PAX PERM
Page 52
Page 53
Page 54
Page 55
If you've noticed an error in this article please click here to report it so we can fix it.
'ION
BY JOHN F. MOON A.M.I.R.T.E.
pRE-PRODUCTION examples of the Pax V range of chassis were exhibited by Dennis Brothers Ltd. at the London Commercial Motor Show last September, but production of this series only started at the beginning of last week. Full details of the range can now be given, therefore, from which it will be seen that it comprises a series of medium-duty chassis for both goods and passenger applications, available with a choice of four power units, three gearboxes, two axles and , seven wheelbases, and with gross-weight ratings extending from 8 tons 7 cwt. as a p.s.v. to 18 tons as an articulated outfit. A six-wheeled version for operation at 20 tons gross is a further project.
One of the basic four-wheelers was recently made available for test, this having had the Perkins 6.354 diesel engine and single-speed axle. Running at a gross weight of just over 12 tons, the Pax V was shown to have a good average performance besides being particularly pleasant to drive. Although more expensive than other British 12-ton-gross machines—the chassis-cab price is f2,125—the chassis has the merit of being built by one of "the old school ", so the general standard of quality tends to be very high, which in turn foretells long, trouble-free life in the well-earned Dennis tradition.
The four engines available are the Perkins 6.354 in its normally rated form, the B.M.C. 5.7-litre unit, developing 105 b.h.p. at 2,400 r.p.m., the Gardner 5LW five-cylinder 7.0-litre diesel, which gives 94 b.h.p. at 1,700 r.p.m., and the A.E.C. AV470 7.685-litre 125-b.h.p. engine. The maximum torque outputs of these four engines are 260, 255, 300 and 325 lb. ft respectively, and the Perkins, Gardner and A.E.C. power units have 14-in, clutches, the B.M.C. engine carrying a 13-in. assembly. Newly developed Dennis gearboxes are used in all cases, these being derivations of the same basic constant-mesh design. The U-type box is a wide ratio unit suitable for torque inputs of up to 250 lb. ft. and is recommended for general heavy-duty applications. Alternative to this box when the Perkins or B.M.C. engines are fitted is the UHtype close-ratio 300 lb. ft. unit which, because of its relatively high fourth gear, is well suited for operation under congested traffic conditions. The third version of this box is the UO-type, which has overdrive top gear and is the only box offered with the Gardner or A.E.C. engines, its input-torque allowance being 325 lb. ft. In all cases the gearbox itself is an aluminium-alloy casting incorporating the bell housing, the withdrawal-fork c2 cross-shaft being carried in p.t.f.e. bearings. From the driver's angle the box has the merit of being very easy to use, the gear-change action being light and positive as a result of the layout of the selector mechanism. Provision is made for fitting Dennis power take-offs on the left side or the top of the box.
A new single-speed rear axle with spiral-bevel gearing has been developed for this range, a feature of the axle being the very rigid differential carrier. There is a choice ot ratios-6-14 or 717 to 1—and instead of the Dennis axle an Eaton 16802 two-speed unit can be specified. The front axle is of conventional design, and p.t.f.e.-faced thrust washers are used at the stub axles. All models have 52-in. by 3-in, front springs, and load carriers have 60-in, by 3-in. rear springs; tractive units, however, have shorter rear springs, these being 45 in. long when Scammell coupling gear is fitted or 54 in. long if a fifth wheel is employed.
In all cases the frame assemblies are of bolted construction, and the standard side-member section is 9-0 in. by 2-5 in. by 0.25 in., low-loaders having 8-062-in.-deep frames rough. Burman recirculating-ball steering is standard on chassis, this having variable-ratio characteristics, with ratio of 26.8 to 1 in the centre position.
Except in the case of the low-loaders, all four-wheeled lassis in the new range have a common braking system, Lis consisting of Girling two-leading-shoe hydraulic units front and rear, boosted by a Hydrovac servo and giving total frictional area of 477 sq. in. The low-loaders have nailer-diameter brakes, but the linings at front and rear -e 5 in. wide, so the total lining area of the footbrake is 74 sq. in., whilst in addition the handbrake lever operates drum brake on the nose of the rear axle which has a Fling area of 52 sq. in. It has been proposed to offer disc rakes at front and rear of the normal-height chassis as ptionaI equipment, in which case the transmission handrake used on the low-loader chassis is employed.
There are several tyre options throughout the range to lit the different gross loadings, including 7.50-20, 25-20, 9.00-20 and 8-25-17. With the Perkins and .M.C. engines the electrical system is 12 V. and 24 V
electrics are used with the two larger engines.
The standard cab offered with these models is the attractive composite-construction assembly introduced in 1960 for the Pax IV range. It is based on an all-steel foundation which is rubber-mounted to the frame at four points and includes the engine cowl and front dash assemblies, which are plastics mouldings. Plastics is employed for the doubleskinned roof assembly and the wings also, the remainder of the cab being of steel, with certain timber framing members. The standard cab has a one-piece curved windscreen which is 6 ft. 2 in. wide and 2 ft. deep, but alternative to this a two-piece screen with either fixed or opening flat panels can be fitted.
An extensive range of wheelbases is offered. The shortest model, for Scammell coupling gear, has a wheelbase of 7 ft., the next largest having an 8-ft. 2-in, dimension and c3
being suitable for fifth-wheel couplings. The wheelbase options for the normal load carriers are 10 ft. 4 in., 12 ft., 14 ft. tin., 14 ft. 9 in. and 17 ft. 5 in. Gross vehicle weight ratings vary according to application and specification, but generally are of this order: passenger chassis, 8 tons 7 cwt.; low-loaders, 9 tons 12 cwt.; normal goods chassis, 10 tons to 12 tons 18 cwt.; and tractive units, 15 tons 10 cwt. to 18 tons.
The Pax V submitted for test was a 14-ft. 1-in-wheelbase model with 9-00-20 tyres, and in chassis-cab form its kerb weight was 3 tons 6-75 cwt., the front-axle loading being 2 tons 0-75 cwt. Iron weights mounted in a tray formed the test load, and it was unfortunate that the tray was a bit too short for the length of the chassis, with the result that the centre of gravity of the load was farther forward than it should have been, so causing the front-axle loading to be higher than desirable. Fully laden, and with two people in the cab, the vehicle grossed 12 tons 3-25 cwt.
Acceleration tests showed the Pax to be quick off the mark when accelerating through the gears, the recorded time up to 30 m.p.h. being good for a vehicle of this weight. I was not surprised at this, knowing the liveliness of the Perkins engine and the fast, easy gear-changing possible with this new Dennis box. The direct-drive acceleration was quite satisfactory, the smoothness of engine and transmission from 8 m.p.h. upwards being particularly marked: in this connection the provision of a damper on the propeller shaft was possibly a help.
c4
Two sets of fuel-consumption figures were taken ow an undulating 10-5-mile course, one run being made witl out exceeding 32 m.p.h. whilst the other was completed accordance with the latest British goods-vehicle speed lin) of 40 m.p.h. The consumption rates recorded were n( particularly flattering, and I would have expected a figui nearer 15 m.p.g. from the slower run. However, road an traffic conditions were not good, as can be gathered froi the relatively slight difference between the two quote average speeds despite a maximum-speed difference of I m.p.h., and in any case the time-Toad-mileage facto] resulting from the two tests are good, besides being remarl ably close. There being no motorway near Guildford, full-throttle consumption run could not be made, but ha it been possible a figure of about 13 m.p.g. would probabi have been obtained.
It was unfortunate that the brake tests had to be carrie out on a wet stretch of concrete road because I feel tha despite rather high pedal effort, good stopping distanc( should be possible with this vehicle on a dry road. As was, the stops from both 20 m.p.h. and 30 m.p.h, wei carried out quite safely, but the distances were aggravate by rear-wheel locking, these wheels being locked for aboi 60 per cent of the total stopping distance from each spee+ Handbrake performance was good for an unassisted brak a meter reading of 27 per cent being achieved• froi 20 m.p.h. A 0-75-mi1e section of Pebble Combe Hill was the scene of the gradient-performance tests, which were carried out in an ambient temperature of 12°C. (53-5°F.). The average gradient of this slope has not, to my knowledge, been clearly defined, but it is obviously in the region of 1 in 10, whilst the steepest section is 1 in 5. The hill was climbed in fine style, spoilt only by slight emission of black smoke: the lowest ratio employed was bottom—engaged for I minute 6 seconds out of a total climbing time of 2 minutes 37 seconds—and the minimum speed at any time was 7 m.p.h., which is equivalent to just below the governed engine speed in bottom gear.
The test vehicle was equipped with the optional radiator blind, the control for which is in the cab, and before making the ascent this was fully lowered. As a result, the recorded temperatures were not high, and the ascent caused the engine-coolant temperature to rise from 54°C. (129°F.) to 79°C. (174°F.). There is obviously ample coolant capacity particularly as the system is pressurized, and if anything I would say the engine is over-cooled, further evidence of this being the fact that the cab heater never really gave out any hot air even when the radiator blind covered half the block.
Brake-fade resistance was checked by coasting the Pax down Pebble Combe in neutral, using the footbrake to hold the speed to just below 20 m.p.h. The brakes were applied for a total time of 1 minute 36 seconds, and at the completion of the descent a full-pressure stop was made. This caused the rear wheels to lock, whilst the Tapley meter reading was 45 per cent. This figure compares quite well with the average meter reading of 63 per cent obtained earlier when the drums were cold, but the required pedal effort had risen appreciably,'even though the pedal travel was not discernibly greater, and later on during the course of the test I noticed this effect again after several miles of fairly high-speed running, which suggests to me that the Pax's brakes are not as fade-free as they might be.
The steepest section of Pebble Combe is at the top of the hill, and the gradient here is reputed to be 1 in 5. Theoretically, this is beyond the gradeability of the particular chassis I was testing, but nevertheless a smooth, full-throttle bottom-gear restart was made on this slope after the handbrake had proved its worth by preventing the vehicle from rolling backwards. Reverse ratio is only marginally higher than bottom gear, so a reverse restart when facing down the hill was carried out successfully
also: on this occasion the handbrake was again powerful enough to hold the vehicle.
Taken generally the Pax V is a pleasant vehicle to drive, and not merely because of the sweetness of the gear change. The clutch is light and smooth also, whilst the ride is above average for a forward-control vehicle of this weight, although allowance must be made for the fact that the test machine was fitted with front dampers, which are optional equipment. The steering is accurate, with a fair degree of castor action, but because of the high frontaxle loading heaviness was apparent at low speeds. Furthermore, some road bumps did cause rather sharp reaction at the steering wheel.
The close-ratio gearbox is very well suited to the power characteristics of the Perkins engine and this resulted in the Pax being lively under all sorts of road conditions, with virtually a ratio for every eventuality. The gear speeds were 8 m.p.h., 17 m.p.h., 26 m.p.h., 38 m.p.h. and 51 m.p.h. It was a pity that the engine's fuel setting was not quite right, because the emission of smoke noted while making the hill climb was sometimes repeated when running along normal roads at normal speeds.
The cab is a mixture of good and bad, though to be fair the good points do outweigh the bad. The interior width averages 6 ft. 6 in., so there is plenty of room for a fairly wide engine cowl without the driver and passenger accommodation being cramped. Nevertheless, I feel that the provision of a two-man passenger seat is a bit optimistic except for occasional use, as the seat cushion actually runs across and rests on the back of the engine cowl, so a passenger sitting on that part of the seat has to position himself at an angle. The general degree of all round vision is good, helped by Tex 5-in. by 7-in, rear-view mirrors, whilst the wiper arrangement is very good also. about 75 per cent. of the screen area being kept clear. The use of a fully adjustable Chapman Leverol seat is a good thing for the driver also, whilst the general positioning of the controls and instruments is entirely favourable.
On the debit side, however, the degree of engine noise that can be heard in the cab tends to be excessive: the engine itself is not an unduly noisy design, and under-cowl insulation would undoubtedly cut out a lot of the unwanted sounds. The door design is such as to give good cab access, but the doors themselves do not contain swivelling quarter lights and the windows are of the balanced-drop type, and so are not as easy to operate as winding assemblies. There are many things about the cab which suggest that it could belong to the " luxury " class, so the omission of such items as sun visors, coat hooks and ash trays is rather strange. Stowage accommodation is limited, consisting essentially of a deep tray on the top of the facia panel. To sum up, the 12-ton-gross Pax V is a good solid design with the minimum of frills which should find its niche in the market for quality vehicles. Taking the long-term view, operation of this model should be economical, mainly because of its sound construction. The list price of the chassis and cab as tested is £2,125 (the cab accounting for £260 of this), and this figure is inclusive of spare wheel, tyre and carrier, flashing indicators, tool kit and front bumper.