The tank was born on the battlefields of WWI where it had to combat not only enemy fortifications, but also mud. Tanks were often lost not from enemy fire, but from being stuck on cross country terrain. Engineers and the military first started thinking about how to improve mobility of their vehicles. Further improvements focused on improving mobility in general, but advanced methods of driving on soft terrain did not progress past experiments. Engineers returned to the issue of improving off-road mobility in WWII. In addition to mud, tanks got a new enemy: snow. This issue was most prevalent on the Soviet-German front where there was more snow than anywhere else for obvious reasons.
Anti-swamp variant
The first attempt to make an improvement to tank mobility was made in 1917. It quickly became clear that tracks used on British tanks have poor traction, which led to tanks getting bogged down. Special grousers were created to solve this issue that were attached on every other track link. The grouser was wider than the track links, so it doubled as a track extender. These grousers were first used on Mark II tanks, but they can also be seen on later models. The grousers were not used on medium tanks at all and do not appear on British inter-war tanks.
A Mark II tank with additional grousers. They somewhat improved mobility on soft soil.
The need for additional improvement in off-road driving was not so great between wars. The British and Americans experiments with wide tracks equipped with wooden pads in the 1920s, but these were not grousers, nor did they move past experiments. Further work focused on improving general mobility and improving the lifespan of track links. In some cases development of tracks was stuck in place, for instance on Christie tanks. These tanks had wide and flat tracks that did not have any kind of grousers and worked quite poorly off-road, Nevertheless, BT tanks retained this track for a very long time.
There was another issue that often goes unnoticed: ground pressure. The BT-2 had a ground pressure of 0.64 kg/cm². the BT-5 was about the same, but the BT-7 had a ground pressure of 0.73 kg/cm². This was not so bad compared to its contemporaries: the T-26 (0.7 kg/cm²) and T-28 (0.66 kg/cm²). However, the T-26's designers thought about ground pressure. The T-46-1, the prospective replacement for the T-26, had much wider tracks that resulted in lower ground pressure: 0.56 kg/cm². This was comparable with the ground pressure of the Pz.Kpfw.I Ausf.B (0.52 kg/cm²). In general, Soviet designers paid close attention to ground pressure. Pressure of 0.9 kg/cm² was considered very bad and this limit was reached very rarely.
Track extensions developed at factory #185. The variant shown in this photo was the most successful one.
Good ground pressure was not always useful. This was usually the case in swamps, a fairly common occurrence on the battlefield. Even wide tracks were not enough here, which was demonstrated in tactical exercises. Because of this work on various equipment designed to improve mobility in swamps started in 1934. Various devices were tried, from special "carpets" to track extensions. Track extensions turned out to be the optimal solution. These were either special track links that were wider than normal ones or wide grousers that could be attached to tracks. These devices were chiefly meant for driving in swamps, but the also worked in snow due to decreasing of ground pressure to 0.27-0.3 kg/cm². This was enough to cross the worst terrain, but track extensions never made it into production. There were various reasons for this including complexity and low lifespan (the extensions often bent during trials).
Track extensions developed at STZ did no go into production, just like factory #185's design.
Work on track extensions died in the second half of the 1930s. The topic suddenly became very important during the Winter War. An order to solve the issue was given at factory #185. Designers Shufrin and Zhukov proposed three different types of extensions. In two cases additional wheels were installed as well. This was an attempt to get rid of the issue of rapid deformation during driving. The second variant, developed by Zhukov, was the most successful. However, it also did not go into production. Work on track extensions for the T-26 tank ended in the summer of 1940. The same fate awaited the design developed at STZ under the direction of N.D. Verner. These were classic non-removable extensions without additional wheels. The Red Army began the Great Patriotic War with a wealth of experience in track extensions designed for driving in swamps and on snow that was never used.
Experiments in snow
There was no time for experiments once the Great Patriotic War broke out. The most important task was to increase production and rebuild factories that were evacuated deep into the USSR. Additionally, combat in the winter of 1941-42 showed that the T-34, the Red Army's main tank at the time, does not need track extensions to drive through deep snow. Even though the T-34's mass gradually increased, it "floated like a swallow" on snow, as Stalin famously said. The tank could navigate snowbanks 40-60 cm deep not only due to its wide tracks, but also a powerful engine. The situation with the KV-1 was worse, as its mass was greater and it did not drive well in snow. In this case wider tracks would not have helped. The only solution was a more powerful engine or reduction in mass.
The use of T-30 and T-60 small tanks in winter showed that their mobility in snow is limited.
The situation with the T-60, which was put into mass production in the fall of 1941, was more complicated. This tank was designed based on the T-40 amphibious reconnaissance tank to replace the T-26. There is a myth that it was light enough to drive on snow crust, but this is not the case according to reports written in 1941-42. The T-60, T-40, and T-30 sank in snow deeper than 30 cm. If the terrain was uneven, the limit was 20-25 cm. Reports like this came from all parts of the front. Work to improve the T-60's armour began in late 1941, which also reflected on its mobility in snow. Even the T-26 worked better in these conditions. According to a report from the Southern Front made in early 1942 the T-26 could drive in snow 30-40 cm deep.
Track extensions for the T-30 and T-60 developed at factory #37.
Department #22 of factory #37 that had been evacuated to Sverdlovsk in the fall of 1941 developed a solution to this problem. Work on track extensions that the factory referred to as "paddles" began in January of 1942. Ironically, the width of these "paddles" was the same as on the T-26: 520 mm. There was little new development needed to be done for the tracks, as the T-40 and T-60 already had openings drilled in them for fitting grousers. The "paddles" could be affixed to these openings. The extender blueprints were numbered 030.37.30, meaning that they were developed for the T-30 rather than the T-60. Each extension weighed 2 kg, and a full set increased the tank's weight by 365 kg.
Trials showed that mobility improved, but the design was fragile.
Trials lasted from March 27th to April 7th, 1942, and showed mixed results. On one hand, ground pressure decreased to 0.26 kg/cm². The depth that the tank sank to in snow was reduced threefold. The tank could drive in snow up to 85 cm deep. The speed dropped by 10%, but compared to the impressive increase in the maximum depth this was nothing. The paddles left much to be desired, however. The design was deemed poor since they had low traction and reliability. 73 paddles broke off after driving for 150 km. The trials commission proposed that the design be improved, but no other paddles were developed for either the T-60 or T-70. This was less of an issue for the latter tank, especially after the introduction of improved running gear. This improvement came not only from a wider track link, but greater mass and a more powerful engine. The T-70 could push its way through 40-50 cm deep snow.
SU-76M produced by factory #40 with track extensions. NIBT proving grounds, December 1943.
As it often happens, solutions were developed in the field. The T-34 could use one half of a split track with a full track attached next to it. Wooden pads could be attached instead of grousers. These improvisations were not long lasting, but these were the most common Soviet track extensions. As a rule, they were used in mud rather than snow.
92 extensions per vehicle were installed.
The first attempt to develop a track extension for winter use was made in late 1943. Soviet industry had abandoned light tanks, but the SU-76 SPG was produced instead using T-70B components. Even though it had wider tracks, the higher mass meant that ground pressure increased to 0.54 kg/cm². The power to weight ratio did not increase compared to the T-70/T-70B. Issues with driving in deep snow were still pressing.
The design of the extension was quite simple. It took 20-30 man hours to install them.
Work on track extensions for the SU-76M began at factory #40 in Mytishi. The idea developed there was reminiscent of the pre-war factory #185 tracks. Instead of making a whole new track, factory #40 decided to go with removable extensions. These were simple devices made from six parts. Each extension weighed 2.6 kg. To install it, two track pins were removed and a "fork" was inserted to replace them. Fewer extensions were needed than in factory #37's design: 92 per vehicle.
Diagram showing how extensions were installed.
The width of the track increased to 550 mm with the extensions. Installation was not very difficult. Trials at the NIBT proving grounds showed that a full set could be installed in 20-30 man hours. Removal took the same amount of time. It was easy to install and remove them, but reliability and effectiveness were more important parameters. A SU-76M with track extensions was put through a set of trials at the NIBT proving grounds.
Typical damage to track links suffered at the proving grounds.
The program called for 500 km of driving: 50 km on a paved highway, 200 on a cobblestone road, 250 on a dirt road. There were also special trials to determine the reliability of the extensions. These conditions were not exactly representative, as the extensions were meant only for driving in difficult terrain. As such, the results were unpredictable.
Here is how these deformations happened. The question is: why?
The extensions were first tried in snow on a dirt road. The conditions were not what the extensions were designed for. The snow was only 10-15 cm deep. According to the report, the vehicle did not drive in deep snow at all, just on a dirt road at maximum possible speed. 8 extensions broke off over 250 km of driving at an average speed of 15.5 kph. Special trials included driving along logs (with predictable results).
Condition of the extensions after the test drive. One must wonder, were the testers told what these extensions were for?
The SU-76M drove for 320 km in total. 42 extensions broke, 34 of them during highway driving. The extension design was rejected, but the trials raise many questions and lead to confusion, reminding the reader of the joke with Siberian loggers and a Japanese band saw. One gets the impression that the trials were done only as a formality. As a result, the work on track extensions for the SU-76M ended.
Trials of winter tracks on the experimental ISU-122.
Work on the Object 242 (future ISU-122) was taking place at around the same time. Experimental winter tracks were installed. These tracks are not listed at all in the reports of their developer, factory #100. Workers from factory #185 worked at this factory, but these tracks had nothing to do with their pre-war work. They were much more influenced by German Winterketten. This was a stamped track link with a reinforcing rib on the outside, like their German counterpart. There were divots on the inside of the extension to save on weight.
The design of the track links was similar to the Winterketten.
Trials of the Object 242 at the Gorohovets ANIOP were held from December 24th to December 30th, 1943. Testing the A-19 gun was a priority, and little attention was paid to mobility trials. However, these trials showed that the idea of Soviet Winterketten was senseless. Even ordinary tracks did not perform that well, prompting the development of longer lasting track links. The performance of extended track links that came under even greater pressure was predictable. Even during the relatively brief driving trials about a quarter of the extensions broke off.
The extended tracks were not long lasting.
No further work was performed on track extensions during the Great Patriotic War. Results of prior trials did not go unnoticed, but also the terrain that the Red Army was fighting in by the fall of 1944 did not require improve mobility in snow. Experiments conducted during the Great Patriotic War ended in nothing. The army solved its problems independently, usually when driving through soft terrain.
Foreign analogues
One can easily get the impression of complete failure when reading about Soviet experiments. The situation was not as one-sided as it may appear. First of all, the T-34 drove well in snow without extensions, surpassing foreign vehicles. Secondly, when similar work was performed in other nations the result was often the same.
German track extensions: Winterketten.
The Germans were the most successful in the creation of winter tracks. This is not surprising, as their tanks had the most issues on soft soil. As mentioned above, Soviet tank builders considered the ground pressure of 0.9 kg/cm² to be very poor. The Germans started out well: the Pz.Kpfw.III Ausf.A had a ground pressure of 0.68 kg/cm², the Pz.Kpfw.IV Ausf.A had similar ground pressure. The issue was that as armour protection grew, so did the mass. The Pz.Kpfw.III Ausf.E/F already had ground pressure of 0.9 kg/cm², and the Pz.Kpfw.IV Ausf.D reached 0.9 kg/cm². It is not surprising that this tank had the worst off-road performance of all tanks that went through comparative trials at the NIBT proving grounds in 1942, especially in swamps.
The Pz.Kpfw.IV Ausf.F fared slightly better. The ground pressure was 0.88 kg/cm², a little higher than the T-34. However, the ground pressure of the Pz.Kpfw.IV Ausf.G increased to 0.93 kg/cm² as a result of installing heavier armament. This was a higher ground pressure than the KV-2 had! This was still better than the Pz.Kpfw.III Ausf.J and Ausf.N, which surpassed 1 kg/cm², or the StuG 40 Ausf.G, which reached 1.04 kg/cm². One can only imagine what happened to these tanks on soft soil, let alone snow.
Unlike Soviet track links, the Winterketten had pronounced ribs, which made them tougher.
This forced the Germans to act. Special tracks called Winterketten were developed in 1942. The Germans did not come up with anything new. The stock Kgs 61/400/120 was given an extension similar to the one installed on T-26 track links at STZ in 1940. To reduce weight the inner side of the extension was hollow and a rib was introduced on the underside. It is not clear when the Winterketten were developed, but this was likely in the summer of 1942. This is when winter equipment for the Pz.Kpfw.III and Pz.Kpfw.IV was significantly improved.
The Winterketten were the same for the Pz.Kpfw.III, Pz.Kpfw.IV, and vehicles on their chassis.
Since the Kgs 61/400/120 track link was standard for the Pz.Kpfw.III and IV, it could be easily installed on these tanks and SPGs on their chassis. The Winterketten first appeared on the front lines in late 1942. Little record of their use survives, but photos show the history of their usage all too well. Mobility definitely increased, but the issues with the track extensions were the same as with the extensions developed in the USSR. They quickly bent and broke off when used. German tanks and SPGs were somewhat lighter than the T-34, which mitigated the issue, but the peak of Winterketten usage was reached in the winter of 1942-43 and then sharply declined. These tracks were used episodically in the winter of 1943-44. Production of narrower tracks called Ostketten that were not as prone to breaking began, gut they were also used very rarely.
No extensive reports on the usage of Winterketten tracks exist, but photos like these demonstrate that they quickly deformed and broke off.
The Americans were experiencing the same issues. The Medium Tank M3 had a ground pressure of 0.885 kg/cm². Its performance in Soviet trials in 1942 was little better than that of the Pz.Kpfw.III Ausf.H. As for the Medium Tank M4, its ground pressure was even higher: 0.962 kg/cm². There were no issues with ground pressure at first (at least that the Americans experienced), but work on wider tracks began in late 1942. This work went in two directions: development of a brand new track link and extended end connectors. Both were built in the end.
The American solution for high ground pressure. This design was used on light and medium tanks.
The 7055614 Extended End Connector was developed and put into production in 1944. There were several versions of this connector. They showed good results in mud, rather than snow. Track links used by tanks with the HVSS suspension showed even better results. For instance, the Medium Tank M4A3E8 weighed 33.7 tons, but had a ground pressure of 0.772 kg/cm².
These track extensions lowered the ground pressure to 0.49 kg/cm².
There was another third variant that was quite similar to Soviet developments from the mid-1930s. In April of 1944 the Ordnance Department launched a track extension program. This increased the width of the tracks to 826 mm. They could be attached to either regular T51 tracks or those with the 7055614 extended end connector. In the latter case the width of the track increased to 940 mm and the ground pressure dropped to 0.49 kg/cm².
They were easy to attach and remove, but they came too late.
Trials carried out in early 1945 were successful. The extensions were mostly meant to be used in swamps, but also worked in snow. An order for 1000 sets was made in February of 1945, but the end of the war put an end to these plans. The Germans ended up being the only ones who put winter tracks into production and used them for their intended purpose, although the results of this usage were mixed.