Forest trailer with loader

Giuseppe Buratti –


In this project a forest trailer with attached loader is studied. It is a particular trailer, used mainly in the woods, with a light structure and able to load with a telescopic crane the trunks just cut. In particular, a Kesla 102 trailer with Kesla 305T loader and Kesla proG26 grapple was analysed. Cranes, grapple and trailer have been modelled in Solidworks respecting the dimensions and masses, data available from the brochures and technical drawings found on the web and the Kesla site. The movement space of the arm and grapple has been analyzed. Subsequently, the load of a trunk was simulated with both a locked trailer and a free trailer, that is able to move with respect to the ground. The data obtained in the two different situations will then be compared by evaluating the measured loads to the actuators. The part concerning the study of the dynamics of the trailer with respect to the ground has proved to be fundamental in order to obtain results that are close to reality, in addition it has been possible to evaluate in which circumstance the trailer can overturn.

Example of forest trailer with loader.

Example of forest trailer with loader.


Once recreated the 3D model respecting size and masses, it was imported into Adams to study it. The objectives of the analysis are:

  • Range of motions: after assembling the model, it has been studied which is the space in which crane and clamp are able to operate in the meridian plane of the trailer. From the technical data sheets the runs of all the actuators with which it was possible to perform the simulations and then compare the results with those in the catalogue.
  • Trunk load simulation: with the trailer fixed to the ground, the load of a single trunk has been simulated under the limit conditions declared in the catalogue, that is a 490 Kg trunk positioned at the maximum distance reached by the arm (8.5 m). The actuator loads were measured during the operation and then compared with the maximum loads they can support (maximum values have been estimated).
  • Introduction of trailer dynamics: by carefully looking at the videos of this type of machinery in operation, it has been observed that introducing flexible components, such as the crane arm, would not have brought great advantages in terms of accuracy of results. The trailer with attached loader, is coupled to the rear of a tractor and during the movements appears as a set of rigid bodies rigidly connected to each other that move oscillating with respect to the ground. The main oscillations are therefore not due to the flexibility of the components or the joints between them, but to the contact between trailer-terrain and trailer-tractor. These contacts have been set in such a way as to recreate movements that are close to reality and therefore the loads to the actuators have been measured.
  • Risk of rollover: by introducing trailer dynamics it is also possible to assess whether the trailer is likely to tip over during loading.


The modelling problem

For modeling we started from the catalogues where the data of trailer, crane and grapple are provided. All the main components of the mechanical system have been designed with respect to geometry and masses to ensure that subsequent simulations are not affected by this type of error. In order not to overload the model, we tried to simplify the geometries as much as possible. In some cases, several subsystems have been combined in a single body and have no direct purpose in these simulations (e.g.: the trailer has been designed as a single frame plus the 4 wheels).

Kesla loader 305T.

Kesla loader 305T.

Kesla grapple proG26.

Kesla grapple proG26.


Kesla trailer 102

The components have been designed and assembled in Solidworks to facilitate their import into Adams.

Forest Trailer CAD model with loader.

Forest Trailer CAD model with loader.

Units settings in Adams:  Length = mm;  Mass = kg;  Force = N;  Time = s;  Angle = deg;  Frequency = Hz

Imported into Adams the model as a parasolid, we proceed creating the links between the parts, operation that must be executed with attention in order to avoid the redundancy of the constraints. The crane could be shaped incorrectly using only revolute joints between the various arms and actuators, but in 3D space this would introduce a high amount of redundancies. Cylindrical and spherical joints are also used, as can be seen from the list of joints.

Inside the mechanism there are specular bodies with respect to the meridian plane (rods, rockers), to simplify the model and reduce the number of constraints these bodies are imported as a single part even if divided, and the constraints on the meridian plane.

List of model bodies and motions

List of model bodies and motions.


Grapple with 2 DOF.

The crane consists of a main open chain mechanism more than the closed chains. Inside it has 5 actuators, a hydraulic motor at the base of the crane that allows it to rotate and 4 hydraulic pistons to make movements on the meridian plane. The grapple has two actuators inside it, a hydraulic motor that allows it to rotate around its axis, and a piston for opening the jaws. In the previous figure are shown all the bodies used in the simulation, in the list of components missing the tires and the road that were inserted as a file .tir and .rdf as will be explained later.

The position of all constraints has been assigned by CAD coordinates to avoid positioning errors.

Joints of model.

Joints of model.

(In this section reference is made to the FOREST_TRAILER_without_trailer_dynamic.cmd model)

Calculation of the degree of freedom of the model with the Grubler formula:

DOF = 6*(N_Bodies)–6*(N_Fix)–5*(N_Rvl)–5*(N_Trnsl)–4*(N_Cyl)–3*(N_Sph)–(N_Motion)

DOF = 6*(28)–6*(8)–5*(9)–5*(5)–4*(5)–3*(5)–(7) = 8

It is obtained that the system has 8 DOF, 6 for the trunk which is free to move in space and 2 rotations granted to the grapple compared to the crane. This result is also found on Adams which confirms the absence of redundant constraints. The trunk, as will be explained in more detail later, is leaning against the ground with a solid to solid contact that allows it to not fall off starting the simulation.

Since all the components have been designed according to the measures in the catalogue, it was sufficient to assign the density to each part to obtain the corresponding mass.

Parts Weight [kg]
Datasheet CAD
Trailer 1760 1755,3
Loader 1370 1364,5
Grapple+Hydraulic motor 130+25 166,1
Tot. 3285 3285,9

Simulations and analysis of results

Range of motions

As anticipated in the objective, the first simulation assesses the crane’s range on the meridian plane. The movement ranges of the various actuators are derived from the charger data sheets:

Motions Range of motions
Hydraulic Motor 1 380°
Piston 1 564 mm
Piston 2 840 mm
Piston Boom 1-2 1700 mm
Piston Boom 2-3 1700 mm
Hydraulic Motor 2 ±360°
Piston Grapple 260 mm

Applying these ranges in series to the actuators, the working space of the arm has been plotted on the meridian plane. The rotation around the hydraulic motor 1, at the base of the Shaft pricipal, has been omitted.

Range of simulation movements

Range of simulation movements.

You can compare the result with the one shown in the catalogue.

Range of catalogue movements.

Range of catalogue movements.

Visually the two curves are similar even though the catalogue one is cut in the area of less interest, that is where the telescopic arm would collide account principal Shaft.

Datasheet [mm] Simulation [mm]
Max Height 10000 9838
Max Distance 8500 8503
Max Depth 5300 4963

The small difference between the obtained and expected values can be due to errors in the CAD geometry design or to the approximations of the values in the catalogue.

Range of grapple movement.

Range of grapple movement.

For the grapple, however, the distance between the ends of the jaws in open or closed conditions and the range of displacement was checked:

Datasheet [mm] Simulation [mm]
Max Opening 1315 1482
Max Closing -810 -751
Range 2125 2233

 Note that the opening and closing values differ from the catalogue values while the range values are about correct. This is due to an error in the choice of zero (initial) position of the simulation.

Trunk load simulation

In this part we simulate the load of a single trunk under the limit conditions declared by the catalogue, that is a trunk of 490 Kg positioned at the maximum distance reached by the arm (8.5 m).

Initial position of the simulation.

Initial position of the simulation.

At instant zero the arm is positioned above the trailer, in the following seconds it will move sideways to reach the trunk, grab it and load it over the trailer. The joints between the various bodies have been reported above. Solid to solid contacts have been introduced to ensure that the grapple is able to grasp the trunk, as well as between the trunk-soil and the grapple-soil.


All contacts introduced in the simulation are of type “Solid to Solid”, normal force is set to “Impact”; and friction is always activated as “Coulomb”. For each contact introduced, the values of stiffness, static and dynamic friction have been calibrated. For the calibration of the stiffness, the penetration between the two bodies was used as a control parameter, while for the friction the tables were used which suggest the values according to the two materials in contact. It has been found that stiffness over 10^5 N/mm give numerical problems (slow calculation, lack of convergence of the solution, physical phenomena not real).

For the remaining parameters the default parameters have been maintained:

Normal Force = Impact

Stiffness = …

Force Exponent = 1

Damping = 10

Penetration Depth = 0,1

Friction Force = Coulomb

Coulomb Friction = On

Static Coefficient = …

Dynamic Coefficient = …

Stiction Transition Vel. = 100

Friction Transition Vel. = 1000

  • Trunk-soil: the stiffness has been set to 500 N/mm with a maximum penetration of about 10 mm, has assumed a compact clay soil. Static coefficient =0,3 and Dynamic coefficient = 0,1.
  • Grapple-soil: the parts that participate in this count are only the two jaws (Wide jaw and Narrow jaw). The stiffness has been set to 1000 N/mm with a maximum penetration of about 1.5 mm, as seen from the simulation the clamp has to touch just the surface of the soil. Static coefficient =0,3 and Dynamic coefficient = 0,1.
  • Trunk-Grapple: the parts that participate in this count are Base Grapple, Wide jaw and Narrow jaw. The stiffness has been set to 1000 N/mm with a maximum penetration of about 6 mm. static coefficient =0.5 and Dynamic coefficient = 0.2. In this contact, the problem initially arose that the trunk tended to slip away from the grapple under the action of dynamic loads. The first attempt was to increase the grapple closure through the actuator, but this would lead to unrealistic excessive penetrations. Another possible solution was to increase stiffness at the same penetration, thus generating more normal strength, too high stiffness led to numerical problems. It was therefore decided to increase static friction.
  • Trunk-chassis: the stiffness has been set to 1000 N/mm with a maximum penetration of more than 10 mm, it is noted that the trunk is dropped over the trailer on which it slams. Static coefficient =0,4 and Dynamic coefficient = 0,2.

 All joints are frictionless inside so as not to complicate the model too much.


The 7 actuators are driven by step functions. This type of function makes the pistons and hydraulic motors move in a similar way to reality. In the pistons the engine is placed in the translational joint, while in the hydraulic engines it is on the revolute joint.

  • Hydraulic Motor 1: STEP(time , 0.5 , 0d , 3 , 65d )+STEP(time , 27 , 0d , 33 , -65d )
  • Piston 1: STEP( time , 7 , 0 , 9 , -67 )+STEP( time ,12 , 0 , 27 , 387 )
  • Piston 2: STEP( time , 14 , 0 , 27 , -600 )+STEP( time , 33, 0 , 36 , -90 )
  • Piston Boom 1-2: STEP( time , 3 , 0 , 8 , 1700 )+STEP( time , 15 , 0 , 22 , -1550 )+STEP( time , 33 , 0 , 36 , 450 )
  • Piston Boom 2-3: STEP( time , 3 , 0 , 8 , 1700 )+STEP( time , 15 , 0 , 22 , -1550 )+STEP( time , 33 , 0 , 36 , 450 )
  • Hydraulic Motor 2: STEP( time , 5 , 0d , 7 , 30d )+STEP( time , 27 , 0d , 33 , 60d )
  • Piston Grapple: STEP( time , 5 , 0 , 8 , 250 ) + STEP( time , 9 , 0 , 11 , -155)+ STEP( time , 35 , 0 , 37 , 128)


The simulation starts with the calculation of the equilibrium position and then follows a dynamic simulation of 38 seconds with 2000 steps. The “GSTIFF-I3” resolution method was used because it was faster. In addition, a test using a more consistent method such as “GSTIFF-SI2”has been performed to verify that the solution is convergent.


For each actuator the force or torque to be exerted in the corresponding direction of movement has been measured. As you can see from the diagrams below, each curve has peaks, particularly between the second 8 and 11. In this interval of time the grapple goes to grab the trunk and then the contact forces are established. Being the system very rigid, since the flexibility of components and joints are neglected, the contacts are real impacts that originate forces that in reality do not occur. These peaks of force will not be considered when the maximum load of each actuator is assessed. In order to verify that these peaks are not due to the type of resolver adopted, a second test with GSTIFF-SI2 and active interpolation has been done, the results remain unchanged.

1 2 3 4

Loads on the actuators (no trailer dynamics).

Loads on the actuators (no trailer dynamics).

For each actuator the maximum load exerted during movement has been measured and shown in the following table. Since no technical data on the performance of hydraulic pistons are available, the maximum force they can achieve has been estimated. Note the operating pressure of the hydraulic circuit (190 bar) and the diameter of each cylinder has been estimated the maximum force. The maximum torque of hydraulic motors are available from the catalogue.

Motions Loads on the actuators
Estimated / Data-sheet Measured
Hydraulic Motor 1 16 E+06 Nmm 9,42 E+06 Nmm
Piston 1 1,8 E+05 N 2,4 E+05 N
Piston 2 1,5 E+05 N 1,56 E+05 N
Piston Boom 1-2 5,4 E+04 N 8 E+03 N
Piston Boom 2-3 5,4 E+04 N 7 E+03 N
Hydraulic Motor 2 11 E+06 Nmm 1,8 E+06 Nmm
Piston Grapple 5,4 E+04 N 3,1 E+04 N

Pistons 1 and 2 are beyond the estimated maximum force values. This is not a problem, however, as it is sufficient to approach the load to the trailer, before activating these pistons, to reduce the tonearm of the trunk weight force. Therefore, the loader kesla 305T is able to load a 490 kg trunk at a distance equal to the maximum arm range (8.5 m) if the trailer is fixed.

Another important fact that would have been interesting to evaluate is the maximum speed of movement of each actuator since this affects dynamic loads. However, this type of data is not supplied either from a catalogue or from data sheets.

Trailer dynamics

In the previous simulation the trailer was fixed while the arm loaded the trunk. This situation only comes close to reality when the trailer is full or almost full. Having a load capacity of 10000 kg and a tare weight of 3285 kg, the trailer will remain practically stationary while the crane moves. When the trailer is empty, the crane movement will have a dynamic effect on the trailer. To study the dynamics the constraint that fixed the trailer was removed and replaced with contacts explained below.

  • Interaction wheels-ground

As a first attempt were designed at the CAD of the wheels and then imported as a solid part into Adams. By introducing contact between the soil and the wheels, we looked for parameters that recreated a real behavior. The result was not positive: with low stiffness the trailer sank on the soil while with slightly higher values began to jump on the ground. Attempts have been made to work on the exponent of penetration, thus eliminating the linearity of the relationship, but with no benchmarks, no good result has been achieved. For the side dynamics of the trailer instead it was possible to obtain a convincing response by setting Static coefficient =0.8 and Dynamic coefficient = 0.76 (friction force Coulomb).

Comparison of different wheel models (contact part vs .tir).

Comparison of different wheel models (contact part vs .tir).

Due to the problems of vertical dynamics it was thought to introduce the wheels as a file .tir. This choice together with the setting of some parameters led to satisfactory results (only the final choices of each parameter will be reported below).

Wheels are been used   “msc_truck_pac2002.tir”   with:

Mass = 100 kg

Ixx = Iyy = 8,74E+06 Kgmm2

Izz = 1,22E+07 Kgmm2

UNLOADED_RADIUS = 0,5 m          (Free tyre radius)

WIDTH = 0,35 m                                  (Nominal section width of the tyre)

ASPECT_RATIO = 0,85 m                  (Nominal aspect ratio)

RIM_RADIUS = 0,3 m                        (Nominal rim radius)

RIM_WIDTH = 0,4 m                         (Rim width)

VERTICAL_STIFFNESS = 5e+005 N/m             (Tyre vertical stiffness)

VERTICAL_DAMPING = 500 Ns/m                   (Tyre vertical damping)

The other data was left in the wheel model of the truck.

The wheel mass data were obtained by drawing a CAD model at the tyre plus rim with respective materials. This type of data was not available in the catalogue. The wheels are fixed to chassis with a revolved joint (this is equivalent to having the wheels not braked). The imported road model is “2d_flat.rdf” and has been superimposed to the soil part. In this way the soil generates the contact forces with the trunk and grapple, while the road generates forces with the wheels. The tyre stiffness adopted is about half that of a truck as this type of wheels have a lower inflation pressure and more shoulder. The compliance of the soil is negligible since the contact zone is wide and its stiffness is of an order of greater size (therefore negligible being springs in series).

The vertical displacement of the wheels was used as the control parameter to check whether the wheel parameters were set correctly. When its value is negative it means that the wheel has deformed radially, when it is positive it means that it has risen from the soil.

(All the following graphs refer to the model FOREST_TRAILER_real_dynamic.cdm).


Radial deformation or wheel lift in relation to the soil.

Radial deformation or wheel lift in relation to the soil.

The wheels on the left, which are on the trunk side, deform radially. The wheels on the right instead lift from the ground. They rise up to 5 cm just as you see in reality. The final vibrations are due to the trunk being dropped into the trailer. An important approximation of all simulations is the absence of suspensions. Since no data were available, it was preferred to neglect their presence. Also because from the videos you can not extrapolate useful information being a hidden component.


Forces on the 4 wheels.

Forces on the 4 wheels.

  • Interaction chassis-soil

The chassis of the trailer interacts with the ground through the two telescopic legs. The chassis is considered infinitely rigid compared to the soil that instead deforms. A compact clay soil with a stiffness of 18000-36000 kN/mm3 is used and the contact area of the two legs on the ground is calculated. It is then possible to estimate a stiffness to have an order of magnitude (about 3000 N/mm). From here, checking how far each leg penetrates the ground or rises, the final value of 5000 N/mm has been established.

Contact Type = Solid to Solid

Normal Force = Impact

Stiffness = 5000

Force Exponent = 1

Damping = 10

Penetration Depth = 0,1

Friction Force = Coulomb

Coulomb Friction = On

Static Coefficient = 0,3

Dynamic Coefficient = 0,1

Stiction Transition Vel. = 100

Friction Transition Vel. = 1000

Leg penetration in the soil.

Leg penetration in the soil.

The left leg penetrates the ground about 10 mm while the right leg has no penetration because it rises as in reality.

Loads on trailer legs.

Loads on trailer legs.

  • Interaction trailer-tractor

The trailer is attached to the rear of a tractor which can move in relation to the ground. This possibility of movement must not be neglected. To recreate this in Adams it was thought to concentrate all the mass of the tractor (5000 kg) inside the green pin (see figure below). This pin is constrained from the ground to stay on the plane parallel to the soil. To avoid excessive lateral movement, 2 parallel springs have been introduced, recreating the lateral stiffness of the 4 wheels of the tractor on the ground. The lateral stiffness is about half of the radial one, as order of magnitude it took 100 N/mm.

Stiffness tractor = 400 N/mm

Damping tractor = 0,8 Ns/mm

It has been observed from the video that longitudinally the moving component of the tractor is much greater than the lateral one. Putting no longitudinal spring and shortening the lateral ones gives a satisfactory result. As soon as the trailer tends to move excessively along its axis, the sine of the force along the side spring intervenes.

Schematized tractor.

Schematized tractor.

Between the green pin and the yellow chassis there is some space and the forces are transmitted by contact.

Contact Type = Solid to Solid

Normal Force = Impact

Stiffness = 10000 N/mm.

Dx=longitudinal displacement, Dz=lateral displacement of the tractor.

Dx=longitudinal displacement, Dz=lateral displacement of the tractor.

The geometry of the pivot prevents the trailer from rising too high.

The model obtained presents 21 DOF: 6 of the trunk, 6 of the trailer, 3 of the tractor, 2 of the grapple and 1 for each of the 4 wheels. There are no redundant constraints.

The simulation starts with the calculation of the equilibrium position and then follows a dynamic simulation of 38 seconds with 2000 steps. The resolution method is “GSTIFF-I3” (the convergence of the solution has been verified with “GSTIFF-SI2”).

As you can see from the animation, the trailer oscillates due to the flexibility of the new system. These oscillations are collected on the loads of all actuators. The model with and without trailer dynamics will be compared below.






Loads on the actuators (with trailer dynamics).

From the graphs you can see how to add to the previous loads those due to the dynamics of the trailer. In particular, one notices the presence of a oscillation with frequency of about 1,2 Hz (obtained with FFT signal analysis). The corresponding mode of vibrating is the roll of the trailer.

Comments loads transmitted to earth: The load on the left leg goes from 4770 N, initial equilibrium condition, to 56170 N, which is 152% of the weight of the whole trailer plus the trunk. The effects of the dynamics on the forces discharged to the ground by the legs are not negligible since they grow of an order of magnitude. The left rear wheel passes from a load of 6708 N (initial equilibrium condition), to 12317 N. This is the most stressed wheel of the 4.

Risk of rollover

In the previous simulation the trunk was positioned next to the trailer. By positioning the trunk next to the crane, it shall rotate 90 degrees from the initial position. In this situation, the risk of the trailer tipping is greater.  The tipping limit can be estimated with a simple analytical account neglecting dynamics. To ensure that the trailer does not overturn, its centre of gravity must not shift beyond the point of overturning. The turning point, shown in red in the figure, corresponds to the point of the trailer leg on which it pivots when it turns upside down. The origin of the absolute reference system is indicated in blue.

Trailer front view.

Trailer front view.

To simplify the analytical calculation we consider the static case on the front plane of the trailer. The overturning point is 1335 mm from the origin. The center of gravity of the trailer with loader, when the crane has the arm all out (without the hooked trunk), has coordinates:

x=1160,7 mm

y=1369,3 mm

z=1130,1 mm

This data was derived with the function “Aggregate mass” in Adams. The coordinate of our interest is the z which must be compared with that of the overturning point. When the coordinate z exceeds 1335 mm the trailer will no longer be in equilibrium and will flip. Without the coupled trunk the trailer is in equilibrium since z=1130,1 < 1335 mm.

As soon as the trunk is coupled, a new trailer+loader+trunk system is created whose centre of gravity moves to z=2080,4 > 1335 mm. According to the analytical calculation the trailer should overturn as soon as it hooks the trunk. This is confirmed by the following simulation:

(Model: FOREST_TRAILER_Rollover.cmd , Simulation: SIM_SCRIPT_Real_Dynamic)

From the catalogue it is stated that the loader is able to lift a trunk of 490 kg to 8.5m distance, but this is not true if the trailer is unloaded because it overturns. The load of wood that can keep the trailer in equilibrium can be estimated as follows:

3285*(1335-1130) + (timber_loading)*1335 – 490*(8500-1335) = 0

  • Timber_loading = 2125 kg

Load compatible with maximum load (10000 kg).

Or the trunk weight can be estimated, which can be raised to 8.5m when the trailer is unloaded:

3285*(1335-1130) – (max_trunk_mass)*(8500-1335) = 0

  • Max_trunk_mass = 94 kg

These results have been confirmed in Adams, the presence of the tractor actually allows us to lift loads a little more.


Crane kinematic analysis confirmed that the model was correctly designed and constrained. By comparing the trajectory that the loader performs on the meridian plane with the data provided by the catalogue, we observe that the maximum deviation is 6%.

The model with trailer fixed to the ground has verified that the loader kesla 305T complies with the maximum performances declared in the catalogue: lifting a trunk of 490 kg placed at a distance equal to the maximum range of the arm (8.5 m). The values of the measured loads to actuators are realistic and compatible with the estimated maximum values.

The introduction of trailer dynamics in the model changes the load on actuators a little. On the legs of the trailer, however, has not negligible effects: on the left leg you measure peaks of forces equal to 152% of the weight of the whole trailer. Introduce the tyre with the model . tir to recreate the dynamics of the trailer has proved an excellent choice despite the wheels are almost stationary.

The limit for the maximum lifting load is not imposed by the crane but by the risk of overturning the trailer. In fact, if the trailer is empty, the maximum lifting load is 94 kg (estimated in the most critical lifting condition for the trailer). To lift the cargo declared by the catalogue (490 kg) the trailer should be partially loaded with wood (at least 2125 kg).


[1] Brochure Kesla downloaded from

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