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Review

Lateral Overturning and Backward Rollover of Agricultural Tractors: A Review

by
Moon-Kyeong Jang
1,2,
Seung-Jun Kim
1,2,
Beom-Soo Shin
1,2,* and
Ju-Seok Nam
1,2,*
1
Department of Biosystems Engineering, Kangwon National University, 1 Kangwondaehak-gil, Chuncheon 24341, Republic of Korea
2
Interdisciplinary Program in Smart Agriculture, Kangwon National University, 1 Kangwondaehak-gil, Chuncheon 24341, Republic of Korea
*
Authors to whom correspondence should be addressed.
Agriculture 2024, 14(3), 334; https://doi.org/10.3390/agriculture14030334
Submission received: 26 January 2024 / Revised: 17 February 2024 / Accepted: 17 February 2024 / Published: 20 February 2024
(This article belongs to the Special Issue Advances in Modern Agricultural Machinery)
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Abstract

:
Tractor accidents caused by lateral overturning and backward rollover during agricultural activities and general driving are common. In this study, various research cases were analyzed to identify the factors influencing the lateral overturning and backward rollover of tractors and to examine their static and dynamic stability. Studies on the analysis of the major causes of these incidents and evaluation of tractor safety were compiled. Test methods, including actual tests and simulations, were categorized, and the characteristics of lateral overturning and backward rollover safety of tractors in different studies were examined. Additionally, safety improvement measures were proposed by identifying and summarizing the causes of accidents involving agricultural machinery. Tractor safety was evaluated primarily by conducting actual tractor and simulation tests. These tests were classified into field tests, tests on scale models, spreadsheet programs, and 3D simulation programs. The primary causes of lateral overturning and backward rollover were unstable center of gravity, extremely high driving speed, and ground conditions. Given the considerable number of studies dedicated to evaluating tractor safety, various technologies aimed at preventing lateral overturning and backward rollover incidents are expected to be applied to tractors in the future. The production and testing of safe agricultural machinery are expected to contribute to a reduction in accident rates.

1. Introduction

The proportion of adults aged ≥65 years is increasing in rural areas of South Korea, reaching 42.3% in 2020; the female population is also increasing [1]. Central European countries, including Poland, Hungary, and the Czech Republic, face a need for countermeasures owing to the rapid increase in the elderly population in agriculture, which plays a significant role in the economy [2]; in 2010, over 50% of all agricultural workers in Europe were aged ≥55, with only 6% aged ≤35 years, indicating a trend toward population aging [3]. To address this issue, the penetration rate of agricultural machinery has been increasing [4,5]. In South Korea, the penetration rate of agricultural tractors increased by 39.65%, with the number of units rising from 65,909 in 2020 to 92,041 in 2021. The global agricultural machinery market, valued at approximately USD 132.5 billion in 2019, is expected to reach USD 160.3 billion in 2024, reflecting an average annual growth rate of over 3.9% [6]. However, owing to the widespread use of tractors, the number of accidents is increasing [7]. Consequently, the accident rate of tractors has been investigated in several countries, including South Korea, Spain, and Turkey [8,9,10,11,12,13]. Reports indicate that more than half of tractor-related deaths are caused by rollover accidents [14]. In South Korea, tractor accidents constituted 12.3% of agricultural machinery accidents in 2020, with farmer injuries caused by lateral overturning and backward rollover accidents in riding-type agricultural machinery accounting for the largest share (34.1%) [6]. Tractors are a major cause of agricultural accidents in South Korea and in many advanced countries [15]. In the United States and EU, agricultural activities are associated with approximately three times more accidents than other activities, and 80% of these accidents involve agricultural machinery, with tractors being the leading cause [16]. In Spain, an analysis of approximately 200 agricultural accidents between 2004 and 2013 revealed that 69% of machinery-related accidents were caused by tractors and that 30% of those were rollover accidents [17]. Similarly, in Turkey, an analysis of 85 deaths resulting from tractor overturning and rollover from 2000 to 2007 revealed that 53 deaths (61.6%) occurred on fields, farmland, and ridges [8]. Results indicate that the causes of lateral overturning and backward rollover of tractors include an excessive tip angle (beyond the static safety limit), unstable and rough ground, loading conditions, and extremely high driving speed [14,18,19,20].
In this study, previous studies on the analysis of lateral overturning and backward rollover of tractors were summarized to enhance safety and identify factors causing tractor accidents. The characteristics of each study were examined and summarized to determine the latest trends in tractor safety evaluation, thereby providing insights into reducing agricultural tractor accidents.

2. Study and Limitations of Lateral Overturning/Backward Rollover Safety Using Rollover Protective Structure (ROPS)

To reduce the number of tractor accidents, the installation of a ROPS, a mechanical structure designed to mitigate impact on the driver during accidents, has been proposed (Figure 1). Myers and Hendricks [21] analyzed the mortality rate resulting from tractor accidents in the United States and advocated for the installation of a ROPS in agricultural tractors to decrease mortality rates. The ROPS originated in Sweden and New Zealand in the 1950s. During 1959–1978, numerous countries, including Norway, Finland, New Zealand, and the United States, introduced regulations mandating the installation of a ROPS, leading to a significant reduction in tractor rollover risks. Moreover, the number of deaths decreased depending on the type and utilization method of the ROPS [22]. Several studies were conducted to enhance ROPS safety. Latorre-Biel et al. [23] developed an energy dispersion disc system capable of dispersing energy when the deformation of the ROPS begins upon impact. They found that the disc could reduce the stress applied to the ROPS and prevent its failure by rotating when deformation begins. Sun et al. [24] utilized the Bullet physics engine to evaluate the relationship between vertical/lateral linear velocity and roll angular velocity based on root mean square error during accidents on embankment slopes and uniform slopes. The estimated error was 0.7, and the collision simulation results of Bullet were consistent with those of the Organization for Economic Cooperation and Development (OECD) simulation for critical ROPS height. Chen et al. [25] demonstrated that the ROPS can be designed by performing dynamic simulations. They also introduced the lateral stiffness coefficient (LSC) as an additional safety criterion for evaluating ROPS safety, highlighting that safety decreases when the LSC is excessively high or low. They noted that driver safety cannot be guaranteed in the event of an accident, even when the ROPS meets static test standards. Ayers et al. [26] emphasized the development of a foldable ROPS for certain tractors without an installed ROPS and highlighted that the ROPS is not properly used in certain tractors. Hunter and Owen [27] mentioned that although the installation of a ROPS cannot perfectly prevent driver injury caused by tractor rollover, it is a crucial safety measure.
Therefore, research is required to evaluate the safety of tractors and decrease the accident rate. Studies on safety are mainly conducted by performing authorized tractor tests and theoretical analysis. Recently, however, safety has been analyzed by employing various methods, such as simulations and scale models (Figure 2).

3. Mathematical Models for Lateral Overturning/Backward Rollover

Theoretical analyses have been performed to assess the lateral overturning and backward rollover of tractors and to develop new models based on established theories such as tractor engineering and chassis dynamics. For theoretical analyses, the variables evaluated by conducting OECD standardized tests are predominantly utilized.
Guzzomi [28] emphasized the need for a new model that allows for better in-depth analyses than existent primary overturn models. Consequently, a new model was devised to analyze situations where both the front and rear wheels on the upper side of a slope are lifted from the ground, leading to lateral overturning. To predict the overturning angle and tire contact force of a tractor equipped with a front axle pivot, a model was presented based on two rigid bodies—the front body consisted of the front axle and front wheels, and the rear body consisted of the remaining chassis and rear wheels. The model analysis results demonstrated that secondary overturning can be prevented by applying brakes to all tires of the tractor. Baker and Guzzomi [29] categorized the center of gravity of a tractor into two parts (front and rear) and analyzed the primary overturning process of the tractor under the influence of the mass of the front part. They noted that safety is influenced by the position of the tractor’s rear center of gravity. When both centers of gravity were considered, an increase in the mass of the front part decreased safety as the rear center of gravity moved to a relatively unstable position. Previati et al. [30] presented three mathematical models for tractor safety analysis: one assuming tires are rigid bodies, another considering the vertical and horizontal stiffness of tires, and a third incorporating suspension on the front axle. They highlighted the significant effect of tire stiffness on vehicle accidents, noting that safety can be overestimated by up to 15% when this factor is disregarded. They stated that the installation of suspension on the front axle significantly improves safety by increasing the static sidelong falling angle by up to 20% and can considerably increase the safety of the tractor when transporting asymmetric implements. The safety of the tractor was notably low when one of the wheels was lifted from the ground. Li et al. [31] analyzed the behavior of the tractor based on its velocity, ground slope, and maximum static friction coefficient. They developed safety indices for lateral overturning and slip, accounting for sensitivity through bounce displacement and acceleration, as well as pitch direction angle and acceleration. Lateral overturning safety was observed to decrease as the tractor’s velocity and ground slope increased, with the maximum static friction coefficient significantly impacting tractor slip. Choi et al. [32] constructed a mathematical model for analyzing the static lateral overturning safety of tractors equipped with asymmetric harvesters and compared it with simulation and actual tests. They found that the developed model exhibited smaller errors than did existent mathematical models and underscored the significant impact of the movement of the coordinates of the tractor’s center of gravity on tractor driving safety. Although experiments were conducted to evaluate the safety of tractors through mathematical models, there were many studies that actually evaluated the safety of tractor manufacturing and structure. In addition, there is a disadvantage that it is not as accurate as tests using actual tractors and simulation tests. The theoretical analysis for tractor safety evaluation is summarized in Table 1.

4. Examining Lateral Overturning/Backward Rollover Factors by Conducting Tests

4.1. Real Test

The OECD test code was developed in 1959 to evaluate the performance and safety of tractors, and agricultural tractors must pass static and dynamic safety tests based on the OECD guidelines. Tests are generally conducted to evaluate the engine output, fuel consumption, and ROPS. The center of gravity and overturning angle are the most important factors in tractor safety evaluation [33,34]. The center of gravity and overturning angle tests, which examine tractor safety under static conditions, require less time than safety evaluations under dynamic conditions, and position changes need not be considered [35]. Tests based on OECD guidelines can increase the export of agricultural tractors through the sale of verified products and enhance safety through improvements in product technology [36].
Chisholm [37] repeated the rollover test 30 times from a 2 m height using the tractor’s mass, track width, cab length, and velocity, as well as the contact of the tire, as variables. Cameras and sensors were used to analyze tractor behavior during the rollover process. The analysis results revealed that the tractor’s vibration and tire friction force significantly affect behavior and that the friction coefficient and camber angle of the tire located on the lower side of the slope influence tractor accidents. Fabbri and Molari [35] stated that the method of measuring the center of gravity height of a tractor under dynamic conditions is complex and inaccurate, and they proposed a method for measuring it under static conditions. They measured variables such as the mass and reaction force of the tractor and the ground slope for the tractor by using a mass system composed of four cubes; they derived the center of gravity height by applying an equilibrium equation and compared it with the result obtained under dynamic conditions. They could easily measure the variables under static conditions and stated that the center of gravity height can be derived relatively easily even though they could not maintain the error of ≤3 mm stipulated in OECD guidelines. Gravalos et al. [38] investigated safety tendencies for the rear track width of the tractor and additional weight applied to the rear wheel. They statically placed a tractor on a specially prepared test bench and measured the reaction force of the wheels under a load. They stated that safety increases as the rear track width of the tractor working along contour lines increases and that the use of weight on the rear wheel located on the upper side of the slope facilitates moderately safe operation of the tractor by significantly reducing the load moving to the wheel on the lower side of the slope. Bietresato and Mazzetto [39] developed a tiltable platform capable of generating various ground slopes to enable static and dynamic safety tests for agricultural machinery. They determined that the center of gravity, which changes during tests, can be identified, unlike in existent safety evaluations, and that relatively accurate safety evaluations would be possible because the centrifugal force is considered. Kang et al. [40] measured the static sidelong falling angle while increasing the track width of a three-wheel riding-type tractor to derive its static sidelong falling angle. When the left and right overturning angles of the tractor were compared, the safety for overturning to the right side was found to be low because the center of gravity (the oil pump and fuel cell) was located on the right side. They also found that safety for backward rollover decreased as the front wheel lift angle increased. Testing using a real tractor is relatively accurate, but the cost of testing and the probability of a safety accident occurring are high. Therefore, it is necessary to be careful when conducting tests using a real tractor to evaluate the safety of lateral overturning and backward rollover. Real test for tractor safety evaluation is summarized in Table 2.

4.2. Scaled Model Test

The safety of tractors is mainly evaluated by conducting authorized tests, which are time-consuming and costly [41]. Safety evaluation through simulations is also time-consuming, costly, and not very reliable, owing to the input of inaccurate properties [42]. Tests based on scale models have been used in various applications, including airplanes, tanks, and agricultural machinery, because test conditions for the external environment can be easily implemented [43]. The model used to evaluate the safety of tractors is shown in Figure 3.
Spencer [44] analyzed conditions in which six different tractors on a slope lose safety when they are equipped with implements and used for towing. Mathematical models were developed considering the reaction force and momentum of the tractors equipped with implements, and they were verified by conducting experiments on actual tractors and scale models. The safety of four-wheel drive tractors was found to exceed that of two-wheel drive tractors, and safety was observed to significantly decrease when tractors moving along contour lines turned toward a downhill slope. Koc et al. [45] presented a method for monitoring the safety of a tractor located on a side slope using a smartphone application. They created tractor scale models using the LEGO Mindstorms kit and conducted tests where rollover was caused while driving. They used the dimensions, acceleration, and angular velocity data of tractors to predict rollover situations. They calculated a safety index using the data and developed and verified a system that can send emergency messages by calculating the index change. Li et al. [46] developed a wheel loader dynamic model with seven degrees of freedom using the Lagrange method. They constructed a 1/16-scale model to test the developed model and verified the safety for turning on flatland and slopes and rollover over obstacles by introducing the lateral transfer ratio (LTR) index. An LTR value of zero indicates that the vehicle is safe because both sides are in contact with the ground, whereas an LTR value of 1 means that it is unsafe because the left or right tire is lifted from the ground. During the turning of the wheel loader, the LTR increased when the slope of the ground increased and when the wheel loader passed over obstacles, which resulted from the centrifugal force, roll direction angle, and vertical acceleration. In addition, Li et al. [47] evaluated lateral overturning safety by deriving the roll angle, LTR, and primary overturning index. They designed an experiment using the Taguchi method for the tire type, slope and roughness of the ground, forward ballast weight, and track width. Tests were conducted by preparing 1/16-scale models. The evaluation results revealed that predicting lateral overturning using the roll angle is difficult and that the primary overturning index can more effectively predict lateral overturning compared with the LTR. Li et al. [48] constructed the scale model of a 2WD tractor and performed an experiment to analyze the effects of the tire stiffness coefficient, ballast weight, and rear axle track width on the safety of the tractor. They created the ground with roughness grades E and F and a ground slope of 10° using a 3D printer and measured the reaction force of the tractor tire using sensors. They then evaluated safety by deriving the front-axle-based safety index and rear-axle-based safety index. The evaluation results showed that safety increases as the tire stiffness coefficient, front ballast weight, and rear wheel track width increase. Jang et al. [49] derived the static sidelong falling angle by employing a tractor scale model constructed using a 3D printer and compared it with that obtained from the authorized performance test. A 1/20-scale model was constructed, and the center of gravity of the actual tractor was implemented by attaching additional structures. They prepared a small platform capable of measuring the static sidelong falling angle and measured the angle at the moment the front and rear wheels located on the upper side of the slope were separated from the ground. A comparison with the static sidelong falling angle of the actual tractor showed that deriving the angle by using the scale model with an error of 2.18% is possible. Tests using a scaled model can be performed relatively easily but have the disadvantage of making it difficult to implement actual phenomena. Additionally, it is relatively difficult to imitate the center of gravity or shape of an actual tractor. Scaled model test for tractor safety evaluation is summarized in Table 3.
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Figure 3. Methods of scaled models: (a) LEGO Mindstorms (Reprinted with permission from Ref. [45]. 2012, Koc, A.B.); (b) self-production (Reprinted with permission from Ref. [46]. 2013, Li, X.); (c) 3D print [49]; (d) self-production (Reprinted with permission from Ref. [47]. 2016, Li, Z.).
Figure 3. Methods of scaled models: (a) LEGO Mindstorms (Reprinted with permission from Ref. [45]. 2012, Koc, A.B.); (b) self-production (Reprinted with permission from Ref. [46]. 2013, Li, X.); (c) 3D print [49]; (d) self-production (Reprinted with permission from Ref. [47]. 2016, Li, Z.).
Agriculture 14 00334 g003

5. Examining Lateral Overturning/Backward Rollover Factors by Conducting Tests

Because safety tests using actual tractors are not only time-consuming and costly but also pose the risk of accidents, simulations have attracted attention as an alternative [50]. Simulation technology for tractor safety evaluation was introduced in the 1960s, and it accelerated research on lateral overturning and backward rollover safety for agricultural tractors [51]. Simulations facilitated studies on implementing various situations, unlike actual tractor experiments, and research has been actively conducted to derive and analyze various factors and identify indicators. In the case of simulation, tests can be performed more quickly and safely compared with tests using actual tractors. Compared with a scaled model test, it has the advantage of being easier to implement for external factors. However, care must be taken when simulating the model because incorrect results may be derived due to incorrect input of properties.

5.1. 2D Simulation

Ahmadi [50] analyzed the effects of a tractor’s velocity, ground slope, and friction coefficient between the wheel and ground on the lateral overturning safety of the tractor when position disturbance of the tractor occurs. In addition, equations for lateral overturning and slip were established for the Mitsubishi tractor, and indicators that determine the stability index were developed. Safety decreased as the tractor’s velocity and ground slope increased, and it was more affected by the slip of the tractor than by lateral overturning. Therefore, the slip phenomenon needs to be considered for the Mitsubishi tractor. In addition, Ahmadi [52] developed a mathematical model for identifying the longitudinal safety of a tractor equipped with a plow when it performs agricultural work on a slope. To lift the plow for plowing and transport, the effects of the force and torque transmitted to the tractor from the plow on the longitudinal safety index of the tractor were determined. No significant difference in longitudinal safety was observed when the plow penetrated soil; however, longitudinal safety decreased significantly, causing the risk of backward rollover when the plow was lifted for transport. Demšar et al. [53] developed a mathematical model for analyzing lateral overturning safety based on the center of gravity, track width, wheelbase, and front axle attachment height of a tractor located on a slope. They found that lateral overturning and backward rollover safety increased as the center of gravity height decreased. They also found that backward rollover safety improved as the wheelbase increased and that lateral overturning safety increased as the track width increased. They, however, stated that the turning radius increased and steering became relatively difficult if the track width increased. Li et al. [54] developed a mathematical model for analyzing lateral overturning when a tractor passes over an obstacle on a slope. The moment the front and rear wheels of the tractor pass over an obstacle was divided into four sections: (1) from the moment the front tire comes into contact with the obstacle to the moment it passes over the obstacle, (2) from the moment the front wheel passes the obstacle to the moment the rear wheel comes into contact with the obstacle, (3) from the moment the rear wheel comes into contact with the obstacle to the moment it passes over the obstacle, and (4) the moment after the rear wheel has passed over the obstacle. The entire process was analyzed by using a mathematical model. The center of gravity moved to an unstable position when the rear wheel passed over the obstacle compared with when the front wheel passed over it, and the direction of the tractor significantly changed when the front wheel passed over the obstacle. Additionally, an increase in ground slope had a considerable impact on the direction of the tractor. Shim et al. [55] developed an integrated implement suitable for the Korean agricultural environment and analyzed its static and dynamic safety when it was attached to a tractor. Its static and dynamic backward rollover safeties were analyzed by deriving the front wheel reaction force when the implement was lifted at static conditions and in a towing operation situation for tractors under 48 kW and those under 92 kW. They reported that backward rollover can be caused by the attachment of the implement, owing to the load movement, and suggested that installing a bucket loader that serves as a weight at the front of a tractor can improve safety. Arote et al. [56] evaluated the lateral overturning safety of a tractor equipped with a bucket loader when the tractor moves on a slope along contour lines based on the lift height of the loader. They introduced a tractor safety index (TSI) and categorized grades as follows: very poor for a TSI of less than 0, poor for a TSI between 0 and 2, good for a TSI between 2 and 4, and excellent for a TSI of more than 4. They found that lateral overturning safety increases as the loader is closer to the ground and the track width of the tractor increases. 2D simulation for tractor safety evaluation is summarized in Table 4.

5.2. 3D Simulation

Park et al. (2002) [57] evaluated the lateral overturning safety of tractors moving on a slope along contour lines based on the ground slope, obstacle height, and timber load conditions by using VisualNastran, a dynamic analysis software program. Timber loading decreased safety, and the risk of lateral overturning increased as the obstacle height and ground slope increased. Huang et al. [58] used dynamics simulation software (Recurdyn) and analyzed the safety of a tractor by performing tire impact analysis when the tractor passed over a cylindrical obstacle and moved uphill (20° and 44°) and downhill (20° and 44°). They found that the maximum impact of the front wheel increased as the uphill slope increased and decreased as the downhill slope increased. When the tractor moved over the cylindrical obstacle, the impact of the front wheel decreased as the driving speed of the tractor decreased. Chowdhury et al. [59] investigated the lateral overturning safety of a tractor equipped with a radish collector. They derived the center of gravity of the tractor by performing mathematical calculations and obtained the overturning angle based on the load conditions and folded position of the radish collector. The simulation results showed that the average left and right overturning angles decreased by approximately 15° when a load was applied on the radish collector compared with when no load was applied, and no significant difference in the overturning angle was observed depending on the folded position of the radish collector. Hwang et al. [60] determined the critical driving speeds of a tractor that cause lateral overturning and backward rollover based on the internal steering angle and front wheel lift angle and compared them with the results obtained through theoretical equations for lateral overturning and backward rollover. The error between the simulation results and the theoretical equations was found to be less than 5%, and the critical driving speeds of the tractor for lateral overturning and backward rollover decreased as the internal steering angle and front wheel lift angle increased. Jang et al. [61] established the lateral overturning and backward rollover tendencies of tractors based on ground slope and obstacle geometry and height. The simulation results showed that the critical speed decreased as the ground slope and obstacle height increased, thereby lowering safety against lateral overturning and backward rollover. Backward rollover occurred at a low-ground slope, and lateral overturning occurred at a high-ground slope. Jang et al. [62] determined relative safety against lateral overturning and backward rollover when implements were attached to the same tractor and analyzed safety based on the change in the center of gravity depending on the attachment of implements. The analysis results showed that the types of lateral overturning and backward rollover changed based on a certain ground slope and that safety decreased as the ground slope and obstacle height increased. Additionally, the safety of the tractor with implements was lower than that of the tractor with no implements. Lysych [63] analyzed the static sidelong falling angle of a tractor equipped with front and rear implements by employing multi-body dynamics simulations. When the rear implement was attached, the static sidelong falling angle decreased as the implement weight increased. When both the front and rear implements were attached, the static sidelong falling angle increased. In a follow-up study, when a tractor equipped with both front and rear implements moved on the ground with single linear, single sequential, group linear, and group sequential obstacles, the lift height of the wheel center and the center of gravity displacement and linear velocity of the tractor were derived. The single linear obstacle did not affect the safety of the tractor. Safety can be completely compromised and an accident may occur owing to vibrations when the tractor passes over group sequential obstacles [41]. 3D simulation for tractor safety evaluation is summarized in Table 5.

6. Discussions on the Safety of Lateral Overturning/Backward Rollover

Studies have presented various methods for reducing the number of accidents caused by the lateral overturning and backward rollover of tractors. Lowering the center of gravity for the tractor chassis and preventing the center of gravity from being excessively biased while driving is crucial. In addition, applying brake and suspension systems to all tires improves safety. An increase in tractor track width can prevent lateral overturning and backward rollover, but an excessive increase is unfavorable for driving and steering. When an implement is attached to a tractor, the excessive lift of the implement may cause backward rollover. Excessive steering for turning may cause lateral overturning, and the substantial lift of the front wheel from the ground may cause backward rollover. When a tractor moves on the ground with a high slope, turning and a reduction in velocity are required. The tractor’s velocity needs to be lowered as ground roughness increases. Safety decreases as the obstacle height on the ground increases, and the tractor’s lateral overturning and backward rollover risk increases when obstacles are sequentially located. Solutions that can improve the safety of tractors are shown in Table 6.
Recently, the number of studies for developing new models through theoretical analyses and comparing them with simulation results has been increasing. This trend may stem from the considerable time and cost involved in evaluating driving safety by using actual tractors or conducting dynamic experiments, coupled with the high risk of accidents owing to unexpected outcomes. Theoretical analyses have been geared toward creating new models based on established theories such as tractor engineering and chassis dynamics to enhance existing theories. Typically, theoretical findings are validated by performing tests and simulations on actual models and scale models. Simulation studies have been focused on the analysis of the impact of the tractor’s center of gravity, driving speed, and ground conditions on safety by incorporating the machine’s actual specifications. Experiments using scale models offer relative ease of preparation and execution compared with other methods; these involve various sensors, such as reaction force, angular velocity, and proximity sensors. In most studies, verifications through two or more experiments have been integrated, although some were based on a single experiment. Factors such as an unstable center of gravity, tractor geometry and velocity, and ground slope and roughness contribute to the lateral overturning and backward rollover of tractors. A comprehensive consideration of these factors is deemed necessary.
Although various causes of tractor accidents have been identified through studies, it is still difficult to perfectly embody actual situations. Therefore, there is a need to develop a tractor accident model that can more accurately embody phenomena that occur in actual fields. In fact, several studies were believed to be able to derive more diverse figures, but they were not achieved. It is believed that new results can be derived by combining multiple studies. It is essential to develop equipment and models that can effectively experiment with various studies, and it is also necessary to develop a platform that can comprehensively derive various results. Additionally, the information derived from research is not reflected in or applied to actual tractors. Therefore, an active attitude is needed to apply the derived contents to actual tractors.

7. Conclusions

In this study, we analyzed research trends on the lateral overturning and backward rollover of tractors, which frequently cause accidents in agricultural fields, and analyzed the main causes of accidents. The evaluation of tractor safety involves diverse approaches, including tests on actual tractors, safety assessment through theoretical analyses, simulations, and experiments using scale models. Tests on actual tractors encompass static conditions involving measurements of the center of gravity and tests on static sidelong falling angles. Factors that impede the safety of a tractor include external factors (ground slope, obstacle, load conditions, etc.), tractor operating conditions (driving speed, plow depth, turning radius, etc.), and the tractor’s center of gravity (wheelbase, track width, attachment of implement). etc.). Therefore, in order to increase the safety of tractors, it is necessary to consider safety when manufacturing tractors, and it is necessary to set operating conditions by considering external factors. There is a need to apply findings to actual tractors based on various research results in the future. The insights from this study are expected to elucidate the latest research trends in tractor safety evaluation and provide valuable information to agricultural workers.

Author Contributions

Conceptualization, M.-K.J.; investigation, M.-K.J. and S.-J.K.; data curation, M.-K.J. and S.-J.K.; writing, original draft preparation, M.-K.J.; writing, review, and editing, B.-S.S. and J.-S.N.; visualization, M.-K.J.; supervision, B.-S.S. and J.-S.N.; project administration, B.-S.S. and J.-S.N.; funding acquisition, J.-S.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through the Machinery Mechanization Technology Development Program for Field Farming Program, funded by the Ministry of Agriculture, Food and Rural Affairs (MAFRA) (RS-2023-00235957). Also, this study was carried out with the support of ‘R&D Program for Forest Science Technology (2023475A00-2325-BB01)’ provided by Korea Forest Service (Korea Forestry Promotion Institute).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 14 00334 g001
Figure 1. Various studies on ROPS safety: (a) sequence of deformation of an energy absorption disc as simulated by using the finite element method (Reprinted with permission from Ref. [23]. 2019, Latorre-Biel, J.I.); (b) ROPS deformation and motion state of the dummy when a collision occurs (Reprinted with permission from Ref. [25]. 2012. Chen, C.)
Figure 1. Various studies on ROPS safety: (a) sequence of deformation of an energy absorption disc as simulated by using the finite element method (Reprinted with permission from Ref. [23]. 2019, Latorre-Biel, J.I.); (b) ROPS deformation and motion state of the dummy when a collision occurs (Reprinted with permission from Ref. [25]. 2012. Chen, C.)
Agriculture 14 00334 g001
Agriculture 14 00334 g002
Figure 2. Tractor safety evaluation methods.
Figure 2. Tractor safety evaluation methods.
Agriculture 14 00334 g002
Table 1. Theoretical analysis for evaluating the safety of tractors.
Table 1. Theoretical analysis for evaluating the safety of tractors.
TargetVariablesReference
Lateral overturningContact force of the tire[28]
Lateral overturningCenter of gravity of front and rear bodies
Potential energy of fixed-chassis tractors		[29]
Lateral overturning and
backward rollover		Tire stiffness (vertical and lateral)
Center of gravity
Ground slope		[30]
Lateral overturningDriving speed
Ground slope
Maximum static friction coefficient		[31]
Lateral overturningCenter of gravity[32]
Table 2. Real test for evaluating the safety of tractors.
Table 2. Real test for evaluating the safety of tractors.
TargetVariablesReference
Lateral overturningMass of the tractor
Track width
Length of cab
Contact of the tire
Tractor’s velocity		[37]
Mass center of the tractorMass of the tractor
Reaction force of the tractor
Ground slope		[35]
Lateral overturningTractor’s rear track width
Additional weight on the rear wheels		[38]
Mass center of the tractorCenter of gravity changes during the test[39]
Lateral overturningTractor’s rear track width[40]
Table 3. Scaled model for evaluating the safety of tractors.
Table 3. Scaled model for evaluating the safety of tractors.
EquipmentMeasurement ItemReference
LEGO MindstormsRoll angle of the tractor
Lateral dynamic stability index		[45]
Self-productionLateral transfer ratio[46]
Self-productionRoll angle of the tractor
Lateral-load transfer ratio
Phase I overturn index		[47]
Self-productionForce-based index[48]
3D printStatic sidelong falling angle[49]
Table 4. 2D simulation study for evaluating the safety of tractors.
Table 4. 2D simulation study for evaluating the safety of tractors.
ToolkitTargetVariablesReference
Microsoft ExcelLateral overturningDriving speed
Ground slope
Friction coefficient of wheel and ground		[50]
Microsoft ExcelBackward rolloverPlow depth
Center of gravity		[52]
Self-productionLateral overturning and backward rolloverCenter of gravity
Track width
Wheelbase		[53]
Microsoft ExcelLateral overturningDriving speed[54]
Ground slope
Microsoft ExcelBackward rolloverCenter of gravity[55]
Microsoft ExcelLateral overturningCenter of gravity[56]
Table 5. 3D Simulation study for evaluating the safety of tractors.
Table 5. 3D Simulation study for evaluating the safety of tractors.
ToolkitMeasurement ItemsVariablesReference
VisualNastranCritical driving speedGround slope
Height of the obstacle
Load conditions
Driving speed		[57]
RecurdynImpact of tireGround slope
Presence of obstacle
Driving speed		[58]
RecurdynStatic sidelong falling angleLoad conditions
Folding conditions of collector’s conveyor
Overturning side		[59]
RecurdynCritical driving speedInner steering angle[60]
Critical angular velocityFloating angle of front wheel
RecurdynCritical driving speed
Rotational angle of the center of gravity
Reaction force of tire
Angular velocity of the tractor
Vertical displacement of the tractor		Ground slope
Height of the obstacle
Shape of the obstacle		[61]
RecurdynCritical driving speed
Rotational angle of the center of gravity
Reaction force of tire
Angular velocity of the tractor
Vertical displacement of the tractor		Ground slope
Height of the obstacle
Shape of the obstacle
Attachment of the implement		[62]
CAE SolidWorks MotionContact force of the tireGround slope
Type of the implement		[63]
CAE SolidWorks MotionLift height of the tires
Displacement of the center of gravity
Linear speed of the tractor		Type of the obstacle[41]
Table 6. Measures to reduce tractor accidents.
Table 6. Measures to reduce tractor accidents.
TargetSolutionsReferences
TractorFour-wheel drive tractors are safer than two-wheel drive tractors
The tractor’s center of gravity must be lowered
Increasing the track width is better
Attaching brakes to all wheels of the tractor is better
Front axle suspension increases safety
A bucket loader should be attached to the front when lifting an implement attached to the rear
The front mass of the tractor should not be excessively increased		[28,29,30,32,44,50,53,55,56]
Operation conditionsWhen attaching the plow, depth should not be excessive
The implement should be lifted cautiously
As the inner steering angle increases, the safety of the tractor decreases
As the front wheel lift angle increases, the safety of the tractor decreases
Using weight on the rear wheel located at the top of a slope increases safety		[46,52,60]
Ground conditionsWhen the ground slope increases, the driving speed must be lowered
The higher the obstacle, the lower the safety
Maximum coefficient of static friction is the main cause of sideslip
The greater the roughness of the ground, the lower the safety
The presence of continuous obstacles reduces safety		[31,37,46,50,54,57,58,61,62,63]
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Jang, M.-K.; Kim, S.-J.; Shin, B.-S.; Nam, J.-S. Lateral Overturning and Backward Rollover of Agricultural Tractors: A Review. Agriculture 2024, 14, 334. https://doi.org/10.3390/agriculture14030334

AMA Style

Jang M-K, Kim S-J, Shin B-S, Nam J-S. Lateral Overturning and Backward Rollover of Agricultural Tractors: A Review. Agriculture. 2024; 14(3):334. https://doi.org/10.3390/agriculture14030334

Chicago/Turabian Style

Jang, Moon-Kyeong, Seung-Jun Kim, Beom-Soo Shin, and Ju-Seok Nam. 2024. "Lateral Overturning and Backward Rollover of Agricultural Tractors: A Review" Agriculture 14, no. 3: 334. https://doi.org/10.3390/agriculture14030334

APA Style

Jang, M.-K., Kim, S.-J., Shin, B.-S., & Nam, J.-S. (2024). Lateral Overturning and Backward Rollover of Agricultural Tractors: A Review. Agriculture, 14(3), 334. https://doi.org/10.3390/agriculture14030334

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