Scientific Weight Reduction Pays Dividends
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L. J. COTTON,
THE 'increase of weight in buses and coaches has become a vicious circle. The advantages of large engines, developed to improve performance, have substantially been offset by the greater use of steel to replace scarce light alloys. Heavier-duty transmission systems have been required, modern refinements for comfort have again added to the unladen weight, and larger-section tyres have been needed to carry the load.
The Bristol-E.C.W. underfloor-engined 30-ft.-long, 8-ft.-wide single-decker has not been afflicted in this way. By good design and collaboration between chassis manufacturer and bodybuilder, a 45-seat integral-construction vehicle has been developed weighing 6 tons 6 cwt, The low weight has enabled a moderate-sized power unit to be employed, yet providing a high performance in keeping with contemporary models, at a lower fuel cost.
Former plans to employ a light-alloy sub-frame have been abandoned because of material shortages, but he present all-welded fabricated-steel structure is to be commended for its scientific design. The underframing, or "chassis," cannot be termed a self supporting unit, because it is necessary to reinforce the structure before it can be tested or delivered to Eastern Coach Works, Ltd., at Lowestoft.
The position of the axles and running components makes for reduced loading at the front and the 9.0020-in. tyres of 12-ply rating satisfy certifying officers. Front-axle loading has been a thorny problem in many underfloor-engined models.
Maintenance engineers have criticized the underfloorengined design because of the alleged need to use a pit when making adjustments or replacements. My own experience of operating such models in the late 1930's a8 contradicts much of the criticism and the Bristol
• supports my opinions. TrolleyS have been designed so-: that the entire transmission system can be re.rnOveci. as a unit, or in separeite components ; without runnkiij the vehicle Over: the Pit, and another appliance is avail
. able for removing the final drive.
The high frame line may help in maintenance br giving adequate space around the components, but this is incidental to the purpose of the frame design. The Bristol-E.C.W. is not available for public sale, but is constructed for member-companies of the British Transport Commission. These operators have demanded that all seats should face forward, and the frame has 'been raised to obviate large wheel-boxes in the body.
ProVision has been made for employing the Bristol:
sixeylindered oil engine, or the Gardner fcinr-. fiVe-' or six cylindered horizontal Unit: The tesf.vehicle was a coach chassis equipped with the 611LW engine... carried on the Gardner conVentional mounting together with the clutch and Bristol five-speed ,overdrive-top gearbox. The gearbox, designed and built in the factory at Brislington, has synchromesh engagement for the second, third and fourth ratios, and overdrive iSolatained by dog engagement of constant-mesh gears. Tooth size is all 5 D.P. with long addendum teeth on constantmesh, third and overdrive-top pairs to ensure minimum noise. A gear-type lubrication pump is employed, supplying oil internally to the main shaft and bushes of the free-running constant-mesh gears.
A single Hardy Spicer shaft runs to the spiral-bevel final drive. In removing the final drive from a complete vehicle, the longest part of the job is often said to be that of draining the oil. Using the specialized equipment provided, the engine and gearbox unit of the Bristol can be changed in just over an hour. A gearbox change takes 40 minutes and final drive approximately 30 minutes after draining the oil.
Further to reduce maintenance• costs, the first batch of production models has rubber-bonded shackle pins, but these are ultimately to be replaced by heavily chromium-plated pins operating in hardened-steel bushes without lubricant.
A Clayton Dewandre triple-servo braking system is employed, and ample brake frictional area is provided in the facings to extend intervals between adjustment. At present, 6-in.-wide shoes are used at the rear, but later a 61-in.-wide shoe will be employed, thereby increasing the total brake frictional area by 27 sq. ins. My tests indicated that, whilst there was sufficient braking effort at the rear, more power would be preferable at the front. As at present leverage cannot easily be modified, servo cylinders of larger internal diameter would have to be provided.
The Bristol L.S.-type vehicle has a wheelbase of 17 ft. 2 ins., which makes for improved riding comfort, and it has approximately equal front and rear overhang (5 ft. ins. and 5 ft. 6 ins. respectively). The sub-frame is Level between the axles and the off-side member skirts the engine. Floor height between the axles is 3 ft. 5+ ins. B9
laden, but the deck is sloped down at the front to provide a platform 2 ft. la ins, above ground level with a full complement of passengers. The rear overhang is also downswept.
The " chassis " was suitably reinforced with 11-in. deep timber beams and truss straps and loaded in readiness for my trials, which were started on a cold morning with ice and hoar frost on the roads. Apart from being exposed to the elements, for which I was totally unprepared, I enjoyed the day's work and thought that the suspension, aided by NeWton Bennett shock absorbers at the front, was above average for general service.
Slave tyres had been fitted in case of damage during heavy braking and I doubted whether a stop-start test could be staged on the slippery 1-in-6 gradient of Dundry Hill. Although the air temperature was 38 degrees F., there were still patches of ice on the corners and steeper sections, but on a straight run the Bristol
climbed the hill in second gear. With care and remarkably little wheel spin, the stop-start test was accomplished in low gear at a section where the Tapley meter registered 1 in 6-1.
As a chassis, the Bristol is overcooled, .although the body, which naturally restricts air flow, may take care of this point. The squat radiator, with 2g sq. ft. frontal area, and having 67 Still tubes of g-in. diameter arranged in three rows, kept the water temperature to 122-130 degrees F. during the climb, but the oil temperature, registered by a Cambridge thermometer inserted in the sump, refused to rise above 75 degrees F. under the heaviest low-speed pulling.
From Dundry we continued the journey through Winford, Redhill and Churchill toward Sidcot in readiness for acceleration and consumption trials, a speedometer check being made en route. The A38 road between Axbridge junction and Highbridge is favourable to consumption trials, its gentle undulations providing steady pulls but without need for gear changing on a model of the Bristol class, with almost 0.6 b.h.p. per cwt. to its credit.
First came the short performance test, in which the Bristol showed its paces by accelerating from rest to n10 30 m.p.h. through the gears in 21 secs. The easy gear change provided by synchromesh on the main ratios has a " preselective " quality and there can be only a split second's difference between those two systems in changing from one ratio to another_ On an open chassis the acceleration was exhilarating, and during my spell of driving 1 found ample reason to praise the gearbox and chassis as a whole.
When making emergency braking tests, the rear wheels locked, on an average, for 43 ft. from 30 m.p.h., giving a total stopping distance of 81 ft. Although sufficient for emergency and passenger safety, the Tapley readings being 58-64 per cent., more braking power on the front wheels would effect an improvement. Two methods of measuring stopping distance were employed. A solenoidoperated Webley pistol was attached to a cross-member and another marking device was linked to a rear brake camshaft. There was a difference of 6-ft. recorded at
30 m.p.h. This, in effect, represented the time delay in the servo system, which, according to the trials, was negligible, the factor being 0.136 sec.
Apart from a transmission unit, I have never before found such an effective hand brake as that fitted to the Bristol. According to the stopping distance recorded by the marker attached to the brake camshaft, the vehicle was brought to a halt in 38-39 ft. from 20 m.p.h. The Tapley meter, which is the customary instrument for recording efficiency, registered 44-45 per cent. The rear wheels locked, which may account for the disparity between readings, but in both cases the hand brake NW.; exceptionally effective.
Good Suspension The consumption test equipment worked on a measured half gallon of fuel and on the first leg of the course 1 instructed the driver to maintain a speed of 35-40 m.p.h. while I took notes and watched the fuel supply. A staff photographer, travelling in front on another Bristol chassis, tried some ambitious shots at high speed and the clarity of the results emphasized the good suspension of the test chassis.
Both on the outward and return runs the Bristol covered 8.3 miles on the measured quantity of fuel, giving a return of 16.6 m.p.g. at 35.8 m.p.h. average speed. Because daylight was fading the consumption trials had to be condensed and the next test was made with one stop to the mile. With a 15-sec. stop at every point, this was a tedious procedure over a 14-mile route, but it was during this trial that the effects of scientific weight saving on fuel consumption became particularly apparent.
With a comparatively light mass to accelerate, less energy is required when starting, and is reflected in con sumption and deceleration rates, there being no overheating of the brakes or tendency to fade. The overdrive was shown to be justified for local operation. The consumption rate of 13.8 m.p.g., making one stop par mile, is good, and above average, for an underfloorengined single-decker carrying the equivalent of 45 passengers.
On the return to Brislington, I made a detour through Axbridge, a village with an extremely narrow street flanked by overhanging "black, and white" buildings, the walls of which have been deeply scored by heavy vehicles running on the pavements in an attempt to pass parked cars. To pilot an 8-ft.-wide chassis through the village was like threading a needle and it added spice to an 86-mile test which had included a great deal of routine, plodding work.
The only criticism I could make of the Bristol during the trials concerned its behaviour during emergency applications of the brakes. Because the model was one of the first to come from the production line, there was no opportunity of determining whether the braking performance would be improved when the " chassis " was complete with its body. It is possible that the temporary strappings reinforcing the frame permitted it to flex more freely than it would if it carried a body. This would affect braking.
In all other respects the Bristol has " gentlemanly " behaviour, its strong features being economy in fuel and accessibility for maintenance. Its load and distribution offer scope for employing single tyres all round, which would reduce weight still further.
The plans to increase the width of the brake facings at the rear axle, in later production models, will not cause any greater tendency to lock the wheels. This will provide a lower unit pressure on the facings and a greater path for heat dissipation, thus affording an improved margin of reserve against brake fade and increased life between adjustment and replacement.
The other modification, to employ chromium-plated shackle pins in hardened bushes, is new in the Bristol design, but considerable service experience has already proved that the rate of wear is negligible. This type of pin and bush was employed in Tank tracks during desert operation.