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Greg W Stephens Page 1 of 4 4/28/2014
Reducing Tire Expense and Failure
Summary
A private fleet with roughly 1000 trailers and three distribution centers initiated a project
to develop a scheduled maintenance program for trailers, improve productivity, reduce
cost of the maintenance function, and specifically reduce tire expense and tire failures. A
similar project for tractor maintenance was subsequently approved.
Although the project was broad, one outcome (not an objective) was to replace
compressed air with nitrogen for tire inflation. This discussion paper is limited to the
project findings and activities related to tire life.
Background
Since the trailer fleet was not equipped with hub-odometers it was unknown what
mileage tires were generating prior to replacement. It was estimated to be about 70,000
miles for a radial tire.
Tire Tread Life
The factors that impact tread wear and tire failure are well understood. The relative
impact depends to what extent each factor is present on the vehicle. The following are
the key factors that cause tread wear all of which are related to friction.
1. Improper tire inflation – every study documents this as the number one cause of
excess tire wear1
.
2. Wheel alignment
3. Improperly installed or bent rims and worn or improperly installed bearings.
4. Wheel alignment on tractor and axle alignment on trailers.
5. Wheel balance
6. Mismatched tread patterns.
7. Tires of materially different tread depth.
8. Load factors. Trailers moving empty have excess tire wear as do overloaded
trailers
9. Speeds in excess of 60 MPH
10. Use of un-dried compressed air (i.e. containing water vapor).
11. Vertical forces due to suspension system characteristics.
Tire failures
There has been extensive research into the causes of tire failures. Still tire failure is less
well understood than tire wear as wear can more easily be measured through controlled
experiments. DOT studies indicate that tire failures are primarily caused by:
1. Road hazards and truck operation: 39%.
2. Heat buildup, caused primarily by under inflation and secondarily by improper
maintenance of brake system: 30%
3. Manufacturing defects: 16%
4. Undermined or multiple causes including using tires older than 5-6 years2
: 15%
1
A random check of 64 tires at the docks of a distribution center on 10/14/10 indicated that over 40% of
tires were under inflated by 15% or more.

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Greg W Stephens Page 2 of 4 4/28/2014
Compressed Air
Many studies have shown that tire life and safety is negatively impacted by the use of
compressed air for inflation. The reason is that compressed air contains oxygen which
oxidizes the rubber in tires and causes the casing and tread to deteriorate. The oxidation
reduces casing strength and changes how tire tread interacts with the road thus
accelerating tread wear.
The concept is similar to using Modified Nitrogen Packaging in the food industry.
Nitrogen preserves the fresh taste of the food while the air in oxygen would allow the
product to go ‘stale’. Nitrogen is also used in a variety of industrial processes where air
would cause oxidation of the either the product or some component of the manufacturing
process.
A similar process occurs in tires inflated with compressed air. Tires (like some foods)
have both single and double chemical bonds. Double bonds are subject to oxidation. In
order to reduce oxidation tire manufacturers add anti-oxidant coatings to the inside of the
casing. However, compressed air ‘uses’ up those anti-oxidants over time. Using up the
anti-oxidants accelerates as fleets ‘top off’ the air in tires to maintain pressure.
The oxidation of tires occurs from the inside-out. As the oxygen ‘attacks’ the rubber air
permeates the tire from the inside creating microscopic holes in the rubber. Over time
holes penetrate the surface of the tire and actually change the physical properties of the
casing and tread. The casing loses strength while the tread softens thus increasing
friction between the tread and the road surface. Friction is the only source of tread wear.
If the compressed air that is being used is not dried, water in the air collects inside the tire
causing the wheel to oxidize and can damage or cause failure of the value stem.
Oxidation of the rim causes small leaks around the tire bead. Freezing of water in tires
can cause valve failure.
Nitrogen
Nitrogen is a totally inert and dry gas. It does not chemically react with tires. Thus while
a commercial tire will lose from 2 to 7 lbs of pressure per month, a nitrogen filled tire
will lose about ½ lb of pressure per month.
Nitrogen inflated tires maintain pressure better and run cooler thus reducing heat buildup
which causes tire failures. Nitrogen inflated tires are safer. Nitrogen is used in military
applications and applications which require the highest tire reliability. Nitrogen is
required by Federal law to be used in airline tires. Since deterioration of the tire casing is
much lower using Nitrogen; airline tires are re-treaded as many as 12 times. There is an
80% probability the commercial flight you are on will be using re-treads.
2
“Comments on Tire Aging: Docket 2005-21276”, 2 JUNE 2008, study performed on behalf of NHTSA
by Strategic Research and Strategies, Inc., Rehobeth, MA.

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Greg W Stephens Page 3 of 4 4/28/2014
Aircraft manufacturers such as Boeing use nitrogen membranes in their On-Board Inert
Gas Generation Systems (OBIGGS) to "top" fuel tanks with nitrogen - an inert gas that
does not support combustion.
Nitrogen Inflation Economic Benefits
There have been a number of studies quantifying the economic benefits of nitrogen
inflation3
. A summary of several of those studies follows:
 Wal-Mart is converting to Nitrogen for its private fleet moving between
distribution centers and stores. Their initial assessment is a 50% reduction in tire
wear.
 Tread wear in the Transport Canada study showed an 85% improvement in tread
life. Nitrogen filled tires delivered roughly 31,000 miles per 1/32 inch of tread
wear while compressed air inflated tires delivered roughly 17,000 miles per 1/32
inch of tread.
 Because Nitrogen inflated tires lose pressure over 5 times slower than
Compressed Air inflated tires, fuel consumption studies sponsored by Transport
Canada for long-haul trucking show a 4% improvement in fuel economy. Other
studies show a 2-6% improvement depending upon how the study was performed.
 Nitrogen inflated tires fail ½ as often as tires filled with compressed air. A 50%
reduction in cost of risk from tire failures and emergency road services should be
realized.
 Nitrogen inflated tires can be retreaded multiple times generating the same
mileage as the new tire at 50-70% the cost. Thus buying a tire at a significant
discount generates savings over and above the increase in tire life on the original
casing.
 Nitrogen inflation is a consummate ‘green’ project. Increased tire life, multiple
retreading of casings, and reduced tire failures all reduce the quantity of tires put
in landfills and the cost of disposal.
3
Studies have been performed by Clemson University, Transport Canada, Ford Motor Company, Akron
Rubber Development Laboratory and the California EPA. Some of the most comprehensive studies were
performed by Lawrence Sperberg of the Probe Forensic and Testing Laboratory of El Paso, TX in 1985
and 1996.

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Observations of Initial Impediments
There was initial resistance to switching from compressed air to Nitrogen. This
resistance came from the following factors:
 Perception that Nitrogen is an ‘exotic’ gas. Air is approximately 80% Nitrogen
 Perception that Nitrogen is unsafe. Unlike air Nitrogen will not support
combustion.
 Nitrogen is expensive. An Ingersoll-Rand Nitrogen generator with a 120 gallon
storage tank costs about $14,000. There are other manufacturers of nitrogen
generators such as Parker Hannifin and pricing is competitive.
 Nitrogen generators are expensive to maintain. Maintenance costs are low and
consist primarily of changing an inexpensive membrane that separates Nitrogen
from air.
 Existing ‘relationships’ with tire vendors.
 Infra-structure investments. Other than the cost of the Nitrogen generator and
storage tank there are no additional investments. The same lines and fittings as
used is compressed air generators work with Nitrogen generators.
Experiments
There were a number of experiments designed to quantify the benefits to this particular
private fleet. These were controlled experiments using trailers with new tires and hub-
odometers to track mileage. Laser tread depth measurement instruments from MTI
Instruments will measure tread depth at an accuracy of 8/100,000ths of an inch.
Shearography and ultrasound inspection was used on tire casings.
Since the project was much broader than just tire inflation the experiments overlapped for
this particular application.
Constraints
In order to generate the maximum potential benefits from nitrogen inflation the chassis
should be maintained according to an appropriate schedule dependent upon operational
environment. In any event, regardless of inflation method, misaligned axles, unbalanced
tires, mismatched tread, excessive suspension wear, worn kingpins, etc. will still cause
excessive tread wear.
Fleet owners that have some maintenance capability in-house will find nitrogen inflation
much easier to accomplish due to the fact that nitrogen is not yet widely used in the U.S.
Owner-operators would find the conversion particularly difficult.
Like any other ‘commercial’ endeavor the players often have a vested interest in the
outcome. Makers of Nitrogen generators want to sell generators. Makers of tires want to
sell more tires.
As an opinion, the most compelling evidence comes from the fact that D.O.T. requires
commercial airliners to use nitrogen and those tires are retreaded as many as 12 times.