Spicer Tire Pressure Control User Manual
Spicer
Questions and Answers About
Reduced Pressure Operation and
®
Tire Pressure Control
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 limiting 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 associated 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 exerted by the vehicle on the ground surface,
measured in pounds per square inch
(psi), can be significantly reduced. This
reduction results in a
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 consider 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
lesser penetration of
ability of the soil to resist it. Since a
longer footprint results in a more favorable 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. Tractio
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 measurable 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 considered. Reduction in tire pressure will result
in a loss of ground clearance. If operating
in areas with deep ruts, high center
n
crowns or sharp break overs this can become
an important issue. It can also be noted however, that higher nominal speed capability
stemming from the improved ride and control
when traveling cross-country at reduced
pressures may allow the operator to traverse
some obstacles that could not be crossed at
lower speeds.
Customers should be reminded that one need
not actually install a tire pressure adjustment
system on their vehicle to determine if
reduced pressures will work in their operation. Manual reduction of tire pressure and
reduced speed operation until tires can be
re-inflated will allow most users to prove the
benefits to themselves with a minimal investment in time or money. It should also be
stated that the use of reduced tire pressure
will not prevent one from ever becoming
stuck again, especially when you consider the
prospects that you will be operating in places
where you may not have been previous
to
using lower pressures.
How are tire pressure settings
developed?
Tire pressures have been developed to provide the sidewall deflections appropriate for
various tire sizes. Of course, one must
remember that reduced operating speeds are
required to use these reduced pressures.
Generally, tire manufacturers specify on-highway tire pressures to achieve sidewall deflections of 10-13%. These pressures allow
operating speeds up the tire design limit. Offhighway deflections are typically 20-22% and
limit vehicle speeds to 35 mph. Emergency
pressure settings may go to 30% deflection,
with a typical speed limit of 10 mph.
The vehicle owner should always select tire
pressures with the assistance of their tire
supplier. The Tire and Rim Association has
developed reduced pressure and speed charts
to assist in selection of tire pressures.
When did Dana start making tire
inflation systems?
Dana is the world’s leading provider of central
tire inflation systems, having acquired the tire
management businesses in 1999.
Over 35,000 systems have been provided
since 1987 to numerous military and
and commercial programs. These systems
are fielded worldwide and were proven
in Operation Desert Storm as a vital
element of tactical wheeled vehicle mobility.
Several important features account for the
success of the product, among them:
- Depressurized control line strategy
which extends seal life, eliminates tire
leakdown when parked and provides
“fail safe” operating capability.
- Speed sensing to assure that tires are
not operated continuously at cross
country pressures when traveling at
highway speeds.
- Simple push button operation using
pre-set terrain based pressure modes.
- Electronic pressure supply sensing to
assure proper integration with brake and
other on-board air systems.
®
The Spicer
was introduced to the commercial market in
1994. It is presently available from Kenworth,
Western Star, Volvo, Mack and Peterbilt.
Dana continues to work toward wider
availability of the system.
Tire Pressure Control was created by building
on the same technology used successfully in
the military systems. The following list
represents some of the many enhancements
that have been integrated into the commercial
version:
- Multiple channel operation, allowing the
- Integration of the wheel valve into the
Tire Pressure Control System,
independent control of pressures on
steer, drive and trailer tires.
hub cap using a rotary joint on nondriven axles to simplify installation and
improve reliability.
- Introduction of diagnostic tools using
either industry standard devices or
personal computers, simplifying
troubleshooting and repair.
- Publication of troubleshooting, and
service manuals as well as driver
instructions and in-cab aids to improve
driver understanding of systems.
- Wheel valve design improvements to
increase system reliability with
longer
trailer combinations and reduce
sensitivity to control system leaks.
- Pneumatic controls design changes to
reduce sensitivity to control system
leaks.
- Introduction of new steer axle products
up to 14,600 lbs. capacity to improve
routing of air to steer wheel ends.
Does Spicer TPCS fit Meritor
drives? Which ones? How about
others?
Spicer drive axle air seal hardware was
developed to fit on the Spicer “R” spindle
configuration. Dana uses this spindle on single axles between 21,000 and 26,000 lbs.
capacity and on tandems from 40,000
through 52,000 lbs. capacity. The “R” configuration meets industry standards for fitment
of wheel end hardware (hubs, bearings, seals
etc.), however individual axle manufacturers
have latitude outside of the specific dimensions for the bearings and the seals to
accommodate manufacturing processes and
other considerations.
Historically, the Meritor “R” configuration has
differed from the Spicer configuration
cally in the width of the outer bearing journal.
It is Dana Corporation’s understanding that
the spindle configuration of the Meritor product was changed in 1999 to accommodate
the hardware required to equip their axles
with Spicer TPCS. The hardware will also fit
on spindles from some other axle manufacturers. We have found that Mack tandems of
38,000 lbs. and 50,000 lbs. will accept the
hardware without modification, however their
44,000 lb. axles will not. Dana will review
specifi-
2
axle spindle geometry from other manufacturers 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 operating 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 braking 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 compressor 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 reduction) in approximately 2 minutes, but the system 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 pressure reserves are maintained and may suspend 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 documented 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 recommended 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 approximation 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 information 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 compatible 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
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