This is most likely caused by snow falling back into

the rut, which is common for deep snow covers

(Fig. 12) and may have happened in this test.

Unfortunately, this effect was not identified early

enough during the testing to be fully investigated.

These data are shown graphically in Figure 13. The

Total average vertical force

25,371 N

25,280 N

data of test 0126 seem to agree well with the theory

Longitudinal forcesa:

that succeeding tires should have decreasing resis-

tance, the average values for the third and fourth

Average left front

454.6

492.7

"axles" being 0.0094 and 0.00685 respectively.

Average right front

634.0

771.0

The implication of this analysis is that, for ve-

Average left rear

368.3

368.3

Average right rear

347.8

456.4

hicles with more than two axles such as the HEMTT

and LAV, a rationale needs to be determined for

Hard surface resistanceb

507.4

505.6

modeling these additional trailing tires. It does not

appear that individual wheel resistance on large

134.9c

Net force (available) w/o gravity

66.6

vehicles will be directly measurable in the near

Average force due to gravityd

869

837

future.

Net force available with gravitye

734

770.4

aFrom

Table 3.

bEstimated using an average coefficient of 0.02.

cA negative value infers that the vehicle would be immobilized

As mention earlier, Richmond et al. (1990) ana-

(no tractive reserve).

lyzed resistance data from a large number of tests

dThe CIV tilt sensor measured an average angle (φ) of 1.96 and

and vehicles, obtaining eq 4 from data for both

1.89, indicating that the CIV was moving downhill. The

wheeled and tracked vehicles. Richmond (1990)

force due to gravity was calculated by multiplying the total

average vertical force by sin φ.

combined the data into groups of like vehicles and

eSince the vehicle was assumed to be traveling at a steady

developed a set of equations of the same form as eq

speed, this value must be attributed to undercarriage drag.

4, but no significant improvement in the correla-

tion coefficients resulted. In these earlier analyses,

only the data from the CIV could be considered as

attributable to the four wheels and the average

leading wheel data. The data obtained from the

undercarriage resistance results in an undercar-

other vehicles were "whole" vehicle resistances

riage resistance that is 22% of the total vehicle

(from snow) values. To try and reduce the data to

that Richmond et al. (in prep) use a multiplier of

fit one equation, the whole vehicle resistance was

1.25 to estimate undercarriage drag for this condi-

simply divided by the number of wheels for each

tion.

vehicle.

Appendix B contains a discussion of the defor-

mation of snow by a wheel and concludes along

Data sets 6 and 8 were obtained by towing the

with eq 6 that wheel width may not be as an

CIV such that the wheels traveled through the ruts

important parameter as arc length and initial den-

created by the tow vehicle. The forces measured

sity. Additionally, the ratio of wheel radius to

may be assumed to be estimates of third and

sinkage should be considered to account for in-

fourth trailing tires. It was further assumed that

creased deformation (more bulldozing) in deeper

snow covers, possibly in the form (4*a/r*π*) *noted

tests 0126g0126i could represent the first and

second wheels; together, these data represent a

earlier. With these ideas in mind, and with the new

complete set of resistance data for axles 1 through

trailing tire information, an attempt was made to

4 (Table 7). Tests 0202f0202h were analyzed simi-

improve the currently used algorithm (eq 4 and 1)

larly.

for predicting motion resistance for wheeled ve-

Comparing the average resistance coefficients

hicles in snow.

between the two data sets shows that the resis-

After several attempts were made to extend the

tance data measured during the 0202 tests (snow

CIV trailing tire data to the HMMWV data, with-

depth 15.8 cm with a density of 250 kg/m3) are

out any clear success, the motion resistance data

much higher than the data from the 0126 tests

base obtained during the Wheels vs. Tracks shal-

(snow depth 12.6 cm with a density of 110 kg/m3).

low snow tests was reexamined. Appendix C con-

13

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