Motion Resistance of Wheeled Vehicles in Snow
PAUL W. RICHMOND
sity of the deformed snow behind each succeeding
INTRODUCTION
axle generally increased, although axle load did
Models of terrainvehicle interaction require
not. Density measurements in the wheel ruts (Table
algorithms that can predict or estimate the resis-
1) were always well below what is considered the
critical density of snow (0.5 to 0.55 Mg/m3) (Yong
tance to motion caused by the terrain. As a vehicle
moves through snow, it must overcome the resis-
and Fugue 1977), the point at which no further
tance caused by snow deformation to maintain
compaction by typically loaded vehicle traffic is
forward motion. There is resistance at each wheel
observed. Additionally, Blaisdell (1987) made some
or track. As a vehicle moves through undisturbed
limited indirect measurements of trailing tire re-
snow, the front wheels move and compact this
sistance using the CIV and found that trailing tire
snow. Succeeding or trailing wheels move over or
resistance in snow could be as high as 84% of the
through the snow compacted by the front wheels,
leading tire resistance.
presumably encountering a smaller resistance
In an effort to increase the accuracy of the
(since the snow has been compacted and moved
motion resistance model for undisturbed snow, a
out of the way). If the vehicle's undercarriage
field study was conducted using the CIV with load
drags, this adds another source of resistance that
cells installed on all four wheels. Prior to this work,
must also be considered.
only one set of load cells was available and these
The motion resistance of a vehicle moving
were installed on the front axle of the CIV (from
through snow is a very complex phenomenon,
1981 to 1991). By use of the data acquired from
studied since the early 1940's. However, a robust,
these and other tests, a further understanding of
simple model of motion resistance due to snow
the motion resistance of wheeled vehicles in snow
has eluded researchers.
was obtained, allowing an improved prediction
Currently, the U.S. Army uses in its mobility
algorithm to be presented.
model an empirical equation for motion resistance
in snow developed by Richmond et al. (1990). This
equation is applicable to a large range of vehicles,
EXPERIMENTAL PROCEDURE
both wheeled and tracked, and does not require
elaborate measurements of snow properties. How-
The experimental procedure was designed
ever, it is currently the weakest link of the overall
around the capabilities of the CIV (Fig. 1 [Blaisdell
1983, Shoop 1992]). The instrumentation of the
Cold Regions Mobility Model (Richmond et al., in
CIV was recently enhanced through the addition
prep). When developing this equation, we ana-
of load cells on the rear axle and a new data
lyzed a large quantity of total vehicle resistance
acquisition system based on a personal computer
data by dividing the whole vehicle resistance by
(Shoop et al. 1991). These triaxial load cells mea-
the number of wheels, thus making the assump-
sure longitudinal, vertical and lateral forces at the
tion that the trailing tire resistance was equal to the
tire/terrain interface. Longitudinal forces repre-
resistance of preceding wheels. This assumption
sent net traction or motion resistance, depending
was based on attempts to correlate data in which
on whether the tire is driven or free rolling. Indi-
motion resistance was measured on one axle
vidual wheel and vehicle speeds are measured
(CRREL Instrumented Vehicle [CIV]), on two ax-
simultaneously with the wheel forces. By towing
les (High Mobility Multipurpose Wheeled Vehicle
the CIV, resistance forces could be measured at all
[HMMWV]), and on four axles (Heavy Expanded
four wheels, and a tow hitch that could be placed
Mobility Tactical Truck and Light Armored Ve-
off-center (Fig. 2) allowed the CIV to be towed
hicle [HEMTT and LAV]) during towing tests.
through undisturbed snow.
This approach was not without basis, as the den-
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