Freund 1

COMMERCIAL VEHICLE SAFETY TECHNOLOGIES: APPLICATIONS FOR TIRE PRESSURE 

MONITORING AND MANAGEMENT 

Deborah Freund 

Federal Motor Carrier Safety Administration 

United States of America 

Stephen Brady 

Booz Allen Hamilton Inc. 

United States of America 

Paper Number 09-0134 

ABSTRACT 

Tire deficiencies often cause commercial motor 

vehicles (CMVs) to be cited for regulatory violations 

and to be taken out-of-service during roadside 

inspections. As part of a major safety technology 

project to assess the state of the practice and potential 

contributions of advanced sensor systems, the Federal 

Motor Carrier Safety Administration (FMCSA) 

sponsored three studies between 2003 and 2008 on 

tire pressure management systems (TPMS). The first 

study focused on obtaining baseline information. 

Fleet records and limited field collections were used 

to develop a database of inflation readings for 35,000 

CMV tires, providing the first large-scale source of 

information on CMV tire inflation in the United 

States. The second study assessed the performance of 

TPMS in a controlled test-track environment. 

Multiple systems were installed on a truck tractor, a 

trailer, and a motorcoach. These were run under 

nominal operating conditions and with tire and 

system faults deliberately introduced. Although all 

the systems functioned at the levels specified by their 

manufacturers, some had limited ability to 

compensate for changes in ambient temperature, to 

reset pressure “alert” thresholds, and to withstand 

repeated tire installation and removal cycles. The 

third study, performed in an operational setting in an 

urban transit fleet, assessed the performance and 

maintainability of tire pressure monitoring devices. 

Three types of TPMS were installed on 12 buses that 

accumulated more than 1.28 million km, in 

aggregate, during the 12-month test period. The 

results of this study pointed to sensor durability and 

data integration challenges that need to be overcome 

for these systems to be used successfully in a severe 

service environment. These studies provided new 

information directly comparing the performance of 

TPMS in controlled and operational settings. Results 

are limited to the particular systems and applications 

tested. Study data are available from the FMCSA. 

INTRODUCTION 

The load carrying capability of a tire is critically 

linked to the inflation pressure. Fleet operators will 

generally select a particular “target pressure” for their 

trucks based on the unique load, operating, and 

environmental conditions in which they operate. If 

not properly inflated, the useful tire life, as well as 

vehicle handling and safety, are compromised. 

CMV tires lose air pressure for a variety of reasons. 

Air can escape between the bead and wheel, as well 

as through improperly tightened valves, torn rubber 

grommets, or valve cores that have been blocked 

open by dirt and ice. Additionally, air molecules are 

small enough to diffuse through rubber (albeit very 

slowly), and an air pressure drop of up to two psi per 

month is not uncommon. Most tire companies 

recommend that tire pressure be checked weekly, 

using properly calibrated tire gauges. However, tire 

pressure maintenance is labor and time intensive. It 

takes 20 to 30 minutes to check all the tires on an 18-

wheeled tractor-trailer combination vehicle and to 

add air to 2or 3 tires that may be low. Due to this fact 

and the lack of time available, tires are often 

improperly inflated. 

Improper tire inflation increases operating costs by 

reducing tire life and lowering fuel economy (an 

underinflated tire “flexes” and has higher rolling 

resistance). For the average fleet operator in the 

United States, improper tire inflation increases the 

annual procurement costs for both new and retreaded 

tires by about 10 to 13 percent. Due to improper tire 

inflation, fuel economy decreases about 0.6 percent 

for typical truckload (TL) and less-than-truckload 

(LTL) operations. According to road-breakdown 

management service providers, weakened and worn 

tires due to improper tire inflation are estimated to be 

responsible for one road call per year per tractortrailer combination vehicle. 

Although most industry stakeholders intuitively 

recognize the importance of proper tire inflation 

maintenance and its impact on operating cost and 
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safety, there was very little empirical data on CMV 

tire pressure maintenance practices. The extent of the 

under inflation problem had never been documented, 

and the costs of improper tire pressure maintenance 

had not been systematically analyzed. Although new 

tire pressure maintenance and management 

technologies have been introduced for the CMV 

market, such as automatic tire inflation systems and 

various types of tire monitoring systems, there was 

little information that fleet maintenance managers 

could use to determine whether those systems would 

provide a reasonable return on investment. 

FIRST STUDY: TIRE PRESSURE SURVEY 

The primary objectives of this project were to 

develop and document the impacts of tire inflation 

maintenance practices on CMV operating costs and 

safety and to provide a quantitative estimate of 

potential benefits of tire pressure monitoring sensors 

and automatic inflation systems. In order to address 

the dearth of comprehensive data on tire inflation 

practices, the study engaged in a cooperative effort 

with the Technology & Maintenance Council (TMC) 

of the American Trucking Associations to synthesize 

existing tire pressure survey data from a wide variety 

of truck and bus fleets and to collect new tire 

inflation field data for the owner-operator segment of 

the trucking industry. The study also gathered 

information from suppliers of tire pressure 

monitoring and automatic inflation systems and 

developed six hypothetical fleet operating scenarios 

to estimate potential costs and benefits from use of 

these systems. 

Tire manufacturers perform fleet surveys for their 

CMV fleet customers to assist them with their tire 

maintenance programs and to determine the 

performance of their tires under various operating 

conditions. Experienced field service engineers use 

calibrated gauges to collect cold-inflation pressure 

data (the flexing of the sidewalls of tires in motion 

increases their temperature and leads to inaccurate air 

pressure readings). These fleet survey data provided 

the most accurate way to assess the relationship 

between a motor carrier fleet’s target tire pressure 

and the actual cold inflation pressure of tires in 

service. The data are considered representative of 

motor carriers that participate in this type of 

maintenance program. 

Conversely, no archival data from tire suppliers was 

available for tires on independent owner-operators’ 

vehicles because owner-operators are generally 

responsible for their own operating costs. The only 

way to obtain cold tire pressure data was to collect it 

at locations in which owner-operator drivers would 

be stopped for at least 3 hours, in order for their tires 

to cool down to ambient temperature. Trucker 

appreciation events held the most promise for this 

data collection. TMC assisted in recruiting a group of 

senior field service engineers who collected owneroperators’ tire pressure data at the Walcott (Iowa) 

Truckers Jamboree and the Reno Truckerfest, both 

held during the summer of 2001. 

A total of 6,086 units (3,261 tractors, 1,300 trailers, 

and 1,525 motorcoaches) and 35,047 tires (18,039 on 

tractors, 7,501 on trailers, and 9,507 on motor 

coaches) were checked, and the pressures recorded. 

The survey data also noted the type of motor carrier 

operation (for-hire LTL, for-hire TL, private, and 

public and private motorcoach). 

Tire pressure survey results 

The results focused upon four important metrics: 

• Proportion of tires 20 percent or more 

underinflated. In general, fleets accept small 

deviations from the targeted pressure. However, 

if a tire is 20 percent or more underinflated, it 

indicates the problem is more serious and is 

likely the result of inadequate maintenance or 

quality control procedures. The survey found 

approximately 7 percent of all tires underinflated 

by 20 psi or more. 

• Proportion of tires within 5 percent of the 

targeted pressure. Higher percentages indicate a 

well-executed tire maintenance program. It is 

generally accepted by the trucking industry that a 

good fleet operator will have 70 percent or more 

of his or her tires within ±5 percent of the 

targeted pressure. The survey found only 44 

percent of all tires sampled was within ±5 psi of 

their target pressure. 

• Proportion of tires 50 percent or more 

underinflated. This degree of under inflation 

would indicate a major tire failure and would 

generally be considered a flat tire. The 

percentage of such tires within a fleet could be 

an indication of either a tire product or tiremounting problem or a poor tire inflation 

maintenance program, as indicated above. 

• Proportion of tires 10 percent or more 

overinflated. A high percentage of such tires 

might indicate that the fleet is systematically 

over inflating the tires to compensate for lack of 

a good quality control program. Since 

overinflated tires also have negative impacts on 

tread wear, this is not considered a viable 

strategy. 
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Other key observations indicated differences among 

fleet operational categories, as well as among fleet 

size. For-hire carriers (LTL, TL, and owneroperators) generally reflected better tire inflation 

maintenance practices than private carriers did. As a 

group, sampled for-hire carriers had 7 percent of all 

tractor tires underinflated by 20 psi or more. In 

contrast, the sampled private carriers had 13.2 

percent of all tractor tires underinflated by 20 psi or 

more. 

Fleets with 50 or fewer power units had 19.1 percent 

of their tires underinflated by 20 psi or more. In 

contrast, fleets with more than 3,000 power units had 

only 2.1 percent of their tires underinflated by 20 psi 

or more. Similarly, motorcoach fleets with fewer than 

50 power units had 11.8 percent of their tires 

underinflated by 20 psi or more, while fleets with 

over 500 power units had only 2.1 percent of their 

tires underinflated by 20 psi or more. 

Study sample data showed that transit bus operators 

had better tire pressure maintenance than chartered 

motorcoach operators did. Only 3.1 percent of transit 

bus tires were underinflated by 20 psi or more, while 

9.4 percent of chartered motorcoach tires were 

underinflated by 20 psi or more. Additionally, 

49.9 percent of transit bus tires were within ±5 psi of 

target, compared with only 34.2 percent of chartered 

motorcoach tires. 

Tractors and trailers had a significant challenge with 

mismatched dual tires. Approximately 20 percent of 

all tractor dual tire assemblies had tires that differed 

in pressure by more than 5 psi. One out of four trailer 

dual assemblies (25 percent) had tires that differed in 

pressure by more than 5 psi. 

SECOND STUDY: COMPARATIVE 

CONTROLLED TESTING OF TPMS 

TPMS can offer safety and productivity benefits to 

CMV drivers and maintenance technicians. They can 

warn the driver and maintenance personnel if tire 

pressure drops to an unsafe level and can provide 

data to aid in problem diagnosis and resolution. They 

can alert the driver to a catastrophic tire failure (the 

loss of a trailer tire may not be noted through noise or 

vibration). Tires run at proper inflation pressures 

wear longer and have longer service lives. Systems 

that automatically maintain tire inflation pressure 

might offer additional benefits of increased fuel 

economy, provided they had a high level of 

reliability, were easy to maintain, and were 

considered affordable. 

The overall objective of this second study was to 

document the performance, accuracy, and operational 

characteristics of several leading-edge technological 

approaches to commercial vehicle TPMS. The study 

focused on the ability of sensors to provide accurate 

tire pressure readings, detect both slow and rapid 

changes in tire pressure, and maintain tire pressure 

under adverse conditions, including partial failure of 

the device. The study examined three types of TPMS: 

dual tire equalizers to balance pressures between tires 

in a dual installation; tire pressure monitors; and tire 

pressure maintenance systems to maintain tire 

pressure at desired levels. The systems were installed 

on a conventional tractor-trailer combination vehicle 

and on a motorcoach. All testing was performed 

under controlled conditions on a high-speed test track 

at the Transportation Research Center in Columbus, 

Ohio. 

The TPMS selected represented a sample of typical 

commercially available systems: two tire pressureequalizing systems, five tire pressure-monitoring 

systems, and two central tire inflation systems. The 

systems and the vehicles on which they were 

installed are listed in Table 1. 
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Table 1. 

TPMS Systems Used in Comparative Evaluation 

 

Technology Tractor Trailer Motor 

Coach Recommended System for Testing 

Dual Tire Equalizers 

Equalizing Systems #1 X X Cat’s Eye 

(Link Manufacturing, Ltd.) 

Equalizing Systems #2 X X Tire-Knight-S 

(V-Tech International, Inc.) 

Tire Pressure Monitoring Systems 

Direct Monitoring System #1 

(valve stem-mounted) X X PressurePro 

(Advantage PressurePro, LLC) 

Direct Monitoring System #2 

(valve stem-mounted) X Integrated Vehicle Tire Pressure 

Monitoring (WABCO) 

Direct Monitoring System #3 

(wheel-mounted) X X Tire-SafeGuard 

(HCI Corporation) 

Direct Monitoring System #4 

(wheel-mounted) X SmarTire 

(SmarTire Systems, Inc.) 

Direct Monitoring System #5 

(tire-mounted) X X eTire 

(Michelin North America) 

Tire Pressure Maintenance Systems 

Central Inflation System #1 X PSI Tire Inflation System 

(Arvin Meritor) 

Central Inflation System #2 X Vigia 

(Gio-Set Corporation) 

Dual Tire Equalizers 

Tire pressure equalizer systems balance dual tire 

pressures by providing a pathway for air to transfer 

between two tires in a dual installation, and they also 

provide an indication of tire pressure. A pressureactuated valve connected by hoses to the valve stems 

of the tires maintains an open position to allow air to 

flow between the tires when the combined pressure 

of the two tires is above a preset level (typically 90 

psi). The pressure actuated valve closes and isolates 

the tires during slow leaks or instantaneous air losses 

(after the combined pressure of the two tires drops 

approximately 10 psi) to prevent both tires from 

going flat. A central fill valve incorporated in these 

devices allows both tires in the assembly to be aired 

simultaneously. The use of equalizers should improve 

irregular tire wear (i.e., cupping) caused by pressure 

differentials between dual tires. Visual indicators are 

also incorporated in the equalizers to provide the 

operator with a quick indication of the tire pressure 

levels during the pre-trip inspection without requiring 

the operator to perform a manual tire pressure check. 

Tire Pressure Monitoring Systems 

Tire pressure monitoring systems consist of a valve 

stem, wheel- or tire-mounted sensor, antennae, 

receiver, and display unit. The battery-powered 

sensors mounted on each valve stem, wheel, or tire 

on the vehicle transmit a radio frequency (RF) signal, 

which includes the tire pressure data, to an antennae 

mounted on the vehicle. A receiver, with an 

integrated electronic control unit (ECU), processes 

the signal transmitted to the antennae and displays 

the tire pressure information on a driver’s cabmounted display. The system also includes audible 

alarms and visible warning lights. 

The study team tested five different tire pressure 

monitoring systems for this project. These included 

one tire-mounted, two valve-stem-mounted, and two 

wheel-mounted tire pressure monitoring systems. 

Figure 1 and Figure 2 display the Tire-SafeGuard and 

WABCO IVTM TPMS hardware respectively. 
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Figure 1. Tire-SafeGuard Wheel-Mounted 

System. 

Figure 2. WABCO IVTM Wheel-Mounted/Tire 

Valve-Stem-Connected. 

Automatic Tire Inflation Systems 

 

Automatic tire inflation systems (ATIS) for tires use 

the air from the vehicle’s air compressor that is stored 

in the air brake reservoirs (tanks) to maintain tire 

pressure at a desired level. The ATIS are plumbed to 

the vehicle’s secondary reservoir that supplies air to 

the front brakes. The ATIS do not take air from the 

primary reservoir that supplies air for the rear brakes 

(which are responsible for the majority of the braking 

power of the CMV). These systems are plumbed 

either through the axle or externally, using a rotary 

union at the wheel hub. They automatically sense the 

tire pressures and inflate the tires when air is lost. 

The benefits of these systems are the elimination of 

manual tire pressure checks and the ability to 

continue operating the vehicle with minor air leaks in 

the tires. ATIS are available for all types of CMVs. 

In this test, one system was tested on the motorcoach 

and one was tested on the trailer of a tractor-trailer. 

Figure 3 shows the detail of the PSI trailer-mounted 

ATIS, and Figure 4 shows the Vigia ATIS mounted 

on a motorcoach. 

Figure 3. PSI Trailer-Mounted ATIS. 

Figure 4. Vigia ATIS Mounted on a Motorcoach. 

Test Vehicles and Data Acquisition System 

The test vehicles were a 2001 Freightliner FLD 

tractor, coupled to a 2001 Utility Trailer 

Manufacturing Co. 2000S tandem axle flatbed semitrailer, and a 2003 MCI motorcoach. To acquire 

highly accurate pressure and temperature 

measurements (i.e., to establish “ground truth” 

against which readings from the various test articles 

could be compared), laboratory grade sensors were 

installed at each test wheel position. This required 

removing the wheels, machining replacement wheels 

for mounting thermocouples and pressure taps, and 

routing wire bundles to the data acquisition system 

(DAS) mounted in the cab of the truck and 

motorcoach. Figure 5 shows the installation location 

of each of the instrumentation packages on the 

tractor-trailer and motorcoach. 
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RFOD 

RFID 

RROD 

RRID 

LFID 

LFOD 

LRID 

LROD 

RFOT 

RFIT 

RROT 

RRIT 

LFIT 

LFOT 

LRIT 

LROT 

RFS 

LFS 

TRACTOR-TRAILER 

RFOD 

RFID 

RTAG 

LFID 

LFOD LTAG 

RFS 

LFS 

MOTOR COACH 

Figure 5. Tractor-Trailer and Motorcoach Instrumentation Package Location.

Additional test equipment consisted of: internal tire 

temperature thermocouples (nine, analog); tire 

pressure transducers (two, analog); custom-fabricated 

dual-flow combination rotary union and slip ring 

assemblies (five); a custom-fabricated pressure 

control manifold; primary and secondary brake 

reservoir and treadle valve pressure transducer (three, 

analog); a digital marker switch for the test driver’s 

use to indicate when a warning was observed; and a 

digital non-contact fifth wheel. Figure 6 displays the 

details of the wheel end rotary union instrumentation 

package. 

Figure 6. Rotary Union and Wheel Installation 

Detail. 

The failure modes test required that the air brake 

systems be monitored to determine whether the 

central tire inflation systems degraded the vehicle’s 

potential braking ability. Therefore, it was necessary 

to monitor the air brake system pressure at three 

points: primary air reservoir, secondary air reservoir, 

and application pressure. Three pressure transducers, 

similar to those used for tire pressure monitoring, 

were spliced in a T into the brake lines at a T serving 

the primary air reservoir, the secondary air reservoir, 

and the application pressure gauge line upstream of 

the gauge. 

The DAS was manufactured by Link Engineering 

Company, Detroit, Michigan. A PC-based laptop 

computer operated the system, stored data as it was 

acquired, and performed real-time analyses. The 

system software supported a variety of interface 

options, ranging from direct user interaction with the 

system during measurement to completely 

autonomous operation based on various preprogrammed trigger events that caused the system to 

begin data collection. The system was also capable of 

issuing prompts to a test driver or an instrumentation 

technician. The Link DAS system received 

information from 16 individual channels at a 

frequency of 10 Hz. The average TPMS test lasted 

about 10 to 30 minutes and generated approximately 

5,000 to 10,000 data points. In total, the testing 

program generated approximately 450 Mb of data. 

The test engineer was responsible for manually 

recording the test identification number and other 

information, including environmental conditions and 

tire(s) under test; as well as any specific warnings or 

indications by the TPMS. This was necessary 

because TPMS were self-contained and were not 

connected directly to the Link DAS, as they did not 

have signal outputs that could be tapped for direct 

recording. 
Freund 7

Test Program 

The test program included seven tests: 

• Functionality Test (Static and Dynamic). This 

was an overview or shakedown test series, 

performed to characterize the operational, 

maintenance, and installation processes for each 

system. 

• Threshold Warning Level Test. This test 

determined the thresholds at which low tire 

pressure warnings were given. In the case of the 

ATIS, the test determined the leak rate (psi/min) 

at which the inflation system could no longer 

keep pace and a low tire-pressure warning was 

given. 

• Loaded Test at High Speed. This test examined 

the effects of tire heating, due to sustained highspeed driving, on the warning indicator/light of 

the systems. 

• Failure Modes Test. This test had two 

components. For central tire inflation systems, 

this test determined whether a large or 

catastrophic air leak depleted the air supply for 

the pneumatic brakes or forced the system to run 

continuously, without giving the driver a 

warning that a tire had lost air. For systems 

where tires were interconnected (dual-tire 

equalizing and ATIS technologies), the purpose 

was to ensure that loss of inflation pressure in a 

single tire did not affect the interconnected tires. 

• Disablement Test. This test determined the 

system’s ability to provide a warning or advisory 

when it was disabled, either by an intentional act 

or because of a failure of a system or component. 

• Operator Interface Evaluation. This test was a 

qualitative evaluation of the effectiveness of the 

driver interface. 

• Gate Reader Evaluation. This test evaluated the 

performance and reliability of the drive-thru gate 

readers, used with tire-mounted monitoring 

systems. The objective was to determine the 

speed and consistency of the gate readers in 

capturing pressures of all 18 tires on the tractortrailer. 

Results of Comparative Tests 

In general, all of the tested valve-, wheel-, or tiremounted systems exhibited base-level functionality 

as specified by the manufacturer of the individual 

systems. The tire pressure values were generally 

accurate to within two to three psi of the values 

measured by calibrated pressure transducers. 

Low-pressure warning thresholds were factory set on 

some systems but user-configurable on others. For 

those systems with factory settings, different warning 

levels, ranging from 12 to 25 percent below target 

pressure, were observed. All tested systems were 

generally within a 2- to 3-psi range of the expected 

warning threshold. 

Many of the tested TPMS used RF communications 

to transmit data between the sensors and the display 

unit or ECU. The relatively long length of typical 

CMVs meant that additional on-board antennae were 

required for some of the systems to receive the sensor 

signals from trailer or tag axle tires. Disconnected or 

damaged antennae could lead to signal loss from the 

sensors. 

The dual tire equalizer systems functioned as 

designed under both static and dynamic conditions, 

and they were found to be effective in balancing the 

pressures between the two connected tires. Figure 7 

and Figure 8 show the equalization system balancing 

tire pressures under static and dynamic conditions 

respectively. The two systems tested prevented a 

total loss of pressure in one or both tires in every 

failure mode implemented, effectively isolating an 

intact tire from the adjacent tire with an artificially 

induced major air loss. Similarly, in a disablement 

test where a hose was cut to simulate damage from 

road debris, one tire of a dual installation sustained a 

total loss of air while the other tire was protected by a 

check valve. Figure 9 and Figure 10 demonstrate the 

effectiveness of the equalizer in isolating a slow and 

fast leaking tire from an intact tire. 

Both tested equalizer systems included a visual 

indicator that provided a gross indication of whether 

tire pressure was within its target range. However, if 

the pressure fell below this range, they only showed a 

“low” pressure condition and did not indicate the 

extent of underinflation. Finally, although the 

indicators provided a good visual indication of tire 

pressure, they could be difficult to read as they were 

mounted on the wheel and could become obscured by 

dirt. 

Despite their relative ease of installation, valve-stemmounted TPMS had several limitations. When tire 

temperatures increased during high-speed driving, 

not all of these systems compensated for the related 
Freund 8

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LFOD LFID

Static Equalization Test - 85psi Underinflated 

Left Front Inner Drive Tire

Figure 7. Static Equalization Test. 

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LFOD LFID

Dynamic Equalization Test @ 45 mph - 85psi Underinflated 

Left Front Inner Drive Tire

Figure 8. Dynamic Equalization Test. 

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S tatic S low Lea k Test - 1 .5 ps i/m in Initia l Le ak R a te

Left Front Inner Drive Tire

Figure 9. Equalizer Static Slow Leak Test. 

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Static Fast Leak Test - 7.5 psi/min Average Leak Rate

Left Front Inner Drive Tire

Figure 10. Equalizer Static Fast Leak Test. 

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RTAG Marker

Dynamic TPMS Functionality Test - 1.5 psi/min Initial Leak Rate

Right Tag Axle Tire

Figure 11. Initial Low Pressure Alarm and 

Second Critical Pressure Alarm Points. 

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RTAG RTAG TEMP. Marker

Loaded Dynamic TPMS Test @ 75 mph

Right Tag Axle Tire

Figure 12. Initial Low Pressure Alarm and Point 

of Low Pressure Alarm Reset. 

 
Freund 9

 increases in tire pressure. One of the two tested 

systems in this study provided temperature 

compensation. One of the tested systems initiated a 

warning when the pressure fell below a preset value 

(~20 percent below target). Figure 11 shows the two 

alarm points on a wheel-mounted TPMS as the 

pressure in the tire bleeds down. However, the 

warning remained active until the tire was inflated to 

a higher value (~15 percent below target). This 

pressure band between the alarm pressure and alarm 

deactivation pressure prevented intermittent warnings 

to the driver. Lastly, valve-stem-mounted systems 

were susceptible to loss because they had to be 

removed during wheel mounting and dismounting for 

vehicle maintenance and inspection. Their relative 

ease of removal could also make them susceptible to 

theft. 

With respect to wheel-mounted TPMS, product 

literature asserted that wheel-mounted technology 

included temperature compensation and typically 

provided the best performance in correcting for tire 

temperature. However, during the high-speed test 

sequences, both evaluated wheel-mounted systems 

had their active warnings disabled when tire pressure 

increased because of increased tire temperature. This 

occurred intermittently in various test runs and on 

different axles. Figure 12 shows the point where a 

low pressure alarm was cancelled due to increasing 

tire temperature causing a corresponding increase in 

tire pressure. However, the test data suggested that 

the systems were generally able to compensate for 

large increases in tire temperature (greater than 20 

degrees Fahrenheit), but were less able to compensate 

when the temperature increases were lower. 

Increasing tire temperatures could prevent a lowpressure warning from being generated. In addition, 

the team found that the wheel-mounted technologies 

could be vulnerable to damage during tire mounting 

and dismounting. 

The evaluated tire-mounted TPMS included 

temperature compensation. A handheld reader 

displayed both the temperature-corrected pressure 

and the uncorrected pressure at ambient tire 

temperature. This system was the only one that 

required the use of a handheld or gate reader to 

inspect tires. No in-cab display was available. Figure 

13 shows the hand reader in the charger and 

download docking station. 

The gate reader clearances were very tight and 

required very slow vehicle speeds, less than five 

mph. However, this particular system was unique 

among those tested in that it was linked to an 

Internet-based tire maintenance and tracking database 

application hosted by the system manufacturer. 

Figure 14 and Figure 15 show the eTire tag and 

installation tools respectively. 

Figure 13. eTire Handheld Reader. 

Figure 14. eTire Tire Tag. 

Figure 15. eTire Mounting Patch and Installation 

Tools. 

In general, the ATIS performed as designed and 

specified by the manufacturers and performed well in 

both static and dynamic conditions. Figure 16 

displays the rate of inflation of the trailer-mounted 

ATIS from various initial inflation pressures. In the 

leak rate tests, the motorcoach ATIS was able to keep 
Freund 10

up with leak rates up to 5 to 8 psi/min. Ultimately, 

this system’s performance was limited by the 

vehicle’s air compressor duty cycle and the 

compressed air supply and storage system design. 

The ATIS tested on the trailer could maintain 

adequate tire pressure with slow leakage rates (less 

than 1.0 psi/min) but did not maintain adequate tire 

pressure for higher leakage rates. This system 

appeared to be limited by its rate of airflow to the 

tires more than by a limitation of the onboard 

compressor and air system. Figure 17 shows the 

ability of the trailer-mounted ATIS to maintain 

pressure at various leakage rates. 

During testing with heavy braking and simultaneous 

tire leaks, the vehicles’ primary and secondary air 

reservoir pressures remained above the level required 

for safe brake operation. The compressor had no 

difficulty recharging the reservoirs without having to 

run continuously. Figure 18 shows the trailermounted ATIS maintaining pressure in the primary 

and secondary reservoirs during brake snub 

maneuvers with a leaking tire. Both tested ATIS 

protected the intact tires from deflating when a 

catastrophic air leak was simulated in one of the other 

tires in the system. In this regard, the systems 

functioned in a manner similar to the dual tire 

equalizers isolation circuits. Figure 19 shows the 

motorcoach-mounted ATIS maintaining pressure in a 

tire with a significant leak while operating at 

highway speeds. 

Vehicle air systems are not optimized to support an 

ATIS with very high leakage rates; therefore, the 

functionality of the ATIS was often limited by the 

vehicle’s air system. Additionally, there may be some 

long-term impact to the CMV’s air system when 

subjected to a high leakage rate from the secondary 

reservoir, which the ATIS utilizes for its supply air. 

These leak rates would cause an increase in the duty 

cycle of the compressor and would increase 

maintenance requirements and decrease compressor 

service life. 

Installation time for systems varied. In general, 

valve-stem-mounted TPMS and dual tire equalizers 

were less time consuming to install (generally, 

several hours), followed by wheel-mounted TPMS, 

tire-mounted TPMS, and ATIS that required up to a 

full day for installation. 

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80 180 280 380 480 580 Time (sec)

Pressure (psi) / Temperature (deg F)

RFIT-85 RFIT-75 RFIT-65 RFIT-55

Central Inflation System Dynamic Functionality Test 

Right Front Inner Trailer Tire

Figure 16. Trailer-mounted ATIS Static Inflation 

From Various Initial Pressures. 

40

50

60

70

80

90

100

110

120

0 100 200 300 400 500 600 700 800 Time (sec)

Pressure (psi) / Temperature (deg F)

0.5 psi/min 1.0 psi/min 1.5 psi/min

2.5 psi/min 5.0 psi/min 10.0 psi/min

Central Inflation System Threshold Warning Level Test 

Right Front Inner Trailer Tire

Figure 17. Trailer-Mounted ATIS With Various 

Leak Rates. 
Freund 11

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

100

110

120

130

140

0 500 1000 1500 2000 2500 300 0 3500 T im e (s e c )

Pressure (psi) / Temperature (deg F)

0

2 0

4 0

6 0

8 0

100

Vehicle Speed (mph)

R F O T - P R E S R F IT - P R E S P r i R e s e r v o ir P R E S S e c . R e s e r v o ir P R E S V eh icle S peed

C e n tra l In fla tio n S y s te m F a ilu re M o d e T e s t P a rt # 1

R ig h t F ro n t In n er T ra ile r T ire

Figure 18. Trailer-Mounted ATIS Brake Snubs With Tire Leak. 

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

100

110

120

130

140

-1 9 9 1 9 9 2 9 9 3 99 4 99 5 99 6 99 7 99 89 9 99 9 1 09 9 1 19 9

T im e (se c )

Pressure (psi)/Velocity (mph)

R TA G P res s u re P rim ary R e s . P re s s u re S ec onda ry R es. P re s su re V e hic le S p e e d B ra k e L in e P re s s u re M a rk e r

Figure 19. Motorcoach-Mounted ATIS Tire Leak at Highway Speed. 

THIRD STUDY: TPMS MINI-FIELD 

OPERATIONAL TEST 

The third study focused upon evaluating TPMS in a 

fleet setting. The study team sought to identify a 

commercial fleet operator (or host fleet) with 

characteristics that would allow for effective and fair 

evaluation of systems and technologies. These 

criteria included: an operating environment and duty 

cycle that could be considered severe for brake and 

tire wear; homogeneity of the fleet in terms of vehicle 

type, make, and model; consistency of operations 

within the fleet relative to driver assignments, routes, 

mileage accumulation, and maintenance operations; 

and a strong commitment by the host fleet in 

evaluating these systems in a controlled study for 

possible implementation in its own fleet. 

The host fleet selected was the Washington 

Metropolitan Area Transit Authority (WMATA). 

WMATA operates approximately 1,500 buses in the 

metropolitan Washington, D.C. area. Transit bus 

platforms were selected for this field test because 

their severe urban, stop/start duty cycle leads to 

accelerated brake and tire wear (thus challenging the 

sensor systems.) In addition, the fundamental brake 

and tire designs were very similar to a conventional 

tractor, thus allowing the results of this study to be 

extended to heavy-duty (class 8) trucks. 

The test fleet consisted of 12 Orion VII series, 2005 

model year, urban transit buses. The buses were a 

“low floor” design, 40 feet long and 102 inches wide, 

and operated on compressed natural gas. Each bus’s 

gross vehicle weight rating was 42,540 pounds. The 

passenger capacity was 41 seated and 36 standing 

passengers for a total of 77 passengers. The curb 

weight of the buses was 30,990 pounds. 
Freund 12

The study team evaluated three TPMS (as well as 

three brake performance monitoring systems, as 

discussed in the companion paper) on 12 heavy-duty 

urban transit buses in revenue service for a period of 

1 year. A control fleet of 12 identical buses was 

operated in a similar manner and used for 

comparison. A maintenance garage located in 

Arlington, Virginia was selected as the test site, 

based on the availability of buses of a consistent age 

and operating environment and because of the 

experience and low turnover of the maintenance 

staff. With the assistance of WMATA and TPMS 

system vendors, the study team retrofitted the 

candidate systems on the buses at the garage. The 

buses operated in an area covering approximately 300 

square-miles south and west of Washington, D.C. 

The majority of miles were accumulated in an urban 

environment with minimal high-speed highway 

travel. The buses averaged 16 miles per hour in 

revenue service and were driven an average of 129 

miles per day. 

Over the course of the 12-month evaluation period, 

the systems were inspected weekly; and system data 

was downloaded as part of the test program. 

Additional data were collected in conjunction with 

WMATA’s various maintenance inspections, which 

included a safety inspection every 3,000 miles and a 

comprehensive preventative maintenance inspection 

every 6,000 miles. WMATA staff recorded all 

maintenance and fueling activities and entered the 

data into a maintenance management database. This 

information was made available to the study team for 

evaluation. At the conclusion of the test, maintenance 

staff were interviewed about their experience 

operating and maintaining the systems. Other than 

the standard data-recording capabilities of the 

candidate systems, no additional (or special-purpose) 

data-logging devices were added to the vehicles. The 

system status displays were located out of the 

drivers’ view per the request of WMATA fleet 

managers. The study team and WMATA technicians 

were responsible for monitoring the systems’ display 

status. This was done to limit driver distraction, as 

well as to reduce the incidence of operators ceasing 

bus service because of information from the displays. 

Limiting vehicle-related information to the bus driver 

(system diagnostic information, dash-mounted 

warning lights, and fuel gauge readings) is common 

in the transit industry. 

Three TPMS were installed in the test vehicles: 

• The WABCO Integrated Vehicle Tire Pressure 

Monitoring (IVTM) System was developed in 

conjunction with Michelin and launched in the 

CMV industry in 2003. Each tire and wheel 

assembly is equipped with a sensor that attaches 

to the valve stem and is secured to the rim by 

two lug nuts. A pneumatic hose runs between the 

sensor and valve stem. Tire inflation pressure 

and temperature data are wirelessly transmitted 

to an ECU on-board the vehicle. 

• The HCI Tire-SafeGuard System consists of 

pressure and temperature sensors, a transmitter, 

and a driver’s display. The measurement sensor 

is strapped to the wheel inside the tire. Data are 

transmitted wirelessly, similar to the WABCO 

system. 

• The Michelin eTire System for CMVs was 

introduced by Michelin North America in 

October 2002. The system includes an RF 

transmitter, pressure and temperature sensors, 

and an antenna, all of which are encased in 

impact- and heat-resistant plastic. The passive 

pressure and temperature sensors (which do not 

require batteries) receive power via RF 

transmissions from an external reader device. 

The eTire unit mounts to the inside sidewall of 

the tire via a molded rubber dock that chemically 

cures to the tire’s sidewall. The system is 

designed to work with a tire pressure gate reader 

but can also be used with a handheld reader. 

Results of Field Operational Test 

To evaluate the performance (or accuracy) of the 

TPMS, manual tire pressure measurement readings 

were taken once a week on each bus and then 

compared with pressures as reported by the TPMS at 

the time of the manual measurement. Each bus was 

inspected approximately 56 times throughout the 

course of the test, resulting in a total of 3,714 tires 

inspected. Occasionally, certain buses were 

unavailable for a weekly inspection because a test 

bus was mistakenly put into service or a bus was 

removed from service and awaiting maintenance. Not 

all of the inspections yielded valid comparisons 

because of problems with the TPMS systems 

themselves or, in a few instances, because of 

problems associated with manually checking the 

tires. 
Freund 13

Key observations related to the on-board monitoring 

systems include the following: 

• The TPMS provided accurate tire inflation 

pressure data when compared to measured (tiregauged) values. System sensors were found to be 

consistent and reliable in reporting tire inflation 

pressures. On average, the systems reported false 

positives (a false low-pressure reading) 6 percent 

of the time or false negatives (a missed lowpressure reading) 2 percent of the time. The more 

frequent issue, found in 17.6 percent of 

inspections, was “no reads,” resulting from 

missing sensors and sensors in the wrong wheel 

location. 

• Keeping track of the individual wheel/tire sensor 

units themselves was a significant challenge 

during the field test. This logistical challenge 

arose because of the high frequency of tire 

changes. The sensor mounting locations were out 

of view of technicians and the lack of fleet-wide 

training on system awareness and operation 

accounted for lost or discarded sensors. Training 

was limited to the host depot, but occasionally 

tire and brake maintenance would occur at other 

depots. Training across the entire system would 

be required to prevent technicians from 

misplacing and inadvertently discarding sensors. 

• The durability of two TPMS sensor designs was 

initially challenged by operating in transit 

service. Failures occurred with the wheelmounted sensors just 2 months into testing. 

Excessive heat build-up caused the sensor 

housing to become brittle, crack, and fail. The 

sensor manufacturer provided a design change 

that consisted of an isolating pad on the bottom 

side of the sensor that contacted the wheel rim. 

This simple solution prevented further failures of 

this type from occurring. Failures also occurred 

with the sensors that adhere to the tire sidewall. 

Specifically, the plastic casings on these sensors 

were found to crack within the first few months 

of operation. The cracked sensor casings were 

determined to be caused by a manufacturing 

batch defect. Replacement sensors were found to 

be significantly more durable. 

• An improvement in adherence to targeted tire 

pressures was not found on the test fleet 

compared to the control fleet. This may be 

because WMATA takes a pro-active role in 

maintaining target inflation pressures to comply 

with the tire vendor’s warranty. Average tire 

inflation pressure was 111 psi for both the test 

and control fleets (target pressure: 115 psi). Tire 

life and fuel economy were also similar in both 

fleets. This was likely the case because the real 

time display of tire pressure readings was 

purposefully not made available to drivers but 

only to maintenance personnel and technicians. 

Therefore, drivers could not act immediately on 

such information to correct any tire pressure 

problems that may have been detected. In most 

commercial truck fleets, such real time 

information would have been provided to 

drivers; and drivers would have had the 

opportunity to act on the information as needed 

(for example, if low pressure was detected, 

adding air to tires at the next convenient time) 

thereby improving the average tire life and fuel 

economy of the fleet. 

• Two tire blowouts could have been prevented 

during the course of the field test had the TPMS 

displays been visible to the bus operators. To 

maximize safety and operational benefits, system 

data need to be available to the driver in real 

time, as well as to maintenance staff. 

• Technicians preferred the wheel-mounted tire 

pressure sensors for two reasons: (1) they were 

easier to install, and (2) tires could be changed 

without removing or disconnecting the system. 

Conversely, the technicians found valve-stemmounted sensors difficult to connect to the inner 

tires on a set of duals. This issue may be unique 

to buses because the wheels and tires are 

surrounded by the structure of the vehicle. Tiremounted sensors required more time to install 

(versus wheel- or valve-stem-mounted systems) 

because of the required tire surface preparations. 

CONCLUSIONS 

Tire pressure maintenance has been a significant and 

persistent problem for operators of CMVs. After 

brakes, tires are the most expensive maintenance item 

for fleets and the most common vehicle-related defect 

cited in crash reports. Tire deficiencies are the 

second-ranked reason for CMVs to be cited for 

defects during roadside inspections. A collection of 

tire pressure readings from over 35,000 tires provided 

the first large-scale source of tire inflation readings in 

the United States. A series of controlled test-track 

assessments of nine TPMS, representing the three 

types in the commercial marketplace, provided a 

comprehensive comparison of the characteristics and 

operation of these systems and suggested 

opportunities for improving their usability and 

performance. A 12-month field operational test of 

three monitoring-type TPMS provided information 

on tire pressure status that was useful for improving 

maintenance practices and detecting low tire 

pressures to perform timely repairs. This information 

had a significant impact on inspection practices and 
Freund 14

enhanced the overall efficiency of operations at 

WMATA. While no firm procurement commitments 

were made, WMATA maintenance managers 

indicated that they would consider using one or more 

monitoring technologies on new vehicle 

procurements and the retrofit of existing buses. A 

study to test the TPMS technologies on commercial 

tractor-trailer fleets is currently underway. 

REFERENCES 

Kreeb, R. M., B.T. Nicosia, and P.J. Fisher, 

“Commercial Vehicle Tire Condition Sensors.” 

Report FMCSA-PSV-04-002, November 2003. 

Brady, S., B. Nicosia, R. Kreeb, and P. Fisher, “Tire 

Pressure Monitoring and Maintenance Systems 

Performance Report.” Report FMCSA-PSV-07-001, 

January 2007. 

Van Order, D., D. Skorupski, R. Stinebiser, and R. 

Kreeb, “Fleet Study of Brake Performance and Tire 

Pressure Sensors.” Performed under Contract 

DTFH61-99-C-00025, Task Order 9, July 2005 – 

July 2008. Report forthcoming from FMCSA.