where ρ0 = density of the snow immedi-

120

ately in front of the vehicle's

L

HML

running gear (Mg/m3)

CAV

L hallenger

BAV (all tires)

contact with the snow (m)

Mradley

80

S 113

HUSV

track (m) (Fig. 3).

HMMWV

5 EMTT

C- Ton

This resistance value is applied to up to

IV

40

four axles or one track set on the vehicles of

interest (including any towed units). For

unknown reasons, predictions of resistance

for the Small Unit Support Vehicle (SUSV)

0

300 are most reliable when only the leading

0

100

200

track set is considered (Richmond et al.

Normal Stress (kPa)

1990). Recent analysis by Richmond (in

prep.) indicates that only the first three ax-

For shallow, undisturbed snow lying on top of

les passing through undisturbed snow are impor-

a strong substrate, we calculate gross traction

tant; we apply this equation to the first four axles

(kN) for one driven wheel or track element *i*

since the HEMTT with four axles was included

(Blaisdell et al. 1990) as

in development of eq 3. The data forming the ba-

sis of eq 3 are displayed in Figure 4. Although

0.823

the trend is strong, the *r*2 value is low (0.39), and

(2)

research refining our understanding of vehicle

where *N*i is the normal stress (kPa) on driving ele-

motion resistance in snow is continuing.*

ment *i *and *A*i is that element's contact area (m2).

The length *a *of a tire or track contact with

The total gross traction for a vehicle is the sum of

snow in eq 3 requires the calculation of vehicle

the traction obtained for each driven wheel or

sinkage *z *(Fig. 3). This is accomplished by the fol-

track. This equation is based on data generated

lowing equation for sinkage in undisturbed shal-

from a wide range of vehicle types and sizes

low snow:

(both wheeled and tracked, with contact pres-

ρ

sures ranging from 10 to 250 kPa) (Fig. 2). The

(4)

correlation coefficient for this equation (*r*2) is

ρf

0.97, with data from the standard military ve-

hicles falling within 9 to 16% of the predicted

where *z*max = maximum predicted sinkage (cm)

value.

in snow occurring under the tire or

The equation for motion resistance (*N*) in un-

track with maximum contact pres-

disturbed snow is

sure *p*max

ρ0 and ρf = initial and theoretical final densities

(3)

(Mg/m3), respectively (Fig. 5).

The final density ρf is a function of the ap-

plied load. In temperate regions with sea-

W

sonal snow, the following ρf values are used

as estimates; they are based on analysis of

field sinkage measurement, as illustrated in

r

b

b

Figure 6:

a

z

a

z

*A new resistance algorithm presented by Rich-

mond (in prep.) for wheeled vehicles in snow will be

incorporated in an NRMM version newer than 2.5.0.

3

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