Swedish tanks Revisited: Strv m/31 & fm/31

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Author: SP15

Welcome back everybody!

It been over three years ago since I started making articles on Swedish AFV’s, in that time a lot of new information has come to light which has made some of my earlier articles obsolete. In this series i will be taking a look back at some of those articles starting with Swedish tanks part II: the history of the Strv m/31 & fm/31.

Background

During the 1920’s the Swedish military started looking into options for the replacement of the Strv fm/21. The fm/21 was originally a German ww1 tank design known as the Lk II, 10 of these tanks had been imported in 1921 under great secrecy as Germany was not allowed tanks under the Versailles treaty. Its been said that they had initially been acquired for the protection of the royal family in case of a revolution, but they soon became a way for the Swedish army to experiment with tactics for armored vehicles. By the time of the mid 1920’s however the Strv fm/21 was already showing its age and due to a lack of spare parts several vehicles had to be cannibalized to keep the others running.

 

Strv fm/21

In 1927 Bertil Burén (commander of the Swedish tank battalion) and company commander Curt Klingspor traveled Europe in search for a suitable replacement for the Strv fm/21. They studied tanks like the Italian Fiat 3000 and the Czech KH-50 but these tanks were either deemed unsuitable for Swedish conditions or were simply unavailable for purchase. These attempts led to the acquisition of a Renault NC 27 in 1928. The NC 27 was a development of the FT-17 which featured an improved suspension and transmission. Under its Swedish designation the NC became known as tank test model 1928 (Strv fm/28). Already during the first round of testing however the Strv fm/28 proved to be both unreliable and unsuitable for Swedish terrain.

 

Strv fm/28

A Swedish alternative

Unable to find a foreign design it was decided that a new tank would be built within the country, and by December 1928 the general staff had agreed on a set of requirements. The requirements called for a tank with a maximum weight of 12 tons with armor protection against 37mm guns. The tank had to be capable of a sustained speed of at least 10 kph in moderate terrain and 20 kph on roads. The armament was to consist of a 37mm gun supplemented by a machinegun. In total Three companies would respond to the requirements, the first of which was Morgårdshammar.

Morgårdshammar

Morgårdshammar was an engineering company which specialized in the building of heavy equipment and machines for the Swedish mining industry. They had actually approached the Swedish military about the possibility of producing tanks long before any requirements were even under consideration. Morgårdshammar didn’t have any direct experience with tanks but they had been in correspondence with Austrian Major and author of Taschenbuch Der Tanks, Dr Fritz Heigl. Heigl was probably the closest you could get to a tank expert at the time, he had compiled a huge amount of information, technical drawings and pictures of tanks from all over the world, and he had even designed an armored car for Skoda.

Morgårdshammar proposal A

Based on their collaboration with Heigl and with some input from Bertil Burén Morgårdshammar submitted two designs in 1929. Proposal A was a conventional design with a crew of 4 and a weight of around 10,5 tons. Its most distinguishing feature was the addition of a mini turret for the commander, Interestingly the tank also made use of a rear driver.

Morgårdshammar proposal B

Due to the unreliable nature of track and suspension design tanks of this period generally suffered from low speeds and poor tactical mobility. To counteract this designers came up with alternative methods of travel, Morgårdshammar’s Proposal B was an example of one such solution. Unlike other designs of its kind however the Proposal B was designed to use railways instead of roads. Though an innovative design Its unclear if this proposal was ever seriously considered.

Morgårdshammar A-4-C

By may of 1930 the Proposal A drawings had been finalized and the design re-named Morgårdshammar A-4-C. The main difference between this version and the earlier proposal was a change in gun caliber from 37 mm to 47 mm and the addition of a Swedish engine.

Aware of the limited funds that had been granted for the purchase of a new tank the company made an offer to construct a soft steel prototype without any guns or optics for just 60 000 Kr, the construction of which was estimated to take 15 months to complete. To put this in to perspective this was 8 times less than what Bofors had asked for when questioned about the possibility of designing a tank. Unfortunately for Morgårdshammar they were quickly running out of time, before the end of the year Fritz Heigl would pass away from cancer leaving them without their main designer.

Heigls death and the fact that Morgårdshammar lacked experience of building tanks would lead to the rejection of their offer by the Swedish military.

Bofors

Bofors had long since been established as Sweden’s leading arms manufacturer. Their history dated all the way back to the 1600’s and by 1930 they had been producing cannons and heavy artillery for nearly 50 years. In the period following ww1 Bofors had secretly been collaborating with the German Kruppworks on the development of several projects that could not be pursued in Germany due to the restrictions set by the Versailles treaty.

Leichttraktor

Thanks to their ties to Krupp, Bofors had been able to offer an already completed design that had been in development by Krupp in collaboration with the Reichswehr. Due to the Secrecy of this project the tank had been named Leicttraktor (light tractor) by the Germans, though in Swedish documentation it was referred to as “m/Bofors”. The tank was demonstrated to a Swedish military attaché in Germany in late January 1930.

The 7,4 ton Leichttraktor was considered to be a fairly modern design for its time. The good mobility in light terrain and on roads was seen as especially impressive by the Swedish military. The tank was also praised for the design of its interior, though the drivers position was criticized for a lack of visibility. The major flaw with the Leichttraktor was the design of its suspension. Due to the way the track was tensioned the tank had a had a tendency to throw the track if the drive wheel was obstructed. The track’s construction was deemed too expensive and did not allow for the mounting of grousers which made it ill suited to Swedish winter conditions. In harsh terrain the tank’s large turning radius also became a problem, and during one of the demonstrations the tank ended up throwing its track which also damaged parts of the suspension.

Landsverk

Established in 1864 Landsverk much like Morgårdshammar was a major Swedish engineering company. Landsverk specialized in the construction of railway equipment but also had experience with production of everything from steam rollers to agricultural equipment. Due to a period of financial instability in 1920 a majority share of landsverk’s stock had been sold to the German based GHH company. With an injection of funds from GHH Landsverk would slowly start to recover by investing into new projects, including the establishment of a military vehicle department in 1928.

Räder-Raupen Kampfwagen M28 GFK

By the time the Swedish army had decided on the development of a new tank Landsverk had already begun work on improving the Räder-Raupen Kampfwagen M28 GFK design by German engineer (and future head of Landsverk’s AFV department) Otto Merker. The chassis of the M28 GFK had been examined by the Swedish army attaché during their trip to Germany in 1930. The attaché considered the M28 GFK to be a superior design to the Leicttraktor mainly due to its more robust suspension, track design and tighter turning radius. The further developed version of this design also promised better armor protection than Bofors offering.

Landsverk B.T 150 II

 

Development

In October 1931 the Swedish army placed an order for 3 conventional and one wheel-cum-track type tank from Landsverk. Internally these tanks would be referred to as L-10 and L-30 but they would be designated as Strv m/31 and Strv fm/31 officially. For the most part the L-10 and L-30 were designed to share the same layout and major components but with different suspensions.

Landsverk L-30

The L-30 improved on the wheel-cum-track suspension of the Räder-Raupen Kampfwagen M28 GFK and featured a slightly narrower chassis compared to the L-10. The transition to wheeled mode took around 30 seconds and could be done while the tank was moving, in this mode the L-30 could reach a top speed of 80 kph on roads. However the heavier suspension didn’t allow for a satisfactory level of armor protection without breaking the 12 ton weight limit.

 

Landsverk L-10

The tracked suspension was identical between the two designs. By getting rid of the hybrid suspension the armor protection of the L-10 was increased to 24 mm compared to 14 mm of the L-30. In general the armor layout was ahead of its time, the front was well sloped and the design made extensive use of welding in its construction which was rare for the time.

For main armament both 37 mm and 47 mm caliber guns had been considered but ultimately a 37 mm was chosen due to its superior in rate of fire and penetration. This gun was an early version of the Bofors 37 mm which would go on to become one of the most widely used anti tank guns of the pre ww2 period. The vehicles carried 4000 rounds of 6,5 mm for the two machine guns and 122 rounds of 37 mm ammunition. The tanks were powered by an Maybach DSO 8 V12 gasoline engine delivering a respectable 200 hp at 3000 rpm, resulting in a power to weight ratio of around 17-18 hp/t. The good power to weight ratio coupled with the substantially improved suspension design gave the tanks a top speed of 40 kph on tracks. For communication the tank was equipped with a two way radio.

Strv fm/31 and Strv m/31 under construction

Service life and Legacy

After some delays the strv fm/31 and the three Strv m/31’s were delivered to the Swedish army in late 1934. By the time of their delivery the strv m/31 and fm/31 were arguably the most modern tanks in the world but unfortunately like many designs ahead of their time the L-10 and L-30 proved to have major teething issues, most of which were related to the drivetrain.

The transmission as well as the tracks were fragile and hard to repair, and changing the transmission required the engine and fuel tank to be removed first. During winter conditions snow would build up in the drive wheel causing the tank to lose its tracks. The extensive use of foreign components in the design meant that spare parts had to be imported.

Strv m/31 and Strv fm/31 at I2

Between 1935 and 1939 the tanks were stationed at the I2 Göta Livgarde tank battalion where they ended up mostly serving as training vehicles due to their reliability issues. During ww2 there were plans to convert the m/31’s to command tanks but nothing ever came of it. Ultimately the strv m/31 and fm/31 ended up being somewhat underwhelming vehicles but the experience gained from their construction would prove to be an important lesson for both the Swedish army and Landsverk. Landsverk would go on to use this experience in the making of the L-60, a modified version of which ended up replacing the Strv m/31 in 1938.

 

Data: Strv m/31

Weight: 11000 kg

Length: 5200 mm

Width: 2150 mm

Height: 2220 mm

Crew: Commander, Radioman/Machinegunner, Driver, Gunner

Engine: Maybach DSO 8 (200 hp at 3000 rpm)

Power to weight ratio: 18 hp/t

Top speed: 40 kph

Trench: 1,8 m

Gradient: 40 dg

Main armament: 37 mm Bofors L/45

Secondary armament: 2x 6,5 mm Ksp m/21-29

Gun elevation/depression: +25/-10 dg

Armor: 24/24/20 mm

 

 

Data: Strv fm/31

Weight: 11500 kg

Length: 5200 mm

Width: 2400 mm

Height: 2220 mm

Crew: Commander, Radioman/Rear driver, Driver, Gunner

Engine: Maybach DSO 8 (200 hp at 3000 rpm)

Power to weight ratio: 17,4 hp/t

Top speed: 40 kph (tracks), 80 kph (wheels)

Trench: 1,8 m

Gradient: 40 dg

Main armament: 37 mm Bofors L/45

Secondary armament: 6,5 mm Ksp m/21-29

Gun elevation/depression: +25/-10 dg

Armor: 14/14/14 mm

 

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4 comments on “Swedish tanks Revisited: Strv m/31 & fm/31

  1. "Light" Tank lover says:

    As my name implies, I always enjoy these kinds of articles, thanks for the good read 🙂




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  2. wolvenworks says:

    so you’re telling me the Loltraktor is actually SWEDISH?




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