The underframe was designed as a lightweight steel unit consisting mainly of mild steel channel section electrically welded together, the whole being stiffened with diagonal members and generously gusseted. Lightweight rolled steel disc wheels were mounted on two axles carried in SKF self-aligning roller hearings in cast steel axleboxes. Wheels and axles were supplied by Owen & Dyson Ltd.

The underframe, showing the rubber bonded metal-to-metal springs sticking up at each corner.

The brakes of normal clasp type, were operated by a straight air system and incorporated an emergency feature. They were supplied by Clayton Dewandre Co Ltd.

Body

The car body was a light steel shell fabricated by welding from top hat channel and "Z " sections.

The floor consisted of corrugated steel section attached by welding to the body longitudinals and cross members. This was covered by a wood composition floor, and finished with a good quality linoleum. Exterior panelling was of 16 swg aluminium for the sides and ends and 18 swg aluminium for the roof panels. Generally, the panels were riveted to the framework and panel joints covered with a moulding of suitable section. Ceiling panels, again of aluminium, were 20 swg except for the end domes, which were formed from 18 swg. Electrolytic action between the aluminium panels and steel frame was prevented by a jointing medium. Where necessary the panels were treated with sound absorbing material to prevent drumming. As drawgear and buffers could not be mounted directly to the underframe, they were carried by the body subframe. They were not designed to withstand normal usage and shunting stresses, but were fitted as a means of towing the railbus in the event of a failure while on the track, for which they were adequate.

Marker lights and headlamps were fitted in according to BR requirements, supplied by BMAC Ltd. Trico-Folbeth Ltd windscreen wipers were fitted.

Suspension

The axlebox suspension followed the Metalastik principle of supporting the axlebox in two chevron shaped rubber-bonded-to-metal springs. The design of the springs and the angle in which they were set allowed freedom of movement in a vertical direction, but exercised the necessary constraint in transverse and longitudinal directions. The springs were set at an included angle of 40 degrees and provided a static deflection of 1 3/16 in. under the maximum design load. The ratio of stiffness of the suspension in vertical, transverse and longitudinal directions was 1:2:7.

Metalastik axlebox suspension eliminated all metal-to-metal contact and the need for lubrication. The springs interposed a barrier of rubber between the tyres and the bogie, or the body underframe, absorbing high-frequency vibration and noise set up by tyres in contact with the rail joints.

The body suspension adopted by AC Cars was of the utmost simplicity. Four interweaved rubber-bonded-to-metal springs were used, one at each corner of the underframe. These springs, which were of the type employed for Metalastik bolster suspension, were set so that the resultant forces were focused at or near the centre of gravity of the body. This arrangement reduced roll and any tendency of the body to dip on braking and permitted the springs to be loaded in shear and compression - the best combination for safety, load/deflection characteristics and long service.

Diagram showing the resultant forces of the body suspension springs.

The ratio of shear to compression loading varied according to the direction of the load. Transversely the springs, which had an inward rake of 10 degrees, were largely in shear. The compression element was higher in the vertical plane, as the springs were tilted upwards at an angle of 16 degrees. In for-and-aft directions, where the greatest stiffness is required, the springs were almost entirely in compression.

Angular settings and rubber hardness were selected to give a static deflection of 2.66 in. under the full load of 19,200 lb. This deflection was considered the most suitable for a light, four-wheel railbus operating on branch lines. Metalastik Springaid buffers attached to the underframe limited the vertical deflection in the event of overloading, and excessive movement due to heavy side loads when traversing cross-overs was also checked by Metalastik buffers. Hydraulic dampers were fitted.

Power Train

Extensive use was made of BUT power transmission equipment and controls, many components of which were exchangable with those on standard DMUs.

The railbus was powered by a B.U.T. 150 h.p. six-cylinder horizontal diesel engine developing a maximum speed of 55 m.p.h. It drove, through a fluid flywheel and freewheel unit a four-speed epicyclic gearbox. A universal joint shaft then took the drive to the reversing final drive unit mounted centrally on one of the axles. The torque reaction was taken by an arm attached to the underframe and incorporated rubber buffers to absorb shock loads. Engine throttle, gearbox and forward-reverse gear were all operated through electro-pneumatic valves, the air supply being drawn from the system after the brake reservoirs had reached a pre-determined pressure. The compressor, of 15 Cu. ft./ min. capacity was not engine mounted as on most DMUs. It was a larger type, and was driven through vee-belts from the input end of the gearbox. Air was drawn through a filter and anti-freezer and connections to and from the compressor were flexible to allow for belt adjustment and vibration.

Combustion air for the engine was taken through a large air filter and then to the manifold by flexible pipe. The engine cooling water was fed through a large radiator mounted at the side of the underframe. The fan was driven by a universal joint shaft from the engine and enclosed in a cowl attached to the radiator body. A header tank incorporating a filler was mounted above the engine to ensure that no steam locks formed in the cooling system. A 50-gallon fuel tank was suspended on one end of the underframe and the filler was easily accessible from rail level.

The generator was also driven from the input end of the gearbox, in which position it was unaffected when the car was freewheeling. J Stone (Deptford) Ltd supplied the generator and control panel. The NiFe Batteries Ltd 200 amp/hour alkaline batteries were suspended at the side of the underframe in a wooden battery box, with a hinged side for servicing.

Interior

The driver's areas were semi-enclosed, the absence of a door enabled the driver to get quickly to a droplight on the offside of the vehicle for purposes such as to making single line token exchanges. Later folding wooden doors were fitted.

Seating was in two saloons, one on each side of the centre vestibule. Each saloon had an offset gangway 1 ft. 5 in. wide at seat level increasing to 1 ft. 9 in. above this level owing to a reduction the width of the seat back. Seat base height was 1' 5" and distance between seat backs was 2' 4". There were four two-passenger seats of fixed type on one side of the gangway, and four three-passenger seats on the other. The seats in each saloon faced towards the ends of the car. Two additional seats, each accommodating three passengers, occupied the corners beside the driving position. Upholstery was of uncut moquette, supplied by TC Firth (1955) Ltd, with Vynide panels on Dunlopillo bases. Luggage racks (light alloy) and seats were supplied by Dean & Son (Yorkshire) Ltd. Warerite panels were supplied by Bakelite Ltd. A portable extending ladder was strapped to a partition.

The Beclawat Zephyr type side windows consisted of a lower portion of 3/16 in. toughened plate glass and a light alloy framed sliding glass ventilator above. The whole unit was glazed directly into the body framing by means of Claytonrite self-sealing weatherstrip. At the ends a small central window was flanked by two larger ones; and those at the corners were curved to follow to the body contours. All these end windows were of ¼ inch laminated safety glass and glazed directly into the body framing in a similar manner to the side windows. Four extraction type ventilators were mounted longitudinally on the roof.

A single sliding door supplied by GD Peters & Co Ltd was situated centrally on each side of the car, controlled electro-pneumaticaly by push buttons in the cabs. Release cocks were fitted adjacent to each door inside and out for emergency and depot use. The doors slid into pockets in the vestibule, which was divided from the saloons by panelled partitions, glazed above waist height. Vertical stainless steel pillars at the inner ends of the partition formed a barrier for the glass as well as being grab pillars. Standard 24v DMU ("bullseye") type lighting fittings supplied by J Stone & Co (Deptford) Ltd were used, spaced to give good illumination, particularly at the doorways.

The railbus was heated by one Smith's combustion unit mounted on the underframe and operated from the same fuel as the power unit. Fresh air was drawn through a filter before heating and then ducted to be omitted at a number of points beneath the seats.

Conventional BUT L-type (Sharps) controls were fitted, as used on almost all DMUs. Fire extinguishers were carried in vestibules and cabs.

Overall length (buffers free) 37' 0"
Length over body 36' 0"
Outside width over panels 8' 11"
Overall height from rail (laden) 12' 2 1/2"
Inside width at rail level 8' 6"
Floor height from rail (laden) 4' 3"
Interior height, floor to ceiling 7' 4"
Wheelbase 19' 0"
Wheel diameter 3' 0"
Tare weight 11 tons (approx)
Seating capacity 46
Maximum speed 55 mph

Roller blinds were fitted, at each end of the car, housed in a box formed by the shaping of the roof and ceiling panels.

After delivery, the ScR requested that power-operated steps be fitted to 79979. This was done before the car moved to Scotland at the start of April '58.