Moving on Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 247

Fig. 22.3 Where camels are unavailable, explorers made do with horses. This early US Geological Survey team struggles across a dune in
Wyoming in 1907. Note that the narrow wheels on this wagon are poorly-designed for soft sand, and have sunk in appreciably. Photo USGS

Often one has little choice in where to go, and thus a low
bearing strength must be accepted. Thus one must use the
other half of the equation—minimize the applied stresses.
Early vehicles, including those before the advent of the
internal combustion engine (see Fig. 22.3 ) often had rather
narrow wheels, which do not fare well in weak soils.
Modern car tires are better, although they are generally
optimized for fuel efficiency and low wear, so tend to
minimize their footprint: fatter tires with lower ground
pressure are better for off-road use. There is a straightfor-
ward way to lower the ground pressure on a tire, and that is
to deflate it. To a first order the walls of a tire are flexible,
and thus the pressure on the ground is the same as that on
the inside tire wall, which is the same as the (gauge)
pressure of the air. Typically for road use on a regular
vehicle, this may be of the order of 20-30 psi, or 2 bar.
Usually, the consideration here is to somewhat minimize the
contact area (and thus maximize the pressure, since the
product of the two equals the vehicle weight) and reduces
the rate of wear of the tire on regular road surfaces.

22.2 Driving on Dunes

Since military encounters with Panzer divisions or hunting
for sandworms are activities not to be expected for the
modern researcher, one of the few areas in which they can
express superiority over their peers is in driving. Some of
the following observations and suggestions may be useful.
First, don't get stuck. An alert deviation to the right or
left for a second or two, or a half-minute stop to survey the
ground ahead may be a worthwhile investment to avoid the
possibility of having to spend 20 min digging out. Beyond
straightforward instinct, and avoiding obviously stupid
things, the most important thing to bear in mind is that the
troubles arise (sometimes unavoidably) when the bearing
strength of the ground is exceeded by the stresses applied by
the vehicle. Thus, depending on the terrain, one can
sometimes maximize the strength of the ground (e.g., by
sticking to stoss slopes). Another tip is that driving on sand
in the morning can be easier because morning dew increases
the sand cohesion.