Timber Wagon: The ÖSTERBY SMEDJA SV5 Forwarder
by Paul Schmit of Luxembourg and Albano Moscardo of Italy
The ÖSTERBY SMEDJA SV5 forwarder was analysed for the transport of firewood logs via horse traction by conducting a test series on different surfaces.
The tests and measurements, which took place in September 2013, showed that, depending on its load and the nature of the soil, this wagon requires a small to medium draft power and is thus suited to a single horse hitch.
The tests also facilitated the evaluation of the interaction between horse and forwarder for different hitch systems; however, no final conclusions could be drawn.
In order to improve the comfort of the horse at work, the activation of the brakes could be improved. Furthermore, a modification of the load securing system could improve safety. Further information about a horse’s acceptable draft power can be found at www.schaffmatpaerd.org.
New equipment for draft horse use in silviculture (growing trees) is commercialized in Sweden at present by five companies, mainly specialized in forwarders and logging arches. In this sector, Sweden is the European leader.
This equipment is primarily adapted to the needs of forest enterprises in Scandinavia. Thus the forwarders are designed for short and small wood, for loading via hydraulic crane or an electric winch, or for manual loading without tools.
This equipment is also adapted to the local topographical conditions. The rocky forests require strong off-road capabilities.
In order to reduce dependence on fossil fuels, heating via wood burning is gaining new importance in both Northern Europe and other European regions. In this context, horse traction is also undergoing a renaissance. One of the goals of the non-profit association Schaff mat Pa?erd is to support this process through the development of new equipment and related studies and publications.
This test report analyses the ease of use of the SV5 forwarder from the Swedish manufacturer ÖSTERBY SMEDJA in the transport of firewood. The designation SV5 is a reference to the fifth development stage of the skogsvagn (wood wagon). The fifth version differs from the fourth, unbraked version by having a braking system mounted on the four front wheels.
The wagon used for the tests was a first prototype of the SV5 equipped with drum brakes from the Italian manufacturer GKN FAD with mechanical activation via levers. The actual version employs a brake system with floating discs from the Swedish manufacturer ISR, entirely incorporated inside of the eight-inch wheels, with hydraulic activation via a master brake cylinder. The wheels of this vehicle, which is also called 8-hjulsvagn (8-wheeler wagon) are mounted on two boogie axles, which together with a low centre of gravity guarantee outstanding off-road performances.
Beside this particular use in forest, this type of wagon is also suitable for other transportation, for example in the maintenance of natural reserves or other structural elements in nature.
In order to guarantee identical measurement conditions for each test and to be able to evaluate the tractive effort required on different soils, the tests were conducted on tracks and meadows, rather than hilly forest ground. Because the topographic, economic and social situations in much of the world do not favour big hitches, this report focuses on a single animal hitch.
Technical Characteristics of the Wagon:
|Model:||ÖSTERBY SMEDJA SV5|
|Tare weight:||318 kg|
|Gross vehicle weight:||2000 kg|
|Weight distribution (empty):||54% front axle / 46% rear axle|
|Weight distribution (loaded):||34% front axle / 66% rear axle|
|Total length:||3000 mm|
|Total width:||1370 mm|
|Total height:||1220 mm|
|Loading length:||2000 mm|
|Loading width:||1280 mm|
|Loading height:||720 mm|
|Wheel base:||1470 – 2370 mm (steplessly variable)|
|Track width:||1120 mm|
|Width of single tree:||800 mm|
|Spring rate of draft springs:||17.5 N/mm|
- 2 rectangular steel 30×50 mm hot-dip galvanized reciprocally articulated around the vertical and horizontal axis
- inner width: 450mm front / 750 mm rear
- length: 2050 mm
Shafts for single tree:
- 2 DN25 stainless steel tubes articulated forficate around the horizontal axis
- inner width: 800 mm front / 1080 mm rear
- length: 2640 mm
Horse in the test:
|Breed:||Swedish Ardennes (ID-Nr SE 23-03-9001)|
|Date of birth:||20.01.2003|
|Collar size:||25″ full sweeny|
Materials and Methods
The tests took place on the 15th, 22nd and 29th of September 2013 on a meadow of the SCHMIT-LAROCHE farm in TUNTANGE (LUXEMBOURG) and tracks in TUNTANGE township.
To evaluate the required tractive effort, the draft force and working speed were measured on the following three soil types:
- Course A (approximately 440 m long): mown meadow with dry soil and short grass
- Course B (approximately 610 m long): dry tarred cart track
- Course C (approximately 840 m long): forest track with dry hard soil
These courses were flat, except course C, which had a small slope halfway along with a gradient of 3.5 – 4.5%. All measurements were taken between 09.00 and 10.30 a.m.
For each soil type, the following three hitch variations were analysed:
- Series 1: traction with leather traces in combination with draft springs
- Series 2: traction with leather traces without draft springs
- Series 3: traction with articulated traction shafts
In the graph legends, the first number indicates the test series (1, 2 or 3), the second number indicates whether the run was empty (1) or laden (2), and the letter indicates the type of soil (A, B or C). Thus, Graph 2.1B, for example, shows the values taken during the second test series with leather traces without draft springs and with no load on tarred cart track. It should be noted that the unloaded measurements were taken in the opposite direction to those with loads.
The measuring device consisted of the following components:
- A Lorenz K-12 force sensor mounted between the single tree or the traction shafts and the draft yoke at the front of the wagon;
- An AHLBORN FUA9192 speed sensor mounted on the rear axle with optical scanning of an impulse disc mounted on the second wheel from the back on the left;
- An AHLBORN Almemo 2690-8 data logger with memory connector and micro SD card mounted on the front frame of the wagon.
As well as the tractive effort generated by the single hitched horse, the steering of the wagon by the horse, the activation of the brakes and the loading/unloading by the carter were examined. The connection between the evener and the double-stitched leather traces with inner-webbed nylon reinforcement was achieved for the first test series using draft springs from the Italian manufacturer EQUI IDEA with a spring rate of 17.5 N/mm, mounted on both sides. The same horse of the Ardennes (SE) breed named Loke, 10 years old and belonging to the SCHMIT-LAROCHE farm, was hitched for each measurement. The angle of draft was set at 15° for each measurement. The wagon’s load was 784 kg of oak cord firewood.
In order to guarantee the statistical significance of the measurements, each measurement was executed twice, with a measurement time per partial measurement between 04 min 31 s and 11 min 59 s. The measured values were recorded with a summary measuring rate of 100 Hz resulting in a frequency of 33.33 Hz for each of the three dimensions (time, force, speed). In order to not exceed the scope of the current report, the graphs below represent just a selection of the results, providing an overview of the characteristics of the SV5 forwarder. Graphs 5 and 6 show the average of the two tests (empty and laden) with no substantial noticeable differences.
Graphs 1.1A and 1.2A show the effect of the wagon load on tractive effort. When empty, the curve of the draft force is very regular and the maximum, recorded at the start, is 1,040 kN. When laden, starting the wagon produces a maximum of 2,303 kN and an average increase in draft force of 47 %. The curve representing the draft force is significantly more irregular, with an increase in standard deviation of 68%.
Graph 4, which displays the results of static tests on different draft springs, conducted by SmP in 2012, shows that the maximum, measured at the single tree when starting the wagon laden, cannot be absorbed by the draft springs as it exceeds the sum of the maximum values of the two draft springs mounted in the two traces of the harness. The limit of the elongation of the EQUI IDEA draft springs is reached at a maximum draft force of 0.808 kN per draft spring, with a corresponding elongation of 45 mm. For safety reasons, the elongation of the springs is limited by two hooks inside the coils.
Comparing Graphs 1.1A and 1.1B shows the effect of the surface on tractive effort. The movement of the empty wagon on grass increases the average draft force by 80% compared to the tarred cart track. Traction on grass is also considerably more irregular, with an increase in standard deviation of 54% compared to asphalt.
Graph 5, which summarizes the results of all measurements, reveals that the effects seen in the hitch with leather traces and draft springs are also seen in the hitch with leather traces without draft springs. For both of these hitch variants, the traction of the empty wagon on meadow requires the same effort as the loaded wagon on forest track. The direct traction of the horse on articulated shafts presents very specific characteristics, which will be discussed below.
Surprisingly, the draft springs did not reduce the tractive effort of the horse. As shown in Graph 2.1B, for example, the traction of the empty wagon without draft springs on the tarred cart track required a draft force of 0.090 kN, whereas the effort with draft springs for the same work came to 0.143 kN. The standard deviations of the two measurements are nearly identical, at 0.067 kN and 0.068 kN, respectively. Without draft springs, a maximum of 0.985 kN was measured at the start, whereas a similar maximum with draft springs during the pull was 1.014 kN. As shown in Graph 5, this effect has also been demonstrated on the other surface types, with the wagon both loaded and empty. According to these results, the traction of the wagon was easier without draft springs.
Graph 2.1C shows the journey along forest track. By comparing these values with those of the other surfaces, it is clear that traction on this type of track falls between that for grass and asphalt. The down hill going on the small slope at halfway is visible in the centre of the curve, where no tractive effort is required. Graph 2.2C clearly shows anew the effect of the load, which results in a higher draft force and thus significantly more irregular movement. The higher effort required to surmount the slope halfway is also clearly visible.
The direct traction of the draught horse on articulated shafts, a characteristic of the hitches in Scandinavia, was initially developed by the Swedish army for moving military equipment in rough terrain. This method of hitching was later used in agriculture and silviculture, not just for equipment pulled on wheels, but also for ground working tools. As expected, this method of hitching displays totally different characteristics compared to traction by traces and single tree.
As illustrated in Graphs 3.1B and 3.2B, the shafts are not only put under traction, they are also nearly constantly under push. Grass was the only surface where the curve for the draft force was positive for the majority of the distance, due to the higher rolling friction and thus a more substantial tractive effort. On asphalt, the lighter rolling and the inertia of the laden wagon, produce considerably more pronounced negative troughs, equivalent to a push of the wagon on the horse.
The values outlined in the red bars of Graph 5 for the traction via articulated traction shafts show the really measured values. It should be noted that the calculation of the average values of the draft forces took also the negative values into account. For the other hitch types, these forces haven’t been measured by the force sensor as they were directly transmitted by the shafts to the breeching of the harness and not to the sensor mounted between the single tree and the draft yoke.
The green bars take account of the positive values for the average value calculation, equivalent to the pure draft forces. From this it results that the articulated traction shafts require a higher effort of the horse than the hitch with traces and single tree. In comparison with the hitch via leather traces with draft springs, the draft force increases by an average of 46% for the empty wagon and 8% while being laden. As against the hitch without draft springs an increase of the draft force from an average of 92% in empty condition to 26% for the laden wagon was measured.
On the hitch with traces and single tree, the adjustment of the breeching straps in a direct and horizontal line between the breeching and the shafts ensures that the push forces are directly transmitted to the breeching. As illustrated in the picture above, which shows the Scandinavian version of the hitch, part of the push forces while braking, stopping or the downhill pull are also transmitted to the back pad and the belly band. The flexible front traces stop the collar from being lifted off the horse’s shoulders during this phase.
As indicated in Graph 6, the average working speed varied little across the different surfaces. A slight increase by an average of 4 – 8% could be measured when returning with a load. This variation is more psychological than technical, and is due to the desire of the horse to return to its herd.
As standard, the SV5 wagon is equipped with a foot pedal to be activated by the driver sitting on the steel floor or the wood loaded to the wagon. To allow the activation of the brakes by the carter walking at the side of the hitch, SmP has mounted a crank pulling a steel cable linked to the brake pedal. Located within easy reach of the carter, this device needs a small activating force and allows for precise setting of the brakes, but requires too much time for activation and deactivation.
The wagon made little noise during work, except during the activation of the drum brakes, which produce a significant grinding noise. Walking on the left side of the hitch affords the carter a good view to the front and the rear and good non- verbal communication with the horse. Thus, with sufficiently short lines, sensitive contact with the bitless horse is assured, and it is possible for the horse to see the carter.
Stability and manueverability are both high. The low centre of gravity in combination with a relatively large track width makes U-turns with pronounced steer angles of the front axle possible without risks. Steering the wagon requires an acceptable effort from the horse when this movement is carried out during rolling. The transfer of the centre of gravity to the rear when the wagon is laden helps to lower the steering force by taking the load off the front axle (see technical specifications).
However, when the horse steers the wagon, the forefront of the articulated traction shafts can pinch the horse’s belly at the inner side of the turn. In this context, the floating shafts combined with the single tree do not have this disadvantage thanks to their increased length. They have also the advantage that they are pushed by the well-muscled shoulders of the horse and not by the ribs. The disadvantage of this type of shaft is that they can get caught up more easily in the forest.
Manually loading and unloading the wagon was easy thanks to the suitable loading height. The wagon’s maximum load could not be verified, as the adjustable stakes at the front and rear of the wagon were not sufficiently high to prevent logs falling off the wagon.
The load on the back pad of the harness with articulated traction shafts at standstill of the wagon was measured at 8.80 kg, and the load for the non-traction floating shafts at 7.60 kg, which is comfortable for the horse.
For each surface tested during these comparative tests, the tractive effort for the horse was light to moderate. As noted during other tests (see SmP 2013 reports in SFJ Vol. 39 Issue 3 and Vol. 39 Issue 4), the oscillations in the draft force increase with the effort by the horse.
The fact that using draft springs does not reduce the average value of the draft force agrees with earlier results from other tests on farm and forest equipment by SmP in Italy, Sweden and Luxembourg, which can be found at www.schaffmatpaerd.org. Even if draft springs can be advantageous in certain conditions, it seems very difficult to adapt the characteristics of the draft springs to the different machinery and tools.
In order to improve the activation of the brakes, the crank could be replaced by a brake lever, which would allow for faster activation. Furthermore, the adjustable stakes could be lengthened in order to increase the maximum load of the wagon, while simultaneously improving safety during transport. The present tests have shown that the loading of the wagon can be increased for the tested surfaces without overtaxing the horse.
Additional tests must be conducted to determine the effect of the floating and articulated shafts on the working comfort of the horse, especially in terms of the support forces of the shafts on the back pad and the steering forces in different working situations. The same applies to a detailed analysis of improvements to draft springs. Comparative tests also need to be conducted on hilly terrain.