Upgrading Horse-Drawn Logging Wagons
by Paul Schmit of Luxembourg
Hybrid Technology for Horses?
Horse-drawn logging wagons manufactured in Sweden and Finland are offered either for a single or a pair hitch. However, for full-time horse entrepreneurs, offering their services to wood owners and other forestry companies and thus relying on the efficiency of their work, even under sub-optimal working conditions, some extra motive power is sometimes required.
Finnish made, big size logging wagons are available with an additional hydraulic drive on one or two wheels of the rear boogie (or tag) axle. The hydraulic pressure is generated by the wagon’s combustion engine in a range from 13 to 25 hp, mounted at the carter’s platform at the front, and usually powering the logging crane by a hydraulic pump. In general, this supplementary power is only activated under very heavy working situations, such as for moving a fully loaded wagon over tree stubs or off a ditch along the forest tracks. As the tread of trailer tires are not designed for traction, tire chains must be mounted on wet or snow-covered ground to assure the needed grip.
Until now, with just a few exceptions in Scandinavia and France, this technology of a hydraulic “extra push” hasn’t really found its way into the European forests. Another option could be an electric assistance. In November 2011, a comparative test of three different draft force measuring technologies from Switzerland, France, and Luxembourg was carried out at invitation of the the Swiss National Stud Farm (SNSF) for the Freiberger horse in Avenches. As a test vehicle, a single-horse forecart with electric assistance was supplied by the Swiss Meterus Sárl from Estavayer-le-Gibloux. This company specializes in hybrid-carriages for tourist transport and hybrid-forecarts, mainly for city works, in the hilly areas of Switzerland and Eastern France.
It proved that the electric assistance was easily adjustable, as the power of the extra electric motor, coupled by a gearbox to the rear axle, is governed by an electronic control unit, which reads constantly the draft force on each singletree and can even visualize parameters for the carter on a screen. However, even if these true hybrid vehicles have reached a high development stage and practicality, their price limits the number of customers.
The fact that electric or hybrid passenger cars, and even electric powered agricultural implements like GPS-guided precision seeders and planters, are promoted by the media and politics now, could tempt us to jump on this bandwagon and further develop other hybrid technologies for animal traction. However, taking into consideration the current discussions about the sustainability of battery production, their life cycle and recycling, as well as the environmental impact of electricity generation in general, we would partly give up some of the main arguments for the use of work horses, which are their 100% renewability on a local level, and their eco-friendliness, compared to any other source of motive power currently available in our high-tech world.
Furthermore, every battery driven vehicle needs a so-called BMS, an electronic battery management system, which not only regulates the charging and discharging of the batteries, but also maintains them in proper working conditions during the time without use. In general, equipment for work horses is characterized by its simplicity and its possibility for on-farm repair. Introducing electronic control systems in horse-drawn machinery would negate these advantages. Therefore, our further research targets should be optimizing pure mechanical solutions.
How Horses Pull
Some research about the draft characteristics of work horses was already done in April 2015. These trials were carried out in Luxembourg with a Swedish made SV5, a so-called “eight-wheeler,” from the Österby Smedja company from Finspång and the Swedish Ardennes gelding Loke. To assure identical measuring conditions, an asphalted field path was chosen as test track and the needed load for the horse was given by a varying slope from 8 to 12%.
Contrary to the measuring setup used for the forest trials discussed here below, where the draft force sensors were mounted on the singletrees, so-called miniature force gauges were inserted into the front tug straps on each side of the horse. The datalogger was saddled on the horse’s back pad.
By using the advanced analysis software MathWorks MATLAB and OriginLab Origin, a Fourier transform of the measured data was carried out. This showed that the frequency spectra for all test runs were marked by a recurrent, and only slightly variable, oscillation between 0,71 and 1 Hz. This prominent value can be directly assigned to the alternating movement of the horse’s shoulders. The higher frequency values were measured with an increased load, which causes in general a pronounced divergence from the horse’s natural pattern of movement.
Moreover, these dominant frequencies were superimposed by other vibrations, creating two other distinctive frequencies, which varied between 1,44 and 1,96 Hz respectively 2,78 and 4 Hz, this depending on the hitch configuration. Besides the traditional steel traction shafts, leather and hemp rope traces were tested as well, fitted alternately with various steel or synthetic rubber draft springs. In general, the draft springs showed an amplifying effect on the frequencies, on average an increase of 20%. This can be explained by the fact the springs themselves act as an oscillation system in interaction with the elastic traces. Finally, it could be concluded that the rolling or dragging characteristics of the equipment to be pulled also influences the draft force oscillations.
All these variables make it very difficult to match the work horse, as a living being, with other assistance systems, whether they are hydraulic or electric powered.
For the time being, the above raised problem of extra power on logging wagons could be solved by adding a third horse to the usual pair hitch and by investigating more about the pulling characteristics of such a hitch, mainly the balancing of the horses.
The three-abreast hitch offers some advantages over other multiple horse hitches as the overall length does only slightly increase, compared to a pair hitch, and a setup with right and left side lines going to each of the horses’ heads can be realized by adding another couple of cross cheeks to standard team lines, if these are long enough. Contrary to ploughing, where the side draft must be considered, this hitch can be centered to a wagon’s longitudinal axis and with correct evener lengths, one could presume that all the horses are perfectly balanced one against the others.
But is this True?
That’s the question, which measuring trials under real working conditions in a Swedish forest near Svenljunga should answer on November 6, 2021. As a logging wagon, a Finnish made mid-size Motti-Hepo wagon from the Åfeltin Työhevoset Oy company based in Ikaalinen was used, which was pulled by the Percheron and Swedish Ardennes/Shire crossbreed horses Granit (left-side/ blue curves), Viola (centre/red curves) and Lotta (right-side/green curves). The equipment and horses are owned by Linus Elmelid, full-time farmer and forestry horse entrepreneur, whose father Bruno raised the question about the balance of the horses during a visit in September 2021. Initial discussions on this topic were already held during a meeting in August 2014 with another Swedish master horseman Lars-Åke Johansson at the Ryasjö Hästgård farm.
Indeed, as the following drawings show, there exist two different hitch options for Scandinavian logging wagons. The variant with traction shafts balances the horses by a so-called “offset” evener composed of 2/3 vs 1/3 levers with an attached pair evener on the short lever and an additional singletree for the third horse on the longer lever. As the centre horse walks in the longitudinal axis of the wagon, the tongue needs an offset. Considering the fact that the whole hitch assembly gets rather heavy, a coil spring usually relieves the front end. On some logging wagons, the eveners can even be raised or lowered by a hydraulic piston to match the horse’s size and to overcome extreme working conditions by varying the draft angle.
The variant with traces, as used on the logging wagon to be tested for this report, balances the three horses by a hitch, which is referred to as “scissor-action“ or “centre fire” evener in the US or “Scottish-pattern” yoke in the UK. Here the centre horse is balanced against the two outside horses by two 2/3 vs 1/3 levers. Instead of an offset-tongue, shafts permit the carter (teamster) to steer and brake the wagon. However, both actions have to be assured by the centre horse, contrary to the traction shafts variant, where two horses steer, and all three horses can brake or reverse the wagon.
Usually, Scandinavian logging crane wagons, are equipped with a dual circuit braking system activating brakes on the front wheels and two rear wheels. This is not only a safety feature, but also lowers the horse’s efforts in sloppy terrain. On steep downhill runs with a fully loaded wagon, it is necessary to have both braking options, the hydraulically activated brakes on the wheels, and the horses, as the wheels tend to slip, especially on wet or snow-covered ground.
After the testing of various Swedish made single-horse logging equipment in a forest near Herrljunga in August 2015, this was the second time that our electronic measuring equipment was in use under real working conditions in Swedish forest. Each of the singletrees on the logging wagon was fitted with a force sensor with a nominal load of 5 kN, and the data logger was fixed to the safety rail on the carter’s platform. The summary measuring rate was set to 100 Hz resulting in a frequency of 25 Hz for each of the four dimensions (time, left-side, right-side, and centre horse draft force).
Each of the four trials, with an average duration of 1 min 40 sec, was carried out twice to statistically secure the measurement results. The first test runs were made with an unloaded wagon. As the summary Graph 5 on page 30 shows, the required tractive effort for the logging wagon with a tare weight of 1350 kg and running on six 11.5/80 – 15.3 14PR pneumatic wheels, is quite considerable, but remains with average value of 0,75 kN per horse in an acceptable “heavy load” range, following the SmP classification.
The following test runs were made with a load of approximately 1000 kg. This additional load increases the required tractive effort of the three horses about 60%. To reach this performance level, a sufficient training condition for the horses is essential. In contrast to farm work, where the physical exertion is usually necessary over a longer time, the forestry work offers the horses the opportunity to regenerate through its numerous breaks when loading and unloading the timber.
During the first loaded runs, the centre horse showed the highest draft effort, contrary to the unloaded runs, where the right-side horse was pulling stronger. As the test track was driven counterclockwise, the unloaded runs could lead to the conclusion that the right-side horse was pulling stronger, as it had to run a longer distance than the inside horse during the turnings, but this assumption wasn’t confirmed by the other trials.
Besides the average values of the measured draft forces, the oscillations, as well as the maximum force peaks, are of interest, as these values are also a measure for the strain of the horses. In general, as mentioned already, the draft force oscillations, characterized by the calculated values of the mean deviations as indicated in the following graphs, increase with the horse’s effort. A correlation to the draft force average values could be proved here, as an increase of exactly 60% could also be determined between the unloaded and loaded runs. This means nothing other than that the horse’s sequence of movements becomes less even, and also less efficient, with greater physical exertion.
As expected, the maximum force peaks increased with the load. During the empty runs, only maximum peaks of 4,5 kN could be measured. As shown in the graphs on following pages, during the loaded runs with more lever on the evener for the left-side horse, the chock loads attended 6 kN per horse, corresponding to an impact of more than 600 kg on the horse’s shoulders. This makes it very clear that this type of work is nothing for beginners, both the horses and the carter, as horses must be steered with foresight and caution between the trees and around or over obstacles like tree stubs or rocks, always considering the extra width of the three-horse hitch.
Theory Meets Practice
As the main evener of the logging wagon is fitted with three different hooking possibilities for both outside horses, two additional trials were carried out with more and less lever for the left-side horse on the evener bar. Contrary to the expectations, the measuring results didn’t show any characteristic differences. This could be explained by the fact that the pulling action of the horses is a highly dynamic process. If the horses would develop a static, thus uniform and straight, draft force, a difference would surely be measurable. But under the working conditions in the forest, the constantly varying position of the horses in the hitch and the very uneven load, due to the strongly changing ground conditions and rolling resistance of the wagon, are predominant here. To be able to determine a striking difference here, an even larger offset on the evener might be necessary.
Many thanks go to Freja, Linus and Bruno Elmelid, Lars-Åke Johansson and Sven-Olof Salomonsson, who made this report possible. As very often in life, the more you deal with something in detail, more new questions arise instead of finding a conclusive answer to the initial question. Much remains to be done in the future and these lines end with the hope that we can face this challenge together, for the horse’s welfare and for the further technical development of our common cause. Please do not hesitate to contact the author, if you wish to contribute in any way.