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The Medium Tank T4/ Medium Tank M1 was the last medium tank to use Christie style convertible running gear, and was based on the Combat Car T4, itself developed from the Christie M1931/ Medium Tank T3.
Four of the Christie-built M1931/ Medium Tank T3s had been given to the Cavalry, where they were known as the Combat Car T1. In 1933 the Rock Island Arsenal then produced a modified version of the design as the Combat Car T4. This was about one ton lighter, at 9.6 tons, used the Christie suspension system, was entirely armed with machine guns and had sloped frontal armour.
T4
The Medium Tank T4 was also built at the Rock Island Arsenal, and was partly based on the Combat Car T4. It was the same length as the Combat Car, but wider and slightly higher. It used the same Christie suspension, with four large road wheels on each side, attached to pivoting arms that were supported on coil springs. The system could be used with or without tracks, and the T4 could reach 35-37mph on wheels and 20-23mph on tracks. The biggest technical improvement on the T4 was the use of controlled differential steering, which replaced the clutch-brake system used on earlier tanks. The clutch-brake system worked OK at slower speeds, but wasn't suitable at the sort of speeds achieved by the Christie tanks.
The T4 carried a crew of four, with the driver and a bow machine gunner in the front, behind a sloping front plate (similar to that on the Combat Car T4, but also with a clear family resemblance to the later M4 Sherman. The T4 was a medium tank in weight only, and the turret only carried one 0.50in and one 0.30in machine guns, each in a separate mount. Armour was also quite thin, ranging from a maximum of 5/8in down to 1/4in in places. The turret was rather ungainly in shape, with a circular rear half, and a step down to the two machine gun mountings.
Sixteen T4s were built at the Rock Island Arsenal in 1935-36. They were tested at the Aberdeen Proving Ground, and found to be underpowered.
T4E1
The T4 was followed by three very different T4E1s. These had the turret removed and a new barbette structure installed on the upper hull, extending out over the top of the tracks. The 0.50in and 0.30in machine guns from the turret were mounted in the front of the barbette, and the machine gun in the hull front was also retained. Another three machine guns were added, one in each side of the barbette and one in the rear, for a total of five 0.30in guns and one 0.50in guns,
On 6 February 1936 the Ordnance department recommended standardizing the T4 and T4E1 as the Medium Tank M1, but the Office of the Adjutant General rejected this on the grounds that the T4 was no better than the Light Tank M2, and twice as expensive. The recommendation was repeated on 30 March 1939, and this time, with war in Europe looking ever more likely, the T4 and T4E1 were accepted as the Convertible Medium Tank M1, Limited Standard. Eighteen of the nineteen tanks produced remained in use at Fort Benning until they were declared obsolete in March 1940.
Stats (T4)
Production: 16
Hull Length: 16ft 1in
Hull Width: 8ft 2in
Height: 7ft 3in
Crew: 4
Weight: 13.5 tons
Engine: Continental 7-cylinder 268hp
Max Speed: 35mph wheels, 15-20mph tracks
Max Range:
Armament: Three machine guns
Armour: 1/2 to 5/8in
Stats (T4E1)
Production:
Hull Length: 16ft 1in
Hull Width: 8ft 2in
Height: 7ft 4in
Crew: 4
Weight: 15 tons
Engine: Continental 7-cylinder 268hp
Max Speed: 40mph wheels, 25mph tracks
Max Range:
Armament: Six machine guns
Armour: 1/2 to 5/8in