The drive device of the invention comprises a ready to fit unit or kit, for installing in a detachable manner on one side of a chassis of a motor vehicle. The unit comprises a flexible crawler looped round a drive wheel adapted to be mounted on a drive hub of the vehicle and round a tension wheel carried by a support frame, which comprises an internal part adapted to be secured in a detachable manner on the chassis remote from the drive shaft, an intermediate part adapted to carry the tension wheel and an external part carrying a bearing for the drive wheel. The invention is particularly of use for transforming vehicles with four drive wheels for crawler operation.