Spicer Tire Pressure Control - Manual

Spicer
®
Tire Pressure Control
Questions and Answers About
Reduced Pressure Operation and
Spicer
® Tire Pressure Control
Is the use of reduced tire
pressures approved by the tire
manufacturers?
The tire manufacturers have approved
operation at reduced pressures when lim-
iting the vehicle speeds. Their industry
group, The Tire and Rim Association
(T&RA), has published guidelines which
outline the tire sizes, loads and pressures
to apply this technology. While major
manufacturers have monitored or taken
part in numerous studies to determine the
safety and efficacy of reduced pressure
operation, most tire distributors are either
unaware of the work or are inexperienced
in applying the guidelines. Over time and
with increasing popularity of tire pressure
controlling systems, the awareness of the
tire industry in the field will grow.
Does low pressure work only in
sand? How about mud, clay,
snow and ice?
Reduced tire pressure enhances the
mobility of vehicles through two different
mechanisms, flotation and traction. In
both cases it is the longer footprint asso-
ciated with reduced pressure operation
that brings about the improvement. (A
steel belted radial tire does not increase
the width of the rubber in contact with the
ground as frequently thought).
To understand the concept of flotation as
it relates to land vehicles, one should
consider the similarity to a water borne
vessel. Just as a loaded ship displaces
water and pushes a discernible “bow
wave” in front of it, a truck on a soft
surface displaces the sand or soil and is
forced to run essentially uphill as it
moves a “wave” of sand in front of it. By
lengthening the footprint, the force exert-
ed by the vehicle on the ground surface,
measured in pounds per square inch
(psi), can be significantly reduced. This
reduction results in a lesser penetration of
the surface by the tire, similar to the
reduction in draft of a ship as its load is
diminished (or the reduced penetration in
snow of a person wearing snowshoes).
This improved flotation allows the vehicle
to move about the surface with less
disturbance to it.
A discussion of traction needs to consid-
er the difference between traction and
friction. Although friction is independent
of area, traction is very dependent on
area. A surface with limited friction will
demonstrate tire slippage with much less
effect on the surface; the tire is slipping
relative to the surface. Limited traction
will show wheel slippage, but with
resulting surface disturbance as the thrust
generated by the tire overcomes the
ability of the soil to resist it. Since a
longer footprint results in a more favor-
able distribution of shear forces in the
surface, the soil is less likely to be
displaced by the torque of the rotating tire
and increased traction results. Traction
increases (as measured by drawbar pull)
of 60% have been demonstrated solely
through the reduction of tire pressures on
a vehicle in a 6x4 configuration. When
reduced tire pressure is combined with all
wheel drive, improvements of over 100%
are readily achievable.
The combination of improved flotation
and traction results in the possibility of
tremendous improvements in overall
mobility. While these improvements are
most striking in coarse grained soils such
as sand, improvements are also measur-
able on other soils such as clay and
muskeg. In simplest terms, if operation of
a vehicle results in surface penetration by
the tires, reduced tire pressures and the
resulting improved flotation may help. If
the surface is hard but slippery, reduced
pressures should not have a significant
effect since friction is the dominant limiter
to mobility.
Uneven terrain should also be consid-
ered. Reduction in tire pressure will result
in a loss of ground clearance. If operating
in areas with deep ruts, high center