Cold Regions Mobility Models
PAUL W. RICHMOND, SALLY A. SHOOP AND GEORGE L. BLAISDELL
If the vehicle can maneuver or tow a trailer
(tractive reserve); and
How fast the vehicle can get where it's go-
Over the past several years several empirical
ing (speed made good).
and theoretical studies have aimed to predict the
The first two issues can be determined at the
mobility of vehicles over winter terrain and the
same time, since a positive value for tractive re-
trafficability of that terrain. Most of these efforts
serve is required for a "go" condition.
were primarily geared toward supporting the U.S.
In its simplest form, mobility can be described
Army Corps of Engineer 's AirLand Battlefield En-
by a single equation expressing the balance be-
vironment (ALBE) demonstration program.
tween traction T and resistance R. Tractive re-
Many of these models, or portions thereof, have
serve or net traction Tnet can be calculated from
also been proposed for incorporation into the
NATO Reference Mobility Model-II (NRMM-II)
Tnet = Tgross Rterrain Rinternal .
(Ahlvin and Haley 1992) and the Condensed Army
Mobility Model System (CAMMS) (Falls et al.
1989). These general mobility models can be used
In eq 1, Tgross is the maximum tractive force that a
to estimate the absolute and relative mobility of ve-
specific vehicle is able to generate on a particular
hicles traveling over a variety of terrain types. In
terrain. Gross traction is generally a function of
addition to being used as military planning and
The tire/track contact pressure;
operational tools, the models can also be used to
The ability of the running gear to engage
compare specific vehicles or to determine specific
with the terrain (e.g. the effect of tire tread
and traction aids, or track grousers and
In this report the interaction between winter
terrain surface conditions and wheels (driven or
The shear strength of the top layers of the
undriven) or tracks is discussed for snow, ice,
freezing/thawing ground and layered combina-
The power available to the tire or track.
tions of these conditions. This report brings togeth-
Motion resistance can be divided into two parts:
er supporting documentation, data and theory
that produced by external forces and that pro-
for individual segments of the cold regions mod-
duced by internal forces. The external resistance
els. A stand-alone program and the FORTRAN
Rterrain is the resistance attributable to the surface
code for CAMMS/NRMM implementation of the
and is a function of the strength of the terrain and
cold regions models are included as appendices.
the vehicle's running gear characteristics. To ob-
tain Rterrain it is usually necessary to determine
the level to which a vehicle sinks below the ter-
rain surface (sinkage, z). Rinternal is the resistance
caused by friction within the vehicle (tire defor-
The term "mobility" is defined as the efficiency
mation or track roller resistance, friction in driv-
with which a vehicle travels from one point to an-
eline components, etc.).
other. Trafficability refers to the ability of the ter-
Speed made good, besides depending on ter-
rain to support mobility. An effective mobility
rain and vehicle characteristics (slip and transmis-
model must be able to predict
sion power curves), is also strongly dependent on
If a vehicle can propel itself between two
operator skill, visibility, terrain roughness and
other parameters. Although important, speed made