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Types A1 RSW and A3 RSW

Types A1 RSW 

WW1 highlighted a need for wheeled military vehicles with an off-road capability in addition to their normal on-road abilities, and the War Office took a practical interest in the development of such vehicles after the war. An effective method of achieving good off-road performance was (and is) through the use of all-wheel drive, and mid-twenties examples of all-wheel drive vehicles were Thornycroft's Hathi heavy tractor and the American FWD lorry. However, at least in the case of the Hathi, the solution to the problem of transmitting power through steerable wheels resulted in a complex and bulky system of bevel gears, and although the system appears to have provided the ideal of constant velocity power transfer, it was a far cry from the simple and compact CV joints of today.

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. 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.

Given that the requirement for a lorry with off-road capability could be met relatively simply by a rigid six-wheeler with an undriven front axle, it became possible for Thornycroft to base its design for such a vehicle on existing components. Thus, in early 1926, the A1 RSW was introduced, based on the four-wheel 1.5 ton (1,524kg) A1 subsidy lorry which had been in production since 1924. All the major units of the A1 RSW were the same as those in the A1 chassis, including the FB/4 engine which produced 36bhp at 1,500rpm, although it could give up to 40bhp. A deviation was the lower final drive ratio of the otherwise standard A1 rear axle selected for the A1 RSW. Also the A1 RSW's transmission included an additional two-speed auxiliary gearbox, not used on the A1, with a low ratio for off-road use, which, with the normal four-speed gearbox, gave eight forward and two reverse speeds. The A1 RSW could carry 2.5 tons (2,540kg) and 1.5 tons (1,524kg) on and off-road, respectively. This fairly simple, conventional vehicle was considered to have not just military uses, but it presented advantages for service in undeveloped countries where roads might be poor or non-existent. Brakes were fitted to both the A1 RSW's driven axles, but, in keeping with contemporary lorry practice, not on the front axle. The A1 RSW followed the A1 in not having a transmission brake.

Engines of the day now seem woefully short of power for what they had to do, and, for the A1 RSW, a mere 40bhp in a vehicle weighing over 4.5 tons (4,572kg) fully loaded for off-road use meant snail-like progress, although the RSW's low gearing enabled it to keep moving over rough terrain, traction permitting. However, the need for low gearing, to counteract the RSW engine's low power, limited the vehicle's off-road speed to about half walking pace in bottom gear at the engine's normal 1,500rpm, with the auxillary gearbox engaged. Even the on-road maximum was a leisurely 18.4mph (30kph) at 1,500rpm in top gear!

Power was transmitted from the auxillary gearbox to the worm gear of the first driving axle's differential by the propeller shaft. This shaft was connected at each end to enclosed metal universal joints - unlike the simpler open fabric joints of the A1 chassis from which the A1 RSW was derived. A short, stiff shaft transmitted power from the first driving axle to the worm of the second axle. This shaft was also coupled up by metal universal joints. A sliding coupling at the forward end of each shaft allowed for distance variations due to suspension movements. Incidentally, Thornycroft's adoption of metal Hooke-type universal joints was part of a general movement away from fabric or leather universal joints for propeller shafts, and Hooke joints eventually became normal on vehicles. Thornycroft's adoption of Hooke joints was prompted by the requirement of the bus licensing authority in the London area (i.e. Scotland Yard), which decided that buses should be fitted with propshafts supported by Hooke-type joints instead of fabric or leather couplings, on the grounds of improved mechanical integrity. Recognising that a wider preference for Hooke joints was developing, Thornycroft decided to adopt them, not only on their bus chassis, but also on their lorries, initially on the Types Q and J.

The A1 RSW's rear suspension complied with two War Office requirements for off-road operation, when driving or braking: 1) no spring distortion, irrespective of the relative movement between the driving axles, 2) even weight distribution on the driving wheels. Simple and ingenious suspension covered by Thornycroft patents met WO requirements. On each side of the chassis, two semi-elliptic leaf springs facing fore and aft were separately fulcrumed at their centres on brackets hung from the chassis frame, one spring above the other. The spring extremities were mounted on the driving axles, the upper spring on top of the axles, the lower spring underneath. Thus, both driving wheels on each side of the vehicle could rise and fall independently of each other without deflecting the springs. The arrangement's symmetry ensured equal weight distribution on the driving wheels. The function of the springs remained the usual one of providing a flexible interface between chassis and wheels.

Either axle could tilt to an angle of 18 degrees, in the opposite direction to its neighbour, and a difference in axle height of 12ins (30.5cm) was allowed. Any tendency to further movement in a downward direction was prevented by slings mounted on the frame and passing under the axles, whilst excessive upward movement was arrested by rubber buffers attached to the chassis frame.

Types A3 RSW

Some months after the A1 RSW's introduction, also in 1926, Thornycroft introduced the very similar A3 RSW.

As usual for their products, Thornycroft produced comprehensive specifications for both the A1 and A3. Differences in these specifications are listed below:

  • The A3 RSW was offered with one final drive ratio. The A1 RSW was also offered with the same ratio, but a lower one was standard;
  • Certain combinations of gear and auxiliary gearbox settings resulted in differences in overall gear ratios between the A3 RSW and A1 RSW, with corresponding differences in vehicle speeds at the same engine rpm;
  • Although not stated in the spec for the A3 RSW, its driving axles were standard 2 ton (2,032kg) A2 lorry items - albeit with a lower final drive ratio than for the A2. The A1 RSW's driving axles were standard 1.5 ton (1,524kg) A1 lorry items, also with a lower final drive ratio;
  • Alternative wheel sizes were available for both the A3 RSW and A1 RSW; however, the only common size was 38 x 7ins (965 x 178mm).
  • The A3 RSW and A1 RSW had different weight distributions;
  • The A3 RSW's chassis weight was 2.5 tons (2,540kg) compared to the A1 RSW's 2.4 tons (2,439kg);

The A3 RSW was demonstrated off-road to staff of The Commercial Motor journal in late 1926. The type had already undergone trials at the hands of the War Office, overseas governments and business interests, and was, by then, in production. The actual vehicle which The Commercial Motor had been invited to see tested was one of an order for 23 for the Sudan government for use in connection with the cotton picking industry. This lorry hauled a two-wheel trailer with a test load of 1 ton (1,016kg), while the lorry itself carried its maximum off-road (test) load of 1.5 tons (1,524kg); chains were placed around the driving wheels to give extra traction.

The Commercial Motor's team was impressed with the way in which the A3 RSW coped on rough terrain during the tests, reporting that "It seemed that nothing short of a wood or 4ft (1.22m) of water need be considered as a hindrance to movement and one became quite exhilarated at what seemed a risk, but never was". Negotiating the test hill was a seemingly impossible task, comprising, as it did, a deep, loose, soil of sand, coarse loam and flints, with no cohesion whatever. The surface would spin away from the wheels and, being "hopelessly irregular", the average gradient of 1 in 3.75 would often be 1 in 2 under the driving wheels where the soil had banked up. Yet the A3 RSW took this "awful surface", with its 1.5 ton (1,524kg) load (without the trailer), in its stride. At one point the lorry was stopped on the steepest part for a photograph, yet it restarted from this position and climbed the last few yards without a moment's hesitation. The axle movements caused comment, and the journal reported that under some conditions the angle taken by the transmission was "simply extraordinary"!

Specification of Thornycroft Type "A3" September 1926

Rigid six-wheeled chassis - to carry a net load of 50cwt (2, 540kg) over good roads and 30cwt (1,524kg) over rough tracks or cross-country, with a body allowance of 15cwt (762kg).


25hp, type "FB4", bore 3¾ins (95mm) x stroke 5ins (127mm), will develop 36bhp at 1,500rpm and up to 40bhp at increased revolutions. RAC rating, 22.5hp. The four cylinders are of the monobloc type with detachable head. This facilitates easy inspection of both inlet and exhaust valves, which are on the near side of the cylinder block, side-by-side, and operated from one camshaft. The valve tappets are adjustable and totally enclosed, by removable covers; the tappets have exceptionally large surfaces in contact with the cams eliminating any possibility of undue wear. The cylinder block is bolted to the crankcase which carries the crankshaft and camshaft, the crankshaft of large diameter being carried in three long die-cast white-metal bearings, the caps of which are bolted to the top half of the crankcase. The bottom half of the crankcase can be removed without disturbing the main bearings, which ensures an easy means of inspection of the main and big end bearings without removing engine from chassis. The big end bearings are gun-metal shells with white-metal linings.

Lubrication of Engine

The oil pump is contained in the base chamber and immersed in the oil; it is driven by skew-gearing from the camshaft, and a large gauze filter is fitted on the suction side. The filtering surface is so arranged that any dirt or carbon falls freely to the bottom of the case and therefore does not tend to choke the gauze, which can be removed and replaced through an inspection door on the side of the crankcase. When the base-chamber is removed, the pump, drive and filter are very easily removed in one unit by undoing one nut. From the pump the filtered oil is forced under pressure to a tunnel cast in the side of the crank-chamber. From this tunnel there are passages to the three main bearings; the crankshaft being drilled, the oil passes from the main bearings to the big end bearings, which are consequently also lubricated under pressure. A certain amount escapes from the main and big end bearings in the form of a spray and lubricates the cylinder walls, tappets and gudgeon pins, camshaft bearings, gear wheels, etc. A very large oil filler cap is provided. No copper pipes are incorporated in the lubrication system except the external pipe to the pressure indicator. A test cock is fitted in the crankcase to indicate the high level point of oil in crankcase and a drain plug is fitted at the lowest point to enable the sump to be cleared out. A pressure gauge or a plunger type of lubrication indicator is fitted, so that the driver is able to know that the pump is functioning. A dipstick is also fitted to enable the driver to ascertain the exact level of oil in the crankcase.

Lubrication of Chassis

Grease-gun system throughout. The system has a quick action bayonet connection and is so designed that in applying the grease-gun, the nipple is self-cleaned.


Damp-proof high-tension magneto. The magneto is driven by a shaft from the timing case and mounted on a bracket with a machined face, which is bolted to the near side of the crankcase. Magneto timing is controlled by a lever on the steering column. To enable the timing to be easily set, the flywheel is distinctly marked.


Float-feed automatic type, very economical and flexible, with great simplicity; a pilot jet is fitted for starting and slow running. We are constantly experimenting to ensure that the carburettor we fit as standard is the most suitable type for Thornycroft engines. The carburettor is mounted on the off-side of the cylinder block, and the mixture is warmed by the circulating water and the exhaust. Controlled by foot and hand accelerator.

Cooling System 

By pump and fan mounted on the front of the cylinder block. The pump is of the propeller type and when out of action does not impede thermo-syphon cooling.


This is similar to the standard "Al" type radiator, but with an increased cooling surface; it is of the vertical gilled tube type, the tubes being 0.3125ins (7.94mm) bore. The radiator is mounted on cup-shaped rubber buffers, and is built up with removable top and bottom vessels, which enables new tubes to be easily fitted. A drain plug is fitted at the lowest point so that the system can be completely drained. A guard is fitted to prevent damage to the radiator.

Petrol Tank 

Twelve-gallon (54.6 litre) brass tank carried on the dash, providing gravity feed to carburettor. The petrol tank is filled from the front of the dash.


Single plate dry clutch engaging with specially prepared asbestos fabric surfaces, combining simplicity with very light rotating parts, which, together with a clutch stop, makes the changing of gears very easy.


Mounted on the rear end of the engine crankcase, ensuring perfect alignment, the gearbox provides four forward speeds, and a reverse, for normal running. On the back of the gearbox, and operated by a separate lever, is an auxiliary gearbox with a ratio of 2.30 to 1, which provides suitably low gears for rough country. The main and auxiliary change speed levers are so placed that it is possible to progressively use eight different ratios between engine speed and a reduction of 10.25 to 1 in the forward direction, and two ratios in reverse. At 1,500rpm of the engine, and with 36ins x 6ins (914mm x 152mm) pneumatic tyres, the gear ratios and speeds are:









with 7¾ Worm Ratio

with 8¾ Worm Ratio








6.02 )












































Between auxiliary gearbox and first driving axle, hollow propeller shaft, with enclosed metallic universal joint at each end. Between driving axles; short solid shaft, with sliding coupling at forward end mounted on metal universal joints, running in anti-friction bearings. Under normal loading conditions, the transmission line is approximately straight throughout.

Driving Axle 

Units Two cast steel casings with semi-floating differential shafts, driven by overhead worm, with a standard reduction of 7.75 to 1; but, if required, a reduction of 8.75 to 1 can be fitted. Worm and differential can the lifted without removing axle from the frame.


Foot brake and hand brake each operates in 16ins (40.6cm) drums, simultaneously on all four driving wheels. The brakes are of the internal expanding, continuous band type, with large friction linings. Adjustment is effected without the use of tools. Operation of the brakes is mechanical, compensation being provided as between fore and aft driving axles, but not between near side and off side wheels.

Steering Gear 

Of the worm and wheel type, with ball thrust bearings at each end of worm shaft, which render the steering very easy. The worm wheel is complete so that the lever can be fixed in a new position on the splined shaft to take up backlash after the wheel has become worn. This vehicle has a turning circle of 56ft 6ins (17.2m) near lock, and 63ft 3ins (19.3m)off lock.


Channel section pressed steel. The power unit is suspended on the three point system, so that twisting of the frame members, due to uneven road surface, has its harmful effect reduced to a minimum. The front suspension bracket is fitted with a rubber block to minimise the transmission of vibration to the frame. Front and rear towing shackles are fitted.

Front Axle 

The axle body is of "H" section steel, drop-forged in one piece. The swivel arms are of nickel steel with hardened thrusts on the pivot pins taking the weight, giving easy steering.

Rear Axle

This has been designed to comply with the War Office requirements for Rigid Six-Wheelers, which demand, in particular, that even distribution of weight, on all four wheels, and freedom from spring distortion, shall be maintained under all conditions of driving and braking, and irrespective of the relative movement of the driving axles. These requirements are made with a view to obtaining equal driving results on all four wheels. If even distribution is not maintained, the more heavily-laden axle tends to dig itself in when negotiating soft or sandy surfaces. This failure to maintain even distribution has proved the most serious defect in existing types of rigid six-wheelers; and the importance of ensuring it will be more readily appreciated when it is stated that with the majority of suspension systems at present in use, the load on one driving axle may easily exceed that on the other by more than 50 per cent, when in a low gear, or when braking. In the Thornycroft design these requirements are fulfilled as follows: Four inverted, semi-elliptic springs are pivoted at their centres to brackets attached rigidly to the frame, and universally mounted at each end to the driving axles. These springs form the only connection between wheels and frame; there are no torque tubes and no radius rods. All driving and braking stresses are transmitted via the springs to the frame, which is suitably designed to take them. The suspension is designed to permit 12ins (30.5cm) difference in driving axle levels; and also to permit either axle to tilt to an angle of 18degs, which corresponds to a difference in wheel levels of 18ins (45.7cm). These movements impose no distortion on the springs. Excess of movement is prevented by means of rubber buffers attached to the frame, and by slings from the frame passing underneath the axles.

Wheels And Tyres 

Disc type wheels bolted to cast steel hubs which run on ball bearings for the front wheels, and roller bearings for the rear wheels. All wheels are suitable for straight-sided pneumatic tyres, which are detachable, and interchangeable, front and rear. The standard sizes fitted are 36ins x 6ins (914mm x 152mm) singles on all wheels, or (a) 38ins x 7ins (965mm x 178mm) singles on all wheels. (b) 36ins x 6ins (914m x 152mm) singles front, twins rear.

Front Mudguards

Are of ample dimensions.


10ft (3.05m) and 4ft (1.22m).


The tracks for all single pneumatic tyres are: Front 5ft 1.625ins (1.57m); rear 5ft 1.5ins (1.56m). When twin tyres are fitted to the rear wheels the tracks are: Front 4ft 8½ins (1.44m); rear 5ft 4.875ins (1.65m).


The main components of this chassis are similar to our "A1" standard type and are therefore readily available.

Ground Clearance 

A large clearance, i.e. 

On 36 x 6 (914mm x 152mm) 8½ins (21.6cm). 

On 38 x 7 (965mm x 178mm) 9½ins (24.1cm).

General Measurements And Weights 

With 65cwt (3.3 tonnes) gross load: 

Front axle (laden) 27cwt (1.37 tonnes). 

Each driving axle (laden) 44cwt (2.24 tonnes).

For cross-country work with a gross load of 45cwt (2.29 tonnes) the axle weights should be reduced to: 

Front axle (laden) 23cwt (1.17 tonnes).

Each driving axle (laden) 36cwt (1.83 tonnes). 


Chassis weight 50cwt (2.54 tonnes). 

Crankshaft diameter 2¼ins (5.72cm). 

Gudgeon pin diameter 1ins (2.54cm).

Valve diameter 1½ins (3.81cm).

Differential shaft diameter 2¼ins (5.72cm)


1 14ins (35.6cm), shifter S.L.0.119D. 

4 double-ended spanners, 0.1875ins (4.76mm) to 0.75ins (19.05mm). 

1 tommy bar, 

1 sparking plug spanner. 

1 hammer. 

1 6ins (15.2cm) screwdriver. 

1 pair of pliers. 

1 magneto spanner. 

1 tappet spanner and feelers. 

1 tommy bar. 

1 half-pint oil feeder. 

1 lifting jack. 

1 4ins (10.2cm) shifter. 

1 spanner for rear axle plug. 

1 ½-link for belt. 

1 grease-gun. 

1 instruction book. 

1 brace for wheel hub nuts. 

1 wheel hub extractor. 

1 leather bag for small tools. 

Tyre levers.

Electric Lighting and Starting 

Provision is made to supply, at an extra charge, either a lighting and starting set, or a lighting set only, the dynamo being mounted in tandem with the magneto and the starter bolted direct to the engine casing.

Tyre pump 

A mechanically-operated pump can be supplied at an extra charge, and provision is made for driving this off the side of the gearbox, the pump being bolted direct to the gearbox.


A belt-driven speed and mileage recorder can be fitted at an extra charge.

Drawbar Gear 

The frame is strengthened so that a powerful spring drawbar gear can be fitted.



Chassis diagram 162kb pdf