Drawbar Pull and Its Effect on the Weight Distribution of a Tractor

Abstract

This article aims to experimentally determine how the tractor's weight distribution changes during loading by drawbar pull, and how the tractor's weight affects its drawbar pull properties. Drive wheel ballasting has a significant effect on the drawbar pull and wheel slip of the tractor - travelling gear losses. To achieve these objectives, we conducted experimental measurements on the tractor Case IH Magnum 370 CVX. The results show that higher drawbar pull is achieved in tractors with a higher weight. The measured increase of drawbar pull was 15,8 kN between maximal and minimal weight (Δ 2320 kg). All variants show an equal percentage increase in the weight on the rear axle of the tractor (+ 6 %). Increasing the tractor's weight affected the drawbar pull as well as the wheel slip. As the tractor's weight increases, there is a smaller increase in wheel slip as the drawbar pull increases. The results confirmed that tractor ballasting is important in order to achieve optimum drawbar pull properties, but it is necessary to keep in mind that the higher the weight of the machine or equipment, the larger the effect on the soil.

Full text

145
ACTA UNIVERSITATIS AGRICULTURAE ET SILVICULTURAE MENDELIANAE BRUNENSIS
Volume 65 17 Number 1, 2017
https://doi.org/10.11118/actaun201765010145
DRAWBAR PULL AND ITS EFFECT ON
THE WEIGHT DISTRIBUTION OF A TRACTOR
Adam Polcar1, Lukáš Renčín1, Jiří Votava1
1
Department of Technology and Automobile Transport, Faculty of AgriSciences, Mendel University in Brno,
Zemědělská 1, 613 00 Brno, Czech Republic
Abstract
POLCAR ADAM, RENČÍN LUKÁŠ, VOTAVA JIŘÍ. 2017. Drawbar Pull and Its Effect on the
Weight Distribution of a Tractor. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis,
65(1): 0145 –0150.
This article aims to experimentally determine how the tractor’s weight distribution changes during
loading by drawbar pull, and how the tractor’s weight affects its drawbar pull properties. Drive wheel
ballasting has a significant effect on the drawbar pull and wheel slip of the tractor - travelling gear
losses. To achieve these objectives, we conducted experimental measurements on the tractor Case
IH Magnum 370 CVX. The results show that higher drawbar pull is achieved in tractors with a higher
weight. The measured increase of drawbar pull was 15,8 kN between maximal and minimal weight (∆
2320 kg). All variants show an equal percentage increase in the weight on the rear axle of the tractor (+
6 %). Increasing the tractor’s weight affected the drawbar pull as well as the wheel slip. As the tractor’s
weight increases, there is a smaller increase in wheel slip as the drawbar pull increases. The results
confirmed that tractor ballasting is important in order to achieve optimum drawbar pull properties,
but it is necessary to keep in mind that the higher the weight of the machine or equipment, the larger
the effect on the soil.
Keywords: drive wheel ballasting, drawbar performance, slip
INTRODUCTION
Tractors are a mobile energy resource intended for
tractive work, which is why its drawbar performance
are essential. The drawbar pull and tractive power
are given particularly by the design parameters
of each tractor. The energy contained in the fuel
(amount of energy depend on fuel composition
and properties (Kumbár and Skřivánek, 2015))
is converted in the engine into mechanical work
intended for tractive work, to drive machines
powered through a power take-off, or an external
circuit of the tractor hydraulics. The effective power
of the engine cannot be entirely converted to either
tractive power or a power take-off. The whole
process can be described by an equation of
the power balance of the tractor (Bauer et al., 2013):
[ ]
etVHHm vswa
P P P P P P P P P PW
δ
=+ + + + +++ + (1)
where:
Pe ...... effective engine output,
Pt ......drawbar pulling power,
PVH ...power transmitted by PTO,
PH ..... hydroelectric generator power,
Pm ..... power loss in transmission device,
Pδ ...... power loss by slip,
Pv ...... power loss by rolling resistance,
Ps ...... power required to climb slope,
Pw .....power required to overcome air resistance,
Pa ...... power required for acceleration.
As shown in equation (1), the conversion
of effective engine power to various useful
components is accompanied by losses. Part of
engine power losses consists of mechanical losses,
part consists of drive gear contact with the ground
(slip and rolling resistance), and part is due to driving
conditions (climbing, acceleration, wind resistance).

146 Adam Polcar, Lukáš Renčín, Jiří Votava
Pertinent parts of this equation (1) includes
the drawbar pulling power, power transmitted
by the hydroelectric generator, and effective PTO
power. At a driving speed of up to 9 m.s-1 on a flat
surface, the climb, acceleration and air resistance are
negligible.
The individual powers shown in equation 1
are given primarily by the design parameters
of the tractor. These design parameters include
the center of gravity and wheel base of the tractor
(expecially for 4×4), its weight, hinge position,
type and condition of tires, engine power, type
of transmission gear, size of active area of belts in
crawler tractors, etc. (Grečenko, 1963) Equation (2)
is an example representing the maximum drawbar
pull Ftmax that can be drawn from a vehicle (Grečenko,
1994):
( )
[ ]
max .
tT
F G fN
µ
= − (2)
It is apparent from equation (2) that the drawbar
pull of the tractor GT, or the ballasting of drive
axles depends on the weight of the tractor and
the difference between the drag coefficient
and the rolling resistance coefficient. The drag
coefficient m represents the perfection of the contact
of the drive mechanism with the ground, as well
as part of the engine driving force transferred to
the ground. It is dependent on the size of the contact
area of the tire or belt, surface, etc. The rolling
resistance coefficient f also depends on many
parameters: the speed, size, air pressure and tire
load on the surface on which the vehicle is moving.
As the above description indicates, the drawbar pull
can be influenced by many parameters. We can most
oen see a change in tire pressure in wheel tractors
and in weight distribution, or the ballasting of drive
axles with ballasts or mounted or semi-mounted
trailers. Each tractor manufacturer offers a set of
ballasts to increase the weight of the tractor, or
change the weight distribution between the axles.
Weight distribution is a variable parameter and it
changes over the course of its work. This change is
a result of the effect of the drawbar pull. Fig. 1 shows
a simplified power diagram for a pulling 4x4 tractor
moving at a uniform rate of up to 9 km.h-1 (the air
resistance and rolling resistance of tires is le out).
The power diagram (Fig. 1) shows a tractor
ballasted by general force applied to the trailer.
The resultant exerted by the tool on the tractor is
labeled F. This resultant is the result of the drawbar
pull Ft and the force arising from the weight of
the machine. The tractor’s weight GT is effective
in the center of gravity. The wheels on the contact
surface are affected by normal force Y1 and Y2.
The ground force and the driving force, Fh1 and Fh2,
move the tractor forward. The size of force Y varies
depending on the drawbar pull Ft. This change can
be described by the torque equilibrium (for item 2)
according to equation 3:
21
0 . . . . . 0
T tt t t
M G d Y a F h F tg c
θ
= −−− =
∑
(3)
The normal force Y1 can be determined by
adjusting equation 3:
[ ]
1
. . ..
T tt t t
G d F h F tg c
YN
a
θ
−−
= (4)
1: Simplified power diagram of pulling tractor (Bauer et al., 2013)
Fh1, Fh2 – driving forces; Ft – drawbar pull; F – final force; GT – weight of tractor’s; Y1, Y2 – reaction
forces from ground (Y1 + Y2 = GT at θ = 0°)

Drawbar Pull and Its Effect on the Weight Distribution of a Tractor 147
If the aggregate machine does not draw any
vertical force (tgθ = 0) then equation (4) is simplified
to:
[ ]
1.
t
Tt
dh
YG F N
aa
= − (5)
The equation shows (5) that the normal
force consists of the static adhesive weight
(member GT.d/a) and the auxiliary weight transfer
(member −Ft.ht/a), which is exerted by the drawbar
pull. We can therefore generally conclude that an
increase in drawbar pull results in a weight transfer
from the front axle to the rear axle. This article
aims to experimentally determine how the tractor’s
weight distribution changes during loading by
drawbar pull, and how the tractor’s weight affects its
drawbar performance.
MATERIALS AND METHODS
In order to fulfill the objectives given above,
the experimental measurements were carried out
on tractor Case IH Magnum 370 CVX. The tractor
New Holland T8.420 Autocommand weighing
17,600 kg was used to create a load force.
The experiment was carried out on a piece of
land aer the harvest of corn silage and worked to
a depth of 10 cm with a stubble plough two weeks
prior to the measuring. According to VÚMOP
(2016), this soil type is classified as alluvial soil.
The land consists of flat to slightly inclined terrain.
The measured soil moisture content was 26 %.
Both internal and external sensors were used in
the tractor during the measurement of parameters.
The reading of data from internal sensors was
conducted via the CAN bus. The electrical signal
from external sensors (drawbar pull sensors
U10M, GPS receivers for measuring the actual
speed of the tractor) was processed by the data
logger CompactRio from National Instruments,
and further evaluated and stored using propriety
soware developed with the LabVIEW program.
The data transmission frequency was 20 Hz.
As the name of the measured tractor implies,
the tractor is equipped with a CVX transmitter. This is
a hydromechanical transmitter, or a transmitter with
continuously variable transmission. For this reason,
it was not possible to perform the measurement in
a specific gear, as it is usually performed (Semetko
et al., 1986). During the measurement, the gear unit
mode in the tractor was set to maintain a constant
velocity. In this mode the gear unit maintains
a constant velocity or constant gear ratio regardless
of the ballasting of the tractor. The speed selected
for the tests was 6 km.h-1 and 9 km.h-1 .
To determine the effect of the drawbar pull on
the weight distribution, we conducted a total of
6 test variants. Each variant presented different
front and rear axle loads. For this purpose, the front
ballast in front tree point hitch and different wheel
ballasts were used. A detailed overview of each
variant is shown in Tab. I.
The tests were so-called „accelerated drawbar pull
tests“. In these tests both tractors move at a desired
speed, while the engine of the tested tractor
operates at the maximum fuel supply. The braking
tractor then steadily decelerated until it came to
a halt. The measured parameters are captured
during the deceleration. Each variant is repeated
for statistical significance. The average values of
the measured parameters are calculated from
the repeated tests. During the testing, the measured
tractor was in 4×4 drive mode. The tractive power
Pt was also calculated for evaluation according to
equation 6:
[ ]
.
t ts
P Fv W
= (6)
where:
Ft ... drawbar pull [N]
vs .... actual speed from GPS [m.s-1]
Other evaluated parameters included wheel slip
calculated with equation 7:
[ ]
1 .100 %
s
t
v
v
δ
= − (7)
where:
vt .... theoretical speed [m.s-1]
RESULTS AND DISCUSSION
As we deduced from the beginning of this
article, increasing drawbar pull creates a moment
of drawbar pull at point 2 (see Fig. 1). This moment
causes the weight distribution to move from
the front axle to the rear axle, overloading it.
I: Tractor’s weight distribution in different variants
variant No. front axle load [kg]/[%] rear axle load [kg]/[%] total weight of tractor
[kg]
17,860/44 9,840/56 17,700
26,530/39 10,420/61 16,950
35,400/33 10,900/67 16,300
47,870/47 8,910/53 16,780
56,570/41 9,460/59 16,230
65,400/35 9,980/65 15,380

148 Adam Polcar, Lukáš Renčín, Jiří Votava
If we know the distance of the center of gravity
from the axis of the rear axle, the height of the hitch
(ht = 550 mm) and the wheelbase (a = 3,375 mm)
we can (using equation 5) determine the reaction
force, or rather the front and rear axle weight (sum
of reaction forces from ground is equal to the total
weight of tractor – see Fig. 1). The average drawbar
pull achieved at a speed of 6 km.h-1 was included
in the equation. The highest drawbar pulls are
achieved at lower speeds (Bauer et al., 2013). Results
in percentages of weight distribution are given in
Tab. II.
As Tab. II shows, the higher the tractor’s weight,
the higher average drawbar pull is. The measured
increase of drawbar pull was 15.8 kN between
maximal and minimal weight (∆ 2,320 kg).
The results also show that the percentage increase
in weight on the rear axle is approximately the same
in each variant. This increase is approximately
6 % of the original or static load on the rear axle.
The greatest increase, 1,302 kg, was found in
the highest drawbar pull. We can therefore generally
say that it is convenient to use a combination of front
and rear ballasts for the tractor’s optimum weight
distribution (also in older types of tractors 4k2).
The additional ballasting of the rear wheels reducing
the wheel slip (see below) occurs due to the moment
deduced from the drawbar pull. Uneven weight
distribution, or overloading of the tractor’s rear
wheels (or overloading of the tractor with front
ballasts reducing the weight on the rear axle) could
lead to pedocompaction. Excessive compaction
of soil results in topsoil with a strained pattern
distribution. This topsoil is characterized by
its compactness, difficult cultivation, increased
capillary porosity, poor infiltration of rainwater
and heavy surface runoff. There is also limited
air capacity and biological activity in this type of
soil. (Jandák et al., 2010) However, as other results
confirm, using additional ballasts has a positive
effect on the drawbar pull properties of tractors.
Fig 2 shows the drawbar performance measured
in a tractor moving at a speed of 9 km.h-1.
The measured values (Fig. 2) show that the tractor’s
drawbar pull properties increase with a higher
weight. These conclusions are confirmed by Bauer
and Sedlák (2000) and Grečenko (1994). The best
drawbar pull properties were found in variant No. 2.
In this variant the tractor was equipped with 650 kg
ballasts in the front three-point hitch and 2,500 kg
ballasts in the rear wheels. The graph also shows
that the drawbar pull of the tractor is approximately
the same in all variants up to 70 kN. This implies
that adding additional weight to the tractor should
be carefully considered depending on what work
we will perform. As stated in the article by Bauer
et al. (2013), increasing the weight of the tractor or
using additional ballasts also increases the rolling
resistance and fuel consumption.
Increasing the tractor’s weight is also reflected
in its slip, in addition to the drawbar pull.
The relationship between the slip and drawbar pull
is shown in Fig. 3.
In large drawbar pull, slip losses constitute
the largest portion of total losses (Bauer et al., 2013).
The greater the driving force of the running gear,
the higher the slip value. In addition, the slip not only
affects the unusable drawbar pull, it also adversely
affects the soil structure and the sward state. The slip
depends on many factors, especially the adhesive
forces between the surface and the tire, the shape
and size of the imprint produced at the tire’s contact
with the ground, and the driving force. The larger
the imprint, the smaller the slip (for the same driving
force). One of the factors is the load on the tire,
which is confirmed by results shown in Fig. 3. As
the tractor’s weight increases, there is a smaller
increase in wheel slip with increasing drawbar
pull. Up to 70 kN the slip is almost linear. From this
point on (at δ = 10 %) it increases steeply. Slip can be
reduced by increasing the load on the drive wheels,
and as the study by authors Šmerda and Čupera
(2011) suggests, by reducing the inflation pressure.
The authors of the study also state that reducing
the inflation pressure not only significantly reduces
slip and increases drawbar pull, but also increases
the tractive power. For optimum transmission of
engine power to the ground we must assess both
the load on the driving axles, or weight distribution,
and the tire pressure.
II: Results of the calculation of the dynamic load of both axles with the average value of drawbar pull achieved by each variant
variant No.
weight static
distribution of front
axle [%] / rear axle [%]
Ft [kN] dynamic load of front
axle [kg]/[%]
dynamic load of rear
axle [kg]/[%]
weight
increase on
rear axle [kg]
144/56 70.7 6,686/38 11,014/62 1,102
239/61 73.54 5,308/31 11,642/69 1,302
333/67 61.83 4,373/27 11,927/73 1,006
447/53 59.8 6,877/41 9,903/59 1,010
5 41/59 60.65 5,762/36 10,468/64 892
635/65 54.9 4,488/29 10,892/71 895

Drawbar Pull and Its Effect on the Weight Distribution of a Tractor 149
CONCLUSION
The results show that the load on the drive wheels is significantly affected by the tractor’s drawbar pull
properties. On the other hand, it is important to realize that high loads also increase tire deformation,
i.e. their rolling resistance. It is obvious that for light pulling work, work with machinery powered
through PTO with low drawbar pull resistance and for transportation, it is necessary to carefully
consider ballasting the tractor. In these cases the use of ballasts is justified to provide steerability of
the tractor with mounted equipment.
Every tractor manufacturer or dealer should have a prepared method for the types of ballasts
the operators should use for different types of work. New Holland has created a detailed guide
0
20
40
60
80
100
120
140
160
180
200
020 40 60 80 100 12 0
Pt [kW]
Ft [kN]
6. variant -15,380 kg
5. variant -16,230 kg
4. variant -16,780 kg
3. variant -16,300 kg
2. variant -16,950 kg
1. variant -17,700 kg
2: Tractive power and drawbar pull relationship at a speed of 9 km.h-1
0
10
20
30
40
50
60
70
80
90
100
020 40 60 80 100 120
δ[%]
Ft [kN]
6. variant -15,380 kg
5. variant -16,230 kg
4. variant -16,780 kg
3. variant -16,300 kg
2. variant -16,950 kg
1. variant -17,700 kg
3: Slip and drawbar pull relationship at a speed of 9 km.h-1

150 Adam Polcar, Lukáš Renčín, Jiří Votava
that describes how to ballast tractor models T7, T8 and T9 in aggregation with different tools. For
example, tractors with high-performance engines (i.e. models T7 and T8) should adhere to the weight
distribution of 40 % on the front axle and 60 % on the rear axle. For “light equipment” (equipment with
a working speed above 8,7 km.h-1) the ratio of the tractor’s weight [kg]/ nominal engine power [HP] should
be close to 45. For “medium equipment” and a speed ranging from 7,2–8,7 km.h-1 , this ratio should be
close to 50 kg/HP. For “heavy equipment” and maximum drawbar pull, it should be 55 kg/HP.
As we’ve mentioned several times, tractor ballasting is important to achieve optimum drawbar pull
properties, but we must keep in mind that the heavier the machine or machines, the higher their
effect on the soil.
Acknowledgement
The presented work has been prepared with the support of IGA MENDELU IP 10/2016: “Verification
of the model force action in a three-point hitch”.
REFERENCES
BAUER, F. and SEDLÁK, P. 2000. Influence of drive wheels load on tractor drawbar pull properties [in Czech:
Vliv zatížení hnacích kol na tahové vlastnosti traktoru]. Zemědělská technika, 46 (1): 11–15.
BAUER, F., SEDLÁK, P., ČUPERA, J., POLCAR, A., FAJMAN, M., ŠMERDA, T. and KATRENČÍK, J. 2013.
Tractors and their Use [in Czech: Traktory a jejich využití]. Praha: Profi Press s.r.o.
GREČENKO, A. 1963. Wheel-Type and Chain-Track Tractor [in Czech: Kolové a pásové traktory]. Praha: Státní
zemědělské nakladatelství in cooperation with Ústředím věděckotechnických informací MZLVH.
GREČENKO, A. 1994. Properties off-road vehicles [in Czech: Vlastnosti terénních vozidel]. Praha: Vysoká škola
zemědělská.
JANDÁK, J., POKORNÝ, E. and PRAX, A., 2010: Pedology. [in Czech: Půdoznalství]. Brno: Mendelova
zemědělská a lesnická univerzita.
KUMBÁR, V. and SKŘIVÁNEK, A., 2015: Temperature dependence viscosity and density of different
biodiesel blends. Acta Universitatis Agriculturae Et Silviculturae Mendelianae Brunensis, 63(4), 1147–1151.
NEW HOLLAND AGRICULTURE. Tractor New Holland weight down [in Czech: Dotěžování traktorů New Holland].
[Online]. Available at: http://www.eagrotec.cz/obrazky-soubory/dotezovani-traktoru_pro-web-37b41.
pdf?redir [Accessed: 2016, August 12].
SEMETKO, J. et. al. 1986. Mobile Power Means of Transport 3 [in Czech: Mobilné energetické prostriedky 3]. 2nd
edition. Bratislava: Príroda.
ŠMERDA, T. and ČUPERA, J. 2011. Tire inflation and its influence on drawbar characteristics and
performance – Energetic indicators of a tractor set. Journal of Terramechanics, 47: 395–400.
VÝZKUMNÝ ÚSTAVU MELIORACÍ A OCHRANY PŮDY (VÚMOP), 2016. eCatalog BPEJ [in Czech: eKatalog
BPEJ]. [Online]. Available at: http://bpej.vumop.cz/00100 [Accessed: 2016, August 12].
Contact information
Adam Polcar: adam.polcar@mendelu.cz
Lukáš Renčín: xrencin@node.mendelu.cz
Jiří Votava: jiri.votava@mendelu.cz