TRACTION PROPERTIES OF TIRES ON TURFGRASS UTILIZED WITH VARYING INTENSITY

Włodzimierz Białczyk¹, Anna Cudzik¹, Jarosław Czarnecki¹, Katarzyna Jamroży¹, Karol Wolski^2
1 Institute of Agricultural Engineering, Wrocław University of Environmental and Life Sciences, Poland
² Department of Agroecosystems and Green Areas Management, Wrocław University of Environmental and Life Sciences, Poland

ABSTRACT

The subject of the present paper are the problems of generating driving forces on recreational grass by traction tires of grass type. The studies examined changes of traction forces generated by these tires on turfgrass utilized with varying intensity. An analysis is presented of changes in the maximum traction forces, friction coefficients, maximum shear stress and compaction. Among the tested tires, the largest traction forces were generated by a tire of 18×9.5-8 for the whole range of changes of perpendicular loads.

Key words: recreational grass, turfgrass, utilization, tires of grass type, traction force, friction coefficient.

INTRODUCTION

Grassland plays an increasingly great role in the environment, and their advantages include very good retention qualities and erosion prevention, stabilization – soil overgrowth. This is especially desirable in mountain conditions. One should not forget about the significant function played by permanent grassland in the vicinity of cities, enabling the inhabitants to relax and creating proper conditions for recreation and sports. Adapting a given grassland to the function it is supposed to perform, the species and variety composition of grass mixtures should be properly selected. Shade-loving species and those that are adapted to moderate utilization have been chosen in city parks so far. This tendency, however, underwent a change after introducing a law on active recreation on the park grass and now – like in sport objects – grasses that are resistant to pressing and friction forces should be selected. The intensity of activities undertaken with the aim of giving the proper form to the recreational grass requires generating high driving forces. At present, the treatment equipment has tires of grass type which are constructed in such a way that – without destroying the plants – the proper traction properties should be kept.

Turfgrass is the surface layer of the soil overgrown with plants' roots, outgrowths and rhizomes that are of grass type [9,11]. Intensive utilization causes a number of changes in the grass eco-system. The species making turfgrass are subject to stress, including low mowing and heavier traffic on turfgrass surfaces. The effect of utilization is not only limited to the aboveground parts of plants, it is also reflected in the underground part. Moving on turfgrass surface increases the soil compaction, which has an effect on the growth and functioning of plant roots. Pressing the soil may cause morphological changes of the roots and disturb their functioning; however, these changes will not always affect the aboveground parts of plants [10]. Despite considerable changes in the morphological structure, the roots – in favourable conditions, i.e. good moisture, abundance of the soil in nutrients and its structure – sufficiently provide the whole plant with water and mineral elements. A lot of grass species find optimum conditions for growth and development of the root system in lightly compacted soil [1]. T. Głąb [3], while studied the yielding of grasses found out that the best parameters of the roots' structure in the context of their role in the characterization of the relation soil-roots were two parameters: root mass (RDM) and root length (RLD). Pressing the soil causes changes in the soil structure. The content of macro-pores decreases and at he same time the proportion of smaller pores with the diameter from 0.005 ľm to 50 ľm increases. Changes in the structure of the soil pores are accompanied by a significant increase of the compaction and bulk density of the soil, exceeding the values commonly accepted as critical for the development of the plants' root system.

Plants sensitive to unfavourable conditions retreat from the grass, changing the botanical composition. The mechanism of resistance to stress conditions was already studied many years ago. The plants' ability to counteract the stress connected with pressing and temporary defoliation caused by the effect of friction forces depends on the species, and even on the cultivar. Different physiological, morphological and anatomic properties of plants were indicated as the factor determining tolerance to unfavourable conditions. The following could have an effect on the resistance of grasses connected with utilization: degree of tissue moisturization, distribution and number of sclerenchyma fibres, turfgrass density and the content of lignin in tissues [2,5]. In 1975, R. C. Shearman and J. B. Beard [6] belonged to the first who conducted studies on physiological, morphological and anatomical properties and proved that grasses differed from each other with such features as healthiness, number of culms, leaf width, the carrying ability of the plant, leaf flexibility, percentage content of water, turgor pressure expressed in percents. Nevertheless, no correlation was shown between all the features of particular species and their resistance to utilization. The analysis of the dependence of tolerance to pressing and cutting on the above-mentioned factors showed that it was only the carrying ability and leaf flexibility that accounted for tolerance to unfavourable conditions.

The condition of grasses and the soil conditions have influence on the generated maximum traction forces. This is confirmed by M. Schreiber and H. D. Kutzbach [7] in their studies on the effect of soil and tire parameters on traction forces. It is thanks to their work that it is possible to predict the behaviour of tires on particular surfaces; however, those studies are of empirical character and they can serve comparative purposes.

Therefore, studies were undertaken which enabled to find out in natural conditions whether changes in soil conditions and in the plant cover would cause significant differences in the size of maximum traction forces generated by tires of grass type.

THE PURPOSE, METHODS AND CONDITIONS OF STUDIES

The purpose of the studies was to show how different levels of utilization affect the changes of:

• turfgrass botanical composition,

• strength parameters,

• traction properties of grass type tires,

• values of longitudinal force (friction coefficients) of grass type tyres.

The studies were conducted on experimental plots situated between the embankments of the Odra river near Malczewskiego street in Wrocław, The plots were set up in 2006 on turfgrass that naturally occurred in the habitat. The bedding was acid brown gleyed soil formed from loamy sand on very fine sandy soil. This kind of soils is susceptible to density. The basic physical properties of the soil were the following: proper density 2.4795 g cm^-3, total porosity 42%.

The botanical composition was established using the botanical-weight method of Stebler-Schröter. The natural turfgrass included the following dominating grasses: red fescue, cocks foot, Kentucky bluegrass, perennial ryegrass, quack grass and soft chess. The percentage of low grasses with a shallow and poor root system in the species composition was 8.1% d.w. (Kentucky bluegrass, soft chess). Low species, with a deep and strong root system (red fescue, blue fescue, fine bent grass) constituted 24.6% d.w. The percentage of plants from the Fabaceae family as well as herbs and weeds, where the species that have deep roots and do not form dense and compact turfgrass was 60.2%, which is presented in Table 2 in the column for 2006. In order to form recreational turfgrass, in the spring of 2007, undersowing of the existing turfgrass was performed with a mixture of Super Sport having the composition that is sown in sport fields (10% of red fescue cv. Leo, 25% red fescue cv. Adio, 25% red fescue cv. Mirena, 30% perennial ryegrass cv. Stadion, 10% Kentucky bluegrass cv. Miracle). These are the species that are characterized by a deep root system and good strength qualities. Definitely the largest percentage in the mixture was for fescues, because they form strong and compact turfgrass. The sowing norm was taken as 12 g·m^-2. The same year, a simulation of pressing was performed and next, after regeneration, the botanical composition was determined. In the following year, the simulation was repeated, strength and traction tests were conducted and again, the botanical composition was established. In the successive years, the species composition was also determined and the changes taking place as a result of intensive utilization of turfgrass were observed.

Weather conditions during the studies as compared to the results of many years were favourable to the growth of grasses since there were a lot of rainfalls and moderate temperatures in spring months, when the most intensive growth occurs. The data presented on Fig. 1 come from the meteorological station of Swojec.

The temperatures of 2008 were higher as compared to those of many years, which – in the case of compacted soil is significant because such a soil warms up much longer than the soil that is not pressed. A very clear difference in temperatures is visible in winter months. The winter of 2008 was mild, which did not cause the freezing of the species with the root mass in the upper layers of the soil. The highest sum of rainfalls was observed for April and August.

Fig. 1. A comparisons of sums of rainfalls and temperatures in 2008 with those of many years

During the experiments, measurements of the green mass were made with the aim of finding out the yearly distribution of the biomass increase. The turfgrass was cut at the height of 50 mm every two weeks. The cut overground parts were picked up and next weighed. Results of those measurements are presented in Fig. 3.

Using a modified method of Weber, the density of turfgass was determined, which means the number of live plants per 100 cm². Those studies were conducted in 5 repetitions enabling a statistical analysis of the results. Fig. 4 presents the arithmetic means from those repetitions.

An additional parameter describing the conditions of studies was the root mass of plants making up the sod on various levels of its utilization. The studies made use of Kopecky cylinders. A sample of the 100 cm³ was taken from the depth of 0.05 m. The roots were rinsed and the sample of air-dry mass was weighed. The values presented in Fig. 5 are the means from 3 repetitions.

The purpose of the studies was to show in what way a change in the intensity of utilization affects – among other things – changes of traction parameters. Hence, regular simulation of the intensity of utilization was performed in order to recreate the pressing caused by vehicles, people and animals. To this aim, a roller was designed and produced on which studs of football shoes were placed and which was loaded in such a way that unit pressures referred to those that of a footballer weighing 70 kg having the shoe size of 42. Assuming that 100% level of utilization refers to the complete damage of turfgrass, the roller was driven so many times that a complete damage of the plant cover was achieved. Additionally, four different levels of utilization were applied, where the number of roller drives was the result of dividing the number of drives completely damaging turfgrass by four. As a consequence, 0% utilization meant the untouched turfgrass, and the following levels of utilization were, respectively, 75%, 50% and 25%. It should be emphasized that each cycle of utilization intensity simulation differed with the number of drives leading to a complete damage of the plant cover, which is 100% of utilization intensity. A model of the studies is presented in Fig. 2. The width of the belt was 1 m. Technological paths of 0.4 m in width were left between particular belts differing in the number of drives. The width of the whole experimental plot was 9.4, while its length was 26 m.

Fig. 2. A model of the studied object

A roller with the diameter of 0.4 m and the length of 1 m was constructed which was used to change the levels of utilization.

Fig. 3. A roller to simulate utilization

The next parameter describing the studied conditions was soil moisture at the level of up to 10 cm. Using a scale-dryer, moisture, which was the means from three randomly selected samples from each plot, was established. The manner of determining moisture was accordant with the Polish Norm PN-EN 13040:2002. Moisture measurement was performed directly before traction tests. Studies presented here were conducted with the moisture of 15%.

3 tires of grass type were selected for the studies. Table 1 includes the characteristics of the used tires.

Table 1. Parameters of the studied tires of grass type

Tire	18×9.50-8	18×8.50-8	18×7.00-8
Tire type	grass	grass	grass
Tire construction	tubeless	tubeless	tubeless
Tread type	turf-saver	turf-saver	turf-saver
Max speed, km/h	30	15	15
PR	4	4	4
Maximum carrying capacity, kG	470	370	350
Maximum pressure, MPa	0.19	0.15	0.11
Dimensions, mm: Height Width Setting diameter	459.7 243.8 203.2	457.2 208.3 203.2	454.7 175.3 203.2

Grass tires are tubeless with the tread protrusions constructed in such a way that the turfgrass should be minimally damaged. Traction forces generated by these tires appear mainly as a result of friction forces, with very little proportion of cutting forces, which makes them suitable for turfgrass surfaces.
The studies were conducted with three different horizontal loads, respectively, of G1 – 760 N, G2 – 920 N, G3 – 1080 N.

The studies established the soil mechanical properties such as maximum cutting stress and compaction. A rotator cutter VANE H-60 by Eijkelkamp company was used to measure the maximum stresses cutting the bedding. The measurements were made at the depth of 0.05 m. To measure compaction, a compaction measurer was used consisting of a conical penetrometer and an electronic register of strength and penetration depth. A cone with the base area of 0.0001 m² and the top angle of 60° was used.

Fig. 4. A measurement stand of traction tests

To measure the maximum traction forces, a measurement stand was used which was constructed at the Institute of Agricultural Engineering in Wrocław (Fig. 4). The drive on the examined tire was realized by means of a double-sided cylinder , which was driven from the external hydraulics of the engine. The overhang of the cylinder allowed for a turn of the wheel by about π/2 rad, which was a sufficient value to perform a full cutting of the bedding. Because the stand made the translation of the examined wheel impossible but enabled only its rotation, the value of rolling frictions was equal to zero and hence the drawing force was equal to the value of the traction force.

RESULTS OF STUDIES AND THEIR ANALYSIS

Table 2 presents percentage proportions of the species making up the turfgrass in particular years of studies. The data from 2006 mean the botanical composition of natural turfgrass, i.e. without underplanted catch crops, while the data of the other years of 2007 and 2008 mean the composition determined after underplanting and after utilization at the level of 100%.

It follows from Table 2 that the proportion of the following species decreased: cocksfoot, soft chess, all species from the Fabaceae as well as weeds and herbs. In 2006, they constituted 45% of the botanical composition, while in 2007 the value was 25%, and in 2008 – 13%. On the other hand, the percentage of grasses increased, especially of such species as red fescue, Kentucky bluegrass, perennial ryegrass – those species dominated in the botanical composition in 2007 and 2008, when intensive utilization was introduced. Grasses, whose proportion increased by 43 percentage points, are those which produce a shallow root system, and a part of them produce outgrowths, thanks to which they are better adapted to stress conditions, including pressing. A pressed soil offers mechanical resistance to the plants' roots: those which have deep roots cannot freely take oxygen, water and nutrients.

Table 2. Changes of the botanical composition in particular years of studies

Species		2006	2007	2008
Grasses	Poaceae	% s.m.
Red fescue	Festuca rubra	3.6	5.1	6.7
Blue fescue	Festuca ovina	2.1	2.4	3.5
Cocksfoot	Dactylis glomerata	3.3	2.3	0.6
Fine bent grass	Agrostis tenuis	1.5	0.2	0.2
Quack grass	Elymus repens	1.4	10.6	12.2
Soft chess	Bromus mollis	3.8	0.7	–
Turf hairgrass	Deschampsia caepitosa.	1.6	0.7	7.8
Kentucky bluegrass	Poa pratensis	3.2	4.6	7.3
Annual bluegrass	Poa annua	0.5	1.2	1.9
Green foxtail	Setaria viridis	3.8	3.1	1.1
Perennial ryegrass	Lolium perenne	4.4	16.9	29.9
Fabaceae	Fabaceae
Bird's foot trefoil	Lotus corniculatus	4.3	3.8	–
White clover	Trifolium regens	2.4	4.2	2.4
Red clover	Trifolium pratense	5.9	0.6	1.5
Smooth vetch	Vicia tetrasperma	4.3	3.9	4.3
Herbs and weeds
Buckthorn plantain	Plantago lanceolata	11.2	12.6	7.4
Grasslike starwort	Stellaria graminea	2.1	1.3	0.6
Common yarrow	Achillea millefolium	12.1	12.6	8.1
Common dandelion	Taraxacum officinale	9.4	7.8	2.4
Nodding thistle	Caraduus nutans	3.3	2.9	1.2
Hedge bedstraw	Galium mollugo	3.2	2.5	1
Total, %		100	100	100

Table 3 presents changes of percentage shares of shallow- and deep-rooted plants which are the result of utilization at the level of 100%. Such a comparison points to an increase of the percentage of species resistant to intensive utilization. The comparison of species with a shallow root system points to an increased percentage of Kentucky bluegrass and perennial ryegrass, whose presence in the dry weight increased to 29.9% in 2008. Most species with a deep root system decreased their proportion in the dry weight from 60.8% to 28.3%, which meant that in 2008 there were half as many of them as in 2006. The presence of blue fescue, red fescue and turf hairgrass increased: these are compact turf species which are well adapted to intensive utilization.

Table 3. Division in respect of the character of particular species and their percentage in botanical composition

Years			2006	2007	2008
Species division	English name	Latin name	[%]
Species with a shallow root system	Soft chess	Bromus mollis	3.8	0.7	–
	Kentucky bluegrass	Poa pratensis	3.2	4.6	7.3
	Annual bluegrass	Poa annua	0.5	1.2	1.9
	Perennial ryegrass	Lolium perenne	4.4	16.9	29.9
	Buckthorn plantain	Plantago lanceolata	11.2	12.6	7.4
	Grasslike starwort	Stellaria graminea	2.1	1.3	0.6
	White clover	Trifolium regens	2.4	4.2	2.4
Species with a deep root system	Red fescue	Festuca rubra	3.6	5.1	6.7
	Blue fescue	Festuca ovina	2.1	2.4	3.5
	Cocksfoot	Dactylis glomerata	3.3	2.3	0.6
	Fine bent grass	Agrostis tenuis	1.5	0.2	0.2
	Quack grass	Elymus repens	1.4	10.6	12.2
	Turf hairgrass	Deschampsia caepitosa.	1.6	0.7	7.8
	Green foxtail	Setaria viridis	3.8	3.1	1.1
	Red clover	Triforium pratense	5.9	0.6	1.5
	Bird's foot trefoil	Lotus corniculatus	4.3	3.8	–
	Common yarrow	Achillea millefolium	12.1	12.6	8.1
	Common dandelion	Taraxacum officinale	9.4	7.8	2.4
	Nodding thistle	Caraduus nutans	3.3	2.9	1.2
	Hedge bedstraw	Galium mollugo	3.2	2.5	1
	Smooth vetch	Vicia tetrasperma	4.3	3.9	4.3
Total, %			100	100	100

The distribution of temperatures and rainfalls in 2008 was favourable to the development and yielding of plants. Fig. 3 presents results of biomass measurements. It follows that biomass increase is not uniform. The growth of grass stems is related to the temperature. The highest increase is in spring months: April-June, when the temperatures are moderate. In summer, when the temperature increases, the growth is inhibited, whereas in autumn it takes place again, which is connected with the accumulation of nutrients and plants preparing for wintering [4].

The highest increase of biomass was observed in May and October. The mean temperatures in May were 14.3°C, the yield was 31.9 kg from the plot's area and it was higher by 12.7 kg, which meant that this was a 65% increase in comparison to the months when the increase was smaller. In October, when the mean monthly temperature was 9.6°C, 27.2 kg of green weight was obtained from the area of the plot. The lowest increase occurred in July and it was at the level of 14.3 kg, which was by 7.9 kg less than the means of other months, meaning that it was lower by 55%. That was caused by high temperatures, the mean value of which was 19.9°C. An increase of the proportion of such species as Kentucky bluegrass, red fescue, blue fescue, perennial ryegrass and white clover contributed to the increased mass.

Fig. 5. Dependence of biomass increase on temperature

Fig. 6 present results of turf density measurements for turf of different utilization intensity. It can be stated basing on the chart that an increase of the utilization level is followed by a decrease of the number of live plants. In the turf utilized with the greatest intensity, the number of live plants dropped to 3 in the area of 100 cm², which constitutes 18% in comparison to 16 per 100 cm² of non-utilized soil. The increase of the utilization level decreased on average by 4 per 100 cm², which is a drop of the turf density at the level of 55%.It can be supposed that the disappearance of the plant cover will have an effect on the maximum traction forces and the manner of generating them will probably change.

Fig. 6. Turf density

Fig. 7 presents results of measuring the root weight of all analyzed turfs. The stimulating effect of utilization on the plants' roots can be observed – their proportion in the botanical composition increased. This confirms the fact that grass roots have their optimum growth in a lightly densified soil. The greatest root mass was observed at the levels of moderate utilization, i.e. 25% and 50%; however, too big densification causes its decrease.On a non-utilized soil, the roots mass was 5.7 g and next it grew to 7 and 6.8 g. A decrease of the roots mass to 3.8 g, which is by 45% as compared to the optimum conditions at the utilization level of 25% and 50%, takes place as a result of intensive utilization. This can be explained by the phenomenon of excessive utilization of the soil.

Fig. 7. Studies of the root mass

The next parameters describing the studied conditions were maximum cutting stresses and compaction measured at the layer of 0.05 m.The former describes the soil strength resistance to cutting, while compaction means the resistance offered by the soil to plant roots. Thanks to that, it is possible to describe the mechanical properties of the ground which directly affect the maximum traction forces and the soil environment, which can support or limit the development of plants.

Fig. 8 presents results of measurements of maximum cutting stresses on a given turf for all 5 utilization levels. The lowest value of maximum cutting stresses equal to 110 kPa was obtained att he level of 0%. The highest value of this parameter was marked at the utilization levels of 50%, 25% and 75% – that was 130 kPa, so an increase by 18% took place. At the level of 100% of turf utilization, the values of maximum cutting stresses dropped to 125 kPa. Such a structure is connected with the distribution of the root mass in the soil profile on particular utilization levels.

Fig. 8. Maximum cutting stresses

Fig. 9 presents results of compaction measurements performed at all levels of utilization. An increase of compaction between particular utilization levels occurreed, on average, by the value of 0.8 MPa, which is 40% from the value of 1.4 MPa at the level of utilization at 0% to the value of 5.55 MPa for 100%. Soil densification with intense utilization leads to the disappearance of the root mass and damage of the plants' overground parts. Probably, this will have an effect on the value of generated traction forces.

Fig. 9. Results of compaction measurements

The main purpose of the present studies was to show whether changing the intensity of utilization of the studied turf would result in a changed value of the generated traction forces, which would have the final effect of changed drawing ability of an agricultural vehicle moving on this soil.

Fig. 10 shows results of measurements of maximum traction forces established for three loads and for all levels of utilization on recreational turf for a tire 18×7.0–8. It follows from the figure that an increase of vertical load always resulted in an increase of the values of maximum traction forces. Depending on the level of pressing, differences between the values of maximum traction forces for particular vertical loads change. The smallest differences with differentiated loading in the gains of maximum traction forces were observed at the utilization level of 0% and 25% because the increase of the maximum traction forces was 53 N in the case of non-utilized turf, and 44 N on the turf utilized in 25%, which constituted an increase of only 8%. The highest differences were found on the turf utilized at the level of 50%. Hence, it can be stated that the highest effectiveness of loading was observed at the utilization level of 50% because the loading of 1080 N resulted in an increased value of maximum traction forces up to 707 N, which on average was by 60% more in comparison to smaller loads.

Fig. 10. Maximum traction forces determined for three loads for a tire 18×7.0-8 at all levels of utilization

Fig. 11 presents results of measurements of maximum traction forces for three different tires on the studied turfgrass with the loading of 760 N and for all levels of utilization. The highest values of maximum traction forces were generated by a 18×9.5-8 tire, while lower ones were generated by the smallest tire 18×7.0–8. The highest values of maximum traction forces were obtained at utilization levels of 25% and 100%. At the levels of 0% and 25% a proportional increase of the values of the studied forces was observed and they were, respectively for the sequence of the studies tires and at 0% utilization level, 309 N, 417 N and 566 N, 25% 436 N, 489 N and 627 N, which meant a mean increase by 30%, depending on the tire size. Together with an increase of the level of utilization, tires with lower transverse dimensions generate the maximum traction forces at a similar level, oscillating between the values 390-465 N, with the differences between the obtained values being 5%, and the values of the forces generated by a 18×9.5–8 tire are bigger than the others by 50% on average, increasing up to 678 N.

Fig. 11. Maximum traction forces determined for tires of three dimensions, all utilization levels on recreational grass with the loading of 760 N

The values of traction forces obtained on the basis of the measurements served to calculate the friction coefficient. According to the Polish norm PN-ISO 8855:1999, it is called the coefficient of longitudinal force defined as the relation of the longitudinal force and the vertical loading of the wheel. Fig. 12 presents the values of this coefficient calculated for tires 18×7.0-8 at all levels of utilization for three vertical loads. The highest values of this coefficient were found in case of utilization at the level of 25% for smaller vertical loads. Loading the system with 1080 N more resulted in the highest values of the coefficient of the longitudinal force at the utilization level of 50%. It can be observed that the coefficient of the longitudinal force was the highest for the loading of 920 N. An exception was the case of the soil utilized at the level of 50% since there the value of the coefficient calculated for the loading of 1080 N was 0.65 and it was by 17% higher than the others. Increasing the vertical load at the level of 50% did not result in any increase of the coefficient.

Fig. 12. Values of the coefficient of longitudinal force for a 18×7.0-8 tire calculated for 3 horizontal loads and all levels of utilization

Fig. 13 presents the values of the coefficient of longitudinal force for all analyzed levels of utilization and three studied tires with the vertical loading of 760 N.The highest coefficient of longitudinal force was calculated in the case of a tire 18×9.5–8, while the lowest proportion of the load during the generation of a traction force was determined for a tire 18×7.0–8. In the case of a 18×9.5–8 tire, a greater part of the vertical load was used to generate the traction force for all utilization levels. The mean values calculated for this tire were by 0.3 higher than for the other tires, which is by 48%. Values of the coefficient increased at utilization levels of 25%, while with 50% utilization, a slight decrease and then an increase at the levels of utilization of 75% and 100% were observed. This regularity differs in the case of a 18×8.5–8 tire since the values of the coefficient of longitudinal force are lower on the bedding utilized with the highest intensity.

Fig. 13. Values of the coefficient of longitudinal force determined for the studied tires at all levels of utilization

Results of studies on maximum traction forces and the coefficients of longitudinal force were submitted to a statistical analysis, which pointed to significant relations between such factors as levels of utilization, loading and tire size, affecting the values of generated traction forces and the coefficient of longitudinal force.

Table 4. Statistical analysis of the results

	Tire size	Level of utilization	Vertical loading
Traction force	0.0013	0.7503	0.0000
Coefficient of longitudinal force	0.1287	0.0381	0.2323

Level of significance α = 0.05

The statistical analysis was conducted using Statistica 8 program by Statsoft [8]. A multivariate analysis at the significance level α of 0.05 was preceded by checking the uniformity of variation. As a result of Leven's test, the hypothesis of nonuniformity of variation was rejected. Table 4 presents statistical analysis of the results.

The multivariate analysis showed a significant effect of loading and the tire size on the analyzed traction forces and the level of utilization on the coefficient of longitudinal force. The level of utilization had no effect on the analyzed traction properties, while it had influence on the values of the coefficient of longitudinal force. No effect of vertical loads and the tire size on the values of the coefficient was found out.

CONCLUSIONS

The studies allowed to formulate the following conclusions:

1. Utilization brings about changes in the botanical composition of the green growth Increased intensity of utilization always results in an increase in the percentage of plants with a shallow root system, and that increase was 50 percenage points. The level of utilization does not affect the values of generated traction forces of the studied tires; however, the effect of utilization is noticeable in the case of the value of the longitudinal force coefficient, and its highest values were calculated for the level of utilization at 25%.

2. The tire size affects the size of maximum traction forces. The highest values of traction forces were generated by the biggest 18×9.5–8 tire, values of the achieved forces were on average by 50% higher than for the other tires. The tire also guaranteed the achievement of the highest coefficient of longitudinal force.

3. An increase in the vertical load of the studied tires always led to an increase of traction forces, while having no effect on the value of the coefficient of longitudinal force. The increase of vertical loading led to the highest increases of traction forces generated on the turfgrass utilized att he level of 50%. The lowest values of traction forces were observed at the level of utilization of 0% and 25%.

REFERENCES

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6. Shearman R. C., Beard J. B., 1975. Wear Tolerance Mechanisms: III. Physiological, Morphological, and Anatomical Characteristics Associated with Turfgrass Wear Tolerance. Agronomy J. 67, 215–218.

7. Schreiber M., Kutzbach H. D., 2008. Influence of soil and tire parameters on traction. RES. AGR. ENG., 54(2), 43–49.

8. Statsoft. Statistica 8.

9. Szweykowscy A. and J., 2003. Słownik botaniczny [A botanical dictionary]. Wiedza Powszechna, Warszawa [in Polish].

10. Taylor H. M., Brar G. S., 1991. Effect of soil compaction on root development. Soil Tillage Res. 19, 111–119.

11. Turgeon A. J., 2005. Turfgrass mangement.

Accepted for print: 5.02.2010

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Włodzimierz Białczyk
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmońskiego Street, 51-630 Wrocław, Poland
phone: (+48) 71 320 57 06
fax: (+48) 71 348 24 86
email: wlodzimierz.bialczyk@up.wroc.pl

Anna Cudzik
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmońskiego Street, 51-630 Wrocław, Poland
phone: (+48) 71 320 57 28
fax: (+48) 71 348 24 86
email: anna.cudzik@up.wroc.pl

Jarosław Czarnecki
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmońskiego Street, 51-630 Wrocław, Poland
phone: (+48) 71 320 57 26
fax: (+48) 71 348 24 86
email: jaroslaw.czarnecki@up.wroc.pl

Katarzyna Jamroży
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmońskiego Street, 51-630 Wrocław, Poland
phone: (+48) 71 320 58 11
fax: (+48) 71 348 24 86
email: katarzyna.jamroży@up. wroc.pl

Karol Wolski
Department of Agroecosystems and Green Areas Management, Wrocław University of Environmental and Life Sciences, Poland
phone: (+48) 71 320 16 51
Grunwaldzki 24A
50-363 Wrocław
Poland
email: karol.wolski@up.wroc.pl

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Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

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