Moving on Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 253

Fig. 22.9 Dismounting (and even, as here, pulling up on the back of the bike) will reduce the wheel loading and thus offer a better chance that
the sand under the wheels will hold without failing. Photo R. Lorenz

A much more problematic issue is if the vehicle has sunk
deep enough that its lower structure is being supported by
the sand—a phenomenon known as 'high-centering'. The
normal force on the bottom of the vehicle means, first, that
the tires carry less weight (and therefore, for a given coef-
ficient of friction, can apply less forward push) but also that
considerable friction exists on the bottom of the vehicle,
meaning more tire push is needed to start moving. There is
little hope for a lone vehicle to get out of this predicament
unless the sand is dug out from the bottom, to put the weight
back on the tires. This, of course, is somewhat arduous; since
the sand is loaded, the grains are locked and it is harder to
dig, and it is difficult to dig efficiently at the shallow angle
necessitated by reaching in from beside the vehicle.

For both the wheel-spinning and high-centering issues
(which should be recognized as separate), the ultimate
recovery method is to get pulled out. A second vehicle (and
for deep desert trips, multiple vehicles that are not fully
loaded should always be employed in order to avoid the
danger that an irreparable failure of one vehicle leaves
people stranded; i.e., the people traveling in N vehicles
should be able to fit in N-1) with wheels on more solid
ground can apply a strong forward force via a towrope,
which may be able to overcome high-center friction and pit
slopes. Of course, digging and channels should still be
employed to reduce these barriers, and care taken to prevent
the second vehicle also getting stuck. Note that towropes
under strain have formidable elastic energy, which can be