In 1926, Thornycroft was producing the A3 rigid six-wheeler to satisfy demand for a lorry with both on and off-road capability, a requirement was identified during WW1. It was decided that the required level of off-road performance for a lorry could be satisfied using a relatively simple transmission, in the form of a rigid six-wheeler with two driven rear axles and a single, undriven, front axle rather than a more sophisticated AWD vehicle. Six-wheelers have the advantage of spreading the vehicle’s weight over six rather than four wheels. Also, important for off-road operation, the two driven rear axles divide the driving and braking torques over four rear wheels, giving better traction than a vehicle with only one driving axle. In addition, wear and tear is shared between both driving axles.
The 1926 version of the A3 RSW could carry 1.5 tons (1,524kg) off-road and 2.5 tons (2,540kg) on-road, and the vehicle was upgraded for 1927 to corresponding capacities of 2 tons (2,032kg) and 3 tons (3,048kg) respectively. However, Thornycroft identified a requirement for a rigid six-wheeler with off-road capability of larger capacity than the A3 RSW, and, in 1927, introduced the Type XB capable of carrying 3 tons (3.048kg) over rough country and 5 tons (5,080kg) on-road.
The XB followed the A3 RSW’s formula, as follows:
The XB’s rear suspension systems allowed a difference of 9ins (22.9cm) in driving axle levels, either axle being able to tilt to an angle of 13 degrees, and each was cushioned on a rubber block at the top or bottom of the tilt. Pnuematic tyres were fitted, and there was no solid tyre option. The vehicle was powered by the newly-introduced MB4 four-cylinder engine giving 60bhp @ 1,800rpm. Clutch and gearbox were built in-unit with the engine, in accordance with Thornycroft’s policy, announced in 1927, that separate clutches and gearboxes should no longer be used.
A long-wheelbase version of the XB was offered in addition to the standard model, although the load capacity of both normal and lwb models was the same. Demand for the XB kept the type in production for the full period covered by this narrative, i.e. 1927 to 1932, with improvements. For instance, in 1930, in common with some of Thornycroft’s other models, the XB was given vacuum servo operated brakes using vacuum from the inlet manifold. Then, in 1931, the on-road load capacity was increased from 5 tons (5,080kg) to 6 tons (6,096kg). When, in 1931, Thornycroft adopted the engaging practice of allocating names to its vehicles, the XB became known as the Amazon while also retaining the XB designation. This became XB/MB4 the following year to indicate the Amazon’s engine type.
Specification of Thornycroft Type “XB”Rigid Six-Wheeled Chassis (Petrol) June 1927
To carry a net load of 5 tons (5,080kg) over good roads and 3 tons (3,048kg) over rough tracks or cross-country.
36.2hp (RAC) type "MB4", watercooled four-cylinder petrol engine, bore 4.75ins (120.7mm) x stroke 6ins (152.4mm), developing 60bhp at 1,800rpm. Compression ratio is 4.4 to 1. The four cylinders are of the monobloc type with two detachable heads. The valves have double springs, and the Thornycroft standard roller tappets, with accessible adjusting nuts, are employed. The pistons are of cast iron with three piston rings, but, if desired by the purchaser, aluminium pistons can be supplied. The connecting rods are of duralumin, the caps being fastened by two bolts. Big end bearings are gunmetal shells with white metal linings. The crankshaft is carried in three main bearings..
Lubrication of engine
Four gallons (18.2 litres) of oil are carried in the sump for lubrication and the oil pump is submerged in the oil. It supplies the oil to a channel which runs end to end of the engine at a pressure of 15lb/in2 (1.034 bar). From this oil channel the lubricant passes to the main bearings and through a drilled crankshaft to the big-ends, the cylinder walls and small-end bushes being lubricated by splash. The camshaft bearings are pressure-fed and the surplus oil passes to the timing gear. No copper pipes are incorporated in the lubrication system, except the external pipe to the pressure gauge on the dashboard. A dipstick is fitted with maximum and minimum marks and a test cock is fitted in the crankcase to indicate the high level point of oil. An oil filter is fitted
Lubrication of chassis Grease-gun system throughout.
A Simms magneto with a vernier coupling and automatic control of the timing is employed. A platform cast on the upper half of the crankcase carries the magneto and dynamo in tandem, and should it be necessary to remove the dynamo for overhauling or adjustment, the magneto can be moved to the forward position and coupled direct to the magneto shaft.
Zenith type, and interposed between it and the cylinder block is a mounting which carries the governor throttle. The governor consists of three balls in a cage enclosed in a casing at the forward end of the camshaft, centrifugal action on the balls causing a coned plate, through a rack and pinion and wire, to affect the position of the governor throttle according to the speed of the crankshaft. The engine of a freight vehicle is governed at 1,900rpm. A control lever on the steering column determines the minimum setting for the throttle.
Water is circulated by a propeller pump mounted on the intake lead with an outside adjustment for the packing gland. The pump shaft is driven by a Whittle belt off the magneto shaft and the fan is mounted on a prolongation thereof.
Single plate dry clutch in unit with the engine, fitted with a clutch stop.
Mounted on the rear end of the engine crankcase, ensuring alignment, the gearbox provides four forward speeds, and a reverse, for normal running. On the back of the gearbox is mounted an auxiliary gearbox with a ratio of 2.50 to 1, which provides suitably low gears for rough country.
The power is taken through a propeller shaft with an intermediate shaft, the rear end of which is supported in a ball bearing attached flexibly to the second cross-member of the frame.
Driving axle units
The two driving axle units are standard axles with a reduction of 7.25 to 1 as used in the KB-type chassis, with overhead worm and with a universal joint between the two axles. An alternative reduction of 8.25 to 1 is available. Access to the worm gear in each axle is obtained by withdrawing the fully-floating drive shafts. The suspension of the two rear axles complies with the War Office requirements for rigid six-wheelers, ensuring equal weight distribution on all four wheels, freedom from spring distortion under all conditions of driving and braking and irrespective of the relative movement of the driving axles. The gimbals to which the rear ends of the springs are coupled are extremely well-supported, and there are neither torque tubes nor radius rods.
Four semi-elliptic inverted springs of ample width and length are employed, and a difference of 9ins (22.9cm) in driving axle levels is possible, either axle being able to tilt to an angle of 13 degrees, and each is cushioned on a rubber block at the top or bottom of the tilt.
Drum brakes are fitted to all four rear wheels. Front wheels are unbraked.
Channel section pressed steel, fitted with outriggers for mounting the body.
The axle body is of “H” section steel, in one piece.
Wheels and tyres. Detachable disc type wheels bolted to cast steel hubs which run on roller bearings, and they are equipped with 38ins x 7ins (96.5cm x 17.8cm) straight-sided pneumatic tyres, singles on all wheels.
Front mudguards are supplied with chassis.
12ft (3.66m) to the forward axle of the bogie and 14ft 3in (4.34m) to the midway position between the two driving axles. A long wheelbase, 3ft 9in (1.14m) longer, can be provided.
Maximum track is 6ft 4.5ins (1.95m).
The ground clearance with a 7ins (17.8cm) tyre is 11ins (27.9cm).
Engine, clutch and gearbox unit: 13cwt 2qrs 18lb (694kg).
Long chassis weight, including engine unit: 83cwt (4,216kg).
Equipment: kit of tools
Provision is made to supply an engine starter, the flywheel having teeth already cut for engagement with the starter. A platform cast on the upper half of the crankcase carries the dynamo and magneto in tandem.
Tyre pump and power take-off
A tyre pump or power take-off, when fitted, is bolted direct to, and driven from the gearbox.
A belt-driven speed and mileage recorder can be fitted at an extra charge.