AROUND SURREY WITH AN ARROW
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-WHEN the Dennis Arrow chassis was first placed upon the market about a year ago, a great deal of interest in its design was displayed by users and maintenance engineers, principally on account of the novel method of construction of the overhead-valve gear and the quickly detachable mountings for the engine-gearbox unit.
Throughout the chassis great attention has been paid to thevery vital maintenance aspect, and Dennis Brothers, Ltd., of Guildford, has realized that it is often far easier to remove the complete cylinder head, engine-gearbox unit, or
rear axle and replace by another assembly from the stores, than to effect the necessary repairs and adjustments at the time. This practice is, of course, be coming more widespread than ever in the case of large operating concerns. The Arrow is particularly well adapted for such maintenance plans.
When we were afforded facilities for testing the Dennis Arrow the vehicle provided took the form of a complete 32-seater bus carrying a handsome Duple body, which was laden with metal weights to give the required total burden equivalent to 32 passengers. Thanks to the Silence of the body, we were able to pay particular attention to the quietness of the chassis. After five minutes' running the most prominent impression was the extraordinary silence which has been attained in the engine and transmission system. At the end of our trial, when nearly 100 miles had been covered, the impression was still more firmly and pleasantly implanted in our minds.
Silence is, of course, valuable in Itself, but to the engineer it is primarily an indication of sound mechanical construction, and promises long life for the mechanism. That silence was not obtained by the use of very small clearances in moving parts which might adversely affect the running was lonseriginnunnwernonmsnoonn proved by the high
speed and general good performance, throughout the trial. There was no evidence of stiffness in the chassis, which, we understood, had covered about 250 miles prior, to our
taking it over at Guildford.
Before starting the road trial the vehicle was carefully weighed and found to scale 7 tons 14 cwt. 1 qr., including the weight of an official of the Dennis Co. and our representative. A study of the detailed weights in the accompanying panel shows that the ballast, plus the personnel, somewhat exceeded 2 tons, which is the total burden representing 32 passengers at the officially recognized allowance of 30 stone per person. The Arrow bus was, therefore, laden to full capacity.
Our first trial was that of general controllability and took the form of a journey through Guildford on to the Hog's Back. At the time there were many street repairs in progress in Guildford, and at two points it was necessary to make more than usually sharp turns around street islands.
Had the Dennis Arrow had the normal 80-ft. turning circle, it would have been necessary to make each turn a two-stage affair, but owing to the fact that the Arrow's turning circle, at its greatest, is 55i ft., the bus negotiated the turns in one sweep. The ratio of the steering—four turns from full right lock to full left lock—was admirably suitable for the weight of the vehicle.
The long climb up on to the Hog's Back called for second and third gears being used for considerable distances at high engine speeds. The most noticeable point in the ascent was the fact that the noise emanating from the gearbox, when second or third speed was engaged, was extremely subdued and the note hardly varied to any extent whether the engine was pulling at 1,000 r.p.m. or more nearly at its maximum of 2,500 r.p.m. Further, there was very little difference in the note coming from the gearbox whether second or third speeds were in use. First gear, also, when it was employed during the day, was commendably quiet.
Expressing our impression briefly, the second and third gears on the Arrow chassis which we tested were as quiet in operation as so-called " silent " indirect gears on certain private cars.
In order to make certain that our• figures were in no way upset by any possible inaccuracy of the speedometer, a check was made and the error was found to be under 2 per cent., which, for all practical purposes, can be regarded as negligible, as tyre pressures and tyre wear may easily upset the most accurate speedometer by more than this amount.
Flexibility on Top Gear.
On top gear the speed range was 5 m.p.h. to 56 m.p.h. During the maximum-speed trial it was noticeable that the engine silence made speed estimation very deceptive. When sitting in any of the seats at the rear of the bus and guessing the speed, we found that our observations were, on the average, 20 per cent. below the true readings.
Acceleration from rest and from 10 m.p.h. to 40 m.p.h. can be most easily gleaned from the accompanying graphs.
In accordance with our custom, braking tests were taken in both directions on a level stretch of road and the mean figures calculated. These form the basis of a graph reproduced on this page. Both brakes were used in conjunction to obtain the most rapid rate of deceleration. The brakes, which were evenly adjusted, brought the vehicle to rest without harshness and in distances well up to modern standards.
On The Mount, Guildford, which has a gradient of 1 in 6 and a poor surface, stop-and-restart tests were effected. The bus could easily be held on this gradient with the handbrake alone. On the vehicle we tested the teeth of the ratchet were somewhat coarse, but -this point is being amended; the certainty of holding power is assured. Needless to say, the foot-brake with the servo in action proved amply powerful to hold the laden bus, and when the servo was put out of action physical effort sufficed to hold the vehicle.
For the purposes of checking the fuel consumption the Autovac tank was used. It was disconnected from the main fuel supply and carefully drained before each trial run. A gallon of spirit was measured into it for each trip, stamped measures being employed. The carburetter setting employed at the time of the fuel test and during all other trials while the vehicle was in our hands was as follows :—Zenith 42 mm., choke 29, main jet 120 c.c., compensating jet 142 c.c., slow-running jet 75 c.c., capacity tube 7, progression jet 160.
Starting from a milestone on the Guildford-London road and travelling towards the Metropolis, we covered 10.4 miles before the engine petered out for want of fuel. No gradient on this route called for a change from top gear.
The main tank was again put into action and the vehicle driven on to theenext milestone, turned around and the Autovac then drained ready for the return run. The main tank
40 SECS
35 30 25 20 15 10 5 0 was put out of action, a gallon of spirit measured into the Autovac and the second run commenced. The return journey included rather more heavy going than the outward one and the distance recorded was 9.9 miles. On two gallons we covered 20,3 miles, which gives 10.15 m.p.g.
On the vehicle tested the sides and windscreen of the cab were fixed at their bases and the floating joint was below the roof. There was, 'therefore, no easy outlet for the fumes arising from the engine. On the Dennis Arrow the hot air passing over the engine and the fumes, arising from it have been carefully directed away from the driver's cabin with the result that long-distance work can be accomplished without that feeling of fatigue which is unavoidable when engine vapours are encountered.
The controls are disposed conveniently, and a very neat feature is the grouping of the electrical instruments immediately below the steering wheel.
Neat Electrical Layout.
The instrument box carries the speedometer as well as the electrical apparatus, and the usual oil gauge is cut out by the use of a green light which is in operation so long as the oil pump is functioning correctly ; a red light comes into action when the dynamo ceases to charge. All the connections to this instrument box are carried through a flexible tube and the question of mounting or demounting N a matter 5 of moments and does not affect the bodywork. The horn switch is to the right of the instrument box and the dipper control in front of it.
Summing up our im pressions of the Arrow, we failed to find any fault with it, with exception of two minor points which relate only to the particular example we tested. One was the fact that the pedal slots were too short—a matter remedied in a few moments—and the other was the hand-brake ratchet ; we have already indicated that this is being modified on later chassis.
When inspecting the chassis from the point of view of one charged with the task of lubricating it, we found that the use of Silentbloc shackles had cut out a large number of items normally needing attention; greasegun points are grouped to facilitate access.
The leading dimensions of the chassis are :—Wheelbase, 16 ft. 6 ins.; overall length, 25 ft. 4 ins.; overall width, 7 ft. 5 ins.; front track, 6 ft. 31 ins.; rear track, 5 ft. 10+ ins. • dash to end of frame, 20 ft.
5 ins. ; ins.; width, 4 ft. 1 ; frame height, 2 ft.; minimum ground clearance, 7 ins.