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Dubravko Horvat1

1 Faculty of Forestry, University of Zagreb, Svetošimunska 25, HR-10000 Zagreb, Croatia.
Telephone: (385 1) 219288: Facsimile: (385 1) 218616; E-mail;


The paper describes the analysis of some parameters used for the calculation of the theoretical traction performances and some other useful properties of four skidders used for wood transportation in mountain forest thinning in Croatia. Those skidders consisted of two adapted farming tractors - Steyr 9078a and Torpedo TD75a. and two medium-sized skidders LPKT40 and Ecotrac-V. These characteristics, important for the dynamic performance of skidders during wood transportation, such as the height of CG, the distance of CG from the rear axle. the height of the lead point (LP). the distance of LP from the rear axle, the turning radius and the winch pull force were measured and compared with those typical of rubber-tyre skidders, determined by the morphological relationship of the database on more than 90 skidders. Calculation of the theoretical traction characteristics will be made by a computer programme developed at the Faculty of Forestry, University of Zagreb, before in situ tests planned for later in the year.


The vehicles used for mountain forest stand thinning should meet the following requirements:

· small dimension, particularly with regard to width and height:
· good mobility, small turning radius; and
· good longitudinal/lateral stability and rear axle load capacity.

During wood transportation in mountain conditions, energetic properties, such as traction efficiency as well as winch pull force, should also not be too small.

Figure 1. Four tested skidders (from left: Ecotrac-V, LPKT40, Steyr 9078a and Torpedo TD75A

Parallel investigations of four skidders (Figure 1) - two adapted farming tractors and two medium-sized skidders, described in this paper, represent the continuation of the research described by Horvat and Sever (19959) and precede the investigation of traction characteristics in in situ tests on standard testing skidding tracks (Sever et al. 1994, Horvat and Sever 1995).

So far, the testing of these four vehicles was performed in two steps: the first step covered technological investigations and. in the second step. technical characteristics were determined or checked. As established by Horvat and Sever (1996), by morphological analysis of some parameters, the engine power of these skidders is near the equalization line (Figure 2). Only the adapted farming tractor, Steyr 9078a, belongs to the group of skidders and not medium-sized skidders.

The same authors have investigated the relationship of B/L and H/L form indexes and have come to the conclusion that, according to these date, the tested skidders belong to the group of medium-sized skidders. It is specific for the Croatian Ecotrac that, due to its small width, it reduces the area of medium-sized skidders (Figure 3). That means new criteria for the manufacturers of forest thinning skidders. This figure also shows that farming tractors enter the skidder area only after being provided with forest equipment.

Figure 2. Engine power vs. tractor mass relationship (Horvat and Sever 1995)

Figure 3. Relationship between B/L and H/L form indexes (Horvat and Sever 1995)

In this paper the third step of investigation is described covering, as said above, the transition to tests on traction performances in exploitation conditions. Because one farming and one articulated tractor have been manufactured in Croatia, the purpose of the whole investigation was to determine the most suitable vehicle for mountain forest stand thinning in Croatia with special reference to the country's products.

Results of the investigation

The computer programme for the forecast of traction performances of a four-wheel drive skidder developed at the Faculty of Forestry, Zagreb, described by Horvat (1989), is based on a common model of skidder dynamic loading (Hassan 1983, Sever 1984, Horvat 1987, Sever 1987, Hassan 1988, etc.) The following dimension characteristics are essential for this model of skidder: position of the CG, position of horizontal rollers, i.e. lead point, and wheel base. Therefore, a comparison has been made between the dimensions of these four skidders and those determined by a morphological analysis of the skidder database described by Sever and Horvat (1994).

Figure 4 shows the CG distance from the rear bridge (A) and its height above the ground B). In the same figure the values of these parameters are indicated, which were determined by a close relationship with the skidder mass established by a morphological analysis. An obvious distinction can be observed between the adapted farming tractors and the skidders, because the latter not only have a more favourable CG (greater distance from the rear axle) but are also very close the general characteristics of this type of equipment. This is the consequence of the core difference in design criteria between special forest vehicles and farming tractors. The big differences in the measured heights of CG and of the skidders lead to the conclusion that the database should be completed since these data were available only for a small number of skidder.

Figure 4 Position of CG

The skidder's lead point position is shown in Figure 5, namely its height on the segment (A) and the distance from the rear axle on the segment (B). Farming tractors have a slightly more favourable characteristic of LP height than the skidders as a result of the installation of a protection board with driving rollers on the hydraulic tractor lifter. The skidder's LP is, therefore, nearer to the rear bridge and the Ecotrac's characteristic is particularly favourable.

Figure 5. Position of lead point

A general conclusion can be made that the skidders, and particularly the Ecotrac, have more favourable dimension properties, from which more favourable traction characteristics can be expected.

In the introduction it was stated that the vehicles for forest thinning have to be as small as possible. Therefore. Figure 6 shows a comparison between the basic outer dimension: length, width and height and their expected dependence on the vehicle mass according to the morphological analysis. The vehicle length (A) measured in a Steyr 9078a tractor was less than expected because it did not have the thrust-board: the Ecotrac is obviously very well designed as far as this parameter is concerned, even though only at the stage of project design. The width (B) of this skidder is also outstanding, while the height (c) is within the expected limits.

One of the effects of the smaller length and width of the Ecotrac is its smaller turning radius as shown in Figure 7.

Figure 8 shows the winch traction forces of these skidders. It can be generally noted that they are smaller than those calculated on the basis of the skidder mass. This can be explained by the fact that a large number of skidders in the database were utilized for final cutting, which leads to the conclusion that this parameter for thinning skidders should be considered independently.

Figure 6. Basic outer dimension of skidders

Figure 7. Turning radius of four skidders

Figure 8. Winch traction force


According to the analysis made, medium-sized skidders show significantly better properties for mountain forest stand thinning that adapted agricultural tractors. The Croatian skidder Ecotrac-V shows the best characteristics and its traction characteristic, both calculated and measured, is expected to be most favourable.


Hassan, A.E. 1983. Weight distribution characteristics of semi-suspended trees. Transaction of ASAE 26(5): 1291-1297.

Hassan, A.E. 1988. Instrumentation research applied to four-wheel drive skidders. JUKEM 13th Symposium on Measurement and Measuring Equipment. Split, Croatia, pp. 579-607.

Horvat, D. 1987. Skidder wheel torque measuring. Proceedings of 9th ISTVS International Conference. Barcelona, Vol. II, pp. 531-541.

Horvat, D. 1989. Prediction of tractive performance of a four-wheel drive skidder. International Seminar ".'Forestry transporting machinery and terrain interaction". College of Forestry Garpenberg, Sweden. pp. 1-12.

Horvat, D. & Sever, S. 1995. Some properties of the skidders used in mountain forest stand thinning. IUFRO XX World Congress. Proceedings of the technical session of subject Group 3.06. Tampere, Finland, pp. 211-216.

Sever, S. 1984. Investigations of some tractors exploitation parameters at wood skidding. Annales pro experimentis foressticis. Vol. XXII pp. 183-303.

Sever, S. & Horvat, D. 1986. Skidder with power of 60 kW. Study pp. 1-182.

Sever, S. & Horvat, D. 1992. Skidders and forwarders database as a source and help in determining morphological relationship. Proceedings of workshop on computer supported planning of roads and harvesting. Feldafing, Germany, pp. 196-200.

Sever, S., Horvat, D. & Tomašic, Z. 1994. A contribution to damage assessment standardization resulting from the research into machine use on variable slopes. Proceedings of interactive workshop and seminar FORSITRISK "Soil, tree, machine interactions. Fedafing, Germany, pp. 1-9.

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