The convertible tank Bt-sv 2

The BT tanks were "convertible tanks". This was a feature designed by J. Walter Christie to reduce wear of the unreliable tank tracks of the 1930s. In about thirty minutes the crew could remove the tracks and engage a chain drive to the rearmost road wheel on each side, allowing the tank to travel at very high speeds on roads. In wheeled mode the tank was steered by pivoting the front road wheels. However, Soviet tank forces soon found the convertible option of little practical use in a country with few paved roads, and it consumed space and added needless complexity and weight. The feature was dropped from later Soviet designs.