Modifications to
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A Popular Passenger Chassis
Features of the Bristol Superbus which have been Revised Recently and were Demonstrated at Last Week's Municipal Confer ence at Yarmouth, IN 1926 the Bristol Tramways and Carriage Co., Ltd., the sales department of which is situated at Tramways Centre, Bristol, introduced a chassis specially designed to meet passenger-transport needs. It was described in our issues of June 15th and October 12th, 1926. Since then it has only been found necessary to modify in detail certain of the chassis parts, and we deal here specifically with recent alterations. They were demonstrated to interested officials of public transport undertakings at Yarmouth last week on'the occasion of the 28th annual conference of the Municipal Tramways and Transport Association.
Of the forward-control type, this Bristol chassis may be fitted with any of the range of three Bristol bodies; the L1 is a 31-seater with a rear-entrance body and a front exit on the near side ; the L2 is a 32-seater with a front entrance and rear exit, both on the near side, and is specially suitable for one-man operation ; the L3 is a 32-seater with a front entrance on the near side and an emergency door at the rear. The chassis provides a lowloading line and one of its features is that the engine and transmission line are offset in the frame to provide a level gangway down the centre of the single-saloon type body.
Details of the Engine..
A four-cylindered engine is employed—this is known as the G.W. type—and has a bore of 4i ins, and a stroke of 5i ins. The Treasury rating is 32.4 h.p. Since its introduction the power output of the model has been materially increased, the present figures are 52 b.h.p. at 1,000 r.p.m. and 75 b.h.p. at 2.000 r.p.m. The engine has a monobloc cylinder casting with a Ricardodesigned detachable cylinder head, duralumin connecting rods and aluminium pistons. The crankshaft is
a nickel-steel stamping with 21-in. main journals.
Transmission is through a plate clutch to a separately mounted gearbox approximately amidships in the frame, and thence by a tubular propeller shaft with mechanical universal joints to the offset wormdriven rear axle.
The Chassis Layout.
In plan the chassis frame is parallel from the front dumbirons to a point just behind the rear of the crankcase, where the members sweep out gradually to the line of the front mounting of the gearbox. Behind this line the frame members are parallel in plan and are horizontal, except where there is an upsweep to clear the rear axle. Tubular cross-members are employed in conjunction with pressedsteel ones ; an interesting detail point is that a longitudinal member is employed beside the gearbox to stiffen up the central portion of the chassis frame. Brakes are fitted to all four wheels, and the effort exerted by the driver's foot is multiplied by a Dewandre vacuum-servo motor.
Turning now to the modifications which have been effected. The carburetter is now of Claudel-Hobson manufacture with a horizontal mixing chamber and is of the power-jet type. it is mounted directly upon the cylinder block and so placed as to avoid any possibility of leakage falling upon any electrical gear.
Self-locking adjustment is provided on the latest type of dryplate clutch. Certain alterations have been made in the design of the front and rear axles. In the case of the former, a nickel-chrome stamping is employed for the axle bed, also for the stub axles and steering arms. The design has been specially calculated to meet the stresses imposed by the use of front-wheel brakes. In the case of the rear-axle casing a six-pinion differential is used; the cast-steel axle casing has the pot mounted considerably out of centre.
When first produced the disc wheels were equipped with 34-in. by 7-in. tyres; the latest models have 36-in. by 8.25-in, low-pressure equipment, single at the front, twin at the rear. In the light of these changes the following road speeds in relation to top-gear ratios are now provided, when the direct drive has an overall ratio of 5i to 1 and the engine revolutions are 2,000 r.p.m.: Top gear, 39 m.p.h.; third, 23 m.p.h.; second, 14 m.p.h.; first, 7 m.p.h. and reverse 10 m.p.h. If the lower set of gears be preferred and the overall ratio on direct gear is 6 to 1 the speeds are respectively—at the same engine revolutions—top,' m.p.h.; m.p.h.; third, 20 m.p.h.; second, 12i m.p.h.; first, 6 m.p.h., and reverse 9 m.p.h.
Chassis Dimensions.
In its latest form this low-loading Bristol chassis has its fuel tank carried between the frame members immediately behind the rear axle.
The chassis now has the following dimensions :—Height of frame from ground 2 ft. Oi in.; overall length, 25 ft. Of in.; overall length behind bulkhead, 20 ft. 61 ins.; overall width, 7 ft. 3 ins.; wheelbase, 16 ft.; front track, 6 ft. 2i ins.; rear track; 5 ft. 8+ ins.; height of body floor from ground, 2 ft. 2+ ins.; ground clearance below chassis (from the front dumbirons to a point 15 ft. behind them), 10i ins; turning circle, 58 ft. 6 ins.