Spicer Tire Pressure Control - Manual (Page 3)

axle spindle geometry from other manufactur-
ers at their request if they provide the
necessary drawing information.
What pressure range can the
system control pressures to?
The Spicer System can reduce pressures to
as low as 25 psi, and inflate to 90 psi.
Practically speaking, 90 psi is the max cold
tire pressure setting because of the air system
limitations on most vehicles, where the
compressor cut in is set to 90 psi. (It must be
noted that although the cold setting is limited
to 90 psi, Spicer Tire Pressure Control allows
unlimited pressure rises resulting from heat
generated in the tire. This is essential in that
systems which do not allow unlimited
pressure rises should rightfully have their
pressures programmed to “hot” pressures
which can be 15% higher than cold). Tire
pressures beyond 90 psi can be achieved,
however since the source pressure is very
close to the tire pressure, actual inflation will
be extremely slow. To say that any system can
deliver pressures higher than 90 psi without
consideration of this fact is somewhat
misleading. Realistically, higher tire pressures
will require that the truck air system pressure
be increased to 145-150 psi and that
pressure reducing devices be installed for
vehicle braking and other systems.
Another detail which is important to note
involves the physics of moisture control.
Spicer Tire Pressure Controls utilize an oper-
ating regime which minimizes the possibility
of moisture accumulating in the tires by
forcing all of the air to be compressed to a
level higher than that to which the tires will be
inflated. Since all of the air going to the tires
will be less than this peak pressure, it will be
even dryer than when it left the on board air
dryer since it has been expanded. Such a
regime is critical for proper long term system
operating reliability and keeping moisture out
of the tires where it can have long term
consequences. Systems which do not use
such a regime may appear to be working
when they are not, as they do not signal
inability to change pressures to the operator.
Finally, the issue of safety cannot be ignored.
The design of Dana’s Tire Pressure Control
demands that vehicle air brake reservoirs be
brought to a high pressure level before any
tire pressure checking or changing operation
be allowed to begin, not merely the minimum
governor cut in level. Other systems may be
programmable to allow higher tire pressure
settings, but pressures above governor cut in
come at the expense of high brake reserve
pressures. Dana considers high vehicle brak-
ing reserve pressures an essential
element of vehicle safety and therefore prefers
to use the margins between governor cut in
and cut out to assure maintenance of these
reserve pressure levels. Dana feels that any
strategy which reduces the average braking
reserve levels or substantially increases the
time required to achieve those levels violates
the intent and spirit of regulations such as
FMVSS 121.
How long does it take for inflation
and deflation?
Any discussion of inflation and deflation
times must consider many factors such as
tire volumes, pressure settings and compres-
sor capacities to adequately address the
issue. One must also distinguish between the
time required to change the pressure in a
given group of tires and the time required for
a system to signal that a pressure changing
operation has been completed.
For example, the Spicer ® Tire Pressure
Control System can, through its wheel valve,
reduce the pressure in a 11R24.5 dual set
from 90 psi to 45 psi (a 3 atmosphere reduc-
tion) in approximately 2 minutes, but the sys-
tem may indicate that the operation has not
been completed for more than three minutes.
Several factors may influence this time. First,
the system checks to assure that brake pres-
sure reserves are maintained and may sus-
pend operations which change pressures to
give this first priority. Second, the system
monitors itself to assure that deflation can
proceed reliably and deflation may be delayed
while the system assures that all deflation
parameters are properly met. Finally, to
assure that tire pressures are balanced
following a deflation sequence, Spicer Tire
Pressure Controls are programmed to go
through a short reinflation period to bring all
pressures within the proper limits.
Inflation times are affected by even more
factors, primarily related to overall tire volume
and compressor output. The Spicer
® Tire
Pressure Control System has been designed
to work on a wide range of trucks and truck
configurations and as such will work with
most any compressor output (presuming the
compressor is in good operating condition).
The issue of inflation time requirements
deserves some discussion. Over the past ten
years, numerous test results have been docu-
mented which show that the use of on-board
controls to adjust tire pressures does not
reduce tire life. Tire manufacturers have
suggested that tire durability will not be
compromised as long as inflation capability
is sufficient to inflate tires to 75% of the rec-
ommended cold highway pressure within 15
minutes of achieving highway speed, and
100% in 25 minutes. This guideline can lead
the person spec’ing the vehicle to an approxi-
mation of required compressor capacity
using the simple analysis below. Dana urges
those who are considering the use of Tire
Pressure Control technology to work closely
with their tire supplier to obtain the informa-
tion needed about tire volumes and operating
pressures as well as suggested inflation times
for their application.
In simple terms, inflation time can be
approximated by dividing the internal volume
of the tires by the compressor output, and
multiplying the result by the amount of the
desired pressure increase. In order for this to
work however, all terms must have compati-
ble units – tire volume in cubic feet (ft
3 ),
compressor output in standard cubic feet per
minute (SCFM) and pressure increase in
atmospheres (atm.). Atmospheres can be
found by dividing the pressure increase in
psi by 14.7. A formula for this can be
expressed as:
{Volume (ft 3 ) / Output (SCFM)} X Pressure
Increase (psi) / 14.7 (psi) = Time (min)
3