WW2 US Medium Tank Prototypes

Chrysler’s Improved Suspension M4A4

United States of America (1942)
Medium Tank – Blueprints Only

The Medium Tank M4A4 Sherman was an improved variant of the M4A3. The goal of the tank was to increase the speed of production of the M4 by using a new multibank engine and with a hull made from 5 pieces instead of seven. The longer and more complex engine would mean an increased length of track on the ground for improved performance of the M4A4 on soft ground, yet despite this, the M4A4 was not adopted by the US Army for use overseas. Early in the development of the M4A4, consideration was given to making us of the longer hull to improve the suspension. This led to the idea of using the ‘Christie’-style suspension from the T4 Medium Tank on this new Sherman. Whilst the M4A4 was built in large numbers and saw extensive service during World War 2 and later, the idea of using this ‘big-wheel’ suspension never left the drawing board.

M4A4 Sherman with the Vertical Volute Suspension System (VVSS) which Chrysler were investigating the replacement of with an improved big-wheel form. Photo: Mark Nash


The design of the M4A4 began in February 1942. This new Sherman was going to be more mobile than the M4A3 by using the 435 hp Chrysler A57 multibank petrol engine. The selection of what was actually 5 engines fitted together created a crowded space within the engine bay, which necessitated a slightly longer hull than the M4A3. This was considered a tradeoff that could add a large number of tank engines into the supply chain which would aid in meeting their production targets. Further, the hull of the M4A4 was simplified, as it was made in fewer parts than the M4A3 (5 instead of 7), and featured a 3-piece final drive housing on the front instead of the single-piece final drive housing on the M4A3. This would improve the speed of repairs and maintenance on the tank although, initially, the complex engine arrangement had been unreliable.

Lengthening the hull by 11 inches (279 mm) in order to accommodate the engine also meant that the suspension would have to be lengthened. The M4A3 had used three pairs of volute spring suspension (Vertically Volute Suspension System – VVSS) and these could be spread out more along the slightly longer sides of the M4A4 or a new suspension system could be considered instead. This prompted a very short study by Chrysler, the design agent for the M4A4, to try and improve the performance of the tank by way of an improved suspension system. The system to be investigated was a modified version of that trialled on the T4 Medium Tank.

Rather than refit the three VVSS units, spaced out along the side, the idea was now to use five large road wheels connected on horizontal crank arms. Springing for the wheels was delivered by means of vertical coil springs mounted on the outside of the lower hull. This has been described variously online as being a ‘Christie Sherman’ or ‘Christie suspension’ but it really is not. The Christie patent for his system had already been sold off by then as well as licensed off to countries like Great Britain and the Soviet Union. One of the dominant features of Christie’s suspension design was the suspension springs operated within a double-wall cavity along each side of the tank. This system was adopted and adapted for use in tanks such as the British A.13 and Soviet BT-5 and remained in use on some tanks through to the end of World War 2. The British Comet, for example, was the last British tank to use a version of this system. This was not the case for this M4A4. Here, the springs would be mounted externally.

Christie, by February 1942, was almost a dirty word in US armor circles and had no formal involvement with the US Army. His last official contact had been with the Ordnance Department in March 1939 and ended when he had stormed out of a meeting in a tantrum when his demand for large orders for his tanks was rejected. He had stomped off saying he would go and see President Roosevelt and with that had ended any prospect of formal consulting work.

Consequently, attributing his name to this design would be incorrect. If there is any doubt on the matter, the somewhat awful book ‘Steel Steeds Christie’ published in 1985 by his son Edward and which makes numerous fallacious claims, makes no claim to this design. The T4 suspension design was certainly based on the work of Christie, but the first conceptualized drawings for a sprung suspension-arm suspension for the M4, prepared by the Ordnance Office in February 1942, had already departed from this arrangement.


The T4 Medium Tank, built by Rock Island Arsenal in 1935 and 1936, weighed just 13.5 tons (12.2 tonnes). Different versions of the T4 were trialed between 1935 and 1940 when it was declared obsolete, but the key feature of the design was the four large road wheels on each side. The suspension of the T4 was certainly based on the suspension designs from Christie, but it did not use Christie’s patents. The track for the T4 was also a short-pitch type track 12 inches (305 mm) wide.

Medium Tank T4 showing the 4 large road wheel design with no return rollers. Source: Hunnicutt

The T4 weighed just 13.5 tons (12.2 tonnes), whereas the M4A4 would weigh 34.9 tonnes (31.6 tonnes), more than double the weight of the T4, so using the same suspension required changes. The T4 used just 4 wheels on each side, which would be inadequate for the extra weight of the M4. Thanks to the longer hull of the M4A4 though, 5 of these large-diameter wheels could be fitted on each side. The second change came about after the initial drawings from the Ordnance Board. Those drawings had shown the five, closely positioned wheels, each mounted on an individual arm with a corresponding spring cylinder angled forwards. To meet the increased weight of the M4, these springs had to be changed too. The solution here was to adopt heavier coil springs and to mount these vertically along the outside of the lower hull of the tank under the sponsons.

First plans for a T4 Medium tank-style suspension on the M4 Sherman, circa February 1942. Note the suspension springs are angled forwards rather than vertical. Source: Hunnicutt


The adoption of the T4 style wheels was also met with the choice of a wider version of the T4 track. This single-pin track was 18.5 inches (470 mm) wide, wider than the standard M4A4 track and the original T4 track, and used a center guide to prevent lateral slippage. With 93 track links per side (compared to 85 on the T4) and the larger, heavier wheel, this new M4A4 was significantly heavier than the original volute-suspension M4A4 by 3,080 pounds (1,397 kg).

The volute-suspension M4A4 used either the T48 or T51 83-link 16.56 inch (421 mm) wide track with a ground contact length of 160 inches (4,064 mm), which was substantially longer than the M4A3 at 147 inches (3,734 mm). Using this T4 style suspension, the track length on the ground was only fractionally longer than that of the M4A3, at just 148 inches (3,759 mm), yet despite this shorter length of track in contact with the ground than the volute-suspensioned M4A4, the wider track made up for this and kept ground pressure to just 14 psi (96.5 kPa).

Front view of the Chrysler sprung swing-arm suspension M4A4 shows the width of the external springs on the sides of the outer lower hull (left), and with the additional width of the spring highlighted in pink (right). Source: Hunnicutt and Author respectively

With the new spring system fitted to the outside of the lower hull, this meant a lot of space was taken up under the sponson on each side. Consequently, the tracks and wheels would be further out than they would be if it had retained the VVSS system. This would have posed some additional issues regarding the transportation of the tank due to its increased width, about 450-470 mm wider than the M4A3 due to the projections of the track and the lack of space in which to add grousers to the inside of the track.

With the new spring system fitted to the outside of the lower hull, this meant a lot of space was taken up under the sponson on each side. Consequently, the tracks and wheels would be further out than they would be if it had retained the VVSS system. This would have posed some additional issues regarding the transportation of the tank due to its increased width, about 450-470 mm wider than the M4A3 due to the projections of the track and the lack of space in which to add grousers to the inside of the track.

M4A4 with VVSS (left) compared to M4A4 with T4 Style Suspension (right) showing the additional width of the M4A4 (not to scale). Source: Hunnicutt
M4A4 with VVSS (top) compared to M4A4 with T4 Style Suspension (bottom) (not to scale). Source: Hunnicutt

One final note of difference between the suspension systems on the M4A4 are the return rollers. Easily overlooked, the VVSS system used a small return roller angled back from the suspension bogie which served to hold the track off from fouling on the top of the bogies. No such rollers were drawn on the T4 suspension units to support the track. The angle of the track, as it descended from the front sprocket to the rear idler, would likely contact the top of the last roadwheel but other than that it was unsupported .

The final product. Five large diameter T4-style road wheels and vertical coil spring suspension on the M4A4. Note that the mantlet is misdrawn and should be further back, towards the turret front. Source: Hunnicutt


Despite the fact that the T4-style suspension was found by engineers at Chrysler to be workable, it was not pursued. The volute system was not ideal but it was simple and reliable. In the short-term, the volute-spring system was retained, although work on improved suspension for the M4 continued. No versions of the Chrysler vertical coiled spring suspension M4 were ever built. Despite 7,499 M4A4s being built, it only saw limited service with the US Army anyway, restricted mainly to training duties. It did, however, find extensive use overseas particularly with the British, where it was known as the Sherman V.

Illustration of Chrysler’s improved suspension M4A4. Illustration by Andrei Kirushkin, funded by our Patreon Campaign.


Dimensions 6.06 m x 2.62 m (hull, 3.07 m to 3.09 m wide over tracks) x 2.74 m
Total weight, battle ready 72,780 pounds (36.29 US tons) (33 tonnes)
Crew 5 (commander, driver, co-driver, gunner and loader)
Propulsion 435 hp Chrysler A57 multibank petrol engine
Speed (road) 35 mph (56 km/h)
Armament M3 75 mm gun in M34 mounting
.50 calibre M2 AA machine gun
2 x .30 calibre M1919A4 machine guns
Armor 1.5 inches (38.1 mm) – 3 inches (76.2 mm) – 107.95mm


Armor Magazine, November-December 1991. Christie’s last hurrah.
Christie, E. (1985). Steel Steeds Christie. Sunflower University Press, USA
Gabel, C. (1992). The US Army GHQ Maneuvers of 1941. Center of Military History, United States Army, Washington D.C., USA
Hunnicutt, R. (1977). Sherman – A History of the American Medium Tank. Presidio Press, USA
Icks, R. (1969). The Fighting Tanks 1916-1933. We Inc. USA

WW2 US Medium Tank Prototypes

APG’s ‘Improved M4’

United States of America (1941)
Medium Tank – Blueprints Only

Developed to meet the needs of both the American and British military during the Second World War, the Medium Tank M4 became one of the most produced tanks in the world. It was reliable, versatile and spawned a number of variants through the course of its production.
However, before the first vehicles were rolling off the assembly line, plans were hatched to improve on its design…

An original concept for an improved M4. Photo: Presidio Press

The M4

The tank started life in 1941 as the T6 and was later serialized as the Medium Tank M4. There were two initial models namely the M4, which had a welded hull, and the M4A1, which had a cast hull. The tank entered service in 1942.
The M4 was armed with the 75mm Tank Gun M3. This gun had a longer barrel length (compared to the previous M2 model) which allowed a muzzle velocity of up to 619 m/s (2,031 ft/s) and could punch through 102 mm of armor, depending on the AP (Armor Piercing) shell used. It was a good anti-armor weapon, but it was also used to great effect firing HE (High-Explosive) for infantry support. For secondary armament, the M4 had a coaxial and a bow mounted .30 Cal (7.62 mm) Browning M1919 machine gun, as well as a .50 Cal (12.7 mm) Browning M2 heavy machine gun on a roof-mounted pintle.
It was well armored for its time, with 50.8 mm (2 in) of frontal hull armor angled at 55 degrees which brought the effective thickness to 88.9 mm (3.5 in). The front of the turret was 76.2 mm (3 in) thick.
Propulsion was provided by a Continental radial gasoline engine, developing 350-400 hp. A drive shaft sent the power from the engine in the rear of the tank to the transmission at the front. This powered the drive wheels and propelled the vehicle to a top speed of 22–30 mph (35–48 km/h). The tank’s weight was supported on a Vertical Volute Spring Suspension (VVSS), with three bogies on each side of the vehicle and two wheels per bogie. The idler wheel was at the rear.

Aberdeen’s Improvement Project

Before the M4 had even entered production, Aberdeen Proving Ground (APG) received a letter from the Office of the Chief of Ordnance, dated December 8th, 1941 (the day after the Pearl Harbor attack). The letter instructed Aberdeen to start work on developing an improved model with increased mobility and protection. Two designs were submitted. These were Aberdeen Proving Ground’s own and another submitted by Detroit Arsenal. Aberdeen submitted line-drawings and a list of characteristics of their initial design on March 13th, 1942. The proposed vehicle had a number of differences from the first models of M4. It did, however, retain the 75mm M3 tank gun and M34 mantlet, as well as the coaxial and bow mounted .30 cal (7.62mm) machine guns.

A head-on view of the design, also showing the thicker tracks. Photo: Presidio Press


The front hull armor thickness of 50.8mm (2 inches) remained unchanged, except for the bulbous final drive housing. At the time of this design, the final drive housing on M4s was made up of three parts bolted together. This new design did away with that, making it one solid piece. Such housings would later appear on subsequent M4 production models. The vertical portion of the housing, originally 2 inches thick, was increased to 3 inches (76.2mm) and the contour increased to improve effectiveness.
The lower side armor (behind the tracks) was also increased from 1.5 inches (38.1mm) to 2.5 inches (63.5 mm). Above the track, on the sponsons, armor was increased from 1.5 inches to 2.75 inches (69.85 mm). The plate was sloped inwards at 30 degrees from the vertical which increased the width the entire hull to 123 inches (10.5 ft) from the original 103 (8.5 ft). The rear plate was also thickened from 1.5 inches (38.1 mm) to 2 inches (50.8 mm).
When this design was presented, it was thought that there would be a large shortage of foundry capacity to produce large castings such as those for the M4’s turret. As such, it was decided to fashion the turret from a number of rolled armor plates welded together. This would give a sharp, angular silhouette to the turret.

A top down view of the design showing the angular shape of the turret. Photo: Presidio Press

Representation of APG’s ‘Improved M4’ in a speculative Olive Drab colour scheme that was common at the time of its conception. Illustration by Bernard ‘Escodrion” Baker, funded by our Patreon Campain.


It was thought that the original Continental engine would be too underpowered for this new design due to the weight increase from approximately 30.5 tons to 42 tons in view of to the additional armor. Aberdeen proposed the use of the new Wright G200 air-cooled radial engine which would develop 640 hp, compared to the previous 400hp. A large bulge had to be drawn into the engine deck to accommodate the engine. The standard transmission used in the M4 was retained, but the drive shaft from the engine was mounted lower in order to increase room inside the tank. It was expected that this new power pack would propel the tank to about 35 mph (56 km/h) which was a substantial improvement over the 22-30 mph (35-48 km/h) top speed of the standard M4.
The weight increase also necessitated changes to the tracks and suspension to support the heavier hull and keep ground pressure to an acceptable limit. Aberdeen chose to use a slightly modified version of the suspension found on the Heavy Tank M6 and the prototype Heavy/Assault Tank T14. This was an early version of a Horizontal Volute Spring Suspension (HVSS). Three bogies were mounted per side, each with two double-wheels. The wheels were 18-inches (45.72 cm) in diameter, apart from the first wheels on the front bogie, and the trailing wheel on the rear bogie. These wheels were larger with a 22-inch (55.88 cm) diameter. The bogies did not have integrated return rollers like the traditional M4 suspension. On this design, there were four mounted directly to the side of the lower hull on each side. The M6/T14’s 25.75 inches (65.40 cm) tracks were also chosen for the tank. Aberdeen surmised that the new vehicle would have a combat weight of approximately 42-tons. Almost 12 tons heavier than the standard M4.

This side profile of the design shows the intended HVSS suspension. Photo: Presidio Press

Detroit Arsenal

The Aberdeen design was not approved for production as there were additional areas that needed further development. Detroit Arsenal continued looking into the possibility of upgrading the M4. They looked into both welded and cast turrets for their design. This turret would have interchangeable front plates enabling it to either carry the 75mm M3 Tank Gun or the 105mm M4 Howitzer or even the M7 3” Gun from the GMC M10 “Wolverine”.
Detroit kept the vehicle’s weight to 30.5 tons, around the same as the standard M4. Armor effectiveness would be increased however in a manner similar to the T14. The hull was made considerably more shallow and the raised ‘hoods’ over the driver’s positions eliminated. This turned the upper plate a perfectly flat, sloped surface. The sponson armor retained the standard thickness of 1.5 inches (38.1mm), but was sloped inward at 30-degrees. This increased the vehicle width to 120 inches (10 ft). As the armor was not increased, the weight of the tank did not climb. As such, it was planned that the standard M4 VVSS suspension would be retained. Three engines were considered for installation on the tank. These were the Ford GAZ, Continental R975-C1, and the General Motors 6046 diesel.

The Detroit Arsenal design. Photo: Presidio Press


The design programs had succeeded in finding numerous potential improvements for the M4 tank, but there were some design choices that were not such an improvement.
Ammunition for the main armament was still intended to be stored in the sponsons. Although this was the perfect place for the loader to access his rounds, it was an extremely vulnerable position. The fuel tanks were relocated from the engine compartment to underneath the turret basket. One can only imagine the catastrophic events that may have occurred should the fuel tanks have been breached and set ablaze.
Though neither the Aberdeen or Detroit vehicles were approved for service, however, the work put into the developments were not in vain, as subsequent models of the M4 would incorporate some of the improvements identified in these projects.

An article by Mark Nash


Total weight, battle ready 42 tons
Crew 5 (commander, driver, co-driver, gunner and loader)
Propulsion 640hp Wright G200 air-cooled radial engine
Speed (road) 35 mph (56 km/h)
Armament 75 mm M3 Gun,
.50 caliber MG HB M2 flexible AA mount on turret
.30 caliber MG M1919A4 coaxial w/75mm gun in turret
.30 caliber MG M1919A4 in bow mount
Armor 1.5 inches (38.1 mm) – 3 inches (76.2 mm) – 107.95mm

Links, Resources & Further Reading

Presidio Press, Sherman: A History of the American Medium Tank, R. P. Hunnicutt.

WW2 US Medium Tank Prototypes

AGF Improved Medium Tank

United States of America (1944)
Medium Tank – None Built

The period starting at the end of the Second World War and going into the new postwar world was a very busy time for US tank design despite the slashing of many programs and budgets from March 1945 onward. The scale of US experiments makes it hard to separate out one design from another as there was so much overlap between armor requirements, speed, and firepower. There was also the switch in the perceived enemy, no longer were tanks being conceptualized to fight the Germans or even the Japanese but instead to fight the armor of the Soviet Union. The enormous number of ideas, concepts, and proposals is simply overwhelming but one project stands out as an example of one of the less extreme ideas.
The ‘Improved Medium Tank’ may date back as far as September 1944 according to ‘Faint Praise’ by Charles Baily. This is based on the date given for the artist’s impression which is the only known image of what this vehicle would look like and which is used into 1945 for the project. The paperwork relating to the design is a little later though and the whole concept stems from the need to develop a new medium tank to replace the older obsolete M4 series. The replacement for the M4 was not a straightforward process though and the Army Ground Force (AGF) had failed to get the T25 Medium Tank into production and they weren’t satisfied with the suggestion from the Ordnance Department for the T20 series tank either. This meant that they did not have a replacement medium tank in development for the army (the T26 was still classed as a ‘heavy tank’ at this time). Clearly, this was an unacceptable situation and the result of this medium tank deficiency was that the AGF simply created their own requirement for a new medium tank instead. This new medium tank can, therefore, be called ‘The AGF Medium Tank’ and a look at the requirements gives a good impression that the AGF really had paid a lot of attention to the needs of combat and of tank crews. They also seem to have allowed some unrealistic expectations to creep into their thought process too as will be apparent in a review of the statistics for the vehicle.

Improved Medium Tank from artists impression 28th September 1944.
The AGF submitted the specification for their 45 ton medium tank on the 6th of December 1944. It was almost immediately rejected on the basis that the specifications were expected to result in a tank of 60 tons because of  “increased bulk requirements due to the demand for increased power, cruising range and larger diameter turret ring…” and was even called “amateurish” but it could not simply be dismissed without some kind of study being conducted. The Ordnance Department, busy with their own program, would not conduct a study of a rival product.
It was on the 2nd January 1945 that the Army Ground Force met in Washington D.C. to finalise their ideas. Their report was ready on the 20th June 1945, just after the war in Europe had ended. Among other vehicles, the report specified that the new medium tank had to be in the 45-ton class. Exactly what the differences between this concept and the initial one are is not entirely clear but some hints can be gleaned from the specifications.
The purpose of the AGF 45 ton Improved Medium Tank was simple; it was to be the “primary weapon for tank units, both in armored divisions and separate tank battalions”. The specified fully laden combat weight of this tank was listed as 90,000lbs, which is 40,823 kg, just over 40 metric tonnes or 45 US short tons. No dimensions are listed explicitly except width which was to be at most 134 inches (3.4 metres) due to transport requirements with height and width only to be the “minimum consistent with efficient construction and design”.
Protection was to be from rolled homogenous armor and “all armor should be as heavy as possible consistent with weight limitations, any reduction in armor thickness should be proportional” with a minimum of 6″ (152.4 mm) for the hull front although 8″ (203.2 mm) was desired. Just to illustrate, the minimum requirement was the same thickness as that of the frontal glacis of the Königstiger! The hull sides were to be 3″ (76.2 mm) thick on the sponsons around the crew compartment, down to just 1.5″ (38.1 mm) thick over the engine and the lower half of the hull sides over the crew compartment was to be just 2″ (50.8 mm) thick. The hull rear was also to be 1.5″ (38.1 mm) thick, with a 1″ (25.4 mm) thick floor under the crew compartment and just ½” (12.7 mm) thick under the engine. The hull roof was to be 2″ (50.8 mm) thick over the crew compartment down to ¾” (19 mm) thick over the rear of the hull.
The turret, which was to have a turret ring with a diameter of at most 80″ (2,032 mm), was to match the frontal protection with 6″ (or preferably 8”) of armor including the mantlet, 3″ (76.2 mm) thick at the sides, 2.5″ (63.5 mm) thick at the rear and a 1″ (25.4 mm) thick roof.


Surprisingly for a future medium tank, this vehicle was selected to have a main gun of not more than 3″ (76.2 mm) caliber stating that the gun must be “specifically designed in view of the space limitation in tanks.” Specific requirements for the gun were that it was to have the smallest possible chamber size, a compact recoil system with a short recoil stroke, and the shortest possible distance from the trunnion to the semi-automatic breech block as possible.
The single-piece ammunition for the gun was to achieve the specified performance of being able to defeat enemy armor up to 8″ (203.2mm) thick at an angle of 30 degrees at a range of 1000 yards (915 m). The firing though would have to cause as little smoke as possible as there was a problem with target obscuration after firing and it had to not cause excessive barrel wear. The dimensions for the ammunition were specified to not exceed 35 lbs in weight (15.9kg) or be longer than 30″ (762mm) inches in length with a total length of 33″ (832.8mm) being the absolute limit.
The penetration requirement of just over 200mm was certainly not impossible with the use of special ammunition like Armor Piercing Discarding Sabot (APDS). In terms of shell size, for comparison, the APDS shell for the British 17pdr gun was just an inch longer at 34″ and could achieve 200mm of penetration, just under the requirements made. Some development might have been needed but effectively the AGF was looking at a gun not that different from the British 17 pounder firing ‘special’ AP ammunition.
The coaxial weapon choices give a little insight into the designer’s mindset as it includes several machine guns and a flamethrower. First, there were to be 2 coaxial machine guns, one .50 caliber and one .30 caliber both of which could be fired independently or together with the main gun, which was also to have a separate electrical firing system installed. Two more .30 caliber machine guns were to be installed “symmetrically located outside the turret in ballistic blisters so mounted as to fire coaxially with the main weapon” and yet another .30 cal fitted in the hull fireable by the driver. Not content with this array of machine guns the AGF stated that “consideration to be given to the mounting of bow machine guns or guns in remote-controlled ballistic blisters” and the artist’s impression seems to indicate the presence of two machine gun blisters on the front of the machine. One final machine gun was a .50 calibre anti-aircraft weapon to be installed on the turret roof fireable remotely by the tank commander and capable of being directed at ground targets if needed, presumably because the other 6 machine guns weren’t sufficient.
At least 6 smoke projectors in an armored housing were to be fitted to the outside of the tank fireable from inside and able to go out to 200 yards (183 m). The final offensive weaponry was in the form of a swivel-type flamethrower on the roof of the bow gunner’s compartment provided with traverse within the limits of his visibility and with a flame burst of not less than 30 seconds up to 1 minute. For defense, it is specified that fragmentation grenades must be deployable by crew to a range of 10 yards (9.14 m) around the sides and rear of the tank without opening any hatches.
No less than 85 rounds of main gun ammunition were to be carried, along with at least 600 rounds for each .50 calibre machine gun, and 2500 rounds for each .30 calibre machine gun, 24 grenades, 6 smoke bombs, and at least 600 rounds of .45 calibre ammunition complete the logistical insight into ammunition supply.


The main gun was to be stabilized horizontally and vertically for firing on the move. It was also specified that the coaxial weapons had to be stabilized at least 90% of the time as well. A built-in 10x rangefinder linked to the gun was to be fitted along with a tank commander’s x4 or x6 sight with “provision… to lay the gunner on the target through the agency of the rangefinder”.

Another feature was that “the tank commander shall be provided with elevation and traverse controls that will follow the same response curves and shall override the gunner’s controls”. Finally, an auxiliary sight adjustable from x3 to x8 power and marked with various graduations to accommodate different ballistics of various ammunition types was used along with a 7 x 50 periscopic binocular sight on a rotatable plate for the tank commander. There was to be an elevation quadrant on the gun mount for the gunner, and all of this was supposed to be ‘rugged, simple, and accurate’ and unaffected by engine vibration.
The 360-degree rotatable turret would be fully balanced and fitted with both a power and a hand traverse with not more than 200 lb.ft. (271.16 N.m) of torque required to traverse it. Other issues for the turret were that it was to be sealed from weather and, like the hull, sealed from harmful gasses. Armored covering would cover the vision openings and the design shaped in such a way as to minimise ricochets from the turret into the roof of the tank. During travel, a sturdy travel lock would be used to secure the main gun.


The improved Medium Tank was to be fitted with power steering and independently sprung wheels with shock absorbers over which would run a center-guide type track with a rubber bogie surface and integral grousers. The track lifetime had to be at least 3,000 miles. It was specified that it also had to be suitable for climbing a 60% grade slope in damp clay.
The engine was to be a multifuel engine which would have to be developed specifically for it and capable of running on standard military fuel, oil and lubricants. It had to produce at least 20 horsepower per ton and cause minimal vibrations. Ease of maintenance was important along with a 5,000 miles (8,047 km) / 500 hours overhaul interval. The transmission would be either an “infinite ratio type” or fully automatic and if a clutch was to be used it had to have a long life and be free from dust. Whichever system was used, it had to have a high reverse speed.
Twenty horsepower per ton with a weight at 45 tons was implying an engine producing 900 hp and such an engine did not exist for them, the engine in the 46 ton M26 Pershing, for example, was just 500hp.
As if the other requirements for the engine were not unrealistic enough the tank was also to have enough fuel for a cruising range of at least 100 miles (161 km) and the expectation was for a top speed on the road of not less than 20 mph (32 kph) sustained.

Miscellaneous Notes

In terms of stowage, this new tank would carry not less than 3 days of crew rations, a minimum of 5 gallons (18.9 litres) of drinking water (preferably 10 gallons / 37.9 litres) along with standard pioneer tools, spare parts and a 12 unit first aid kit in an armored external box.
The radio would be fitted as specified by Army doctrine along with a fixed fire extinguisher system and a positive pressure ventilation system for the crew. In the event of a failure of this system a door could be opened from the crew compartment to the engine to draw in air through the hatches for the crew that way.
Easily openable hatches were to be wide enough for crew in winter uniforms, seats to be adjustable, non-skid surfaces were to be used inside along with no sharp corners or projections. All instruments had to be operable while wearing gloves, and an escape hatch was to be fitted in the hull floor. Other features would include a red/white interior light, illuminated instrument panels, and the maximum use of alloys to save weight.
Exterior lighting was to be exactly that of the M4 series and the tank was to be easily towable with a temperature operating range from -10F (-23.3 C) to +120F (48.9 C) and wind speed up to 40 mph. With winterization the vehicle would be suitable to operate down to -40F (-40 C).

Artist’s impression of the ‘Improved Medium Tank’ from AGF Report June 1945. The relevance of the square squares drawn on next to each machine gun is not known.


Looking at the specifications for the tank it is easy to see that a considerable amount of experience has gone into them. Crew comfort from seats to avoiding sharp corners and the use of gloves etc. all feature and yet still the AGF managed to add in a simply ridiculous amount of pointless items such as the superfluous machine guns scattered around the machine. The 3” gun specified might have been suitable for 1944 when it was first being considered but it was already going to be outdated by the time it could have entered production if it was approved.
Whatever the thinking was it was redundant. A new board under General Stilwell had convened in November 1945, known officially as the War Department Equipment Review Board (unofficially known as the Stilwell Board), to consider many of the same issues the AGF had. By the 19th January 1946, the Stilwell Board completed its own report and the idea for this tank was effectively dead. Any final hints of it completely disappeared by December 1950 when the requirements changed with the release of the Army Equipment Development Guide and the Army moved in a different direction with its medium tanks. The AGF’s Improved Medium Tank was consigned to history and mostly forgotten, the last grasp at a WW2 tank design.

AGF Improved Medium Tank specifications

Dimensions length not specified, width 3.4m, height – not specified
Total weight, battle ready 40,823kg (90,000lbs.) est.
Crew 5
Propulsion  20 horsepower per ton, ~900hp desired
Speed (road) Not less than 32 km/h (20 mph)
Range Not less than 161 km (100 mi)
Armament 3″ (76.2mm) gun capable of penetrating 8″ (203.2mm) at 30 deg. at 1000 yds. (914.4m) with at least 85 rounds
Coaxial .50 calibre machine gun with at least 600 rounds
Coaxial .30 calibre machine gun with at least 2500 rounds
Symmetrically mounted .30 calibre blister mounted external machine guns with at least 2500 rounds each.
One .50 calibre AA machine gun with at least 600 rounds.
At least one .30 calibre hull machine gun (possibly two in blisters) with at least 2500 rounds each.
Smoke projectors with 6 bombs.
24 fragmentation grenades.
At least 600 rounds of .45 calibre ammunition for crew weapons.
Swivel type flamethrower with 30 to 60 seconds of fuel.
Armor Hull front – 6″ (152.4mm) min, 8″ (203.2mm) desired
Hull sides upper – 3″ (76.2mm) on sponsons around fighting compartment
Hull side lower – at least 2″ (50.8mm) around fighting compartment
Hull sides behind FC – 1.5″ (38.1mm)
Hull rear 1.5″ (38.1mm)
Hull top front over FC – 2″ (50.8mm)
Hull top rear ¾” (19mm)
Hull bottom front under FC – 1″ (25.4mm)
Hull bottom rear ½” (12.7mm)
Turret front 6″ min., 8″ desired including mantlet
Turret sides 3″ (76.2mm)
Turret rear 2.5″ (63.4mm)
Turret top 1″(25.4mm)
Total production None
For information about abbreviations check the Lexical Index

Links, Resources & Further Reading

Patton – R.P. Hunnicutt
Firepower – R.P. Hunnicutt
Army Ground Forces Equipment Review – June 1945
Report of the Army Equipment Board – 1945
Faint Praise: American Tanks and Tank Destroyers during World War II – C.M. Baily

Impression of the AGF Improved Medium Tank by David Bocquelet
An impression of AGF’s ‘Improved Medium Tank’ by Tank Encyclopedia’s own David Bocquelet.

Has Own Video WW2 US Medium Tank Prototypes

Medium Tank T6 – The Birth of the Sherman

United States of America (1941)
Medium Tank – 1 Built

On the 2nd of September 1941, a single tank was completed and drove under its own power for the first time at Aberdeen Proving Ground (APG) in Maryland. It was the first of nearly 50,000 and the first U.S. designed tank to employ the now standard concept of a three man turret crew. Only 13 months later it would see its first action in the North African desert in the second battle of El Alamein with the British Army and would remain in frontline service for another 30 years in various forms with a variety of countries. The tank was the T6 and it would become the legendary Medium Tank M4 or as the British named it, the Sherman.

Design Development

On the 31st of August 1940, the United States Army Armored Force submitted detailed characteristics for a medium tank to replace the Medium Tank M3, even though the design of the M3 hadn’t been finalised and was not expected to be in production before the following summer. The M3 was a stop gap design and its shortcomings were already apparent. The most obvious shortcoming of the M3 was the location of the main 75mm gun in a limited traverse hull mounting, so the highest priority was given to designing a suitable fully rotating turret for 75mm main gun. The Armored Force also listed in its requirements a lowering of the overall height of the tank in comparison to the M3 and a provision for anti-aircraft protection.
As the design process of the M3 was fully occupying the design team at Aberdeen, the new tank had to wait until the M3 design was completed. On the 1st of February 1941, the Chief of Ordnance released a directive to proceed immediately with the design of the M3 replacement.
At a conference at APG on the 18th of April 1941, the major features were confirmed for the new design. The basic chassis of the M3 was to be retained including the lower hull, engine, gearbox and final drive, suspension and tracks, most of which had already been carried over from the previous Medium Tank M2. Two main reasons for the M2/M3 carryovers were an ease of transition on the production lines from the M3 to the new tank and because the M2 chassis dimensions were already designed with mass transportation in mind. The original M2 was built to fit a standard gauge railway flatcar. The U.S. was under no illusion that it would be fighting a major war in the continental United States, so priority was given for a mass produced vehicle able to be shipped across oceans with as little alteration to existing transportation infrastructure as possible.
Although the design was based around the continued use of the Wright R975 radial engine, the engine bay was designed with enough space to accept larger engines in the future as they became available.

The new upper hull was to be either cast or welded and was to use as many existing components from the M3 design as possible. The turret ring was increased to 69” (1752.6mm) and the armor thickness was to be a maximum of 3” (76.2mm) on the glacis. The twin fixed .30 caliber machine guns (operated by the driver) were to be retained from the M3 design as well as a new bow mounted .30 caliber machine gun (operated by the co-driver, also known as the bow gunner or BoG) on the right hand side of the glacis.

Two escape hatches were to be placed (one on either side) of the hull sponson plates to allow the crew the ability to bail out of a damaged vehicle on the leeward side of any incoming fire.
The turret was to have a removable plate to allow the fitting of a selection of armament combinations. Five possibilities were considered:
(1) One 75mm (2.95”) gun M2 with a .30” (7.62mm) coaxial machine gun.
(2) Two 37mm (1.45”) guns M6 with a .30” (7.62mm) coaxial machine gun.
(3) One 105mm (4.13”) howitzer with a .30” (7.62mm) coaxial machine gun.
(4) Three .50” (12.7mm) machine guns mounted for high angle anti-aircraft fire.
(5) One British Qf 6pdr (57mm) high velocity gun with a .30” (7.62mm) coaxial machine gun.
Although not listed in available reference material, one other combination was obviously considered at some point as a picture depicts the wooden mock up with a different style turret with a 75mm M3 main gun and a 37mm M6 coaxially mounted. The commander’s cupola from the M3 with its high angle .30 caliber anti-aircraft machine gun was to be retained. It was hoped that this design would allow a single vehicle design to be equipped to meet a number of tactical missions.
The crew was to be reduced to five with a driver and co-driver in the front of the lower hull and three man turret crew in the arrangement that would become the standard for all future U.S. tank designs. The gunner was to be on the right of the gun at the front of the turret with the vehicle commander behind and slightly above him under the cupola. The loader was on the left of the main gun with the .30 caliber coaxial machine gun to his front. The turret crew would share a single roof hatch and the two hull crew would share a single roof hatch on the left side of the hull and a floor escape hatch just behind the co-driver’s seat (these were obviously augmented by the two sponson escape doors).
The T6 full size wooden mockup, to aid speed the lower hull and running gear was reused from the wooden mock up of the M3.
The T6 full size wooden mockup, to aid speed the lower hull and running gear was reused from the wooden mock up of the M3.
In May 1941 the Ordnance Committee recommended a full size wooden mockup and a pilot tank to be built. This was approved in June 1941 and the new design was designated Medium Tank T6. Once the wooden mock up was completed, Aberdeen was instructed to build a pilot model using a cast upper hull. The casting of both hull and turret was to be done by the General Steel Castings Corporation of Granite City, Illinois. At the same time the Rock Island Arsenal was instructed to build a welded hull version but this was delayed because of modifications to the design to reduce the number of plates and the length of the welded joints.

The T6 Pilot

The pilot T6 was completed at Aberdeen on the 2nd of September 1941, and was inspected by representatives of both the Armored Force and the Ordnance Department. The pilot used a donor lower hull, suspension and tracks from an M3 to aid speed to the project.
The T6 pilot on presentation day
The T6 pilot on presentation day, original features that were soon removed include 1) the M3 commander’s cupola 2) side grab handles and 3) the circular armored cover for the antenna base. Picture credit: sherman_minutia
As built, the T6 had side doors but without the vision devices as seen in similar doors on the M3. The bow mounted .30 caliber was linked to a sight rotor in the upper hull casting and both hull crew positions were equipped with direct vision slots with hinged armored covers. The driver also had a periscope mounted in the hatch above his head and his seat was height adjustable allowing him to drive with his head out of the hatch when open, or lower down with the hatch closed using either the periscope of direct vision slot for observation. The twin .30 caliber machine guns were operated by the driver with a limited elevation.
A pistol port with a protectoscope (an armored glass vision port) was located on each side of the turret, and the gunner’s sight rotor was linked in the same way as the bow machine gun. This linkage only moved the top mirror in the periscope which if damaged could be replaced from within the tank by way of a mirror magazine that was integral to the sight.
The gun mount (T48) was designed for the longer barreled M3 75mm (2.95”) gun (L40) but as none were available the shorter M2 75mm (L31) gun was installed. This was breech heavy in this mount and prevented the gyrostabilizer from working correctly. Counterbalance blocks on the muzzle were required to remedy this and were installed shortly after presentation day. The gyrostabilizer was essentially the same as the unit used on the M6 37mm (1.45”) in the M3 Medium Tank.
The T6 was fitted with two radios, one in the front right of the hull, operated by the co-driver (an SCR 506) and one in the turret bustle for use of the commander (an SCR 508). The radio brackets in the turret were also designed to take the British No. 9 radio set.
The co-driver’s position showing twin bow machine guns and the linkage bar to the rotor sight with the SCR506 radio behind.
The co-driver’s position showing twin bow machine guns and the linkage bar to the rotor sight with the SCR506 radio behind.
At a conference on 3rd September, a number of key changes to the T6 were agreed for any further production models, these included;
(1) Removal of the M3 style commander’s cupola to be replaced with a commander’s split hatch that would become standard on the M4.
(2) Removal of the hull sponson doors, as they were considered to be too great a compromise of the armor integrity and also restricted the amount of main gun ammunition that could be stowed in the sponson racks.
(3) A rotating periscope would be retrofitted in the turret roof above the loader’s position.
(4) The rotor mount bow machine gun was to be replaced with a ball mount.
(5) If possible a .50 caliber (12.7mm) anti-aircraft machine gun be installed.
(6) A gun shield should be added to protect the gun mount from splash damage.
The conference confirmed that the T6 would be standardized as the Medium Tank M4.
The M3 style cupola was removed very soon afterwards as it only appears on pictures taken on the 15th of September and is gone in photos from the 16th of September.
Further changes include the addition of an armored cover for the M3 air intake on the engine deck. A change in the casting of the rear of the hull and the replacement of the pepperpot exhaust, which were ill suited to the task and caused problems of serious overheating of the rear decks of M3s. These changes required an adjustment in the tool stowage positions.
The rear deck showing the unarmored air intake inherited from the M3.
The rear deck showing the unarmored air intake inherited from the M3.
The co-driver also received a roof hatch with the removal of the rotor sight. The hinges of the hull hatches were moved forward from their original position at the rear of the hatch. The circular armor protection for the antenna mount above the SRC 506 radio was replaced with a hull ventilator and another ventilator was added just behind the turret and another fitted in the turret roof.
The welded hull pilot constructed by the Rock Island Arsenal has become somewhat of a mystery, no photos have (to date) been found of the completed pilot and only one picture seems to exist of a scale model of the vehicle. Interestingly, it includes the co-drivers hatch and does not have the side sponson doors and also included the later M34 gun shield.

It should be noted that the T6 was not the only horse in the race to replace the Medium Tank M3. In June 1941, three months before the T6 was completed, the Canadian built pilot of the Cruiser Tank Ram was completed and arrived for testing at APG in August. Before the first M4 had rolled off the production lines, 110 Rams had been built by the end of February 1942.
The Ram used the entire lower hull and running gear of the M3 with little or no changes. It utilized a smaller turret ring than the T6, only 60” (1524 mm) as opposed to the 69” (1752.6 mm) in the T6, which left the turret described as “cramped” and unsuitable for anything larger than the QF 6pdr fitted. As such, the Ram was considered unsuitable for combat by both the Americans and the British after trials at APG, but it served successfully as a training vehicle and as a base for a number of specialist vehicles.

T6 Medium prototype by David Bocquelet


On the 11th of December 1941, the welded hull version was designated the Medium Tank M4 and the cast hull the Medium Tank M4A1. Construction of the initial production pilots commenced in November 1941 and full production of the Medium Tank M4 began in February 1942. The last known reference to the T6 was in February 1947 in a picture showing it fitted with a potential field modification for additional armor protection for the differential housing. It is likely that the T6 was scrapped during the Korean War era “scrap drives,” but there remains a small hope that it survived and is still waiting to be rediscovered in some forgotten corner of APG.
The Canadian built Ram pilot undergoing automotive trials at APG.
The Canadian built Ram pilot undergoing automotive trials at APG.


Sherman: A History of the American Medium Tank, R.P. Hunnicutt
The T6 Medium Tank on The Shadock
Armored Thunderbolt, Steven Zaloga
AFV/G2 T6 Mockups, Chris Benedict

Medium Tank T6 specifications

Dimensions 5.6 x 2.6 x 2.9 m
18’4” x 8’6” x 9’6”
Total weight, battle ready 27.2 tonnes (60,058 lbs)
Crew 5 (Commander, Driver, Co-Driver, Gunner and Loader)
Propulsion Wright (Continental) R975 EC2, 9 cylinder radial
Speed (road) 38.6 km/h (24 mph)
Range 193 km (120 miles)
Armament 75 mm Gun M2 with 75 rounds
5 x .30 MG M1919A4 machine-guns with 10,000 rounds
Armor Maximum 76.2 mm (3”)
For information about abbreviations check the Lexical Index

This picture dated 7th April 1941 (although this date cannot be confirmed and May would be more likely) of the wooden mock up showing the little known 75mm and 37mm combination and a wider turret than the final design. The cut out in the upper right front of turret is possible a gunner’s sighting periscope. Although the twin bow machine gun apertures are present there is no flexible bow machine gun and no obvious vision device in the front right hull it would seem at this stage no co-driver was planned. The driver’s hatch appears to roll backwards unlike the later hinged hatches.

In this right side view of the early version T6 the right hand side sponson door is missing further adding to the argument for no co-driver.

The last known picture of the T6 taken on the 18th February 1947 with a prototype field expedient armor upgrade for the differential cover.

The new turret design with the pistol port and Protectoscope vision device.

The only known picture of the Rock Island Arsenal welded hull pilot (albeit a scale model) showing some of the design changes from the original cast hull pilot.

A practical demonstration of the floor escape hatch located behind the co-driver’s seat.

Rear view shower the pepperpot exhausts (1) that would soon be discontinued due to overheating of the rear deck area and the original straight edge casting that would be changed on the production models.

This picture clearly shows the reason for the removal of the forward hull mounted antenna which could easily be entangled with the main gun when traversing the turret.

Tank-It Shirt

“Tank-It” Shirt

Chill with this cool Sherman shirt. A portion of the proceeds from this purchase will support Tank Encyclopedia, a military history research project. Buy this T-Shirt on Gunji Graphics!

American M4 Sherman Tank – Tank Encyclopedia Support Shirt

American M4 Sherman Tank – Tank Encyclopedia Support Shirt

Give ’em a pounding with your Sherman coming through! A portion of the proceeds from this purchase will support Tank Encyclopedia, a military history research project. Buy this T-Shirt on Gunji Graphics!