Abstract: There is disclosed a conversion unit for attachment to an all terrain vehicle to convert it into a track laying vehicle. A subframe is attached to the rear of the track laying vehicle, and two idler wheels are mounted on the subframe. Spring strut means extend downwardly from an upper portion of the vehicle to the subframe and bias the latter downwardly. The unit is quickly and easily attached to an all terrain vehicle and as easily removed. It is possible to improve the luggage carrying capacity of the all terrain vehicle and to improve its safety. No special tools are required for installation or dismounting of the unit.