CAPE TOWN BUYS BRI 1 H SIX-WHEELED BUSES. T HE important and
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rapid developments which are occurring in the progress of British rigid-frame sixwheelers for passenger work are evidenced by the fact that the Cape Electric Tramways, Ltd., are having built for them by Karrier Motors, Ltd., a number of W.L.6-type chassis suitable for 40-seater single-deck bus bodies. These chassis closely resemble the type which will be employed by Birmingham for their double-deck six-wheeled Karrier buses, but they will, of course, be strengthened in certain details to cope with the extra load, and they are the same in their main characteristics as those on order for Salford.
The actual chassis for Cape Town, which we examined last week, will suffice to cover the other models. It is of particular interest, as it represents the vary latest in Karrier six-wheeler design, and it replaces the offset-drive model which we described in detail in our issue dated October 20th, 1925, and which was found to possess certain disadvantages. These have been eliminated without sacrificing the low-Yrame level, which is actually 2 ft. I in.
Preliminary details of the W.L.6 chassis were included in our issue dated August 10th, but it is ex • tremely difficult to obtain an adequate conception of what a chassis will be like merely from a preliminary, Specification. Actually, it is an exceedingly fine-looking and sturdy job, cleanly designed from front to rear and
• embodying many excellent points which make it 'particularly suitable for its allotted tasks. Its weight, for a vehicle of this class, is light, being only 31, tons, whilst the ground clearance for 14 ft. 6 ins, from the dumbirons is 101 ins, and under the rear axles 6 ins.
The frame is built up of side members, 10 ins, deep at their maximum, with a long insweep to the dash. Light pressed-and-welded steel brackets are bolted to these side members to carry the body.
The chassis is of the forward-drive type, the driver being accommodated at the side of the engine, where he is protected by a neat scuttle dash in which is mounted an instrument board carrying a speedometer, oil-pressure indicator, brake-pressure indicator and other fittings. The change-speed lever is at his right and the hand brake lever at his left, whilst ignition and throttle-control levers are mounted on the steering column.
The same six-cylindered power unit is employed on 'this chassis as on the previous model. It is a monobloc with a monobloc detachable head, the bore and stroke being 100 mm. and 140 mm. respectively. The R.A.C. rating is 37.2, but the unit is capable of developing a maximum b.h.p. of 70 and 50 b.h.p. at the normal speed of 1,250 r.p.m.
A difficulty is sometimes experienced with six-cylindered engines in obtaining satisfactory carburation, and in this case two Zenith instruments are provided, both being at the near side, where are all the valves, which have adjustable tappets enclosed by quickly detachable covers. There is an oil filler on the timing case and an accessible oil filter, whilst hot-spots are provided by webs on the inlet manifolds which mesh with others on the exhaust manifold.
A centrifugal water pump is driven from the timing case, in conjunction with a dynamo, by a totally enclosed chain, and cooling is assisted by a four-bladed castaluminium fan on an eccentric spindle mounting, which 032 permits adjustment of the link belt. A starter ring is bolted to the flywheel, although in the case of the vehicles for Cape Town starters are not actually fitted.
A large oil pump, located in the bottom of the swap and driven by spiral gearing from the camshaft, forces the lubricant to the large main and big-end bearings.
On the suction side-of the pump is a strainer of large area, which is additional to the filter to which we hav& already referred. Oil from an adjustable relief valve lubricates the timing wheels and the chain driving the magneto.
The whole power unit is carried in a sub-frame three point suspended in the chassis and free from torsion. The flywheel is open, but is protected at its upper portion by a neat pressed-steel shield which is bolted to the steel dash.
The external cone clutch is faced with friction fabric, and is also provided with a fabric-faced clutch brake mounted across the sub-frame, the carrier being slotted at its bolt holes for adjustment purposes. A direct clutch-withdrawal gear is employed, the ball beariag for this being a deep-grooved radial type, which is free from rattle. Immediately behind this bearing is a flexible-disc joint, the rear spider of which is splined on to a short shaft supported by a Skefko bearing in a cross-member. There is a second joint of the same type immediately in front of the gearbox.
Exceptional care has been taken to render the gearbox silent while i& operation, and although its shafts cannot be said to be long, yet each is provided with a centre bearing which prevents any risk of whip. It provides four speeds forward and embodies the feature of a reverse gear which is completely idle when not in use, thus assisting in the prevention of noise. This feature is obtained by using a double pinion for the reverse, and this slides into mesh. with the pinions on both the primary shaft and the layshaft.
The gearbox itself is a single, practically oval, casting in iron, which obviates drumming. The main cover is at the front and carries the forward bearings, whilst the bearings at the rear are in separate cap housings. The box is supported above a cross-tube at the front end and hung from a second cross-tube at the rear.
To prevent throwing unnecessary stresses upon the gearbox, the transmission brake drum is mounted separately. The short shaft carrying this drum is also carried on a tubular cross-member and is connected to the gearbox through the medium of two fabric flexible joints and to the forward axle of the semi-bogie by a tubular propeller shaft haying two cross-type universal joints in which Timken taper roller bearings are employed. The peculiar advantage obtained by the use of these bearings is that the whole shaft can be adjusted until it is perfectly true, a feature which is extremely difficult to obtain in any other form of joint. A plunger joint is, of course, incorporated.
Reverting to the gearbox, the striking mechanism for this has three spring plungers mounted at 120 degrees. A single ball in the box itself is also used to lock the three rods, so that in the event of anything becomigg damaged the simultaneous engagement of two gears is prevented. There is also a calliper locking device at the front control, whilst the change-speed lever itself is of a special type which enables easy operation of the mechanism. In this arrangement a lever on the sliding shaft is hinged to the hand-lever proper, which can float in the gate, but, at its lower end, has a ban sliding between two plates which do not restrict the forward and backward movements, but act as a fulcrum for the ball when the lever is moved sideways.
Another feature of the gearbox is that it can be detached merely by disconnecting the joint spiders, c33
undoing two two straps and sliding back the whole box out of the front support.
A speedometer drive is provided at the end of the gearbox.
Internal-expanding shoes are emplOyed in the transmission brake, as these reduce the overall outside diameter, but the drum can be removed for refacing the shoes without disturbing the universal joints. This brake is hand-operated.
Several alterations have been made in the arrangement of the semi-bogie. The " spectacles" frame has been retained, as this is not only a neat way of carrying the axles, but affords enormous strength, and to avoid tipping of the bogie under heavy driving stresses tHe fukrum of the two semi-elliptic springs has been dropped considerably ; in addition, these springs, although of the inverted type, are underslung, being attached direct to the rear-spring brackets and by links to the forward brackets. This permits springs of extra length to bie employed.
The rear end of the forward worm shaft is spline, as is the forward end of the rear worm shaft, and on • these splines are mounted the sleeve halves of special universal joints, which take the }jail ends of a coupling shaft, the actual drive between sleeves and shaft being through blocks.
Covering these joints and the shaft is a special torque member consisting of one sleeve sliding over the other, the arrangement being similar to that employed on the previous model, as this has proved extremely satisfactory. It has a gland at the end of the outer sleeve, which prevents loss of lubricant. This driving and braking-torque device prevents rocking of the axles, but permits them to move out of parallel with each other ; the distance between the axles is controlled by two neat ball-ended radius rods, which are set close above the rear springs and connect the spring brackets as between axle and axle.
Each axle of the bogie has a horizontal banjo made of pressed-steel halves welded together. The combined brake-shoe carriers and spring brackets are also welded to the axle easing.
Exceptional attention has been paid to the matter of effective braking. The hogie-axle brake drums in the leV.L.6 model have been increased to a diameter of 18 ins., as compared with the 15 ins, of its prototype. The shoes in these drums are faced with friction fabric, which is secured in the well-known Karrier way by T-headed bolts, which force a portion of the material into grooves in the shoes, thus permitting easy replace-. tnent, as no riveting is required.
Enormous braking power is provided by using the Westinghouse pressure system. The compressed air for operating the brake-actuating diaphragms is contained in a reservoir mounted just inside the off-side frame member. The supply of air is obtained from a Westinghouse compressor driven from the front end of the gearbox layshaft through the medium of an internal-cone clutch. This clutch is arranged to operate automatically. It is thrown out of action at just under 100 lb. pressure, and again connects the compressor to the drive when the pressure falls to 75 lb. A safety valve close to the reservoir is set to blow off at 100 lb., and there is a valve chose to the same point from which a connection can be made to the tyres for filling these.
An important modification has been made in the construction of the compressor. It was found that the big-end bearings on these were apt to wear out too rapidly. Therefore, a tank has been provided under the compressor and equipped with a filler and dip stick. A small pump at the front end of the compressor shaft draws oil from this tank and forces it into the crankcase of the compressor, and it returns to the tank through a large overflow pipe; thus a constant oil level is maintained.
Accessibility is a matter which has received due consideration. For instance, in the gearbox an the pinions are renewable individually, and by removing the front cover the whole layshaft, complete with its pinions and centre bearing, can be withdrawn. The axle gearing is also readily reached.
The vehicle having forward drive, the worm and sector steering gear is mounted above the off-side dumbiron and steering is facilitated by the provision of an extra-large wheel.
The tyre equipment is 36-in. by 8-in. Dunlop pneumatics and the wheels are detachable steel discs mounted on hubs running on taper roller bearings.
In this chassis no leather gland washers are employed ; even the sliding joint at the front end of the propeller shaft has a simple cast cover, in which grooves are cut to form oil seals.
A grease-gun lubricating system is employed throughout.
Autovac feed is utilized for the two carburetters, the fuel being obtained from a petrol tank with a capacity of 25 gallons, fitted into the chassis on the near side.
The equipment includes a 12-volt lighting set as standard, and a bracket to hold a spare wheel is provided at the end of the frame.
The gear ratios are as follow :—Top, 7.25 to 1; third, 12.02 to 1; second, 20.95 to 1; first, 33.4 to 1; reverse, 45.4 to 1; and the corresponding speeds at 1,250 r.p.m. are :-18.48 m.p.h., 11.12 m.p.h., 6.38 m.p.h., 4.02 m.p.h. and 2.94 m.p.h.
The leading dimensions of the chassis are :—Wheelbase, 17 ft. 6 ins.; track, 6 ft. 3 ins.; turning circle, 60 ft.; overall length, 27 ft. 9i ins. ; overall width, 7 ft. 3 ins.; dash to end of frame, 22 ft. 8 ins.