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Experimental 6X6 Artillery Tractor

September 7, 2014

by Roger V. Lucy

In late 1942, the workshop at No.1 Proving Ground in Ottawa initiated the idea of improving the traction of the Ford 6×4 truck by converting it to a 6×6, with a self-locking differential on the rear bogie. The idea was formalized at the Army Technical Development Board (ATDB) as Project 48 in December 1942, and assigned to the Department of National Defence’s Directorate of Mechanization (DMech). Its aim was to produce a low-silhouette, high-mobility, lightly-armoured 6×6 vehicle for use as an armoured personnel carrier, ammunition carrier or a self-propelled mount for light anti-aircraft guns, anti-tank guns, field artillery, or mortars. Three pilots were authorized, one for each role. Another possible role considered was as the chassis for a medium armoured car.

The first vehicle, shown here with a cargo holder attached to the front bumper. MilArt photo archives

The first vehicle, shown here with a cargo holder attached to the front bumper. MilArt photo archives

Three obsolete 6×4 Ford trailing-axle chassis were selected for conversion, one as an unarmoured 4×6, and the other two as armoured 6×6 vehicles. Thornton 4-wheel rear bogies, with self-locking differentials, were fitted. This US-made bogie gave the wheels a high degree of movement when traveling over broken ground.

The locked differential greatly improved traction, by ensuring all four wheels were powered, at all times. In addition, greater gear reduction was provided and heavier shock absorbers, adapted from the Ford Scout car. The clutch, transmission, and 95 HP, 239 cubic inch (4 litre) engine of the Ford Field Artillery Tractor were fitted in the prototypes, but the eventual use of a larger engine and tires was contemplated. The frame was redesigned so the driver and his mate were seated on each side of it, with their heads just above the engine cowling, allowing the gun mounting to be placed as low as possible in the vehicle. The wheelbase was 131 inches (3.3 metres), and overall dimensions were 211 inches (l) x 82 inches (w) x 55 inches (h) (5.25 metres x 2.05 metres x 1.4 metres). Ground clearance was 20 inches (50cm). The maximum weight of the two armoured prototypes was 16,000 lbs (7230 kg).

This is the second vehicle built and one of two sent overseas for testing. Here it takes part in trials to find a suitable tractor for the 6-pdr anti-tank gun. Here it is shown negotiating the long ascent - a trial segment it ultimately failed. MilArt photo archives

This is the second vehicle built and one of two sent overseas for testing. Here it takes part in trials to find a suitable tractor for the 6-pdr anti-tank gun. Here it is shown negotiating the long ascent – a trial segment it ultimately failed. MilArt photo  archives

The 6x6 was also used as a trial tower for the trails of the 25-pdr MARS gun. See article. MilArt photo archives

The 6×6 was also used as a trial tower for the trails of the 25-pdr MARS gun. See article. MilArt photo archives

The 6×6 could attain a top speed of 60 mph (100 km/h), or 50 mph (80 km/h) when towing a 6-pounder anti-tank gun. The Canadian distributor for Thornton bogies, H.V. Welles of Windsor, Ontario, carried out the conversions. One of the completed 6×6 prototypes was sent to the John Inglis plant for trials as an SP mount for the Quad 20 mm; the other underwent extensive road trials at Ottawa’s No.1 Proving Ground. Canadian views of the vehicles’ purpose were decidedly mixed. The Deputy Master General of the Ordnance and DMech saw them as test beds only. They did not consider that their mass production would be feasible in Canada. (The bogies were made in the USA, which had its own demands on them, and the Canadian automotive industry had little enthusiasm for self locking differentials). H.R. Crang, of the ATDB, was, however, a keen advocate. While visiting the UK in early 1943, he tried hard to persuade Canadian Military Headquarters (CMHQ) and the War Office to let the vehicles undergo trials in the UK. In March 1943, DMech reported that the vehicles had met all expectations, both as a General Purpose Light Armoured Vehicle and as a 20 mm SP. In July the Army Engineering Design Board (AEDB) concluded, at the end of the trials at No.1 Proving Ground, that they did not offer sufficient advantage over other vehicles to merit further development as a gun tractor and that the GMC 6×6 would better fit this role.

Although lower the 6x6 provided sufficient space for the 6-pounder gun crew. MilArt photo archives

Although lower the 6×6 provided sufficient
space for the 6-pounder gun crew. MilArt photo archives

In April, on the basis of Crang’s recommendation, CMHQ requested that the vehicles be sent, without delay, for evaluation in the UK, as the British were very interested in 6×6 vehicles. In August, following the completion of their Canadian trials, the two 6×6 vehicles were returned to H.V. Welles for refurbishing,  before being sent overseas. They arrived in the UK at the end of October. During Christmas week, 1943, one of the prototypes was tested, along with a number of wheeled and tracked vehicles, in trials to select a 6-pounder tower. These revealed the strengths and weakness of the 6×6. Its 14mm of armour provided good splinter protection, and its low silhouette gave good concealibility. Unlike several vehicles tested, it could carry the entire six-man gun detachment in relative comfort, as well as 72 rounds of ammunition (although, it was recommended that six ammunition racks be removed to allow all the crew to stow their kit). It was also the fastest vehicle tested, but – despite being able to pull the gun up a 50% grade with power to spare – its cross-country performance was deemed inadequate. It got stuck on the hummock test (and lacked a winch to extract itself ), and on the long hill test, where it stalled one quarter the way up a 1 in 7 slope. The two-foot (60cm) fording depth was deemed inadequate, and while it was easy to steer, the very low steering ration gave it a huge 72-foot (21 metre) turning circle. Other criticisms were that its design it did not allow for easy maintenance; access to the engine was poor to impossible, while the grease nipples on the suspension were completely inaccessible. In January 1944, the 6×6 was deemed completely unsatisfactory as a tower (rather unfairly, when compared to the other wheeled vehicles in the trials). The final blow appeared to be the change, in February 1944, to 21st Army Group’s requirements for 20 mm guns. Lacking any apparent requirement, the 6×6 project was declared complete by the ATDB on 14 March 1944.

The project took on a new lease of life in the summer of 1944. In April the 6×6 had been tested again by the Wheeled Vehicle Experimental Establishment (WVEE) at Farnborough, and compared to a 6×6 Studebaker truck. The Canadian vehicle’s cross-country performance, ability to handle mud, and 12-ton tractive effort compared very favourably to the Studebaker’s. Although its tires were deemed too small, the single-tired rear bogy performed better than the Studebaker’s double tires, and the 6×6 had very good ground clearance. WVEE recommended that a more powerful engine and a self-locking differential for the front wheels be installed. At about the same time, the USA advised the War Office that it was ending production of the M5E1 half-track, the chosen prime mover for the 17-pounder anti-tank gun. The War Office’s own gun tractor design, the Bedford Traclat – a half-track prime-mover based on the German Sd.Kfz 7 – would not be coming into production before late1946. An interim armoured gun tower was, therefore, required. In July the War Office asked DND to develop a new 6×6 prototype, incorporating the WVEE’s proposed modifications: a larger engine (the Chevrolet 140hp was under consideration), heavier clutch, improvements to the steering, and larger tires.

6-pdr towing trials. MilArt photo archives

6-pdr towing trials. MilArt photo archives

FILE4428

Subsequently, the British suggested a number of other changes to the design: a 144 inch (3.6 metre) wheel base, a 12 foot (3.6 metre) body, and a tractive effort at least equivalent to the 10 ton 100hp Albion 6×6 artillery tractor. The vehicle was to be able to carry a two tonne payload, and almost 1,700 lbs (775 kg) of armour (10mm front, 6mm at the sides). DND estimated that it would take six months to convert the Canadian design, and 18 more to put it into production. The Directorate of Vehicles and Small Arms (DVSA) was instructed to determine if the Canadian auto industry could take on this project, and if so, when it could be completed. In November DND decided to base their design on GMC’s 3-ton, 145-inch wheelbase, 6×6. The AEDB estimated an unarmoured prototype would be ready by late January 1945, and the vehicle could go into production eight months later. By this time, the Ministry of Supply was beginning to look ahead to postwar requirements. In light of differences between North American and UK vehicle and manufacturing standards, British policy was moving toward sourcing post-war vehicles, as far as possible, domestically. In the end, Canada proceeded on its own with a new (and long-term) project, No. 908, approved by the ATDB in April 1945, to develop a 6×6 Interim Artillery Tractor. One version was to be an armoured tower for the 17-pounder.

Under McNaughton, the CAO had been looking at mobile anti-aircraft defence since the summer of 1941. Two Oerlikons were borrowed from the Admiralty to investigate mounting on an armoured chassis. In November, a British firm, Stohert & Pitt of Bath, was given a contract to develop a mock-up turret. Consideration was also given to mounting the guns on a Loyd Carrier chassis. This work seems to have been taken over by the British Ministry of Supply’s Directorate of Tank Design (DTD) eventually resulting in the Crusader AA tank. While the protection of armoured columns “under all conditions” could only be met by  using a fully tracked vehicle, protecting road convoys required a wheeled vehicle that could keep pace with road traffic. In Canada – with development of the Inglis 20 mm now underway – the design of such mobile mountings came under consideration. On 4 December 1942, the AEDB approved Projects 41 and 42 to develop respectively quad and twin mounted 20 mm guns on a wheeled chassis. Responsibility was assigned to DVSA and DArty with assistance from the AEDB. The Armstrong Wood and John Inglis companies of Toronto provided commercial input.

The third, and last,  vehicle built in the series. This one was built as a SP mount to transport the Inglis 20 mm Quad AA gun. Canadian Army policy on the types of AA guns deemed acceptable for overseas use saw this gun. MilArt photo archives become redundant and the vehicle found itself without a cause. MilArt photo archives

The third, and last, vehicle built in the series. This one was built as a SP mount to
transport the Inglis 20 mm Quad AA gun. Canadian Army policy on the types of AA guns deemed acceptable for overseas use saw this gun. MilArt photo archives
become redundant and the vehicle found itself without a cause. MilArt photo archives

FILE4848FILE4852

General Service (GS) specifications drawn up on 19 December 1942, called for a vehicle, which offered a stable platform for the operation of quadruple or twin 20 mm guns. These were to be installed in a Canadian-designed or other suitable gun mount. The vehicle should have sufficient speed and cross country performance to operate with armoured formations. Armour protection would be confined to protecting the driver and co-driver. Armour was confined to that integral to the Inglis mountings. No predictor equipment was required; nor were out-riggers unless they proved necessary to provide stability; 500 rounds per gun was required, ready for instant use and a No.19 wireless set would be fitted. Project 42, the twin mount, was cancelled in February 1943, after CMHQ advised the MGO that “the superiority of the Quad Mount as developed by the John Inglis Company, over any two gun mount was so great that no further consideration was given the twin.” A pilot quad 20 mm vehicle with an unarmoured cab, aeroplane shock absorbers and a ring for the mount was ready by 20 March 1943. Further work was held in abeyance as CMHQ wanted to test the Inglis quad mount in the UK before a decision on the SP was made.

In August 1943, successful road trials were held using one of the high mobility 6×6 chassis, the quad mount and ‘drill purpose’ guns. The trials included a 650 mile (1,030 km.) road test and 100 miles at 30 mph (50 km/h) on the Ottawa No.1 Proving Ground’s test track. On 22 August, firing trials were held with the guns mounted on a GMC 6×6 at the Inspection Board’s small arms test range at Long Branch. Firing tests, broadside and to the rear, firing 10 rounds per gun simultaneously, showed minimal movement in any direction. Dispersion of the rounds on the target (at a range of 1,000 yards -900 metres) was 18 inches (45 cm) and the gunner noted very little vibration to the sight.

Reports on these trials were forwarded to the UK, but, despite several reminders, no response was received until 18 February 1944, when CMHQ advised DND of 21st Army Group’s Oerlikon only policy. The vehicle was eventually demonstrated in the UK in May 1944. The British Army was not impressed by it; however, the RAF showed some interest.

On 14 March 1944, the ATDB declared the project to be completed. Canada did, however, mount 250 Quad Polsten guns on the Ford 132-inch wheelbase, 3-ton, 4×4 truck. Sixty were sold to the UK and, in early 1945, 72 were shipped to First Canadian Army, in line with a revised 21 Army Group policy that replaced two 40 mm troops in each light anti-aircraft regiment with 20 mm SP mounts. By the time the vehicles were delivered to the units, the war was virtually over.

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For more information on the Canadian experimental projects order "Secret Weapons of the Canadian Army" from Service Publications

For more information on the Canadian experimental projects order “Secret Weapons of the Canadian Army” from Service Publications

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