In terms of carrying capacity, the 2 ton (2,032kg) Type BT was the smallest lorry in Thornycroft's range until the introduction of the 1.5 ton (1,524kg) Type A1 subsidy lorry. Also, the BT and its artic derivative were the only Thornycroft lorries with three-speed gearboxes produced between 1919 and 1926, the period covered by this narrative. All Thornycroft's other lorries produced during this time had four-speed boxes.
The BT was added to Thornycroft's lorry range at some time between 1913 and 1916. According to Thornycroft's specification for the BT, the vehicle was built along established Thornycroft lines, comprising a conventional front engine/rear wheel drive layout with brakes on the rear wheels only and a transmission brake. The BT also had solid tyres and a four-cylinder engine. Early vehicles had 3,469cc units giving a very modest 20/24bhp, but by 1924 the BT had the improved AB4 engine with ioe valves, an increased capacity of 4,531cc and a power output of 35hp.
Thornycroft had an eye for the beneficial effects of good publicity on sales, and a BT lorry was awarded the Dewar Trophy by the Royal Automobile Club for the most meritorious performance during 1921 after a 1,260 mile (2,027km) road trial. A 3.5/4 ton (3,556/4,064kg) articulated 6-wheeler version of the BT was exhibited at the Commercial Motor Exhibition in late 1923, in particular for overseas use where axle weights were restricted by local regulations or road conditions. Both tractor and trailer were offered with solid or pneumatic pneumatic tyres. Ball and socket joints were used on the turntable. BT lorries, including the short-lived artic, were discontinued in 1924.
Specification of Thornycroft 40cwt (2,032kg) Chassis, Type "BT" March 1924
For a net load of 40cwt (2,032kg) in a lorry or van body.
Chassis Weight, in running order, 43cwt (2,184kg).
Gross Load, including cab and body, 50cwt (2,540kg).
Total Laden Weight 93cwt (4,724kg).
Axle Weights (Laden).
Front, 24 cwt (1,219kg).
Rear, 69cwt (3,505kg).
30hp (type AB/4). Four cylinder, monobloc with detachable top, inlet valves over the exhausts. Bore 4ins (101.6mm) x stroke 5½ins (139.7mm). Ignition. Damp-proof high-tension magneto.
Of improved float feed type. A pilot jet is fitted for starting and slow running.
The carburettor throttle is controlled by a lever on the steering column and also by a foot accelerator. A separate throttle is fitted above carburettor controlled by a governor which prevents engine exceeding a predetermined speed. The magneto is controlled by a second lever on the steering column.
Placed on the dashboard where it is accessible for easy filling, and allows of a constant gravity feed to the carburettor under all conditions. The capacity is 14½ gallons (65.9 litres), and a very large filler is fitted which does away with the need for a funnel.
Of the cone type. A pressed steel cone lined with ferodo engages the cast-iron flywheel. This is of large diameter, ensuring smooth running, absence of slip, and maximum life of clutch lining. The clutch springs are easily accessible for adjustment. A large and efficient clutch stop is fitted to facilitate gear changes. Between the clutch and the gearbox, a leather disc-type universal coupling is fitted.
A one-piece casting with large inspection cover. Ball bearings on primary and secondary shafts with ball thrust behind brake drum. The drive on top is direct with two indirect gears and a reverse. At 1,000rpm of engine the road speeds are as follows: Reverse, 4.15mph (6.7kph); first 4.15mph (6.7kph); second 8.3mph (13.4kph); third, 14.6mph (23.5kph).
Operated through a gate type quadrant.
Live axle fitted with overhead type worm gear. The central portion of the axle is composed of a cast-steel casing of ample strength into which is fitted the differential worm gear, carried in a self-contained housing which, without taking the load off the road wheels, can be removed bodily at any time. Standard ratio 7.25 to 1.
Two brakes are fitted: a foot brake, of the contracting type, operating on a drum at the rear end of the gearbox, and a hand brake of the expanding type, which is applied on the rear wheels direct. The drums are of very large diameter. Both brakes can be adjusted for wear by hand without the use of tools.
A sheet metal mud-shield under the fore part of the chassis protects the mechanism from dust and mud.
The axle body is of H-section steel, drop forged in one piece. The swivel arms are of nickel steel, with ball thrusts on the pivot pins taking the weight, giving easy steering.
Channel section pressed steel. Strong, but light. The engine and gearbox under-frame is suspended on the three-point system, so that twisting of the frame members due to uneven road surfaces has no harmful effect whatever.
This is fitted with ball bearings and ball thrusts, which help to render the steering is very easy.
The minimum width of road for reversing direction without using reverse gear: Standard chassis 38ft (11.6m), 14ft (4.27m) wheelbase chassis 45ft (13.7m).
Nominally 35½ins (902mm) diameter over tyres, on front and back. Of cast steel with solid rubber tyres. The wheels are carried on phosphor-bronze floating bushes of large size.
Solid rubber singles on front wheels; twins on back wheels, all of 90 x 741mm fitting size.
Minimum clearance underneath chassis, fully laden, about 9ins (229mm).
Standard, 12ft (3.66m); extended wheelbase chassis for coaches or omnibuses, 14ft (4.27m).
Front wheels 5ft 2½ins (l.59m.). Centre-to-centre of twin tyres on back wheels 4ft 8ins (1.42m).