Estimating productivity of forest equipment and also developing productivity models are imperative to managers for estimating production costs of logging and to develop a cost efficient logging plan. This study determine the production rates and productivity models of Timberjack 450C skidder in two skidding directions (uphill and downhill) in a part of Hyrcanian mountainous forests (Part of forests related to Farim Forest Company). In order to assess skidder productivity, time study method was used. Skidding cycle time and the primary factors affecting skidding performance for two skidding directions, were analyzed descriptively. For the purpose of developing the productivity models for the two skidding direction, multiple regression analysis, using the least-squares method, was used to test the correlation among the skidding cycle times and probable effective parameters on it. In order to determine system productivity based on productive time and gross time, average skidded volume per turn divided by average skidding productive or gross cycle time. The related average production rates to downhill and uphill skidding were 17.17 m3/hr and 12.77 m3/hr, respectively. Two regression equations were generated between skidding cycle time and harvested stand attributes for two directions, separately. In downhill skidding, there were strong positive correlation among skidding distance, number of logs skidded per turn, winching distance and cycle time. Number of logs skidded per turn, winching distance and an interactive variable DS (skidding distance multiply by slope); all were entered in the uphill regression model. Also, related times to each elements of skidding cycle and delay times were determined. All delays (technical, operational and personal) accounted for about 17% of total skidding time.
