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Recommended Practice

Proposed RP 235A (T) VMRS 017 GUIDELINES FOR TIRE INFLATION PRESSURE MAINTENANCE

PREFACE The following Recommended Practice is subject to the Disclaimer at the front of TMC’s Recommended Maintenance Practices Manual. Users are urged to read the Disclaimer before considering adoption of any portion of this Recommended Practice.

PURPOSE AND SCOPE The purpose of this Recommended Practice (RP) is to demonstrate the importance of inflation pressure and its effect on tires and tire service life. It addresses all aspects of the relationship between inflation pressure and tires, and provides equipment users with a practical guide for better understanding the issues Underinflated Correct Overinflated and costs that stem from failing to address this issue Inflation properly in everyday fleet operations. This RP applies to Class 2-8 commercial vehicles in light-, Fig. 1 medium- and heavy-duty service. fatigue and break. Also, excessive heat will develop INTRODUCTION and deteriorate the tires. This RP offers comprehensive guidelines on proper tire inflation pressure maintenance. It covers air Overinflated tires are more vulnerable to tread sur- pressure and its relevance to: face cutting, impact breaks, punctures, and shock • footprint damage. Changes in inflation pressure affect the • irregular wear tire’s footprint, which is the area that contacts the • load road. (See Fig. 1.) • tire temperature • maintenance INFLATION PRESSURE AND IRREGULAR WEAR • sealants Both over and underinflation change a tire’s footprint, • steer axles affecting tire traction and leading to irregular wear • dual assemblies patterns. For example, medium truck tires rotate • equipment about 500 times for every mile travelled. In 100,000 • fuel miles, each part of the tread gets pressed against the • nitrogen pavement about 50 million times. • automated systems • determining correct pressure. Irregular wear can result from uneven abrasion of the • tire damage. tread against the pavement. Therefore, tires should be maintained at a consistent shape throughout their INFLATION PRESSURE AND FOOTPRINT life. Proper inflation pressure gives tires the right Maintaining the correct tire inflation pressure is criti- shape for slow, even wear. When inflation pressure cal to the successful operation of truck tires. In- falls below the rated value, slippage of the tire tread creased flexing due to underinflation causes heat in contact with the ground becomes greater—accel- buildup within the tire components. If tires are erating tire wear. Fig. 2 shows the relationship underinflated, the body ply cords in the sidewall will between these two factors.

Tire Wear Life

Heel and Toe Wear/ Block Edge Wear

Proper Pressure Fig. 2 Fig. 4

UNDERINFLATION/OVERLOAD underinflation condition, the decrease is about 50 Underinflation will cause irregular wear. It is a cause percent. (See Figs. 5 and 7.) Overloading will cause: of shoulder cupping/scallop wear, alternate lug wear • Excessive deformation of sidewalls, increas- and will accelerate multiple flat spotting, diagonal ing the chance of cutting. wear, rib punch and heel/toe wear. (See Figs. 3 & 4.) • Excessive temperature and stress increasing fatigue of rubber and cords, and weakening OVERINFLATION/UNDERLOAD adhesion between rubber and cords. Overinflation will cause and accelerate both shoul- • Excessive movement of tread that acceler- der and heel/toe wear. (See Figs. 3 & 4.) ates wear and/or creates irregular wear.

INFLATION PRESSURE AND LOAD INFLATION PRESSURE AND TIRE TEMPERATURE Tire life is greatly affected by load. Therefore, it’s Heat is generated inside of a tire through sidewall important that inflation pressure be properly con- deformation (flexing) and tire tread contact with the trolled in relation to the load. A constant 20 percent ground. The lower the inflation pressure, the more a underinflation/overload decreases the life of the tire tire will deflect or flex, resulting in an increase in tire by 30 percent. In the case of a 40 percent overload/ temperature. (See Fig. 6.)

Rib Depression Wear Full Shoulder Wear

Erosion / River/ Channel Wear

Fig. 3 Fig. 5

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 2 The higher the outside temperature, the greater the internal air pressure build up. The greater the percentage of underinflation, the more mileage lost. Fig. 8 Fig. 6

Tire service life is influenced by differences in the internal air chamber. Air pressure inside a tire rises as heat is generated during normal operation. Heat causes air to expand, and the inflation pressure increases. For example, tests show that air inside a 11R22.5 tire can reach 160°F or more, depending on air pressure, road temperature, ambient air temperature, altitude, and other factors (see Figs. 8 and 9).

Inflation pressure can rise 10-15 percent as a result of normal operation. This is considered hot inflation Although air pressure changes with altitude, the pressure. In some cases the actual hot inflation changes are small and not a factor in the air pressure could be higher than the maximum inflation pressure maintenance programs. pressure marked on the sidewall. This is not a safety issue. Tires are designed to withstand this normal Fig. 9 pressure buildup. ! WARNING : Never bleed inflation pressure from a hot tire. When adjusting inflation pressure, the vehicle Choose the appropriate cold inflation pressure for should be parked for 3-4 hours. If the truck must be the loaded condition and maintain that pressure. moved, be sure it has been driven less than one mile.

Continuous operation above 230°F will damage a tire as rubber begins to revert and loses its strength.

If a vehicle is equipped with a tire pressure monitoring system that provides an over-temperature alert, the driver should stop and investigate the cause.(See Fig. 8.)

INFLATION PRESSURE MAINTENANCE Tire inflation pressure should be checked on a regular basis and the fleet should have a structured tire maintenance program. Tires lose pressure for a variety of reasons such as punctures, and cuts, in Fig. 7 addition to loss of inflation through the tire body,

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 3 Fig. 10 between the bead and wheel as well as through improperly tightened valves, damaged rubber grom- mets, or valve cores that have been blocked open by dirt/ice, or other debris. Look for air leakage and Fig. 11 replace lost valve caps. deflection—vary considerably between tire manufacturers and from one tire model to another. Tires should be equipped with metal valve caps. Do not use the plastic caps that come with many new Air molecules are extremely small, and gradually valve stems. Inflate-thru valve caps are acceptable escape through the tire body. Because air molecules and offer labor savings as well. (See Fig. 10.) For are small enough to diffuse through rubber, tire more information, refer to TMC RP 234, Proper inflation pressure can drop up to two psi per month. Valve Hardware Selection Guidelines. INFLATION PRESSURE COMMUNICATIONS Most tire companies recommend that tires are As part of its overall tire maintenance program, each checked with a calibrated pressure gauge for correct fleet should establish inflation pressure settings for inflation pressure once a week in addition to the pre the tires on its vehicles. These standards should be and post trip tire checks. If this is not possible due to specified in writing to all persons that routinely check the fleet’s operation, tire checks should be sched- the tires. This includes all drivers, maintenance per- uled as often as possible. Many fleets use scheduled sonnel, yard personnel, and servicing tire dealers. It tractor preventive maintenance (PM) events as the is a good practice to mark the vehicles with correct interval for checking tractor tire inflation pressures inflation pressure and pick an arbitrary time interval for checking trailer stickers and placards tire and dolly tire inflation pressures such as weekly, (see Fig. 12). TMC 30 days, 60 days, etc. also recommends that a system be in place Whatever interval is chosen, all vehicles should have to verify that the speci- a tire inflation update decal placed in a visible loca- fied inflation pressures tion on the vehicle. This decal should be made so are being maintained that the date of the inspection can be written on the in all vehicles. This decal by the tire technician. This decal will advise would include actual anyone servicing the vehicle when to check tire tire inflation measure- inflation pressures and what the correct cold pres- ments and calibration sure should be. Decals can be obtained from TMC, of all pressure gauges. Fig. 12 and tire manufacturers or fleets can have them printed themselves. (See Fig. 11.) Fleets should also clearly indicate to all appropriate personnel what procedures should be followed when Accurate tire gauges should be used to check pres- a tire with low inflation pressure is found. Any tire that sures. Ballpeen hammers or “Tire Billys” should not contains an inflation pressure that is 80 percent or be used to hit tires as an inflation check. No one, less than the fleet recommended tire inflation pres- despite many years of experience as a driver or sure should be treated as a flat tire and a potential technician, has ever been able to consistently and sidewall zipper condition, and be immediately re- accurately gauge tire inflation pressure in this man- moved from service. The tire must be demounted ner. Visually observing/evaluating the deflection of and inspected by a qualified tire service technician to the sidewall is not an accurate gauge of tire pressure determine whether it should be scrapped, repaired either, since tire constructions—which affect sidewall and/or returned to service.

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 4 SEALANTS ments for retread and repair operations on these Various products are marketed in the aftermarket to types of tires. improve the inflation retention qualities of tubeless tires. These sealants may be applied to new or used INFLATION PRESSURE AND STEER AXLES tires and are often applied as part of a repair or Timely and accurate pressure maintenance is critical retread procedure. Care should be taken if selecting for steer axle tires. In most cases, steer tires require a tire sealant as the chemical composition of seal- maximum cold inflation pressure to support the load. ants varies widely. Since tires can lose up to two psi per month, it is commonplace for steer tires to fall below the recom- The use of some sealants, both liquid and powder mended air pressure. Underinflation/overload on the forms, may negatively impact valve stem grommet steer axle will cause irregular wear at earlier stages life and even degrade the performance of valve of tire service life and worsen rapidly and may lead to stems and valve cores. Sealants may corrode wheels an unexpected air loss (blow out). which can affect pressure retention at the bead seat. Be aware that the use of sealants with certain chemi- AIR PRESSURE AND DUAL ASSEMBLIES cals and petroleum-based materials may void the tire An inflation mismatch of greater than five psi will warranty. Contact the supplier or the tire manufac- result in two tires of a turer to confirm warranty coverage if sealants are dual assembly being used. Sealants cannot be used in tire/wheel assem- significantly different in blies that contain tire pressure sensors. circumference. Be- cause they are bolted Tire sealants are generally formulated to stop pres- together, they will sure from leaking from the tire at a puncture, but in cover the same most cases do a poor job of keeping water from amount of road in a entering and rusting the area around the injury thereby single revolution. The enlarging the damaged area. Sealants may keep a larger tire drags the punctured tire in service, but in many cases the smaller one. Very fast casing may not be retreadable. Fleets choosing to or irregular wear, es- use sealants should also expect to be charged a pecially on the tire with removal fee by their tire dealer prior to repair or less inflation, will re- retreading. In some states, certain types of tire sult. (See Fig. 14.) Fig. 14 sealants are classified as hazardous waste and require specialized disposal. Testing has shown that a five psi difference creates a 5/16” difference in tire circumference. Two tires Some manufacturers produce tires with a self-seal- with different diameters cannot cover the same dis- ing layer between the tire liner and ply. This layer tance in the same number of revolutions unless generally runs they’re bolted together. In that case, this condition across the will lead to both rapid and irregular wear. crown of the tire and seals In a single mile, this 5/16" difference causes the any nail holes smaller tire to be scuffed 13 ft. In 100,000 miles, the or similar tire is scuffed 246 miles. (See Fig. 12.) punctures in that area to prevent air Correct loss. (See air pressure Fig. 13.) Fig. 13 Since this sealant is an integral part of the tire, it does not need 5 PSI under to be removed prior to retreading. This saves time in correct pressure the retreading process and eliminates any environ- mental concerns with sealant disposal. The tire manufacturers supply any special handling require- Fig. 12

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 5 Access to the inner tire of the assembly is critical to maintaining air pressure in dual assemblies. In order to ensure access to the inner wheel valve, wheel hand holes need to be aligned and valves of the two tires should be as near as 180° apart as possible. Mismatched wheel hand holes are the most common cause for poor inner tire air pressure maintenance. Because inflation controls tire shape, always use the recommended cold air pressure, regardless of ambient temperature.

AIR PRESSURE AND EQUIPMENT There are many steps that must be taken to ensure correct tire air pressure maintenance. The first begins with the maintenance facility. The facility must have a properly maintained air compressor, air lines equipped with filters, and air pressure regulators. The connection for the air line used to fill the tires should never be taken from the bottom of a run, where water may accumulate. Lubricators must not be plumbed into air lines that are used to inflate tires. 11 An air dryer is the best way to remove unwanted 10 7 moisture from the air lines. If a dryer is not installed, 3 8 1 3 1 air lines must be drained on a regular basis. Even 9 with an air dryer installed, drains should be checked 3 periodically. Air lines should be free of moisture at all 6 times when inflating steel radial tires. Any moisture 4 that is in the air lines will permeate the tire’s liner and attack the steel cords. 2

Fleet tire shops, maintenance facilities, tire dealers, 5 and truck stops are not the only sources of air to adjust tire air pressure. Service trucks and on-board air hoses are other sources frequently used for 4 compressed air. Although these systems are por- 3 table, the equipment should be maintained to the 2 same standards found within the maintenance facilities.

No. Qty. Description Remarks TIRE GAUGES 1 2 3/4" tee Technicians, tire maintenance personnel, drivers, or 2 2 3/4" ball valves 3 4 3/4" nipples Length not critical anyone responsible for checking tire air pressures 4 2 3/4" - 2" bell reducers 5 1 2" pipe Length not critical should be equipped with good-quality truck tire pres- 6 1 3/4" pipe Length not critical sure gauges that are checked regularly for accuracy. 7 1 1/4"-3/4" reducing bushing 8 1 1/8"-1/4" reducing bushing Adjustable pressure gauges are also available. 9 1 Schrader type air tank valve 10 1 regulator Speedair 6ZCX31A 250 psi max, Grainger No. 6ZC31 or equivalent

11 1 gauge Ashcroft 1008, 0-200 psi liquid-filled, Grainger No. 2C454 or equivalent.

Fig. 16: Truck Tire Dual Foot Service Gauge Fig. 17

2.5 2 1.5 1 0.5 0

% rate change -0.5 -20 -10 0 10 20 Fig. 18: Air Gauge Checking Station -1

Using the Master Gauge -1.5 Truck tire service gauges should be checked weekly -2 at a valve stem adapter built into the air line. This adapter should be checked weekly against a master -2.5 gauge. Service gauges that are off by five percent or Difference from the standard inflation pressure (psi) more should either be recalibrated or discarded. Fig. 19: Inflation Pressure and Fuel Use Fig. 17 features a schematic and parts list for a typical master shop gauge set up. Fig. 18 shows an above a test target pressure resulted in a two percent air gauge checking station. variation in fuel consumption over that 40 psi range on a 18-wheeled combination vehicle. (See Fig. 19.) The regulator should be set to a convenient pressure (probably 100 psi for most shops). Regulator manu- MAINTAINING TIRE AIR PRESSURE facturers recommend that pressure always be in- Tire air pressure maintenance is labor and time creased from a lower setting to the final setting for intensive. It takes approximately one minute per tire consistent results. The technician should then check to check, adjust, and record tire air pressure. There- the service gauge by applying it to the Schrader-type fore, an 18-wheeled vehicle can take from 15-20 valve. A master gauge (shown in Fig. 17) has a rated minutes to maintain. Multiplying this number by the accuracy of +/- two percent in the middle of its range. number of vehicles in a fleet, and inspecting them That is, if a 0 - 100 psi gauge reads 100 psi, the actual weekly results in a large expense in labor, time, and pressure will be between 98-102 psi. Master gauge equipment. Without tire air pressure maintenance check stations can be purchased as a complete irregular wear, rapid wear, non-retreadable casings, assembly from tire supply distributors. and numerous, expensive road failures are inevi-

Compressor Capability When selecting an air compressor for a maintenance facility, it is important to consider the operating air pressures for the fleet. When the recommended tire pressures are higher than 110 psi, the air compressor must be capable of generating higher pressure in a timely manner.

INFLATION PRESSURE AND FUEL Proper inflation pressure for a given tire size and load is critical to maintaining a proper stress distribution in the tire. This, in turn, reduces flexing and heat buildup that can waste fuel and shorten casing life. In Class 8 vehicle field testing the effect of inflation over a 40 psi range, on all tires, from 20 psi below to 20 psi Fig. 20: Proper On-Vehicle Inflation Procedure

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 7 table. How much the fleet is willing to pay for good, such as leaking valve stems, punctures, cuts, etc., economic tire performance is the compromise be- prevents the guarantee of better pressure mainte- tween thorough weekly pressure maintenance and nance using nitrogen for tire inflation. Therefore PM-only checks and adjustments. regular pressure maintenance still remains critical and tire inflation check intervals must not be ex- Who performs a fleet’s air pressure maintenance will tended if nitrogen is used. influence costs and performance: • The fleet’s own technicians on weekly or RECOMMENDED INFLATION PRESSURE other basis. MAINTENANCE PROCEDURES • The fleet’s own technicians during refuel- The following constitutes TMC’s recommended pro- ing/safety inspection. cedure for inflation pressure maintenance: • The fleet’s drivers during each pre-trip in- 1. Check the inflation pressure using a reliable, spection. calibrated gauge, and record the date, tire • The fleet’s tire dealer as an added paid position, tire ID number, and inflation pres- service to the fleet. sure for future reference. Whenever possible, • The fleet’s drivers during each post-trip check the inflation pressure when the tire is inspection. cold (ambient temperature; tires have not been run for at least 3 hours). If pressures NITROGEN must be checked when the tire is hot due to Nitrogen molecules make up about 78% of the earth’s running on the road, skip to Step 4. atmosphere. Nitrogen gas is colorless, odorless and 2. If the actual (cold) pressure measured is generally inert. It does not burn, does not support greater than 80 percent of the target pressure combustion and is only slightly soluble in water. It is determined, inflate or bleed the tire as neces- relatively inactive chemically, but many of its com- sary to meet the target inflation. Go to step 7. pounds display marked reactivity. At high tempera- 3. If the actual (cold) pressure measured is 80 tures it reacts with some of the other elements to form percent or less of normal pressure, consider nitrides and are larger than oxygen molecules. There- the tire to be flat. Deflate the tire, remove it fore, nitrogen is slower to migrate through the casing from the vehicle, and repair or replace it as than air and since nitrogen molecules do not carry required. A leaking valve or a nail hole will moisture, nitrogen may reduce rust in steel cables continue to lose air if not repaired. and belts that is caused by moist air permeation. ! DANGER : Re-inflation of a tire/wheel as- The down side is that sembly that has been operated in a run flat or nitrogen is more ex- underinflated condition (80 percent or less of pensive than air not normal pressure) can result in injury or death. only because of added initial equipment costs The tire must be removed, demounted and but also because of inspected for punctures or other damage. The the maintenance re- tire may be damaged on the inside and may quired by nitrogen rupture with explosive force during the infla- generation systems. tion process. If the above procedure indicates Nitrogen inflation sites a condition that can be repaired, locate the may be difficult to find cause of the air loss, repair it, and bring the unless a fleet has its pressure back to the fleet’s target value. Be own nitrogen genera- sure to test the repair for leaks. Go to Step 7. tion equipment. 4. Using a regular tire gauge, cold inflation pressure cannot be determined by measuring the Nitrogen is not re- tire when it is hot. However, if pressure must Fig. 21: Nitrogen quired and may not be measured when the tire is hot, look for Inflation System always bring the ex- consistency between tire-to-tire measure- pected benefit. While ments. Tire inflation pressure changes with it is true that the physical properties of nitrogen will temperature. There is about a two psi in- reduce the pressure loss due to air permeation, the crease for every 10°F increase in tire tem- existence of several other possible sources of leaks perature. It is not unusual for the difference

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 8 between the cold inflation and the hot pres- garage. Provide ample time for the tires to sure to be as much as 20 psi. If a vehicle's warm up inside the garage before checking tires are measured when hot, expect the pres- and adjusting tire pressure. It tire pressure sure readings to be higher than the target has to be checked outside in low temperature inflation pressure established in Step 1. Don't conditions, reduce the moisture content in the be too concerned about the actual values if inflation air by using airlines equipped with they fall within the above approximations. moisture and oil remover filters. Look for consistency in inflation. Both steer 9. Make sure the valve is securely fitted with a tires should read about the same, and all drive sealing valve cap. Flow-through caps are tires should read about the same, and all recommended, since they make pressure trailer tires should read about the same. (After checks easier. running on the highway for a few hours, it takes about three hours for truck tires to cool AUTOMATED SYSTEMS down to ambient temperatures) Many automated systems to handle air pressure 5. If the actual (hot) pressures measured across maintenance are currently on the market or on the the same axle are within five psi of each other drawing board. These systems save labor and time and they are all higher than the target pres- but at some extra cost and complexity and these sure determined in Step 1, tire pressures in systems often need special maintenance to remain these tires are probably acceptable. Never operational and effective. bleed hot tires to reduce inflation pressure. Instead, check these tires when cold at the See TMC RP 239, Commercial Vehicle Tire Inflation next opportunity to ensure that cold pressures and/or Monitoring Systems Guidelines for more in- are correct. Go to Step 7. formation. 6. If the actual (hot) pressures measured across the same axle differ by more than 20 percent DETERMINING PROPER INFLATION PRESSURE from each other, the low pressure tire(s) may The correct tire air pressure for a vehicle is deter- have an air loss problem and should be treated mined by the loads carried by the individual tires. as a flat. Otherwise the tires can be inflated to Using the load and inflation charts in the tire match the other pressures. Determine if there manufacturer’s data book or the Tire and Rim is a problem with the tire before reinflating. A Association’s Yearbook, the load capacity at the leaking valve or a nail hole will continue to lose required cold pressure for a single tire or for each tire air if not repaired. Locate the cause of the air of a dual assembly can be determined. loss, repair it, and bring the pressure back to the target value. Be sure to test a repaired tire For example, referring to Fig. 22, a tire size 295/ for leaks. If one or two tires have pressures 75R22.5, used as a single, (steer position), load much greater than the others across the axle, range G, will carry 6,175 pounds at 110 psi cold the cause may also be overload due to unbal- inflation. Therefore, two such tires on a steer axle will anced loading, or heat exposure from unbal- carry 12,350 pounds when each tire is inflated to 110 anced brakes or sunlight. psi cold. Note, that this same tire at 105 psi cold 7. Extreme cold weather operations may require inflation will carry only 5,980 pounds or a pair will adjustments to the above procedure since carry only 11,960 pounds on a steer axle. If the steer pressures measured in a heated garage may axle load requirement is 12,000 pounds, then 105 psi not correlate the same way with cold ambient cold, in this example, is underinflated and the tires temperatures as discussed above. Company are overloaded. representatives can offer specific recommendations for operating in these conditions. Looking at the same 295/75R22.5 load range G, for 8. Care should be taken when checking tire use in duals, shows a load capacity of 5,675 pounds inflation pressures when the outside ambient at 110 psi cold. Since tires used in duals are gener- temperature is below freezing (32°F). Mois- ally fitted on drive and trailer axles and the maximum ture in the inflation air may freeze in the valve legal load capacity for a tandem axle set up is 34,000 core and cause the tire to leak or go flat. To pounds, the per tire maximum will be 4,250 pounds. prevent this from happening, when the out- Using Fig. 23, it is evident that in dual applications side temperature is below freezing, the tire only 75 psi cold air pressure is required to carry the pressure should be checked inside a heated 4,250 pound load per tire. A higher air pressure may

Fig. 23: Pressure Variance with Load Range be used, but nothing lower. Most fleets use a higher No tires should be operated at any inflation greater inflation pressure (such as 100 psi) for drive and than the maximum indicated on the tire sidewall or in trailer axle tires to compensate for minor dual mis- a load chart from the Tire and Rim Association or the matches and extended maintenance checks. tire manufacturer. Overinflation does not add strength to a tire. For operations requiring loads and pres- Note that the 295/75R22.5 load range H, as a single sures that would fall outside the available load limit tire for example, can carry more load than load range charts, consult the tire manufacturer. Further, do not G but does so at a higher pressure, 120 psi versus exceed maximum load or inflation capabilities of 110 psi cold inflation. However, the load range H tire wheels with radial tires. This information is available at 110 psi has the same capacity as the load range in wheel manufacturer data books and is also stamped G tire. Conversely, a load range G tire overinflated on the wheel. If the tire manufacturer or a tire to 120 psi will not carry the load of a load range H tire association recommends a higher inflation than the at 120 psi since the load range G tire’s maximum one stamped on the wheel, consult the wheel manu- inflation pressure of 110 psi will be exceeded. facturer.

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 10 inflation pressure that can be used for this off highway run. To do this: 1. Locate the proper table in the Tire and Rim Association Yearbook or the manufacturer data book. 2. Find the tire size on the table. 3. Find the tire load on the table. 4. Locate the permitted tire pressure for these conditions. In this case, the low speed off highway segment can be run with 74 psi in the trailer tires. Before entering the highway system, these tires must be inflated to the Fig. 24: Maximum Tire required pressure for highway service. The Inflation Pressure Labeling standard load and inflation table shows that this should be 90 psi.

Special Applications AIR PRESSURE STANDARDIZATION For special applications that run at slower speeds Many fleet operators prefer to use one tire inflation and heavier loads, specific load and inflation tables pressure for all the tires on the vehicle as this have been designed to address these operations. simplifies tire inflation maintenance. The inflation pressure required for steer axle tires is usually the Example: A tractor / trailer is equipped with 11R24.5 one chosen. This is a common practice. If separate tires with dualed tires on the trailer. The trailer has inflation pressures are maintained for single and dual just been loaded with logs and has a two mile haul on tires, it may be wise to choose a slightly higher an unpaved road before it gets to the main highway. pressure for the duals in order to provide them with The vehicle will not exceed 35 mph on this off an inflation “margin” that would provide sufficient air highway segment. The load per trailer tire is 5500 pressure if inflation updates are not consistently pounds. It is necessary to determine the minimum done or are not done on a timely basis.

Fig. 25: Reduced Tire Inflation Pressure Limits

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 11 Whatever inflation pressure is selected for the drive, dolly, and trailer positions, assuming it is adequate to carry the load, all tires should be uniformly inflated (within 2 psi). Unequal inflation of tires in a dual installation, especially in trailer and dolly service, can lead to irregular wear.

Effects of Speed Speed affects the load carrying capacities of tires. If vehicle operation speeds exceed 65 mph, load carrying capacities of tires not rated at 75 mph or above are seriously affected. Not only must additional inflation pressures be added to tires, but the carrying capacity of tires decreases as well. Steer tires are especially affected. Therefore, the service load and Fig. 26: Speed Restriction Labeled on Tire minimum (cold) inflation pressure must comply with the following limitations unless a speed restriction is exceed 30 miles per hour. The fleet wants to run indicated on the tire or the tire manufacturer rates the more than the 5,675 pounds maximum load for this tire at 75 mph or above. tire. The fleet manager looks at the table in the Tire and Rim Association Yearbook to see what load and Tables 1 and 2 show the effects that speed has on inflation can be run. tires. However, these tables apply to tires only. 1. Most major tire manufacturers’ 295/75R22.5 Consult the rim/wheel manufacturer for rim/wheel LR G on/off highway drive tires are not speed load and inflation capacities. restricted. Because of this, the fleet should use the appropriate table (see Fig. 27) found The following examples help explain how to use in the Tire and Rim Association Yearbook to Tables 1 and 2. determine the maximum permitted load for these conditions. Example: A truck is equipped with 295/75R22.5 LR 2. Find the maximum speed. G dualed drive tires with an on/off highway tread 3. Note that up to 6,365 lbs. (5,675 x 1.12 = 6,365 design. It is in an application in which it will not pounds) can be run by increasing the inflation

Fig. 27

© 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 12 TABLE 1: LOAD LIMITS AT VARIOUS SPEEDS FOR TRUCK BUS TIRES RATED AT 65 MPH USED ON IMPROVED SURFACES FOR METRIC, WIDE BASE TIRES, AND CONVENTIONAL RADIAL TIRES

The service load and minimum cold inflation must comply with the following limitations unless a speed restriction on the tire or the manufacturer rates the tire at 75 mph or above.

METRIC AND WIDE BASE RADIAL TIRES CONVENTIONAL RADIAL TIRES SPEED RANGE INFL. PRESSURE INFL. PRESSURE % LOAD CHANGE % LOAD CHANGE (MPH) CHANGE CHANGE 7%1 thru 75 -i12 +%5 ps -i12 +5 ps

6%6 thru 70 -i4 +%5 ps -i4 +5 ps

5e1 thru 65 Neon Neo increas Neon No increas

4%1 thru 50 +e7 N%o increas +e9 No increas

3%1 thru 40 +e9 N%o increas +e16 No increas

2%1 thru 30 +i12 +%10 ps +i24 +10 ps

1%1 thru 20 +i17 +%15 ps +i32 +15 ps

6%thru 10 +i25 +)20 ps +)60% (1 +30 psi (1

2%.6 thru 5 +i45 +)20 ps +)85% (1 +30 psi (1

C%reep thru 2.5 +i55 +)20 ps +)115% (1 +30 psi (1

C)reep +)75% (2 +)30 psi (2 +)140% (1) (2 +40 psi (1) (2

S%tationary +i105 +)30 ps +)185% (1 +40 psi (1 Limitations (1) Apply these increases to Dual Loads and Inflation Pressures (2) Creep is motion that does not exceed 200 feet in a 30-minute period. NOTE: The inflation pressure shown in the inflation tables are minimum cold pressures for the various loads listed. Higher pressures should be used when required by the above speed/load table and/or when higher pressures are desirable to obtain improved operating performance. For speeds above 20 mph, the combined increases of A and B should not exceed 20 psi above the inflation specified for the maximum load of the tire. TdThe maximum load a.and inflation capacity of the wheel must not be exceeded.

pressure to 120 psi (110 + 10 = 120 psi). pressure must be increased by five psi. Therefore, 4. Ensure the rim can handle the increased only 5,086 pounds can be carried on each steer tire inflation pressure. (10,172 on the axle) inflated to 105 psi, which is not enough to carry the 11,500 pounds on the axle. Example: A tractor-trailer is equipped with 295/ These tires are rated at 5,980 at 105 psi according to 75R22.5 tires that are speed rated at 65 mph. It the load and inflation chart. However, with a 12 normally carries 11,500 on its steer and 34,000 on its percent reduction in carrying capacity and an in- tandem tractor and trailer axles with 100 psi in all of crease of five psi, these tires will be at their maximum its tires. However, its fleet operation has changed inflation pressure but will still only be able to carry and the vehicle must now operate at highway speeds 10,524 pounds on the steer axle. Therefore, the fleet over 70 mph. The fleet manager looks at the speed manager must either change the tires out or reduce and load limits table to see how this will affect the load the load that is placed on the steer axle. the tires can carry. (See Fig. 28 on the following page.) VOCATIONAL CONSIDERATIONS The fleet should consult with their tire supplier for air The load that the vehicle can now carry on its steer pressure recommendations relating to their specific tires must be reduced by 12 percent and the inflation vocational requirements. In certain fleet vocations/

This table applies to tires only. Consult rim/wheel manufacturer for rim/wheel load and inflation capacities.

METRIC AND WIDE BASE RADIAL TIRES CONVENTIONAL RADIAL TIRES SPEED RANGE INFL. PRESSURE INFL. PRESSURE % LOAD CHANGE % LOAD CHANGE (MPH) CHANGE CHANGE 7e1 thru 75 Neon Neo increas Neon No increas

6e6 thru 70 Neon Neo increas Neon No increas

5e1 thru 65 Neon Neo increas Neon No increas

4%1 thru 50 +e7 N%o increas +e9 No increas

3%1 thru 40 +e9 N%o increas +e16 No increas

2%1 thru 30 +i12 +%10 ps +i24 +10 ps

1%1 thru 20 +i17 +%15 ps +i32 +15 ps

6%thru 10 +i25 +)20 ps +)60% (2 +30 psi (2

2%.6 thru 5 +i45 +)20 ps +)85% (2 +30 psi (2

C%reep thru 2.5 +i55 +)20 ps +)115% (2 +30 psi (2

C)reep +)75% (3 +)30 psi (3 +)140% (2) (3 +40 psi (2) (3

S%tationary +i105 +)30 ps +)185% (2 +40 psi (2 Limitations (1) For speeds above 75 MPH, contact tire manufacturer. (2) Apply these increases to Dual Loads and Inflation Pressures (3) Creep is motion that does not exceed 200 feet in a 30-minute period. NOTE: The inflation pressure shown in the inflation tables are minimum cold pressures for the various loads listed. Higher pressures should be used when required by the above speed/load table and/or when higher pressures are desirable to obtain improved operating performance. For speeds above 20 mph, the combined increases of A and B should not exceed 20 psi above the inflation specified for the maximum load of the tire. TdThe maximum load a.and inflation capacity of the wheel must not be exceeded. applications, tire manufacturers sometimes recommend air pressures higher than those indicated by the load /inflation tables.

Fig. 28