25, •1957) and the Mastiff fourwheeler (September 12, 1958), and this pair of thoroughbreds has now been joined by a 20-tongross single-drive six-wheeled chassis to be known as the Mastiff Express Freighter.
This outstanding vehicle is announced today, and a road test revealed truly remarkable fuel economy under it variety of different operating conditions. This economy in terms of gross ton-M.p.g. and timeload-mileage factor figures is even better than was obtained with either the Trusty eight-wheeler or the Mastiff four-wheeler, whilst in respect of acceleration, braking, hill climbing, and general handling, this new chassis is well up to the standard now expected of the Thornycroft factory.
The new chassis, other than with respect to the cab and frame, makes use of standard components already fitted to . various other Thornycroft models. The engine is the QR6 as used in the Trusty range, and this unit—the gross power output of which is 139 b.h.p.—is mated with the Trusty five-speed constant-mesh gearbox, which is available with overdrive sixth gear for additional economy.
Front and driving axles and the front suspension are taken from the Mastiff 4 x 2, whilst the trailing axle
The trailing axle for the new 6x2 is made up from the outer sections ofa Trident driving axle, a tube replacing the normal differential ease.
has been ingeniously built up from the Trident driving axle, the original tubes, hubs and braking equipment being retained but the normal differential ease being replaced by a tube to join the halves of the axle and so form .a dead assembly.
The Trusty balance-beam rearbogie suspension is incorporated in the new chassis. Whilst the braking follows Mastiff 4 x 2 practice in that it is an air-hydraulic system. A difference here, however, • is that a split-circuit is employed, there being separate air-hydraulic actuators and master cylinders for the front and trailing axles and for the driving axle.
A new chassis frame has been evolved for this vehicle and is available with one standard wheelbase only —15 ft. 101 in. it is not unlike that of the Mastiff 4 x/2 but the material
thickness is in. compared with 7132 in., and the side-:thernbers have a An entirely new cab has been developed for the 6 x 2 model and, as usual with present-day Thornycrofts, a high proportion of plastics is employed. The cab is in two basic sections. The cab base is in four parts. The floor is of steel, and to it are attached the plastics wings and the lower front panel. •
The upper section of the cab is an all-plastics assembly, incorporating a few metal and timber fillets which are laid-up in the manufacturing process and which cannot be seen in the• finished product.
Styling is effective, its more noticeable features including the use of paired headlights, deep wrap-round windscreen panels and a neat radiator grille. • Interior appointments include full-width shelves above and below the windscreen and a large pocket ahead of the passenger seat, two coat hooks, two ashtrays, a large toolbox under the mate's seat, and even a small light inside the steering column shrouding panels which illuminates the cab floor.
The test vehicle was the first of the new models to have been built and on it had been mounted a Thornycroft light-alloy 21-ft. 4-in, platform body. The kerb weight was 5 tons 19f cwt., a figure which allows a 14-ton payload to be carried. The body weighed 8 cwt. 7 lb. and was of somewhat substantial proportions.
Weight boxes carrying concrete and steel blocks had been lashed to the body so that the total running weight, with myself and Thornycroft's Mr. G. K. Martlew aboard, was 20 tons 31 cwt., the distribution being such that with nobody in the cab, all the tyres were equally loaded.
A fuel-consumption meter had been provided and this was used during a 15-mile fuel-consumption run along the crowded A30 road between the west end of the Basingstoke by-pass and Blackbushe airport.
Particularly heavy traffic was encountered and, although Mr. Martlew endeavoured to drive at well over 30 m.p.h. wherever possible, slow-moving heavy vehicles kept our hill-climbing speed down to 20 m.p.h. These low-speed ascents did, however, give me an opportunity to study the exceptional low-speed engine-torque characteristics, the power unit pulling strongly at speeds below 10 m.p.h. in both direct drive and overdrive.
Despite the really unfavourable conditions, an average speed of 29 m.p.h. was maintained over the course and a consumption rate of 10.25 m.p.g. recorded. After braking and acceleration tests, the return journey was made under even worse conditions and this gave 10.9 m.p.g. at 30 m.p.h., the overall figure for the 31-mile round trip being 10.6 m.p.g. at 29.5 m.p.h., results which are good in view of the circumstances.
For the braking and acceleration tests, a little-used stretch of level, though narrow, road on the south side of Blackbushe airport was used.
The braking tests showed commendable retardation characteristics, the distances recorded being little more than those obtained with the Mastiff 4 x 2 last year, though, because of the greater number of tyres on the eight-wheeler, not quite so good as were achieved with the Trusty in 1957.
Slight lag in the system was noticeable, accounting for a difference of some 20 per cent. between the maximum and average figures obtained, and stops from both 20 m.p.h. and 30 m.p.h. caused all the rear wheels to lock, melting the tar of the road surface in the process. The retarda tion rates were sufficient to start to move the test load forward and bend the body headboard, a point to be watched with any powerful braking system.
For the standing-start acceleration tests up to 30 m.p.h., second, third and fourth gears were employed and exceptionally good times were obtained, these again being little worse than were achieved with the Mastiff 4 x 2. Indeed, because of the favour able power-to-weight ratio of the 6 x 2, far better direct-drive times were returned between 10-30 m.p.h., this again emphasizing the outstanding high-gear performance of this vehicle. The engine pulled smoothly during these latter tests, and there was no transmission roughness.
Having returned to Basingstoke at the end of the second half of the first fuel-consumption test, the 6 x 2 was taken up Farleigh Hill, a 1.4-mile incline with an average gradient of 1 in 27.
The climb was made in an ambient temperature of 66° F. and the vehicle seemed to fly up the hill, the time of ascent being only 4 min. 19 sec. The minimum speed at any time was 8 m.p.h., this being observed when in second gear.
r14 Because the cooling-system thermostat is set to open at 170° F., and because the pressurized cooling system has a water capacity of 61 gal., the thermostat did not open during this climb, with the result that the radiator top-tank coolant temperature was the same after the climb as it was before it-120° F.
A descent in neutral made to check the fade resistance of the brakes occupied 31 min., and, of this time, nearly 2 min. were spent in fifth gear, driving at full throttle against the brakes to compensate for the reduced gradient near the bottom of the slope.
At the end of this descent, a fullpressure stop from 20 m.p.h. gave a Tapley-meter reading of 55 per cent., which is a drop of only 15 per cent. compared with the figures obtained earlier in the day with cold drum& This indicates good anti-fade characteristics. The two near-side rear brakes were smoking profusely at the end of this test, but the other linings were merely smelling without issuing forth smoke.
I then drove the vehicle back to the steepest section and stopped it there, whereupon the multi-pull hand brake was applied. The brake only just managed to hold the vehicle on this incline, but the drums were still hot, of course, from the descent made immediately before.
Mr. Martlew then suggested that I would manage to get away in second gear and, although I had doubts about this, it proved possible. As soon as the clutch was fully engaged, the engine speed dropped sharply but the low-speed torque was sufficient to continue to pull the vehicle slowly up the hill.
Several more fuel-consumption tests were then made, the first of these being an unladen 30-mile run over the Basingstoke-Blackbushe course. This was made using the overdrive ratio the whole time (including when starting from rest) and the average speed for the round trip was 31 m.p.h., the fuel-consumption figure being 19.75 m.p.g. This run was made at a weight of 6 tons 131 cwt., the weight boxes having been left on, which accounts for the running weight being more than the kerb weight .
Lighter Traffic
On the second day of the test, I elected to make another laden fuel run over the same course in the hope that traffic might be lighter and, as luck would have it, little traffic was encountered in both directions. The result was that an extremely fine performance in respect of speed and fuel economy was achieved, the resulting overall average figures being 30.6 m.p.h. and 13.4 m.p.g.
One of the main reasons for the big difference between these figures and those obtained on the previous day was that, because of the absence of traffic, the hills on the route could be approached at a run, speeds of up to 45 m.p.h. being employed. Thus the momentum of the Thornycroft was sufficient to carry it over such hills in either fifth or sixth gears.
As a final fuel test, the 6 x 2 was driven as fast as possible over a five-mile stretch of the Winchester bypass, the one set of traffic lights on this road being fortunately with us, so that the run was made non-stop at an average speed of 41.5 m.p.h. This produced a fuel-consumption rate of 9.2 m.p.g., which is very good for a vehicle of this weight.
These two fuel-consumption results show that this latest Thornycroft chassis thrives on high-speed operation and that, when given its head, the Mastiff can show most other traffic a clean pair of heels with a consumption rate more to be expected from a vehicle of half the gross weight.
This sort of performance is needed for motorway operation, although an even higher axle ratio (5.375 to 1) is available for those who require to work continuously at still greater speeds. This high-ratio axle gives a top speed of 60 m.p.h., compared with the 52 m.p.h. yielded by the 6.14-to-1 axle fitted to the test vehicle.
Of particular importance for such high-speed operation is the good speed performance in fourth and fifth gears. These ratios give 30 m.p.h. and 40 m.p.h. respectively, speeds which are invaluable when overtaking slow traffic.
Although I have somewhat naturally emphasized the performance and fuel economy of the chassis at high speeds, the original fuel-consumption figure obtained under bad conditions shows good economy and indicates that no higher figure than 10.5 m.p.g. can be feared with this vehicle when running soh), as intended by the designers.
Consideration of the laden and unladen figures indicates that operators carrying goods in one direction only can expect to ,receive overall average consumption rates of about ' 16 m.p.g under normal service conditions.
Apart from a slight tendency to wander at high speeds and, under certain conditions, to pitch slightly at over 45 m.p.h. (a fault which could be cured by fitting dampers at the front axle, these being listed as optional equipment), the general handling was beyond criticism.
The gear change gives action far better than that to be found with most synchromesh boxes, despite its constant-mesh gearing: The braking system has entirely progressive characteristics, the first 4° of movement of the treadle-type pedal giving 5 p.s.i. line pressure and the next 17° building up to 75 p.s.i., after which— at approximately 60 lb. pedal pressure—maximum line pressure of 105 p.s.i. is obtained. Under normal conditions the steering was good, the hydraulic servo being of great value when manceuvring in confined spaces. Although the steering was not dangerously light, its finger-tip action made for fast negotiation of roundabouts and sharp corners, and fortunately the suspension was up to this so that roll never became unduly pronounced.
Even over unfavourable road surfaces at high speeds, the suspension was beyond reproach, whether the vehicle was laden or empty. The engine behaved quietly and smoothly at all times. The cab is most comfortable and the all-round range of vision complete, even the 51-in.
diameter mirrors giving a better field of rearward vision than their size would suggest.
As the test on the first day was completed in darkness, this afforded a good opportunity to study the effectiveness of the paired-headlamp system. When on " head " all four lights are on full beam, giving a good spread, and when on " dip" the inner lamps go out and the outer lamps change filaments so that there are two good beams thrown in the direction of the kerb.
Optional Equipment
The chassis tested was completely standard in all respects, the only optional equipment fitted being thesixth (overdrive) gear ratio, power steering, wind-up spare-wheel carrier and cab heater and demister. Other optional equipment includes 10.0020-in. (12-ply) tyres, additional fuel tanks, front towing eyes or towing jaw, single headlights, rear drawbar gear, twin Windtone horns, and the Thornycroft NS6 I09-b.h.p. six cylinelered oil engine.
This alternative engine, which is the standard unit in the Mastiff 4 x 2, is offered primarily for operators who can expect to do most of their running in level areas, and although it would not necessarily give any great advantages in respect of fuel economy it would cost less than the standard QR6 engine.
From the maintenance aspect, the new chassis follows normal Thornycroft lines, the various procedures having been detailed in road test reports on the Trusty eight-wheeler and the Mastiff 4 x 2.