Improved mobility for crawler vehicles on steep slopes and rough terrain depends on the ground contact area and the even distribution of ground contact pressure. Forest microtopography, assumed as the basis for undercarriage design of forest vehicles, was measured using a device that we developed. The measurement data was then used in an attempt to optimize the undercarriage specifications. The vehicle's ability to pass over rough terrain was determined by comparing its thrust force, which was derived from the ground contact ratio in the measured microtopography data, with the required thrust force calculated using a Microsoft Excel macro program. Weight, height of the center of gravity, crawler width and lug height of the assumed vehicle model, and cohesion and internal friction angle of the soil were applied as constant parameters. Ground contact length, roadwheel interval and suspension stroke were applied as variables shifting within the probable range. Although the program determined the optimum undercarriage specifications, its validity is difficult to prove due to the lack of a verification method. However a problem of mobility of a real vehicle could be clarified by comparison between the calculation result and the examination result. Future tasks include improving the program and proving its validity.

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Circular roads have two functions: improve traffic flow by shortening the route distance and provide an alternative route to a destination. High-density road networks in forests consist of many circular roads, and it is important to assess their function during route planning. We evaluated the alternative functions of a high-density road network by a reliability analysis that is used to analyze public road networks. The following results were obtained. The alternative function was evaluated qualitatively in terms of the number and efficiency of alternative routes. The efficiency increased with shorter length of the alternative routes. Further, it was found that the reliability of the network was related to its structure: a network with a ladder-shaped structure exhibited high reliability. It was confirmed that the alternative function of circular road networks can be evaluated effectively by the reliability analysis. This analysis can also be used to evaluate the productivity and safety of logging in a forest with a high-density road network, because these are guaranteed by a high reliability network in which a driver is not forced to take a detour or reverse the vehicle. The reliability analysis can also be applied to forest roads as they play an important role in local traffic systems, especially in the case of a disaster; this analysis contributes to the development of road networks with a reliable alternative function.

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The effect of "bypass notch" and "uneven hinge thickness" on directional felling and job safety was observed using video recording, and was analyzed using the finite element method (FEM). Video observation showed that the hinge of trees with a bypass notch broke quickly even when the tree inclined slightly and the direction of felling could not be controlled. FEM analyses clarified that "barber chair" tended to occur and could cause accidents in the case of low backcut. Bypass notch should therefore be avoided in the case of low backcut to conserve the hinge. It was confirmed by video observation that trees with an uneven hinge thickness did not always fall in the direction of thicker hinge. FEM analyses showed that a slight right-angle displacement to the felling direction occurred when an uneven thickness hinge vented. However, the effect of the displacement on the final felling direction is not clear. To control the felling direction by changing the hinge thickness unevenly is not a reliable method in actual felling operations and is not appropriate for job safety.

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