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Visser, R., H., Berkett, 2015: Effect of terrain steepness on machine slope when harvesting. Int. J. For. Eng. 26(1): 1–9. https://doi.org/10.1080/14942119.2015.1033211 Visser, R., K., Stampfer, 2015: Expanding Ground-based Harvesting onto Steep Terrain: A Review. Croat. j. for. eng. 36(2): 321–331. Weise, G., 2002: Was kann der Rückezug wirklich? KWF Forsttechnische Informationen 1+2: 4–6. Weise, G., 2003A: Load Distribution Plans for Forwarders. Agrartechnische Forschung 9(1): 7–10. Weise, G., 2003B: Load Distribution Plans for Forwarders. Landtechnik 58(1): 30–31. Wang, X., L., Gao, P.D., Ayers, S., Su, C., Yuan, 2016: The influence of the lift angle on the center of gravity: measurements for zero turning radius mowers. Appl. Eng. Agric. 32(2): 189–199. https://doi.org/10.13031/aea.32.10548 Wong, J.Y., 2014: Mechanics of off-road vehicle – Terrain interaction. In Road and off-Road Vehicle Systems Dynamics Handbook, 1st ed.; Mastinu, G., Ploechl, M., Eds.; CRC Press: Boca Raton, FL, USA, 697–726, ISBN 978-0-8493-3322-4. Wronski, E.B., N., Humphreys, 1994: A method for evaluating the cumulative impact of ground-based logging systems on soils. J. For. Eng. 5(2): 9–20. https://doi.org/10.1080/08435243.1994.10702656 Summary In the example of an eight-wheeled forwarder, a simulation model is presented for assessing the mobility of the forwarder when transporting timber uphill and downhill. The presented model shows the changes in forces during timber forwarding by a nominally loaded forwarder due to a wide range of changes in influencing factors: 1) the slope of the terrain, 2) the direction of timber forwarding, and 3) the soil bearing capacity expressed by the cone index. By incorporating the criteria/limitations of timber forwarding derived from previous research, the forwarder mobility model gains practical meaning; that is, the theoretical approach brings the reality of timber forwarding closer. The mobility model shown is based on easily measurable or available data but also on the position of the centre of gravity, which is relatively easy to determine by the shown method of lifting the forwarders’ axle with the use of portable scales and is usually unavailable to read in the manufacturers’ catalogue. All known criteria/restrictions of mobility of forest vehicles are primarily related to their movement uphill. The limitation of the skidder’s mobility when skidding timber downhill has been firmly determined, while there are no limitations in the literature for forwarding timber downhill. Gradeability of a nominally loaded forwarder downhill, where the thrust force takes the value of zero, did not prove to be a good indicator or a limitation of timber forwarding. Determining the mobility limitations of the forwarder during timber forwarding downhill will be a research challenge in the future. Key words: forwarder, centre of gravity, axle load |