Geoscience Reference
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Fig. 22.4 Hybrid vehicles such as half-tracks or here, a snowmobile,
are useful compromises between the low ground loading and superior
traction of a long track at the rear and independently-movable steering
wheels or skis at the front. Here the first author uses a snow machine in
fieldwork in Barrow, Alaska. Photo by Kevin Hand, with permission
To reduce the pressure, and thereby reduce the proba-
bility of soil failure, one can let air out of the tire. How far
one can do this depends on the tire—many modern tubeless
tires rely on the pressure to hold the tire onto the wheel rim,
so dropping it too low may cause the tire to fail. Low tire
pressure will of course increase wear on the tire, and one
should make sure to have a means of re-inflating the tire
when returning to the road. When in the field, note also that
low pressures (especially if combined with high speed) will
increase the probability that large rocks or similar obstacles
will damage the wheel itself (and thus also the tire).
One should, of course, consult the vehicle owner's
manual or the tire documentation for guidance, and the
authors and publisher of this topic accept no liability for
what may happen in the field. But a general indication is
that one can usually go down to 10 psi (about 0.6 bar) for
short periods with modest risk, but one should never go
below 6 psi. To obtain ground loading comparable with a
human foot (\0.2 bar), requires tracks as used on many
military vehicles: specially-designed wheeled vehicles with
large ('balloon') tires are an intermediate case.
Beyond the vertical reaction force exerted by the surface
to stop the vehicle sinking, the ground must also supply any
horizontal forces to the tires in order to effect acceleration,
deceleration or turning. Therefore, moving in a straight line
at a steady speed is to be preferred, and four-wheel drive
vehicles will spread these side forces for acceleration over
all four tires rather than two. It is difficult in sand to effect
rapid speed or direction changes, and it is generally unde-
sirable to make such changes suddenly, as transition from
simple rolling may lead to a build-up of a mound of sand in
front of the tires. Turns should be made with as wide a
radius as possible, and it is better to coast to a halt by
depressing the clutch pedal than to brake hard.
 
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