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 pres- sure 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 ap- plies 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. Issued 10/2003 © 2008—TMC/ATA Proposed RP 235A (T) Ballot Version — 1 Revised x/xxxx Shoulder Wear/ Shoulder Step Wear 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 tempera- ture, 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 monitor- ing 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 regu- lar 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 manu- facturers 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.