Preselective Gearbox in New Thornycroft Coach
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FIVE new Thornyeroft coaches in the service of the Bristol Co-operative
Society, Ltd , are equipped with Nynchromesh and pre selective mechanism adapted to the higher ratios of the gearbox and a steering, wheel which is adjustable for height. The chassis, designated the SG!1■1R6 type, are fitted with the maker's six-cylindered direct-injection 7.88-litre oil engine, which develops 100 b.h.p. at 1,750 r.p.m.
Drive is transmitted from the engine to the gearbox through a fluid coupling, tubular propeller shaft and a small single-plate clutch. The clutch is necessary to allow engagement of first and reverse gears, which are not preselective.
Two levers are provided in the cab for gear selection. One, located to the left of the driving seat, makes provision for engaging reverse, first, second and " locked " second positions. The second lever, mounted on the steering column and operated by the right hand, is provided for preselcctive engagements of second, third and fourth gears.
When starling on hills, first speed is engaged by means of a clutch and the left-hand lever, and a change to second
gear is made in a similar manner. The right-hand lever is used to preselect third gear, which may be engaged, when required, by operating the clutch pedal and releasing the accelerator pedal. The change front third to fourth gear is again preselective and the gear is engaged by moving the gear lever, operating the clutch pedal and, releasing the accelerator.
When changing down in the preselective range, it is necessary only to preselect the gear and release the accelerator pedal. When descending long hills, second gear is selected by the right-hand lever and the " locked " second position is engaged by the lefthand lever.
Starting in "Second"
For normal driving, second, third and fourth gears are employed and it is not necessary to use the left-hand lever, except when negotiating severe bills and a change to bottom speed is required. The hydraulic coupling permits the vehicle to be driven away in second gear on flat territory.
The gear ratios are 5.12, 3.06, 1.64 and 1 to 1 forward, 6.55 to 1 reverse and 5.21 to 1 final drive.
Further to improve comfort in driving, the height of the steering column is adjustable vertically to the extent of 3 ins. Mantes cam and double-roller steering is employed.
Other features of the chassis are Clayton Dewandre air-pressure braking, a multiple-pull ratchet-pattern hand brake, and 24-volt starting and 12-volt lighting equipment.
Luxurious 33-seater bodies have been built on these chassis by Longwell Green Coachworks, Ltd.. Bristol. As the coaches will be used mainly for Tours. deep windows have been provided and the front part of the roof may be opened. The seating is handsomely finished with moquette tiod hide.
Total weight of the :Mattis and body is 6 tons In cwt.