On March 25th, 1931, the US Wheel Track Layer Corporation and Bureau of Ordnance signed a contract to build five Christie M.1931 tanks, later expanded to seven. American infantry received three Convertible Medium Tanks T3 and cavalry received four Combat Cars T1. It seemed that the long struggle between Christie and the American military finally ended with Christie's victory, and a large order will follow the first batch of tanks any day now. However, history took a different path, and the Christie suspension was a dead end for the American tank building school. However, the evolution of American convertible drive designs led to several interesting vehicles, one of which was the Convertible Medium Tank M1, which was standardized for service.
The Fire Brigade at Work
In order to understand one issue that plagued Christie, one must travel back to 1924, when a serious conflict between the inventor and the American military took place. It's well known that none of the many designs developed by the Front Wheel Drive Motor Corporation from 1917 to 1924 were accepted into service. Frequently, the issue wasn't with Christie, but with the sluggish bureaucratic machine, where the left hand didn't know what the right was doing. The military often didn't know what it wanted. For example, the 155 mm SPG was redesigned three times and still rejected in the end.
Experimental prototypes built during this period cost the treasury $175,000, a sizeable sum at the time. Christie took the blame. In 1924, the Bureau of Ordnance broke off its relationship with the Front Wheel Drive Motor Corporation, and General Williams cursed Christie. Christie responded by calling his payment «blood money», but didn't stop there.
The packet of patents purchased from Christie in 1920 cost the military $100,000. It included rights to all currently designed types of vehicles and their components. In addition to the patents, his contract gave the military rights to future inventions, a clause which cost Christie dearly.
The use of the Convertible Medium Tank T3 and Combat Car T1 in the military revealed a number of drawbacks. Many of these drawbacks had nothing to do with Christie. For example, the one man turret installed on the tanks differed little from the turrets of the Light Tank T1 family. The fact that the commander couldn't be responsible for everything at once and that the turret had to fit two people was fair, but Harry Knox's tank had the same problems. The same can be said for the driver's compartment, which was designed for one man in Christie's tank. The idea to make it fit two people only came about in the spring of 1932, when the Light Tank T1E4 began trials, which used solutions borrowed from the Vickers Mk.E. In other words, the tank wasn't bad, but requirements changed.
Another problem with Christie was that he had a very stubborn character and often became obsessed with new ideas. For example, in 1932, he became obsessed with the idea of a flying tank, which resulted in the Christie M1932 Airborne Tank. It was the fastest tank in the world, and it was praised in the press and shown off to Congress. The military didn't need a new tank with an uncertain future, it needed improvements of already existing designs.
In June of 1932, the Bureau of Ordnance recommended the preparation of documentation for an improved version of the Medium Tank T3. At the same time, the head of the US Wheel Track Layer Corporation was informed that $200,000 will be allotted in the next fiscal year for the production of the improved Medium Tank T3E2. Christie tried to insist that only the US Wheel Track Layer Corporation has the rights to build the Medium Tank T3E2, but an unpleasant surprise lay in store for him, one that he himself prepared in the past. Turns out that the Bureau of Ordnance could legally use his inventions as it pleased.
On October 14th, 1932, the military finished working on specifications for the Convertible Medium Tank T3E2. The request for tender was sent to multiple companies, including the US Wheel Track Layer Corporation. The results were predictable: by November 28th it was clear that Christie would not receive the contract. Instead, the contract for five tanks and the sum of $200,000 went to American LaFrance.
Ironically, the contract went to the company that the Front Wheel Drive Motor Corporation, Christie's previous company, competed with on the fire truck market. Back then, in the mid 1910s, American LaFrance won, and now history repeated itself. The loss of this contract spelled bankruptcy for the US Wheel Track Layer Corporation. The sale of the M1932 to the USSR only postponed the end while aligning the US military against Christie.
Meanwhile, the tender didn't mean that the infantry would get tanks that were superior to Christie's. For American LaFrance, the victory was accidental. Founded in 1832, American LaFrance Fire Engine Company specialized in fire engines from the beginning. By November of 1932, the company has not produced a single armoured vehicle. It's not surprising that American LaFrance worked closely with Army organizations, especially the Watertown Arsenal, while working on the Convertible Medium Tank T3E2. The development of the tank dragged on, and the prototype was only completed in late 1933.
Instead of modernization of Christie's tank, his competitors ended up with a completely new one. The differences began in the front, which, according to the military's demands, was widened to fit two people. The assistant driver, sitting to the right, received a Browning M1919A4 machinegun. The turret also fit two people and received an elongated shape. It was noticeably larger, which was a bonus for the crew. The tank kept the armament of the Convertible Medium Tank T3, but for some reason, three machineguns were added to the sides of the turret and the rear hatch. American LaFrance engineers retained the commander's cupola, but it was now placed in the center of the turret and was not especially comfortable to use.
Even though American LaFrance had experience with 12 cylinder truck engines, it picked the Curtiss D-12 435 hp aircraft engine for the tank. Like the Liberty L-12, this engine was widely used, and no issues were foreseen. According to the military's request, access to the engine and transmission was improved, although not by much. The transmission could be serviced through the removable shutters in the rear.
As for the hatches in the front of the tank, they were rather unusual. The driver's two-piece hatch repeated the shape of the predecessor, but it was only meant to improve visibility on the march. The driver entered and exited the tank through another hatch in the hull roof.
The suspension was also radically changed. The idler started to resemble a shrunken road wheel. The drive sprocket changed the most. The tank now had a more conventional «star». The track links also changed. American LaFrance engineers discarded the idea of a Galls chain and returned to the same gearbox as the M.1931 had.
Thanks to the more powerful engine, the Convertible Medium Tank T3E2 was faster than its predecessor. Its maximum speed was 56 kph on tracks and 93 kph on wheels. However, these numbers were obtained on the proving grounds. In practice, they were not as fantastic. During army trials, the American LaFrance tank was significantly less agile.
All five tanks ended up in the 67th Infantry Regiment (Medium Tanks), along with Convertible Medium Tanks T3. The designs ended up very raw, and around 60 changes were made to them during the duration of their service. After modernization, these tanks received a new index: Convertible Medium Tank T3E3. The military turned down further work with American LaFrance.
American cavalry declined Christie's services before the infantry did, back in February of 1932. The reasons for ending the partnership with the US Wheel Track Layer Corporation was simple: money. The cost of the Combat Car T1 was so high that it was impossible to purchase the necessary amount of tanks. The cavalry first attempted to make an alternative for Christie's tanks in 1931.
The result was the Combat Car T5, later renamed to Combat Car T2. It was developed by the Bureau of Ordnance along with the Rock Island Arsenal. Harry Knox became the author of this curious design. Often, the Combat Car T2 is passed off as an attempt to use Christie's patents, but in reality it was a self-made design, which the cavalry quickly rejected, realizing that it has no future.
A conference held in February of 1932 formed the requirements for an improved cavalry tank. The limit on its mass was 9 short tons (8163 kg), maximum speed on wheels had to be 64 kph, and maximum speed on tracks 35.2 kph. The crew, just like withe the Convertible Medium Tank T3E2, was increased to 4 men. Two machineguns (12.7 and 7.62 mm) would be used in the turret, and another one in the hull. A heated debate erupted over the mass, as a result of which it was lowered to 8.5 short tons (7710 kg). The maximum speed on tracks was increased to 48 kph.
The design, which received the name Combat Car T4, had a lot in common with the Convertible Medium Tank T3E2, especially when it came to the driver's compartment. Overall, the design was different, and had little in common with the initial Christie tank.
Unlike the Combat Car T2, Knox has nothing to do with this tank. An unexpected counterpart turned up: Major Gladeon Marcus Barnes, at the time the chief of the Aberdeen proving grounds. Barnes' specialization was artillery. but the talented engineer turned towards armoured vehicles in the early 1930s. Taking Christie's suspension as the basis, he eliminated one of its greatest drawbacks: the tall springs. He placed them at an angle, also changing the swing arms, which allowed the hull to be much lower.
The suspension wasn't the only change. While the predecessor to the Convertible Medium Tank T3E2 was obvious, the Combat Car T4 that arrived at the proving grounds in August of 1933 was completely different. With a similar width, it was half a meter shorter and 30 cm lower. The mass was within the required 8.5 tons, but the armour was only 9.5 mm thick around the perimeter. Thanks to the reduction of space required by the suspension, the upper front plate of the hull was sloped.
Instead of a large aircraft engine, the tank received an 11 L Continental R670 air cooled engine, which could produce 264 hp of power while being much lighter and more compact than the Liberty L-12. The transmission was included in one compact assembly with the engine. Thanks to the smaller dimensions, the engine compartment roof could be sloped, which helped reduce mass.
The running gear was also nothing like its predecessor. The road wheels, idlers, and drive sprockets were designed anew, the tracks were new as well. The track holders were also very original: half of the track was held on the fenders, the other half underneath, with the ends resting on the drive wheel and the idler.
Trials at the Aberdeen Proving Grounds showed that the characteristics of Barnes' tank were even better than required. The maximum speed was 80 kph: the cavalry tank was even faster than the Combat Car T1. It's not surprising that the Ordnance Committee approved the design on June 7th, 1934.
At the same time, there were several issues. The 1118 mm turret ring diameter was too narrow. The committee ordered that it should be increased to 1420 mm. The tank that began trials in late August of 1934 had a new horseshoe shaped turret. The idea of a coaxial machinegun was discarded, and the tank had two separate machineguns. The mass of the tank grew to 9 tons, but its characteristics remained at the previous level. The converted vehicle was later indexed Combat Car T4E1.
It seemed that the tank would make it into mass production. The cavalry insisted that Combat Cars should be used to equip mechanized units. In reality, the Combat Car T4E1 remained experimental. At the time, the Combat Car T5 was undergoing trials, a design very similar to that of the Light Tank T2, designed by Harry Knox. Even without the lobbyists in the Bureau of Ordnance, the Combat Car T5 had two significant advantages. At 7.5 tons, the mass was within revised Bureau of Ordnance requirements. Knox's tank also cost half as much as the Combat Car T4E1. For the American economy, which was just climbing out of the Great Depression, this was an important factor.
In addition, the Combat Car T5 could reach a speed of 72 kph on tracks, almost as fast as its competitor was on wheels. Its armament was the same, and the armour was a little bit thicker. It's not surprising that the military did not choose the convertible drive tank.
This was not the end of the Combat Car T4E1. In the winter of 1935, the vehicle was converted into the Combat Car T4E2. The turret was replaced with a large casemate. Three machineguns were added to the existing three: one per side and one in the rear. The mass of the tank grew to 13 tons.
Even though the Combat Car T4 was a dead end, it left its mark on the American tank school. Its further evolution went along the lines of the infantry tank. In April of 1934, the Combat Car T4 was demonstrated to the infantry command, which was satisfied with the design. Around this time, the trials of the Convertible Medium Tank T3E2 began, proving that the design was unpolished and lacked a future. It was decided that an infantry tank on the Combat Car T4 chassis with improved armour would be designed.
This tank entered trials in 1935. Its shape was reminiscent of the Combat Car T4E1, but it had many differences. The driver's hatch was a different, greatly improved, design. The sides of the upper front of the hull and forward observation ports were different. It had no additional machineguns or toolboxes on the fenders. The new two-man turret was a distant relative of the turret from the Combat Car T4E1. Its main difference was the characteristic «step» on top which served as a commander's cupola.
The armament was installed differently. While the cavalry tank had its Browning M2HB on the left, the infantry tank had it on the right. Protective shields which served as gun mantlets were added.
The suspension differed from that of the Combat Car T4E1. All wheels, including idlers and drive wheels, were designed anew. In wheel mode, the tank was propelled by a Galls chain. Since the mass of the tank grew to 12 tons, the engine was supercharged to 268 hp. The new tank also received mufflers on the roof of the engine compartment.
The tank was indexed Convertible Medium Tank T4. Despite the improved engine, its characteristics were more humble than those of the cavalry tank: 61 kph on wheels and 38 kph on tracks. The tank became wider and taller, and its armour was thickened to 16 mm. Nevertheless, the tank proved reliable, which was enough for mass production. American infantry received 16 Convertible Medium Tanks T4 in 1935-36.
In addition, the army received three Convertible Medium Tanks T4E1. These vehicles were the infantry equivalent of the cavalry's Combat Car T4E2. As with that tank, the Convertible Medium Tank T4E1 received a roomy casemate with four 7.62 mm Browning M1919A4 machineguns around the perimeter. One Browning M2HB 12.7 mm machinegun was mounted in the front.
The Convertible Medium Tank T4 family was the largest among American convertible drive vehicles. Despite the weak engine, they had decent mobility. Like other convertible drive tanks ordered by the infantry, they were stationed at Fort Benning.
The first attempt to standardize these tanks was made in February of 1936, without success. The second attempt was on March 30th, 1939, and this time it was successful. The tanks received the index Convertible Medium Tank M1, Limited Standard. The Medium Tank T4 became the only convertible drive tank that was officially accepted into service. However, that was only its swan song. A year later, the tanks were written off as completely obsolete. By that point, tanks on the Light Tank T2 chassis were the standard in the American military.
One Convertible Medium Tank T4 survives to this day. Today, it can be seen at the National Armor and Cavalry Museum, Fort Benning.
Translated by Peter Samsonov. Read more interesting tank articles on his blog Tank Archives.
- T.3 Christie: Armour in Profile #4, Peter Chamberlain, Steveson Pugh, Once Read Books, 1967
- Sherman: A History of the American Medium Tank, R.P. Hunnicutt, Presidio, 1978
- Stuart: A History of the American Light Tank Volume I, R.P. Hunnicutt, Presidio, 1992
- A Yankee Inventor and the Military Establishment: The Christie Tank Controversy, George F. Hofmann, Military Affairs, Vol. 39, No. 1 February 1975
- Author's photo archive