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AFTER conducting many interesting road tests of petrol and oil-engined vehicles, not forgetting several battery-electric vans, I recently realized one of my greatest ambitions--to drive a trolleybus, and to subject it to one of my standard tests, sofar as it could be applied to a vehicle confined to a limited route in the Metropolis.
So unlike is a trolleybus to an internal-combustionengined vehicle in the rate and manner of its acceleration, braking and in its general smoothness of control that I can describe my experience only as a thrill. Accompanied by Mr. C. C. Oakham, the traction engineer of British United Traction, Ltd., Mr. W. G. Hobson, technical officer of the English Electric Co., Ltd., and other senior officers representing the manufacturers and operator, I conducted the test on one of the B.0 T. three-axled 8-ft.-wide models supplied to the London Transport Executive early last year.
The test vehicle was powered by a MetropolitanVickers 115 h.p. motor, driving through an open propeller shaft to the rear bogie. Both the centre and rear axles are equipped with offset underslung worm
drive, with a differential between the two axles. Of special interest in the bogie are the torque-reaction members, which provide equal loading to both axle,, under all conditions of surface or traction. The efficiency of this arrangement was thoroughly tested.
Non-regenerative Control An important feature, so far as comfortable travel and longevity of the chassis and body are concerned, is the method of control, which in this particular model was by the English Electric S.D. system. This nonregenerative control system for the single motor combines the advantages of a predominantly seriesmotor characteristic for acceleration with the features of a shunt generator for rheostatic braking To obtain these conditions, the motor has a comparatively strong series field and a weak shunt field. When motoring, the series field is used in conjunction with a weak value of shunt field. After cutting out the series-starting resistances, the shunt-field circuit is opened, and still further acceleration is obtained by series-field diversion. There are 11 starting notches.
During rheostatic braking the generated energy is absorbed in the starting resistances on the vehicle, only the motor shunt field being used. This ensures a high degree of stability c8
A special feature of the rheostatic brake is the feeding of the shunt field through a portion of the braking resistance. By this means the excitation of the shunt field is automatically decreased when higher valuesof current are flowing in the main braking
circuit, and vice versa. • The operation of the brake pedal first open-circuits the control to the line and resistance contactors, so that in no circumstances can current be fed to the motor when braking is required. The two steps of rheostatic braking are followed progressively by the application of the Westinghouse air-pressure braking system, with an intensity proportional to the degree of depression. Rheostatic braking is effective down to a speed of about 4 m.p.h.
The tact that rheostatic*braking is always obtained before the friction brakes take effect, and is retained when the friction brakes are applied, results in prolonged life for the brake facings and drums, besides minimizing the frequency of brake adjustment and maintenance.
Adjustment for wear on the BUT, chassis should be spaced at long intervals, because the total effective facing area of the foot-operated brakes is 810 sq. ins., and with the LLin.-thick material renewal should be required at intervals of not less than 18 months. A further measure to increase the period between docking is the Tecalernit Syndromic lubrication system which, with a 40-point feed, provides lubrication to all the wearing parts.
Through the courtesy and team work of the officials of the B.U.T. and London Transport Executive, I was able to board the vehicle at the Holloway depot, knowing that a considerable number of man-hours had been expended in arranging a ballast load.
Normally, the B.U.T.standard chassis weighs 5 tons, but with special features, such as Nife batteries, motor generator set, brackets and other details peculiar to the model supplied to the L.T.E., the chassis weight of the test machine was 5 tons 14 cwt. The overall length of 30 ft. affords sufficient space for a 70-seater body. Accordingly, the test chassis, equipped with a Metropolitan-Cammell-Weymann body, carried a ballast load of 4 tons 1 cwt., which, together with the observers, was representative of full loading.
One of the many advantages of the trolleybus is that there is no warming-up period, and within a few minutes of leaving the depot we were ready for a first trial at Despard Road, near the foot of Highgate Hill. A brief stop was made to permit a service vehicle to get well
ahead and ensure that we had a clear 'run up the incline. Acceleration from rest was smooth and rapid, • and within 15 secs.. from starting we .had reached -a balancing speed of 20 m.p.h. This is a line achieverrient, Considering that the average gradient is in the region oil in 12i. Nearing the top of the hill, where the gradient is Tin 15, the speed rose to 23 m.p.h.
A test of the rheostatic braking system was Made on the descent of Highgate Hill, and with the first notch engaged the speed was brought down from 30 m.p.h. to 22 m.p.h. The second notch of the rheostatic control reduced the speed to a steady 20 rn.p.h:
Initial acceleration trials, made in the outer regions near Barnet, produced a result of .30 m.p.h. in 2L8 secs. It is probable that the voltage was low at this point, and subsequent readings, taken nearer Central London; showed a brisker performance. Up to this point the trials had, by judicious pauses, been conducted without hindrance to service vehicles, but the braking tests, with comparatively long halts to measure stopping distances, required that the trolley poles should be hauled down at each halt.
Checking Bogie Movement
With emergency applications of the brakes, the vehicle was brought to rest smoothly and without time delay in the system or any suspicion of suspension bounce when• coming to rest. Although --sharp deceleration was attained, sonic measure of its smoothness is afforded by the fact that the ballast load remained unmoved during the trials. Applied on a smooth and damp macadam surface, the trolleybus came to rest in 36 ft. front
• 30 m.p.h. without the slightest trace of wheel lock—an excellent test for the B.U.T. bogie torque-reaction system. I could hardly credit the readings obtained during initial brake tests from 20 m.p.h., but a series of trials from this speed failed to disprove a retardation equivalent to 31.9 ft. per sec. per sec.
A check on acceleration showed an improvement over the earlier trials, so the tests were resumed. To recapitulate, the test machine was fully loaded, the gross running weight being 14i tons, and it was a standard vehicle withdrawn from service. Bearing these facts in
e I 0 mind,-the,faultlessly smooth acceleration from rest io 20 m.p.h. in 6 secs. may be regarded as an achievement reflecting credit on the design and maintenance. From 25 m.p.h. the acceleration curve flattened and 30 m.p.h. was reached in 20 secs. Even so, this constitutes a record for any passenger vehicle tested by any representative of "The Commercial Motor."
The results of the acceleration trials, shown plotted against those of high-powered, compression-ignitionengined chassis, emphasize that the trolleybus forms no hindrance to the general flow of traffic on the road; indeed, its performance is invariably regarded with envy by operators of internal-combustion-engined chassis.
Many onlookers in Finchley must have been surprised to see a trolleybus climbing a gradient with both the overhead trolley poles pulled down. Such was the case,
few minutesafter the end of the acceleration tests, „ ,
vhcn I decided to try the manteuVrabilitY.Of the trolleyius, relying:SOlely On . the: batteries for tractiOn. gormally the batteries are ,Provided . for lighting, nanceuyring in the depot when empty or pulling into the ide of the road should there be an overhead power 'allure.
My test was made with a load equal to a full comple
nent of passengers, and with part-charged cells. It vould have been difficult to reproduce a worse eonlition, but for all this, the B.U.T. rolled .tip the incline it a steady 2 m.p.h. Adistance of 650 yd. was covered )efore the trolleybus came Enally to rest.
Technique in Driving After the overhead system had been reconnected,! took wer the controls under. the :expert guidance of the chief 'riving instructor of the L.T.E. I found that the controls vere planned for. simplicity, in driving. 'At the same ime, the driver, has to observe the post markings tenoting overhead "breakers," the drill being to release he contactor control pedal while passing across the gap. [his prevents arcing, or burning, of the overhead wires.
Off-shoots and junctions in the overhead system lemand care, but, taken as a whole, it is easier to master he controls and observe the rules of trolleybus driving echnique than to learn to drive the conventional nternal-combustion-engined vehicle. The trolleybus 'river has a spacious, comfortable and, above all, quiet :ab.
The light action of the controller pedal, steering and 'rakes convinced me that trolleybus driving was an mperience not to be missed. After driving from liolders Green to Finchley, I was permitted to filter hrough the turnabout at Tally Ho! Corner and through ill the accompanying traffic to Holloway. I accelerated apidly from rest, but gross abuse was required before he overload tripped. The noise that accompanied this !xperiment suggested that drivers will not be tempted O misuse the controls too frequently.
Next, I depressed both controller and brake pedals
simultaneously, but because the initial movement of the brake pedal isolates the overhead system, the brakes came into action to bring the vehicle to rest. I then applied the brake pedal more than 20 times, lowering the air pressure in the reservoir. It was not possible to exhaust the system, because the compressor replaced the air faster than it could be used.
Air Braking System
The compressor motor was then switched off, so that could observe the action of the low-pressure warning arrangements. A large number of brake applications— so many that lost count of them—reduced the pressure to 46 lb. per sq. in., at which point the visual warning indicator rose to its stop position, and the warning bell buzzed loudly behind my ear. It took 21 minutes to raise the air pressure in the reservoir from zero to the maximum working pressure of 85 lb. per sq. in.
Before returning to the depot, I found that the balancing speed on level ground had been omitted from the tests, and. I had no reluctance in continuing to drive to a suitable stretch to complete the trials. Observing the recognized stopping points during this trial, I found that a speed of 35 m.p.h. could be reached under favourable conditions. This also appeared to be the balancing speed on level ground.
In trolleybus operation, the rise of temperature in the rear axles can be plotted against the distance travelled. Because of the `relatively short distance of my test, the axle temperatures were still rising when I returned to the depot. Maximum temperatures recorded in London service have been 185 degrees F.
After the trolleybus trials, I took control of a tram for a short distance to compare the performance, driving comfort and technique. The high unladen weight of the tram afforded an inversely poor acceleration and the braking effort left much to be desired. Judged by the draught-free cab of the trolleybus and the associated comforts of an adjustable driving seat and light, wellpositioned controls, the tram lacks many amenities to reduce driving fatigue.