Thornycroft’s rigid six-wheelers were suitable for on-road use, however, some of them were also able to travel off road. This capability was given by additional features in the rear suspension complying with War Office requirements for rigid six-wheelers, ensuring freedom from spring distortion under all conditions of driving and braking and irrespective of the relative movement of the driving axles. The War Office requirements also included equal weight distribution on all four rear wheels, a feature which all Thornycroft RSWs possessed whether suitable for off-road operation or not
1Introduced in 1929, the A5 RSW had an off-road capability in common with the existing A3 RSW (1926) and XB RSW (1927). These three vehicles used Thornycroft’s proven off-road design, a more detailed look at which is available in the XB’s description. The design comprised a relatively simple transmission in the form of two driven rear axles and a single undriven front axle, the latter feature being consistent with giving the required off-road performance. The rear suspension was supple enough to allow either driving axle to tilt to an angle of 18 degrees, and to accommodate a 1ft (0.305m) level difference between axles. The A5 could carry 3.5 tons (3,556kg) on-road and less off-road and was offered in standard and subsidy forms. Also, the A5 was the most powerful of Thornycroft’s three RSW off-roaders when it was introduced, thanks to its 70bhp 5,703cc six-cylinder engine.
Older than the A5 and of longer wheelbase, the XB RSW off-roader remained in production for a year or so after the A5 was withdrawn in 1931. Perhaps demand for the A5 was less than Thornycroft expected.
Specification of Thornycroft Type “A5” Rigid Six-Wheeled Chassis (Petrol)
To carry a net load of 3.5 tons (3,556kg) over good roads and a reduced load over rough tracks or cross-country.
33.8hp (RAC), watercooled six-cylinder petrol engine developing 70bhp, with a bore 3.75ins (95.3mm) x stroke 5.25ins (133.4mm).
In unit with the engine.
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 2.30 to 1, which provides suitably low gears for rough country.
The power is taken from the gearbox to the driving axles by propeller shaft.
Driving axle units
The two driving axle units have a reduction of 7.75 to 1. Drive is transmitted to each differential unit by an overhead worm. The two rear axles use Thornycroft’s patented bogie suspension, which 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. All three axles are supported by leaf springs, and either driving axle is able to tilt to an angle of 18 degrees. Permissible level difference between driving axles is 1ft (30.5cm).
Vacuum servo assisted drum brakes are fitted to all four rear wheels. Front wheels are unbraked.
Wheels and tyres
Detachable disc type wheels equipped with pneumatic tyres, singles on all wheels. Standard size is 36ins x 7ins (91.4cm x 17.8cm), and 36ins x 8ins (91.4cm x 20.3cm) is available at extra cost;
67.5cwt (3,429kg) including engine unit.