Terrestrial Trafficability Schemes 
Although the density of snow at 6 to 18 pounds per cubic foot is 
considerably less than that of the red clays at approximately 89 pounds per 
cubic foot, their consistency and shear strengths are quite similar. Conse- 
quently, it is felt that the experience gained in testing and operating vehicles 
in soft wet snow can be of assistance in determining the best type of wheel 
or track for use on the ocean bottom (Bekker, 1956, p. 160). 
Extensive experimentation with both tracked and wheeled vehicles 
has revealed that for a vehicle to be successful on soft snow, it must have a 
low, evenly distributed ground pressure, a long narrow bearing area, and small 
relative sinkage (Mellor, 1963). 
The average ground pressure of a vehicle is the vehicle weight divided 
by the total ground bearing area, and is often quoted as an index of the vehicle’s 
potential for oversnow travel. Since penetration into the soft surface corresponds 
more closely to the peak stress than to the average ground pressure, the latter 
can be a misleading index. The distribution of pressures beneath many types of 
tracks is far from uniform; therefore, they usually sink deeper than might be 
expected on the basis of a calculated uniform pressure. 
The weight of most tracked vehicles is carried on their running wheels, 
or bogies, which themselves bear on the section of track laid on the ground or 
snow. With the vehicle stationary, its flexible track is deflected sinusoidally, 
depression being deepest beneath wheels. The track arches up in the spaces 
between wheels. When the vehicle moves, the peak stresses pass over all the 
snow in the path. Asa result, sinkage is increased and with a slack track, the 
wheels are always running uphill on a sinusoidally distorted track. A tight 
track tends to decrease the uphill angle. Figure 31 (Mellor, 1963) shows 
schematically the stress distributions beneath four common track types. 
When a track or wheel sinks a major fraction of its vertical dimension, 
it will push, or bulldoze, snow ahead of it, shearing the snow horizontally. 
This consumes energy that could otherwise be used for tractive effort. Because 
bulldozing resistance is proportional to track width, a narrow track is desirable 
to minimize this loss. As a large bearing area is also necessary to provide low 
ground pressure as discussed above, a long narrow track is indicated. There is 
a limit, however, on the length of track that can be used on a vehicle with one 
pair of tracks. The moment of turning resistance is directly proportional to 
track length, vehicle weight, lateral friction, and load distribution. It has been 
found that if the ratio of length to gage (width between track centerlines) is 
greater than 1.7 to 1.9, the vehicle will not turn (Bekker, 1956, p. 76). To 
eliminate this restriction, some vehicles employ two or more pairs of tracks 
in an articulated configuration. The length-to-gage ratio of some of these 
units is between 3 and 4. Examples of this type of vehicle are tabulated in 
Table 3. 
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