A drive assembly for a track-laying vehicle adapted to travel in a horizontal direction along the ground has a horizontally elongated frame having two opposite ends and upper and lower edges extending parallel to the direction between the ends, a drive wheel at one of the ends, a support fixed on the frame between the ends, and a guide on the frame extending between the fixed support and the other end along a line forming an acute angle with the direction and extending downward toward the other end. A movable support displaceable parallel to the line in the guide carries an idler wheel at the other end. A spring braced between the supports urges the idler wheel parallel to the line away from the drive wheel. A plurality of ground wheels are mounted along the lower edge between the drive and idler wheels and an endless track looped around the wheels extends parallel to the direction along the idler wheels. A drive connected to the drive wheel rotates the drive wheel and displaces the track around the wheels.