A track system for use as a wheel replacement on typically wheeled vehicle is disclosed. The track system comprises a drive wheel configured to be mounted to the vehicle, a support frame, front and rear idler wheels respectively pivotally mounted at the front and at the rear of the support frame, road wheels pivotally mounted along the support frame between the front and read idler wheels, and an endless track mounted about the wheels. The position of the front and/or rear idler wheels is adjustable in order to modify the overall shape of the perimeter of the track system generally defined by the endless track, thereby changing the riding behavior of the track system. A method to change the position of the adjustable idler wheels is also disclosed.