LOW-BRIDGE BU DRAWBACKS 11 LAS ITS AFETY
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Disadvantages May Outweigh the Good Points When Consideration is Given to Operating These Vehicles in Urban Areas, Says Municipal Manager
By P. A. C. Brockington, A.M.I.Mech.E. ALTHOUGH the, high capacity and stability of the low-bridge double-decker are regarded as good features by Mr. A. Witcomb Smith, general manager and engineer of West Bromwich Transport Department, to employ such a vehicle in urban areas would, he considers, have a number of disadvantages.
Accidents to pedestrians, in which the person is thrown under the bus, have been few, but the danger of such accidents Warrants careful consideration of the value of a comparatively high ground clearance.
The reduced clearance of the lowbridge bus could result in more serious injuries to the pedestrian involved. The introduction of a lowbridge type with a front entrance would necessitate alterations to the existing shelters in the town and Mr. Witcomb Smith believes that a front entrance tends to increase the accident rate.
One Safeguard
He agrees, however, that conductor-operated doors offer a safeguard to passengers so long as they are always closed at the appropriate time. But this is far from being a certainty, and he doubts whether in practice they add appreciably to the safety of passengers.
Typical bodywork features of lowbridge buses include longitudinal seats over the front wheel-arches. These are normally combined with a raised platform, and could represent an additional travelling hazard to some passengers, particularly to the aged.
Mr. Witcomb Smith also points out that the space occupied by a front entrance and staircase is the "best riding arei " in the vehicle, and therefore he prefers a rear entrance and staircase, apart from considerations of safety_ Fuel Saving A number of the West Bromwich buses is equipped with M.C.W. and Willowbrook lightweight 60-seal bodies, which give an unladen weight of between 7 tons 3 cwt. and 7 tons 4 cwt. This compares with the unladen weight of 8 tons 3 cwt. of buses with the more orthodox bodies.
The saving in fuel consumption is greater than 1 m.p.g. Records taken over an extended period show that the consumption rate of the lightweight buses is 9.8 m.p.g., and that the heavier vehicles on similar services average 8.73 m.p.g.
West Bromwich technicians have no objection to a rear engine and agree that double-reduction drive has n20 the advantage that it operates with reduced mechanical stresses. This, however, is a complication which might increase maintenance costs, they believe.
The relatively short life of backaxle worm drives compared with prewar designs, is another factor which has raised maintenance costs. Formerly, a bus averaged about 200,000 miles before an overhaul of the rear-axle assembly was necessary, whereas since the war the mileage has been reduced to about 80,000 to 100,000.
General use of progressive springs is regarded as a long-overdue development, and a system that -offered a reduction in spring main
tenance costs would also be welcomed. Enhanced passenger comfort with continual variations in loading and reduced body maintenance are the envisaged advantages of progressive springing, which could, it is assumed, be of the lightweight leaf type, combined with chromiumplated shackle pins.
The Gregoire system, with an .auxiliary coil spring, might, it is suggested, be a preferable alternative if first cost and weight were acceptable. The characteristics of air-sack springing have been studied, but apprehension is felt regarding its suitability for double-deckers.
Doubt on Springs If'• employed on such vehicles it is feared that failure of the pneumatic -control would seriously impair stability. Doubts are also expressed by Mr. Witcomb Smith regarding the durability of rubber springs.
Supplement 1 detergent lubricants have been employed for about two years, and have obviously improved the life of new and overhauled engines, but experience is too limited for exact comparisons to be made between similar engines operating on detergent and straight mineral oils.
Buses normally run up to 200,000 miles before a complete engine over
haul is required. Cylinder heads are removed after 75,000 miles, and formerly it was customary to replace worn pistons and rings at the same time, but it is now hoped that the use of detergent oil will obviate this necessity.
Most of the buses are equipped with Gardner engines, and the maker's recommendation regarding lubricating-oil grades is strictly observed, an S.A.E.20 lubricant being employed throughout the year. Mr. Witcomb Smith has reviewed the results of other operators who have employed lubricants of lower viscosity and has carefully considered adoption of the same policy.
This has been rejected, however.
because of the maker's warning that an oil of lower viscosity would not be suitable. Rates of cylinder-bore wear have increased since the war. and this is attributed in part to siliceous dust in the atmosphere and acidic elements produced by factories in the area.
Samples of abrasive -dust taken from inlet manifolds indicate the need for thorough air filtration, which is not provided by most conventional types of filter. Oil-bath filters are fitted to some of the buses, and it is conceded that they may eliminate most of the air contaminants. However, such filters increase the amount of maintenance required, partly because engine vibration frequently causes mounting brackets to fracture —the result of their weight and shape. An improved lightweight filter which could be easily serviced would offer obvious advantages.
All the engines are equipped with thermostats, the use of which is believed to enhance their life. Radiator blinds under the control of the driver might, it is considered, give improved results if the driver continually made adjustments in accordance with temperature changes, but this would involve too many practical difficulties.
Thermostats give a certain amount of trouble, possibly because of the hardness of the mains water. In Mr. Witcomb Smith's opinion they should be of more robust construction.
Lead-acid batteries have, on average, a useful life of between four and five years, and replacement costs are heavy. Nickel-iron batteries have been in service for six years, but the economy of using this type is still in doubt on account of high initial cost.
Dual-valve air brakes will be increasingly employed, as it is considered that they eliminate the risk of complete failure of the braking system. Woven brake facings are used almost exclusively because of the greater efficiency they provide. Bonded facings increase the mileage
interval between replacements from 12,000 to nearly double this figure, and would, therefore, be used if the efficiency obtainable were adequate. .
Automatic adjusters have proved satisfactory, but it has been shown that the close clearance which is maintained between the facings and the drums tends to reduce heat dissipation.
Cutting Costs Experiments with the single-coat painting process give hope that it will enable overall painting costs to be reduced. The process offers an immediate saving of 125 a vehicle, and the finish of buses which were painted by this process three years ago is comparable with that of othei vehicles for which conventional painting methods were employed at the same tittle. Buses are normally repainted every five years.
The interchangeability of tyres between front and rear wheels would be of great advantage, enabling covers regularly to be changed round and so improve tread life.. Tyre failures are a small proportion of the total causes of breakdowns, but it is considered that tubeless tyres would be an .asset, particularly if the gain derived from obviating inner-tube replacement after deflation (which is generally necessary) were not offset by -additional damage to the cover.
A fully adjustable seat and a heater are standard cab fittings. The half. cab is preferred to other types on account of the better vision provided, including a view of the near-side kerb, and because the driver is separated from the passenger compartment.
Although power steering would reduce driver fatigue it is a complication that will probably be avoided on practical grounds. Nearly all the buses in the fleet are fitted with a hydraulic coupling and preselective gearbox, which have proved highly satisfactory in simplifying gear changing and in maintenance costs.
Ideas on Gears
Mr. Witcomb Smith would favour electro-pneumatic control of gear changing and would prefer this type to fully automatic control. He doubts whether a satisfactory method of automatic operation could be evolved for stage-carriage work.
In December and January, dynamo output is normally insufficient to keep the battery fully charged and Mr. Witcomb Smith is, therefore, particularly interested in experiments with A.C. generators. It is envisaged that they would provide a surplus charging capacity at all times and obviate the trouble with cut-outs which is now experienced.
In the interests of easy maintenance,,screwed-on body panels with a
wood packing are preferred to the Pop-riveted type. Wood screws give a greater location latitude when replacements are made and thus enable appreciable pillar variations to be accommodated.
Stainless-steel stanchions are fitted in preference to the aluminium type, because aluminium tends to blacken from contact with the hand and through acidic constituents in the air.
Wiring Innovation
A recent innovation is the use of Ega p.v.c. wiring conduit which, it is estimated, will last the life of the vehicle. The conduit is fitted inside the body below the seat rail where it cannot be affected by moisture or weathering, and is virtually protected from damage by abrasion and impact. It is preferred to flexible metal conduit on account of the improved insulation provided.
Some of the buses have covered over 700,000 miles in 18 to 20 years. According to the.present replacement policy, renewals are normally made at 15-year intervals. This gives an average yearly mileage of about 34,300. Of the 115 buses in the West Bromwich fleet, 114 are of Daimler manufacture, including 103 doubledeckers.