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29th November 1957
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Page 46, 29th November 1957 — BRITAIN'S: MOST POWE
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yal Scot Six-wheeled silt for South African 200-b.h.p. Horizontal Top Speed is 67,rn.p.h., wed Economy is Good

T. . HE most powerful passenger chassis to have been built in Great Britain,. the Albion Royal Scot oil-engined six-wheeler, has been evolved .primarily to meet the specific requirements of the South African Railways, and 22_ have been: ordered, Powered by a 200-b.h.p. engine, it has a Maximum speed of 67 m.p.h. and its acceleration rate up to 40 m.p.h. is outstanding for a vehicle running at almost 17 tons gross weight

Although this type of chassis cannot at present operate in Great I3ritain, it may become practicable when motorWays are built. A fully equipped luxury body for 50 or so passengers could be provided and, even assuming an average speed as low as 50 mph,, a journey from London to Glasgow would take only eight hours and be cheaper than either rail or air services. , The Royal Scot is a six-wheeler. with single tyres all round, because of the limitations on wheel loadings which exist in many parts of South Africa and Rhodesia. Similarly, there is no third differential on the driving bogie, because the buses ate likely'to be operated over unmetalled. muddy roads where traction would not be possible if a third differential were fitted.

Adequate Axle Articulation

Four-spring, balance-beam rear suspension is employed, rather than the heavier and more expensive type of twin inverted semi-elliptic spring layout. Nevertheless, the suspension gives adequate articulation for normal rough going. with a diagonal articulation figure of plus or minus 8 in. The overhead-worm-drive rear axles have 8-in, worm centres, and because of the large tyres the minimum ground clearance, which is under the sump, is 13 in.

The power unit is the Albion' EN.90I horcizontal unit. Its normal rating is 230 b.h.p. at 1.900 r.p.m., with a torque output of 700 lb.-ft. at 1,200 r.p.m. For use with the South African vehicle, however, the unit has been down-rated to

al2 develop 200 b.h.p. at 1,800 r.p.m., and the different injectionpump setting has reduced the torque output to 630 lb.-ft. An advantage of this de-tuning is that no further downrating becomes necessary if the chassis are operated in mountainous areas, such as Johannesburg which lies at 6,000 ft. above sea level.

Unlike most British underfloor-engined designs, the radiator is mounted vertically at the front of the chassis, the fan being driven through a two-piece jack-shaft and twin V-belt. This radiator position lIas been adopted to improve the ground clearance, give a better flow of cooling air through the block and Minimize the chance of the block becoming fouled or* damaged by dust, stones, or water splash::;.

Burgess Air Cleaner

The engine, which is supplied with air through a large Burgess oil-washed air cleaner with centrifugal oil-bath precleaner, is mounted as a unit with a Z.F. six-speed constantmesh gearbox. This German box is used in the absence of a suitable British assembly which would cope with over 600-lb.-ft. torque. The complete engine-gearbox assembly is Mounted on rubber tt three points, and twin starter motors are fitted.

A dual-circuit layout is employed for the air-pressure brakes. The pedal actuates a double air valve, one section of which supplies air to the frontand rear-axle brakes. whilst the other section actuates the centre-axle brakes and the trailer brakes. Additionally, a hand-reaction valve, mounted on the steering column, is provided for indepen-i dent operation of the trailer brakes.

A Z.F. exhaust brake is fitted to the engine. This is manually actuated, but has been interlinked with the hydraulic clutch-actuating Mechanism in such a way that if the clutch pedal is depressed when the exhaust brake is applied the exhaust brake is released automatically. Thus there is no fear of the driver "losing his engine " through trying to change gear with the exhaust brake in action, This brake gives additional fade-free retardation, but it is something of a lu-xury, because the rear brake drums are fully exposed to the air stream and, as tests proved, brake fade is unlikely to occur on the longest gradients. Nevertheless, the presence of an exhaust brake does give additional safety on a high-speed vehicle such as this.

The Manes cam-and-double-roller steering gear incorporates integral valves for the steering servo, this equipment also being of Mantes design, manufactured by the Adamant Engineering Co., Ltd., Luton. Power for the servo is provided by a Plessey pump, which shares the belt drive of the cooling fan. The hydraulic system, which is of the constantflow type, incorporates a relief valve to limit the degree of assistance provided when the vehicle is stationary.

The chassis has an overall length of 36 ft. The bodywork is to be provided by Bus Bodies, Ltd., Port Elizabeth, South Africa, and will be 8 ft. 21 in. wide. These " country-bus " 1313 bodies seat 61 passengers; eight in the first-class saloon at the front and 53 on the five-abreast seats in the main compartment.

There are separate entrances for the two sections. Little luggage accommodation is provided in the bus, the bulk of it being carried in a fourwheeled trailer designed to run at a gross weight of 51 tons. Thus the bus, which has an official gross weight rating of 15+ tons solo, will, with trailer, operate at 21+ tons gross train weight.

As supplied for test the chassis was loaded to a gross weight, including driver and observer, of 16 tons 19+ cwt. This is over a ton more than the maximuni recommended solo weight and is much more than the complete vehicle could gross in service.

The tests were conducted north of Glasgow and on the way out to the test area the chassis was taken to Cleveden Road, Kelvindale, where it was stQpped and restarted on the 1 in 61 slope. When the chassis was stopped the multipull hand brake was only just capable of holding it, which suggests that greater leverage would be an advantage.

A re-start in second gear was attempted. On the first three occasions this produced only excessive clutch slip without any forward movement, but a fourth effort was successful and the chassis slowly pulled away. This performance was repeated several times, but it seems that the clutch is only just capable of transmitting full engine torque. I understand, however, that stronger springs are to be fitted as soon as they become available. A smooth bottom-gear re-start using only quarter throttle was also made.

A long level stretch of road was used for the braking, acceleration and fuel-consumption tests. The braking figures obtained were somewhat disappointing and indicated a lag in the system which accounted for a difference of 15-20 per cent. between maximum deceleration recorded on the Tapley meter and the average deceleration 'shown by the stopping distances. Because of the length of the air lines to the rear brakes, however, to reduce this lag appreciably would not be easy.

No wheel locking occurred during these tests, but the geometry of the bogie suspension is such that the centre axle tends to lift when braking violently. This is shown by the increased wear on the rear tyres, although some of this would be caused by the absence of a third differential and normal scrub.

Standing-start acceleration figures were taken up to 50 m.p.h., there being an insufficiently long stretch of completely level road to enable accurate figures to be taken up lo 60 m.p.h. Between 0 and 40 m.p.h. excellent figures were recorded. Even above this speed the acceleration rate is completely adequate for a vehicle of this type and will allow it to keep up comfortably with the normal traffic stream on South African roads.

As might be expected with the high torque output of the engine, the direct-drive acceleration between 10 m.p.h. and 40 M.p.h. was also good, and the smoothness of the engine and transmission in this gear at 9 m.p.h. was noteworthy. Indeed, the engine suspension is extremely good, and only when overrunning the governor is any vibration transferred to the chassis frame.

Four fuel-consumption tests were conducted. During the first the chassis was kept down to 30 m.p.h., and during the two successive tests the maximum speed was limited to 40 m.p.h. and 50 m.p.h. respectively. For the final test full throttle was used whenever the opportunity presented itself. Frequently the top speed of 67 m.p.h. was reached; this occurred at an engine overrun speed of 2,000 r.p.m.

All the figures show remarkable economy when compared with the average fuel consumption rates returned by passenger vehicles of this size manufactured on the Continent and in the U.S.A. Reference to the time-load-mileage factors given in the data panel show that the Royal Scot is most economical at between 40 m.p.h. and 50 m.p.h., but even under continuous full-throttle conditions excellent fuel economy is apparent.

Only at 30 m.p.h. does the engine cease to operate economically, but this is of little importance, as the vehicle will be driven at about twice that speed in service.

Following the high-speed fuel-consumption' run, during which it became necessary on several Occasions to brake sharply because of traffic conditions, I noticed that the rear brake drums were stilt cool enough to touch comfortably and even the front drums were not excessively :hot. This is because the 24-in.-diameter wheels allow plenty of air space around the 16f-in.-diameter drums. "

A controlled brake-fade test was conduetcd' on ,the 1:2mile gradient of Gleniffer Brae, Paisley: This hill ha's an average gradient of I in 13, with d maximum slope of 1 in 8. After a four-minute descent, which was made in. neutral with the foot brake applied to restrict the maximum: speed to 20 m.p.h., a " crash " stop from 20 ni.p.h..shoOred that the braking efficiency had been reduced by a -mere:5 per• cent.

The ascent of Gleniffer Braes was conducted in an ambient temperature of 44° F. At the bottom of the hill the temperature of the water in the radiator header tank was 149° F. and of that in the engine wafer jacket 150°F. The climb took 4 min. 50 sec., equivalent to a.speed of 15 m.p.h. The lowest gear employed was third, and it was possible to change up into fourth on several occasions.

At the top of the hill the thermometers showed. that the header-tank water temperature had risen by 16° F. and the .water-jacket.temperature by 21°F. The cooling system was 'thus proved to be 'adequate, 'making allowance for the fact that the thermostat would not be fttllY open. The water pump has a cieliVery rate of about 150 g.p.M. at maximum engine speed. Oil cooling is the .use of a heat exchanger.

A second ascent of the hill occupied a, slightly shorter time. following which a descent Was., made to test the efficieney of.,the exhaust brake. Third eear wasP engaged and at no time -did it become necessary to touch the foot brake, the highest speed observed being 15 m.p.h.,-eqhiwa. lent to 1.300 engine r.p.m. Indeed,:it became necessary. to ' release the brake addto " use the 7acceleralor tp, maintain al5

motion on the less steep sections. The difference in ratio between third and fourth gears is great enough, however, to have required occasional use of the foot brake if the descent had been made in fourth gear.

The Royal Scot is an exhilarating and easy vehicle to drive. The steering servo takes all the hard work out of handling a vehicle of this size and weight, and even at speeds above 60 m.p.h. there was no trace of wander or instability. Gear changing causes no hardship, although there is some play in the linkage, and fast changes can be made up or down without any difficulty. The clutch action is light.

It is difficult when testing an open chassis to assess the suspension characteristics. The Royal Scot was driven over various rough-surfaced and pot-holed roads and it seemed to ride satisfactorily, although the driver, being 'ahead of the front axle, tended to get shaken about more than the observer, who was sitting to the rear of the front wheels.

Similarly, it is impossible to judge how much engine noise will be heard in the body—this is a matter for the bodybuilder and his method of insulation—but the exhaust note is loud by British standards. I understand, however, that this characteristic is not likely to be unpopular in South Africa.

The Royal Scot is a completely new venture in the British commercial-vehicle export market. Albion sixwheeled chassis have been supplied to the South African Railways before and since the war, but these had conventional-mounted petrol engines and nothing like the speed and acceleration performance of this new chassis.

It is a fine example of British engineering designed to meet requirements of a rigid specification and there is no reason why similar chassis could not be supplied to other countries. It shows that, although we have no use for a vehicle of this size in Great Britain, it can be built in a way that makes it competitive with similar vehicles produced overseas.

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