with zero slip and with unrestricted slip. The NRMM
snow depths greater than 38 cm using eq 32. The
deep snow resistance predictions are close to the fi-
finite element model falls between the NRMM mo-
nite element results for a free slipping wheel, as
tion resistance prediction equations for deep snow, as
would be expected for a wheel being dragged through
does the measured data. The bottom part of Figure 62
deep snow, while shallow snow results are best repre-
shows the corresponding sinkage, which the finite
sented by the simulation with zero slip.
element model (with zero slip) underestimates,
The bottom of Figure 63 shows the measured
particularly for the deeper snow.
sinkage data falling between the zero slip and unre-
The NRMM predictions are also a function of the
stricted slip models. Measured slip in shallow snow is
initial snow density. Although the material model
was designed to simulate 200-kg/m3 snow, NRMM
near zero, but higher slip measurements are not un-
predictions based on 240-kg/m3 snow yield an excel-
common as the snow gets deeper. Realistically the
model should behave somewhere between these two
lent match to the finite element resistance, as seen in
extremes. This part of the physics of the interaction is
the top of Figure 63. This would indicate that the
not accurately reproduced in the model, partly be-
FEA model is more representative of a slightly
cause the tread, which is designed to resist slippage,
denser snow.
is not yet accurately modeled.
Figure 63 also shows model results for a wheel
Figure 63. Finite element simulations at zero slip and at unlimited
3
slip and NRMM motion resistance predictions for 240-kg/m snow.
48
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