SOV - SU-31 (systém BGS-2)

On January 29, 1942, a draft of tactical and technical requirements for a preliminary design of a universal self-propelled chassis based on the light tank T-60 was drawn up. The preparation was provided by S.A. Ginzburg, at that time deputy head of the 2nd department of the NKTP of the USSR. According to the requirements, a universal chassis was to be created (chassis No. 31 ("Шасси 31")), which later served as a base for armored vehicles of various purposes, including an anti-aircraft tank. A similar development was required for the platform of the latest light tank, the T-70 (chassis No. 32 ("Шасси 32")).
It was planned to use several components from the light tank T-60, but several modifications had to be made to the original platform. The width of the platform was increased and thus the length of the torsion bars.
The higher priority of chassis 31 is also evidenced by the fact that three versions were planned, chassis 32 was only planned in two versions.

On chassis 31 (SU-31) the following versions were planned:
BGS-1 (БГС-1 (Боевая Гусеничная Машина 1)) with a 25 mm anti-aircraft gun 72-K
BGS-2 (BGS-2 (Combat Tracked Vehicle 2)) with 37 mm protilietadlovým kanónom 61-K
BGS-3 (BGS-3 (Combat Tracked Vehicle 3)) with 76,2 mm divíznym kanónom ZIS-3

On 22 May 1942, a meeting was held at Plant No. 37, where the priority programs were approved, namely the BGS-2 on the platform of Chassis No. 31 and the BGS-5 (76.2 mm self-propelled gun with modified tank gun ZIS-5) on the platform of Chassis No. 32.
The BGS-2 on chassis platform No. 31 (SU-31) was a self-propelled gun with a 37 mm 61-K anti-aircraft gun, which had already been introduced in the Army. Therefore, this project did not have a main gun problem, as was the case with the BGS-1, whose planned 25 mm anti-aircraft gun 72-K was still under development and was not expected to be available anytime soon.
A blow to this project was the decree "ГКО № 1958сс" signed by Stalin, which envisaged the production of the T-34 and T-70 tanks. In another decree, "ГКО №2120сс", Plant No. 37 was charged with the production of T-34. This meant that the T-60 platform, and thus its derivatives, had nowhere to be produced.
Nevertheless, the development work did not stop and on 18.8.1942 SU-31 and SU-32 passed gunnery trials at the Sofrin polygon. On 21.8.1942 the trials continued at the Kubinka polygon. A light tank T-70 took part in the tests. Paradoxically, the prototype of chassis No. 31 (SU-31) demonstrated better off-road performance than the remaining pair of platforms (SU-32 and T-70). The evaluation committee therefore recommended the introduction of the SU-31 platforms. However, during the tests, problems were found with undersized chassis parts, problems with the engine pair and in the transmission layout.
The SU-31 and SU-32 program was transferred to Plant No. 38, where they were redesignated SU-11 and SU-12. Also, due to the termination of production of the T-60 and due to the planned modification of the T-70 platform to produce a reinforced design better suited than the problematic T-60, the project was canceled. Priority was given to the SU-32 (SU-12) platform, which became the basis for the later SU-76 self-propelled gun.

zen.yandex.ru
https://warspot.ru/8037-na-puti-k-su-76
TsAMO RF, RGASPI, RGAE

August 18, 1942 passed the SU-31 (BGS-2) shooting tests at the Sofrin polygon.

zen.yandex.ru
https://warspot.ru/8037-na-puti-k-su-76