Military Off-the-Road Tires F2000IS002 Horst W. Stumpf , G. H. Hohl

F2000IS002
Seoul 2000 FISITA World Automotive Congress
June 12 -15, 2000 Seoul, Korea
Military Off-the-Road Tires
Horst W. Stumpf1,2, G. H. Hohl2,3
Technikum Joanneum GmbH, Austria1
Austrian Society of Automotive Engineering2
Austrian Federal Army3
ABSTRACT
During the last years more military tasks have been transferred from tracked vehicles to wheeled armoured vehicles. Studies
show that in most cases off-road mobility can be obtained by wheeled vehicles. It is a disadvantage of conventional pneumatic
tires that they are not serviceable after a puncture. An off-road vehicle requires modern radial tires with cross-country profile,
which must be able to convert the output of the vehicles engine into tractive effort on the ground. In military operations various
types of tired vehicles are expected to operate over a great variety of surfaces in cross-country transport. Reliability is of particular importance.
KEYWORDS: Tire, Tire Testing, Off-the Road Tire
INTRODUCTION
TIRE CONSTRUCTION TYPE
•
During the last years more military tasks have been transferred from tracked vehicles to wheeled armoured vehicles.
Studies show that in most cases off-road mobility can be
obtained by wheeled vehicles. But one cannot deny that a
certain amount of hesitation regarding the suitability of
pneumatic tired vehicles lies in the vulnerability of the tires,
as they are very often exposed to the effects of enemy attack.
Apart from direct hits by infantry weapons, artillery shrapnel often puts tires out of action and immobilizes the vehicle.
It is a disadvantage of conventional pneumatic tires that
they are not serviceable after a puncture. In many cases
there is not enough time for mounting the spare wheel.
These considerations require tires with emergency capabilities after puncture which offer complete safety after burst
and which can be run flat for considerable distances at relatively high speeds. A vehicle provided with such an emergency running system, should be able to continue its mission on and off the road without essential decrease of its basic functions such as load carrying capacity, transmission of
torque, directional and cornering stability, ride comfort,
steerability acceleration and maximum speed [1].
An off-road vehicle requires modern radial tires with cross country profile, which must be able to convert the output of
the vehicles engine into tractive effort on the ground as
shown in Figure 1[2]. All off-highway operations do not
need the same degree of traction. As a result, separate tread
designs are used for different degrees of tractive effort.
•
•
•
In the construction field, as typified by dam, waterway
and highway projects which require movement of large
quantities of earth and rock. In this type of service,
speeds as high as 40 to 80 km/h, length of haul to 16 km,
and size as a means of transport of loads and equipment
to 75 m3 capacity, are generally expected [3].
In the logging, mining and petroleum industries - heavier units such as mobile cranes and self-contained pumps
and power plants, are used. This demands tire types with
high flotation characteristics and load-carrying capacities.
In military operations various types of tired vehicles are
expected to operate over a great variety of surfaces in
cross-country transport. Reliability is of particular importance.
In general transportation into newly developed areas
without adequate highways or railways - such vehicles
must have flotation and mobility capability under heavy
loads, without the need for extensive preparation and
maintenance of roads or tracks.
TIRE SIZES AND TYPES
The growth of off-road operations has brought about a great
diversification in tires to meet all service requirements.
Tires have become larger both in cross-section and in rim
diameter. Larger tires permit higher loads per tire without
sacrificing flotation. With the same tire loadings, inflation
pressures on the wide-base tire can be reduced. The wider
cross-section gives improved traction and flotation and the
lower unit ground pressure can improve the resistance to
damage from stones and other objects. The wide base principle can be extended into low section-height tires. The low
section-height shape makes possible a wider cross-section
for improved flotation without increasing overall diameter
or tire weight, as would have been necessary if conventional
tire shapes had been maintained as shown in Figure 2.
Radial ply
Bias ply
Figure 1 - Tire construction type
1
j

−

K
H = (A ⋅ c − Fz ⋅ tan Φ )⋅ 1 − e


j ... Shear displacement
K




(4)
... Soil deformation modulus
H ... Thrust
For a rotating tire or track Janosi and Hanamoto proposed a
modification which accounts for the changing soil deformation along the contact length: Eq (5).
L


K − s⋅ K
H = A ⋅ c + FZ ⋅ tan Φ ⋅ 1 +
⋅e
− 1
 s ⋅ L

L . .. Tire contact length
(5)
(
)
s
... Overall wheel slip
The definition of the mechanical soil parameters took place
in a natural soil bin on sandy loam. The test run was made
on tillaged soil. Figure 3 shows a typical example for a diagram in which the shear strength is plotted versus shear velocity. An increase of shear velocity entails an improvement
of shear strength, which depends on the cohesion c and the
angle of internal friction Φ.
M = Distance between centers
Figure 2 - Tire and rim measurements
LONGITUDINAL AND LATERAL FORCES
The slip definition used for military engineering is eqivalent
to the definition in automotive engineering, since the tractive properties are of main interest [4]. Consequently there
are two different slip definitions for the propelled and the
braked wheel: Eq (1) and Eq (2).
v − va
sT = th
v th
v th ... Theoretical driving speed
τmax
(1)
0 < vs < 1,2 m/s
Figure 3 - Influence of shear velocity on shear strength [8]
Hidden line calculated
v a ... Actual driving speed
v − va
s B = th
(2)
va
This slip definition is useful when high positive and negative slip values are obtained in a test run and have to be presented in one diagram. A disadvantage of the definition is
the unsteadiness at zero slip and the neglect of steering
movements of the gauge wheel [5].
Many equations and methods were established for traction
prediction more than twenty five years ago [6]. Because the
soil used in this investigation had significant levels of cohesion and angle of shearing resistance, the main approach
used was based on Micklethwaite's equation as modified by
Janosi and Hanamoto for a driven wheel or track: Eq (3) [7].
H max = A ⋅ c + FZ ⋅ tan Φ
In consideration of the shear velocity vs the Mohr-Coulomb
equation Eq (6)
τ max = c + σ ⋅ tan Φ
τ max
... Area of tire and soil contact
c
... Soil cohesion
FZ
... Tire load
(6)
[c, σ , τ] ... N / mm
is put in concrete terms for sandy loam, before passage Eq
(7),
(7)
τ max = 0,009 + 0,04 ⋅ v s + σ ⋅ tan 29°
and after passage Eq (8):
(8)
τ max = 0,016 + 0,026 ⋅ v s + σ ⋅ tan 32°
Terrain vehicle mobility is strongly affected by the performance of the tires [9]. Here the influence of several tire
parameters have already been investigated in a number of
field and laboratory tests and by the use of simulation models. Most of the calculation models do not take effects of the
tread profile into account.
One of the earlier methods for predicting the motion resistance of a rigid wheel is that proposed by Bekker [10]. It
can be seen from Eq (9) that to reduce the compaction resistance RC, it seems more effective to increase the wheel diameter D than the wheel width b.
2
H max ... Maximum thrust
A
... Shear strength
(3)
Φ ... Angle of internal resistance
Janosi and Hanamoto proposed to describe the asymptotic
curves of shearing versus soil deformation: Eq (4).
2
2n+ 2
1
RC =
(3 − n )
2n + 2
2 n +1
⋅ (n + 1)
1
⋅ b 2 n +1
1
K
 2 n +1
⋅  C + K Φ 
 b

However, in this simulation the tread design was still neglected. The effect of the tread profile was replaced by a
pure surface function between tire and soil as shown in Figure 4 [11].
 3F  2 n +1
⋅ Z 
 D
RC = Motion resistance due to terrain compaction
(9)
D = Diameter of tire
= Width of tire
b
K C = Cohesive modulus of terrain deformation
K Φ = Frictional modulus of terrain deformation
FZ
= Normal load
n
= Exponent of terrain deformation
Terrain vehicle mobility is not only a military problem but
is also important for earth-moving and farming process. For
wheeled vehicles the tire as interfacing element of vehicle
and ground takes a strong effect on off-road mobility. The
effect of different tire parameters have been investigated in
a number of field and laboratory tests.
The relationship between the total tractive force and the slip
when part of the tire tread sliding on the ground is expressed by Eq (10).
λ ⋅ µ p ⋅ Fz − K ′ ⋅ s 2
Fx = Fxs + Fxa = µ p ⋅ Fz −
(10)
2⋅ L ⋅ K ′⋅ s
(
Figure 4 - Influence on deflection
The material description of the soil model is based on an
elasto-plastic material law: Eq (6).
Soil strength varies with the rate of compaction as shown in
Figure 5.
)
K ′ = L ⋅ kt ⋅ λ
Fx = Tractive force
Fxs = Tractive force developed in thge sliding region
Fxa = Tractive force developed in the adhesion region
Figure 5 – Deflection on a driven tire
µ p = Peak value of coefficient of road adhesion
Figure 6 shows the influence of tire pressure.
λ = Longitudin al deformation of thetire tread
prior to ntering the contact patch
L = Contact length
k t = Tangential stiffness
Rolling resistance from the tire is not a vector but it is a scalar. Because rolling resistance is the conversion of mechanical energy into heat through the rolling tire: Eq (10). That
relationship is very important for tire testing.
dQ
FR =
dl
P − Pout
Q
FR =
= in
(10)
vR
vR
PR = Pin − Pout = Q
Q = Loss energy
Pin = Ingoing power
Pout = Outgoing power
v R = Rolling speed
Moreover models for the simulation of tire and soil interaction have been established and improved. Recently, with the
increase of computer capacity, the Finite-Element-Method
(FEM) as a tool for various problems, has turned out to be
suitable for the investigation of tire-soil interaction as well.
Figure 6 – Influence of tire pressure
TREAD PATTERN FOR OFF-ROAD TIRES
Figure 7 shows different tread pattern designed for different
requirements. Tires must possess the characteristics of durability, load carrying ability, speed capability and low operating temperature - to the degree which service requirements demand. High tensile cord materials must be used as
reinforcement to provide sufficient strength and flexibility
to the tires, and tire material should be characterized by
toughness and high abrasion resistance and low heat generation during operation.
3
surface. An effective method to reduce the soil pressure is
an increase of the contact area between tire and ground, for
this the tire becomes wider and the flanks become softer. To
characterize a tire running on a soft ground measurements a
absolutely necessary. All calculations can only give a first
prediction.
ACKNOWLEDGMENT
This paper is based upon studies that were conducted at the
Automotive Engineering department of Technikum Joanneum GmbH in Austria. The work was part of a project designed to install an off-road tire measuring and FEMcalculating system. The authors acknowledge the contribution of the Austrian Federal Army.
REFERENCES
[1] Hohl, G. 1989. Tests of off-road tires with emergency
capabilities. International Society for Terrain-Vehicle
Systems, 4th European Conference, Wageningen: 100107.
[2] Stumpf, H. 1989. Bereifungsmöglichkeiten für PkwGeländefahrzeuge. ÖVK Symposium: Geländefahrzeuge in Theorie und Praxis, Wien: 109-132.
[3] Yong, R., Skiadas, F., Skiadas, N. 1984. Vehicle traction mechanics, Elsevier, New York: 45.
[4] Haken, K. 1993. Konzeption und Anwendung eines
Meßfahrzeuges zur Ermittlung von Reifenkennfeldern
auf öffentlichen Straßen. Dissertation Universität
Stuttgart.
[5] Armbruster, K. 1991. Untersuchungen der Kräfte an
schräglaufenden angetriebenen Ackerschglepperrädern.
Dissertation
Universität
Stuttgart,
VDI
Fortschrittberichte, Reihe 14, Nr. 53.
[6] Keen, A. 1998. Traction prediction on a sandy loam
soil for a single wheel tester. Proc. of the 5th AsiaPacific Regional Conf. ISTVS, Korea: 393-400.
[7] Keen, A., Craddock, S. 1997. A method for determining the tire-soil contact of a rolling tire by using the dynamic measurement of the radial and tangential tire deflection. 7th European Con. ISTVS, Italy.
[8] Kutzbach, H., Schlotter, V., Barrelmayer, T. 1999. Investigations on longitudinal and lateral forces on tractor
tires. Proc. 13th International Con. ISTVS, Germany:
397-404.
[9] Fervers, C. 1996. Phenomena of tire-profile on different soils. Proc. 13th International Con. ISTVS, Germany: 337-344.
[10] Wong, J. 1993. Theory of ground vehicles. John Wiley
& Sons, New York, 2nd ed.: 150-163.
[11] Aubel, T. 1996. FEM-Simulation der Wechselwirkung
zwischen Reifen und nachgiebiger Fahrbahn. TÜV
Bayern.
Figure 7 - Off-road tread patterns
(a) Representative tread pattern designed for maximum
traction requirements, as in agricultural tractors, earth
moving or excavation operations used.
(b) Variation of this pattern in which deep grooves have
been added to the traction elements to reduce rubber
mass and provide lower tire operating temperatures in
high-speed service.
(c) Medium traction design used mainly in military operations.
(d) Design which is supposed to provide extra traction in
off-road operations whilst still performing satisfactorily
if used part of the time on the highway.
(e) Widely-rolling wheels
(f) Free-rolling wheels
(g) Unusual service conditions sometimes demand a special
choice of tread pattern. In muskeg and deep snow operation, high flotation and traction are required and
hence terra tires are used. In desert operations maximum traction is necessary but the use of an aggressive
design is not always recommended because the tire may
tend to dig-in, and become stuck when a no-go situation
is encountered.
(h) In this case, a ribbed design could be used where the
lack of traction elements is offset by the large ground
contact area. Snow creats problems similar to soft sand.
CONCLUSIONS
This review has included mainly theoretical studies of tire
shape, tread pattern and structural properties of the soil or
sand. The lateral force-slip angle performance is represented in consideration of inflation pressure, tire load and
4