C.R. SILVERSIDES
THE DEVELOPMENT of the articulated wheeled tractor for use in logging was peculiarly a North American phenomenon. It was the end result of a relatively short evolutionary development which came into being immediately following the second world war, causing the introduction of wheeled vehicles into the forest as a replacement for animal power and timber but, as their numbers declined, a replacement for this form of power was required. Tracked tractors were applied successfully, but had limitations as to speed and economy.
In Europe, where forest operations have been integrated to a high degree with those of agriculture, wheeled farm tractors have been used fairly successfully in as a more suitable and economic machine than tracked tractors. The wheel is not to be found in nature and, following its invention by man, was really only used successfully in conjunction with paths and roads. The successful application of wheels in the forest was only made possible by wartime and immediate postwar developments in machine and tire design.
TABLE 1. - COMPARISON OF UTILIZATION RATES AND CRAWLER TIMES FOR WHEELED AND FOR TRACKED VEHICLES
|
Vehicle |
Empty/loaded |
Ground |
Speeds meter/minute |
||
|
Min. |
Mean |
Max. |
|||
|
Timberjack |
Empty |
Cross-country |
28 |
53 |
78 |
|
200 - 215 |
|
Main strip road or main haul road |
50 |
105 |
172 |
|
Crawler tractors |
Empty |
Cross-country |
30 |
43 |
71 |
|
|
|
Main strip road or main haul road |
42 |
55 |
87 |
|
Timberjack |
Loaded |
Cross-country |
26 |
48 |
76 |
|
200 - 215 |
|
Main strip road or main haul road |
45 |
75 |
137 |
|
Crawler tractors |
Loaded |
Cross-country |
32 |
47 |
66 |
|
|
|
Main strip road or main haul road |
38 |
52 |
75 |
SOURCE: Skogsarbeten (1).
Horses had been widely used for skidding and hauling logging for many years. In North America this integration did not exist. The machines designed and sold there for open field operations were unable to stand up to the much more arduous conditions of logging. Again, by their very design, they lacked the maneuverability and mobility desired by the logger. What the logger really required was a substitute for a horse, a machine capable of high tractive effort, one highly mobile under forest conditions and one economical to operate.
C.R. SILVERSIDES is Director of Woodlands Development, Abitubi Paper Company, Toronto, Ontario, Canada.
Early in the development of a replacement for animal power, wheels showed promise over tracks, although the argument of wheels versus tracks has involved engineers and operators for many years. If a wheeled vehicle can do the job at hand it is usually to be preferred to a tracked vehicle for lower first cost and lower operating and maintenance costs. Usually too, the utilization rate and the trip times for the wheeled vehicles are much better than for crawler vehicles: see Table 1 (1).¹
¹ Numeration refers to the bibliography to be found at the end of this article.
Where the terrain is beyond the capabilities of wheeled vehicles then crawler tractors should be used. It has been established that with 4 X 2 tractors (four wheels with only two wheels driving) maximum obstacle-crossing ability exists with much larger wheels in the rear than in the front, with the weight distribution largely toward the rear (2). The maximum performance of such a machine is still only about 75 percent of the poorest 4 x 4 vehicle. With equal sized wheels front and rear, the performance of 4 x 2 vehicles is less than 33 percent of the poorest standard 4 x 4. On this basis, 4 x 2 vehicles should not be considered for cross-country work.
The performance of a 4 x 4 vehicle is almost exclusively determined by (a) the ratio of the wheelbase length to the diameter of the wheels (S/D), and (b) the weight distribution. Figure 1 shows the relative obstacle performance for a series of machines.
The flexible tracks used for vehicles during the second world war were expected to have a wide application in industry. This application has not materialized to date. One reason is that in a military vehicle the load is relatively constant. A logging tractor usually carries or tows a load in one direction. Tracks are usually tensioned in the unloaded condition and, when the vehicle is loaded, the track tension increases and the performance deteriorates as the friction losses in the tracks increase to an unacceptable level. If the tracks are tensioned in the loaded condition, there is a risk of shedding them when the vehicle is unloaded. New designs in tracks and track suspension offer considerable hope that some of these factors will be overcome, and that suitable tracks with their basic advantages and with most of their disadvantages overcome will be developed.
FIGURE 1. - Maximum obstacle performances of military four-wheel vehicles.
NOTE: Rear wheel is the limiting factor in all cases:L/D = Wheel base/wheel diameter
h/D = Obstacle height/wheel of diameter
U0 = Coefficient of friction
FIGURE 2. - Comparison of area of contact for track and wheel.
NOTE: W = Weight in lb
P = Ground pressure in lb/sq in, when b and L are inches
There are several general principles involved which give the relationship to wheeled and tracked machines and which are of interest and value to loggers:
(a) The soil thrust generated by a vehicle is directly related to the length of the ground contact area and to the slippage. Figure 2 shows that the main difference between the wheel and the track has to do with the length of their contact area (3). In analyzing the effect of each, a track is considered to represent a segment of a very large wheel. All other things being equal, the difference in thrust and slippage between a wheel and a track is caused only by the length of the ground contact area. The larger the ground contact area the greater is the thrust produced with smaller slippage. The travel efficiency, therefore, of a low wide tire will be inferior to that of a high relatively narrow tire, and vehicles should be designed with this principle in mind. This also illustrates that a tire cannot replace a track under critical soil conditions because its ground contact cannot be as long as a track.(b) Pneumatic tires find their best application in sandy soils. In such cohesionless soils, which generally have a high bearing capacity, a rubber tire with a higher unit bearing surface than a track will generate a high thrust. This explains why pneumatic tires are used in desert operations where dry sand is predominant.
(c) In cohesive soils such as plastic clays the weight of the tractor is a liability as it is not a thrust generator in such soils, while total contact area and light unit bearing pressure are. In these soils track performance is superior to wheels.
Sometimes the surface of an otherwise firm soil will become slick and vehicles, particularly wheeled machines, cannot gain sufficient traction to move themselves forward (4). In such a case the vehicle may simply spin its wheels, not moving forward and not sinking appreciably. Few tracked vehicles appear to be seriously impeded by surface slipperiness. Surface slipperiness effects are magnified when associated with slopes because of the reduction in normal load against the slope.
In any discussion of off-road transport the two terms "mobility" and "trafficability" are often used synonymously.
They are not synonymous, although they may be considered to be interdependent.
Mobility of a vehicle may be defined in terms of the time required by the vehicle to pass over a given route without restrictions or perhaps as the absolute ability of a vehicle to traverse a specific route or a specific type of terrain.
Trafficability of a soil is the ability of a soil to withstand vehicular traffic. It is often expressed as the number of traverses a particular vehicle can make in the same path before immobilization.
It follows that soils and vehicles must be carefully matched to achieve optimum results. There is no sharp cutoff between wheeled and tracked vehicles. In some difficult soils the best wheeled vehicles will outperform the poorest tracked vehicles. Conversely, the best tracked vehicles can outperform the poorest wheeled vehicles from all points of view in medium to severe soil conditions. In summary:
(a) Wheeled machines have greater speed and mobility than tracked vehicles. Track machines work best up to 3 mph while wheeled machines are effective in the 2 to 7 mph range.(b) At crawler work speeds, tracks often develop up to 1½ times more traction and drawbar pull than a wheeled machine of the same weight.
(c) The cost of running gear makes up a significant part of the total cost of a machine, 8 to 10 percent for rubber tires and 20 to 25 percent for crawler tractors.
(d) In cohesionless soils (sands), tires give long service at low cost, while crawler tracks wear rapidly; on rock, wheels tend to wear rapidly while tracks stand up well.
(e) The relative life of the two types of machine is clearly illustrated by the United States Army specification that the optimum life for track-laying vehicles is set at only 6,400 kilometers (4,000 miles) while that for wheeled vehicles is 32,000 kilometers (20,000 miles).
(f) From tests made by the United States Corps of Engineers the following characteristics of a good cross country wheeled vehicle have been developed (5):
(i) the wheels should be as large in diameter as possible;
(ii) the wheelbase should be as short as possible;
(iii) there should be no overhang, front or rear;
(iv) the total vehicle length should be as short as possible (it appears that a square vehicle - same length as width would approach the ideal);
(v) the ground contact pressure should be as low as possible.
FIGURE 3. - F.W.D. Blue Ox skidder.
The first wheeled tractors used in logging were essentially four-wheel drive trucks. These heavy rugged units outperformed farm type tractors, but even these were unable to perform as required within acceptable limits of cost, and with reliability. Such a machine was the Four Wheel Drive Blue Ox, named after a mythical North American creature noted for his pulling power (Figure 3). Its main period of use was 1956 to 1960 and there are few, if any, of these units operating in the forest today. The weaknesses of this type of machine lay in its use of conventional Ackermann steering and the rigidity of its frame.
The need for a suitable wheeled unit to operate off-road in the forest was recognized by the Canadian Pulp & Paper Association, and Woodlands Mechanization Project was established in 1954 to design and develop such a machine. This development culminated in 1955 in the Mark V Bonnard Hauler (Figure 4), the first articulated forest tractor in eastern North America. This development was sold to Clark Equipment Limited, Benton Harbor, Michigan and appeared as the PL-75.
Another development concurrent with the above was that made by Dwight Garrett, Eunemclaw, Washington. The result of this program was the Tree Farmer, the forerunner of the currently successful Tree Farmer and Timberjack articulated skidding machines.
SKID STEERING
In the development of wheeled machines for use in the forest an attempt was made to replace crawler tracks with wheels on conventional crawler tractors. The steering systems remained the same. Figure 5 demonstrates that when such a unit is steered into a curve there is a forward force associated with the outer wheels and a backward or negative force associated with the inner wheels (5). In a regenerative braking system the power is transferred from the main sprocket which is being braked to the outer sprocket which is being accelerated. In the nonregenerative system the inner track is braked and the sprocket power has to be absorbed by that brake. Although it was possible to turn within a short distance, the tires scuffed badly when forced sideways and obstacles such as boulders, stumps, and windfall trees, greatly hindered its movement.
Concurrent with the attempt to convert track into wheeled skidders there was the introduction of the integral arch concept to tractors which was adopted in all the later developments. The integral arch consisted of a fairlead above and slightly to the rear of the winch of the tractor which permitted the lifting of the front ends of logs or trees clear of the ground, but in a manner that transferred the load imposed to the running gear to give the desired traction. This principle is not new but has reached its highest level of development with the articulated steered machines.
FIGURE 5. - Forces in steering tractor at low speed. Vertical arrows illustrate resistance of the ground to turning. Horizontal arrows illustrate driving force on the track to negotiate the turn. Source: Hanning (5).
ACKERMANN STEERING
The standard automotive steering system, as used in trucks, is called Ackermann steering. It uses two pivot points (one at each front wheel) and this produces a more constant wheelbase and track than does articulated steering. Some machines have been built incorporating four-wheel steering, but even this has not produced the mobility of articulated steering. During a turn the actual width of a vehicle is not the measure of the minimum possible distance between obstacles. Because of the sweep of the vehicle, which is the actual area covered by a vehicle during a turn, the width of the vehicle is increased to some effective width greater than its actual width. This effective width is related directly to the length of the vehicle and inversely to the radius of the turn.
The secret of the successful design of the new family of articulated tractors was in its method of steering. This permitted the machine greater flexibility in use, the ability to turn in its own length, and higher available drawbar pull than any wheeled machine developed heretofore.
FIGURE 6. - A plan view of the Brown one-row cultivator, 1916 (Deere and Co.).
ARTICULATED OR FRAME STEERING
Articulated steering is the steering of a vehicle (tracked or wheeled) consisting of two units of a single pivot point system in which the pivot is not located over the axle of either unit. In an articulated vehicle, steering is accomplished by bending the vehicle about a pivotal point, and in consequence, is often referred to as frame steering This concept is not new as it existed as long ago as 1916, being used in a one-row cultivator machine by John Deere &; Company (Figure 6).
It has not been possible to establish quantitatively just why articulated steering succeeds where conventional steering fails, but comparative tests show that vehicles with frame steering are capable of extricating themselves from ruts, mud holes, and other obstructions. No particular increase in drawbar pull is apparent with articulated steering over conventional units on a straight pull. The merit in the frame steered vehicle appears to be its ability to "step" or "wiggle," and each time the vehicle is steered a slight forward motion is obtained. It is in this maneuver that it is felt the net tractive effort or drawbar pull is somehow increased. Here drawbar pull is considered as the tractive effort developed by a vehicle in excess of motion resistance (net tractive effort). Since the drawbar pull is apparently increased (motion resistance reduced) during the steering maneuver, it can be concluded that there must be something peculiar to articulated steering that produces this increase.
It has been found that, when negotiating a rutted condition, a machine with articulated steering can move the front end of the vehicle out of the rut into adjacent soil, whereas this cannot be done with Ackermann steering (7).
A 90° change in direction can be made without forward motion of the vehicle. The location of the steering trunnion at or near the center of the vehicle also results in almost perfect tracking of the front and rear wheels. This feature is important in avoiding obstructions, and, at the same time, enables the vehicle to be readily steered either forward or rearward as the steering characteristics of the vehicle do not change with direction (8). This characteristic makes possible the ready use of dual controls which enable the operator to drive forward or backward with equal ease; and this greatly reduces the need for maneuvering and turning in the forest (Figure 7).
One factor noted in tests is that articulated steered tractors are considerably more unstable when turning uphill than skid or four-wheel steered machines.
Articulated wheeled tractors are used in two principal ways:
(a) to carry loads of wood on the rear unit of the machine (to forward);(b) to drag long length timber behind it with the front end of the timber raised off the ground (to skid).
FIGURE 8. - Koehring Forwarder in operation.
FIGURE 9. - The Packjack Forwarder (Timberland Ellicott Ltd.).
AS A FORWARDER
One of the original developments of the articulated tractor, that of the Canadian pulp and paper industry was as a short wood transporter. The function of the machine was to back up to a prepared stack of wood in the forest, and to raise it up and carry it out to a road or prepared landing. This development proved successful, but was replaced while still in the development stage by a machine which had a hydraulic boom and grapple mounted on it to permit the loading of small piles or individual sticks. It had been found that the productive capacity of the original machine was restricted by the size of the piles of wood prepared in the forest. It was not always possible to put up piles which would give an optimum size load for the forwarder. In the case of the later machine it was always possible to bring an optimum-sized load out of the forest.
Forwarders have proved very successful on operations where wood is produced in short lengths 240 cm (8 feet).
A variety of forwarding articulated tractors are now available. The unit illustrated in Figure 8 was the original design, and to date it has proved to be the most successful unit available. A few details on this unit are in order.
General specifications - Koehring Forwarder
|
Gross weight |
22,500 kg (50,000 lb) |
|
High flotation tires: diameter |
197 cm (6 ft 6 in) |
|
Overall dimensions: wide |
310 cm (10 ft 2 in) and |
|
long |
716 cm (23 ft 6 in) |
|
Minimum ground clearance |
69 cm (27 in) |
|
Center frame articulated steering |
|
|
4-wheel drive |
|
|
Differential locks |
|
|
Floodlights |
|
|
All-weather cab |
|
|
No men on ground |
|
|
Forwarding capacity |
3.5 cunits |
|
Average payload |
3 cunits |
|
Loader capacity |
0,5 cunit |
|
Loader reach |
426 cm (14 ft) high at |
|
|
300 cm (10 ft radius |
This particular machine is designed to handle 240-cm (8-ft) length wood. With its large diameter wheels and 69 cm (27 in) ground clearance and articulated chassis, it has a high degree of mobility over a wide range of rough ground and deep snow.
The general requirements that have been established for an efficient forwarding tractor are high mobility, an efficient loading and unloading system, a large payload which can justify long forwarding distances, and an ability to operate around the clock throughout the year, in all weather. Figures 9, 10 and 11 illustrate three other versions of forwarding tractors that are under development.
Considerable effort had been made to develop a suitable bundled wood forwarder with the rigid frame wheeled tractors. These were for the most part unsuccessful mechanically because of the forces imposed on the tractor while carrying up to 3 tons on its back and moving cross country.
The same method of forwarding bundled wood has been recently successfully applied to articulated wheeled skidders. In this instance, because of the articulation the load is carried, well balanced, on the rear half of the tractor and little stress is placed on the machine as a whole. In Canada, this technique is called "pack-sacking" (to carry in a bundle on one's back) (Figure 12).
The disadvantage of this method of moving wood in short lengths from the stump to the roadside is that it still only handles piles of the size prepared in the forest.
Some present models of tractors have both the integral arch for skidding tree lengths, as well as the packsack for forwarding bundled short wood. This makes the units more versatile.
AS A SKIDDER
Articulated steered wheeled tractors have found their widest application in logging as skidding machines. Trees are felled and limbed, or not, as the case may be and attached by means of a light cable choker to a main cable attached to a powered winch at the rear of the tractor. By winching in the main cable, the trees are drawn together with their forward ends raised off the ground, under the integral fairlead arch at the rear of the tractor.
Skidding whole trees is not yet widely practical, although it is currently being done on an experimental basis in several countries.
Skidding of tree lengths, trees with the tops and limbs removed, is now widely considered to be a conventional method of logging. In North America trees are universally skidded butt foremost while skidding top first is common in Scandinavia. Each method has certain advantages and disadvantages.
Skidding top foremost
(a) Directional felling of trees is possible, which permits easier choking.(b) When choking tree tops, flexible light tops permit moving manually to assist this operation.
(c) Tree tops being small permit a number of trees to be choked at a time.
(d) Shorter chokers can be used.
(e) At the landing, tree tops are relatively uniform but the* butts will be irregular. This may act to slow down the cross-cutting operation at this point.
(f) Tree tops are flexible and this may result in poor loading of rear end of tractor, the bulk of the weight being on the ground still. This can reduce the tractive effort possible by the tractor.
Skidding butt foremost
(a) Trees for the most part are felled parallel to one another.(b) Longer chokers are required to reach the butts of trees, as these are generally too heavy to move together manually.
(c) Skidding butt foremost seldom permits the choking of more than one tree per choker, unless the trees are small.
(d) Winching butts foremost up under the fairlead of the tractor transmits the greater part of the load to the rear wheels of the machine and permits maximum tractive effort.
(e) Tree butts will be relatively uniform in alignment when the load is dropped at the landing. This facilitates the cross-cutting operation at this point.
(f) Skidding full trees butt foremost in cold weather often results in the removal by breakage of a large number of branches, which reduces the limbing operation at the landing.
A brief description of a typical skidding operation may be of interest. Assuming a five-man crew, the distribution of manpower will be:
Two men - felling, limbing and topping trees
One man - operating the skidding tractor
Two men - cross-cutting trees on the landing and piling wood.
The job layout is normally rectangular in shape away from the landing or haul road so that the fellers operate along the long face of the forest stand. With the use of power saws stumps are cut low, and there is seldom need to clear out specific skidding trails for the tractor. In Canada, at least, the trees are generally felled at right angles to the direction of skidding. This permits the butts of the trees to be choked without interference by the tree tops. The tree lengths roll sideways and forward when winched in, and their ends clear the stumps. It has been found that, if the tree tops are left lying at right angles to the direction of travel of the skidder, they can be driven over and help to level the terrain; they do not become caught up in the load, as is the case when they lie parallel to the direction of skidding. It is normal for the tractor operator to follow a favorable path from the forest but, if ground conditions are poor, it may be desirable to pass over new terrain each trip. If limbs and tops are thrown into holes and thrown clear where ground conditions are favorable, the fellers can influence the efficiency of the tractor.
The operation of the tractor controls the whole operation to a high degree. For this reason the operator should be a man with a feel for the machine, with judgment, and with initiative. He must know the capabilities of the tractor as to obstacles, slopes, and maneuverability. He should know when he must drop his load on the ground, advance while paying out the cable, and then winch the load forward into position again. Any machine can be destroyed by abuse, and the good operator is one who has brought out a maximum volume of wood at the end of the day with a minimum of delays for operating or mechanical reasons. A slow steady performance is much more desirable than a rapid erratic one.
Usually the operator fastens the chokers to the trees to be skidded, although often one of the fellers will assist. If the basis of payment is a sum payable to the whole crew for the day's output, there will be much more co-operation among the crew to maximize its output. Prechoking trees has been attempted widely but to date it does not appear economic. Prechoking consists of having an extra set of chokers and fastening these to prepared tree lengths while the tractor is taking a load out to the landing.
Cutting to length on the landing and piling the wood is important, and it can often be a bottleneck to the whole operation. There is a trend, in Canada at least, to have a skidding tractor equipped with a front-end or rear-end loader to pile the logs mechanically. The landing men must select the products from the tree lengths, that is, sawlogs and pulpwood, must cut them to length accurately and pile them properly to permit scaling and loading (Figure 13).
The factors which have been established as having the greatest effect on productivity are tree size (important because of its strong influence on load size), load size, and distance traveled loaded. These points are well illustrated in Figures 14 and 15, which are representative only but are based upon extensive studies conducted by the Pulp and Paper Research Institute of Canada (10). It was found that the high travel speed of the wheeled skidders and the relatively high proportion of total time devoted to choking and unchoking the load meant that productivity was only slightly affected by distance (Figure 14). Productivity was found to be sensitive to the load carried. Loads should be in excess of 0.6 cunits. It was found that, at a constant distance, an increase of 0.1 cunits in load volume would increase production per effective machine hour by 0.12 to 0.23 cunits (Figure 15).
FIGURE 14. - Effect of volume per load and distance loaded on productivity.
FIGURE 15. - Effect of volume per load and distance loaded on productivity.
Figure 16 illustrates a new development which is expected to grow rapidly. It is a chokerless skidder. By means of an articulated hydraulic powered boom and grapple, the skidder operator loads the rear bunk of a wheeled tractor with the butts of trees. The bunk is hydraulically operated to clasp and hold the butts in place. This type of machine will eliminate the use of skidding cables and chokers and will eliminate the need of a man on the ground to choke the trees.
There is every indication that articulated steered wheeled tractors will find everincreasing uses in the forest beside forwarding and skidding wood. Figure 17 illustrates several possibilities.
The appendix lists coefficients of frictions established when skidding both full trees and tree lengths under a variety of conditions commonly found in forest operations (11).
(1) Skogsarbeten. 1964. Ekonomic, No. 9, Estockholm.
(2) RETTIG, GEORGE P. 1958. Obstacle performance of wheeled vehicles. Detroit, Michigan, OTAC, Research and Development Division, Land Locomotion Research Branch, Ordinance Corps. Report No. 29.
(3) BEKKER, M.G. 1965. A proposed system of physical and geometrical terrain values for the determination of vehicle performance and soil trafficability. Volume 2, Interservice Vehicle Mobility Symposium, Hoboken, N. J., Stevens Institute of Technology.
(4) U.S. DEPARTMENT OF THE ARMY. CORPS OF ENGINEERS. 1962 Operation Swamp Fox I: terrain and soil trafficability observations. Washington, D.C. Technical Report No. 3-609.
(5) HENNING, W.W. 1953. Steering of track-type vehicles. S.A.E. Transactions, 61.
(6) GRABAN, W.E. 1964. Terrain evaluation for mobility purposes. Journal of Terramechanics, 1 (2).
(7) HARRISON, W.L. et al, 1959. Mobility studies. Detroit Michigan, OTAC, Research and Engineering Directorate, Research Division, Land Locomotion Laboratory. Report No. RR-5.
(8) McCOLL, B.J. 1962. The Bonnard prehauling unit. Pulp and Paper Magazine of Canada, 50 (1).
(9) BELL, JAMES. 1965. Fundamentals of mechanical logging: the short wood system, engineering features. Montreal, Canadian Pulp & Paper Association, Woodlands Section. Index No. 2337 (B-1).
(10) BENNETT, W.D., WINER, H.I. & BARTHOLOMEW, A. 1965. Measurement of environmental factors and their effect on the productivity of tree-length logging with rubber-tired skidders. Montreal, Pulp and Paper Research Institute of Canada.
(11) BENNETT, W.D. 1962. Forces involved in skidding full tree and tree-length loads of pulpwood. Montreal, Canadian Pulp & Paper Association, Woodlands Section. Index No. 2162.
Coefficients of comparative frictions
|
Test no. |
Species composition of loads |
Trail description |
Skidding coefficients |
|
|
Full tree |
Tree length |
|||
|
2 |
Mixed² |
Hard-packed snow |
170 |
.173³ |
|
3 |
Jackpine |
Gravelly loam |
.626 |
.487 |
|
4 |
Jackpine |
Silty sand |
.541 |
.460 |
|
5 |
Black spruce |
Organic (80%) |
.343 |
.279 |
|
6 |
White spruce |
Organic (89%) |
.452 |
.417 |
|
8 |
White spruce |
Organic loam |
.506 |
.340 |
|
8 |
Balsam fir |
- ditto |
.7574 |
.340 |
¹ Trail description.Texture. (e.g., gravelly loam). This is the average of laboratory analyses of the surface soil samples taken before each trip, in each area (24 samples for each species).
Moisture. Moisture percentage is the average for all samples taken on each skidding trail (24 samples for each species).
Cone index. Cone index was measured before each trip (24 readings for each species).
Stumps. The basal area of each stump on the trail was measured, and the percentage of the trail area occupied by stumps calculated before the trials started in each area.
² In all other areas individual loads were composed of one species only.
³ The only case where full tree loads were apparently easier to skid than tree length loads, although the difference is not significant. Two factors were probably the main contributors to this: the smooth, hard trail, with no obstructions, and the difficulty in assembling full trees without losing many branches (they were very brittle due to the extreme cold (to - 30°F).
4 The only evidence of a species effect. Actually due to a combination of the saturated organic/loam muck of the trail and the unusually branchy and heavily foliaged condition of the balsam fir. No species effect was observed in any tree length loads, i.e., the skidding coefficient was the same for all species in any one area.
NOTE. Specifications of machines mentioned in this article will be included in a Forestry Equipment Note which can be obtained on request from the Forestry Equipment Section, Food and Agriculture Organization of the United Nations, Rome, Italy.
