Abstract: The invention relates to an arrangement in a forestry machine (1), in which there are front and rear units (2, 3) and bogie wheels (14, 15) mounted on a set of swing axles (12), as well as a crawler mat (20) running around the bogie wheels (14, 15) and an auxiliary wheel (16) outside the set of swing axles (12), and in which the auxiliary wheel (16) is essentially permanently mounted in bearings in the front and/or rear unit (2, 3), so that, when the set of swing axles (12) turns, the tension of the crawler mat (20) is maintained by means of flexible elements connected to its perimeter, which are principally formed of the flexibility of the auxiliary wheel (16).