[Care and Use of Synthetic Rubber Tires] [From the U.S. Government Publishing Office, www.gpo.gov] Care and Use of Synthetic Rubber Tires OFFICE OF DEFENSE TRANSPORTATION Washington 25, D. C. "Uncle Sam, Now Fighting Two Wars, Cannot Afford the Loss of Use of a Single Car or Truck Because of Damaged Tires.” J. M. Johnson Director, Office of Defense Transportation TABLE OF CONTENTS Foreword ..._________________________________________________________________________________,_____ 3 The Heavy-Duty Tire Problem ______________________________________._____'__________________________,_ 4 How to Identify Synthetic Rubbers................„.......................................... 4 Synthetic Rubber Casings and "War Tires” _________________________'._______________________________ 4 Casing Cord Materials __________________________________________________:...................... 5 Characteristics of Synthetic Rubber Tires: Heat ___________________________________________________________:__T______________ 5 Cuts and Tears ________________________...._______________________________________ 5 Relation of Load to Tire Heat: Government Test Fleet.............................................................. 5 Tire Manufacturers’ Tests «____________________________________________________:... 6 Synthetic Rubber Tubes _________________;______________________________________________... ..— .___ 6 Good Operating Practices: Breaking in New Tires .................... .’.............I_____________________ 7 Mating of Dual Tires ................................. ....;____________......_ 7 Tire Mating Tolerances ___________________________________________________________ 8 Trailer Axles ___________________________________________a______:................... 9 Rims _________________________...........................*..................... 9 Comparative Air-Loss System for Checking Tires For Punctures ...................... 10 How to Check Tires by the Comparative Air-Loss System ____________________________ 11 Running Gear Conditions_____________________„ .............................. 11 Recapping _________________________---------,—...................................... 11 Tread Cut Repairs ___________________........................ :_________________.._ 12 Mounting of Repaired Casings ........ ....___________________________________________ 12 Flaps_______...____________________—______________________________________________12 Casing, Tube and Flap Lubricants _______________________________*__________________ 13 Mounting Tires on Flat Base Rims _________________________________________________ 13 Mounting Tires on Drop Center Rims _______________________________________________ 13 Valve Caps _______________________________________________________________________ 13 Tire Valve Stems __________________.......__________'_____________________________15 Spare Tires on Vehicles...............................:____________.______________ 15 Tandem Rear-Drive Axles ..........................................................L— 15 General Operating Suggestions---------------------------------------------------------------------- 15 Tire Fires------------------------------------------------------------------------ 16 Published Information ________________________________4-------------------------------------------- 16 2 OFFICE OF DEFENSE TRANSPORTATION HIGHWAY TRANSPORT DEPARTMENT THE CARE AND USE OF SYNTHETIC RUBBER TIRES FOREWORD The prevention of damage to tire casings is the most important problem heavy-duty tire users face at the present time. This is as true of natural rubber tires as of new synthetic rubber tires; for if the tire casing is maintained intact, long tread mileages can be obtairied, but if the casing is damaged or prematurely worn out, all of the tire is lost. Before the war, extravagant misuse of truck tires was economically feasible. Today, when facilities for the manufacture of tires for civilian use are limited, it is necessary to reduce tire casing damage to a minimum in order that an average of more than one recap per casing may be obtained. Casings consistently run loaded above their rated capacity can, of course, seldom be recapped because overloading damages the cords and casing assembly. In this Bulletin, tire use and maintenance practices which have the greatest effect on tire casing life are discussed. Current information about the peculiarities of synthetic rubber compounds now used in tire and tube manufacture is also included. From this information about the nature of the synthetic materials now being used, a truck or bus owner, operator or tire maintenance man can gain a better understanding of the limitations of synthetic rubbers and hence can do a better job of saving available tire casings. For the sake of brevity, factors which influence only tread wear are not discussed in this Bulletin. We refer those who are interested in these items to the SAE-ODT maintenance methods booklet, "Steering Maintenance” and the many publications and excellent training material offered by tire manufacturers and others. Save the casing and you save the tire. W—J. Cumming, Chief Maintenance Section Highway Transport Department Automotive Materials Officer 3 SYNTHETIC RUBBER TIRES—THEIR CARE AND USE The Heavy-Duty Tire Problem Admittedly, synthetic rubbers at their present stage of development do not have all the desirable and valuable characteristics of natural rubber for use in heavy-duty truck and bus tires. Yet, of necessity, synthetic rubbers must continue to be used increasingly from now on until after the war. Furthermore, because synthetic rubber tire casings are more difficult to manufacture and owing to greater military demands, heavy-duty tires will be scarce for months to come. This production shortage makes it imperative for bus and truck operators to find ways to prevent the premature destruction of their new synthetic rubber tires as well as the natural rubber tires now mounted and in use. In order to obtain satisfactory service from synthetic rubber, heavy-duty tires, truck and bus operators must learn to recognize and accept the differences and deficiencies of synthetic rubbers. For example, they must face the fact that in hot weather synthetic rubber tires will not carry loads equal to those carried by natural rubber ones of the same size. Operators who report that new synthetic rubber casings have failed after a relatively short period must accept criticism rather than sympathy, for they have probably abused their valuable tires. In this Bulletin, the deficiencies of synthetic rubber are discussed together with such tire maintenance practices as are essential for the successful use of synthetic rubber tires. How to Identify Synthetic Rubbers The synthetic rubbers used in tire casings and tube manufacture are identified by code letters and colors. Where the code letters cannot be molded into the product, a band, dot or other mark of colored rubber is molded into the product to identify the type of synthetic. Type of synthetic Government rubber symbol Code letter Code color Buna S Butyl Neoprene GR-S GR-I GR-M S I ... M Red Light Blue Yellow Synthetic Rubber Casings and "War Tires" Synthetic Rubber Casings.—Synthetic rubber tire casings are maked with code markings to indicate the general type of construction and compounds used. To identify the type of synthetic rubber used, a colored dot, approx imately 1" in diameter or 1" square, is vulcanized in each sidewall of the tire. The code colors of the dots correspond to the code schedule for synthetic rubbers. In addition, in each tire sidewall are molded code numbers in accordance with the code given below which identify the type of casing construction and compounds used. These code figures and letters are sometimes superimposed on the colored dot. (See WPB Order R—1 for complete details.) In the code schedule below, the reference to the type of rubber in the casings refers to the friction (cementing rubber) compound, which is used to bind the plies of cords together to make the casing. For example, in the S-3 casing, 100% GR-S synthetic is used as the friction compound between and through the plies, whereas, in the S-5 tire, natural rubber is used as the friction compound. This means that the friction and cord materials in the S-5 casing are of the same quality as those in prewar tire. The values of the other code markings indicate the various mixtures of natural and synthetic rubbers which are permitted. S-3 100% GR-S tread on 100% GR-S casing. S-4 90% GR-S and 10% natural rubber distributed throughout the casing as the tire manufacturer desires. S-5 100% GR-S tread on natural rubber casing. S-6 70% GR-S and 30% natural rubber distributed throughout the casing as the tire manufacturer desires. S-7 35% GR-S and 65% natural rubber distributed throughout the casing as the tire manufacturer desires. S-8 93% GR-S and 7% natural rubber distributed throughout the casing as the tire manufacturer desires. Synthetic heavy-duty tires are generally made with rayon cord construction but the casing material (can be identified as indicated in the section on casing cord material below. "War Tires."—A large number of "War Tires” are still in use, although the manufacture of this type of tire is now restricted to special services. These war tires contain no synthetic rubber, but are made with various mixtures of natural crude rubber and reclaimed natural rubber. War tires may be made with either cotton or rayon cord and the casing fabric material can be identified as indicated in the section on "Casing Cord Material.” War tires are marked near the serial number with the phrase, "War Tires” and letters designating the rubber compounds. The meaning of the code letters AA, AB, etc. is explained below. The first letter in the code marking refers to the friction compound between the plies and the second letter refers to the composition of the tread. "A” compounds are approximately equal to prewar quality. 4 Crude rubber Reclaim rubber V ulcanizing compounds Friction Compound A 83.5—88.5% 0-10.7% 5.8—16.5% Friction Compound B : 73.0-78.0% 2-21.2% 5.8-25.0% Tread Compound A 71.0-73.0% O- 4.0% 25.0-29.0% Tread Compound B 57.5-59.5% 7.5-17.1% 25.4-35.0% Casing Cord Materials The materials used in making the cord fabrics of casings are the same as those used in prewar tires, namely cotton or rayon. Most of the large heavy-duty tires which are used in over-the-road service are now made of rayon cord construction. Rayon cord casings are usually marked with either the word "rayon” or the letter "R” molded into the outer sidewalls of the casing. Tires not otherwise marked are usually of cotton casing construction. The tire vendor should be consulted in regard to specific trade markings. Characteristics of Synthetic Rubber Tires Heat.—Heat from any source is the chief factor to be considered in the use of synthetic rubber tires, as heat is particularly destructive to them. As the temperature of the tire is increased, the capacity of the synthetic rubber tire to carry a load is decreased. A tire company representative recently made the following pertinent comment about tire temperatures: ' “In regard to operating temperature, a tire is somewhat like a man. If a man’s physical temperature remains normal at 98.6 degrees, he may live 80 years. If it goes up to 102 degrees, he may last 80 days. If it goes up to 104 degrees, he may last only 80 hours. “With tires, the critical operating temperature is about 240 degrees at the hottest place in the tire. If load, speed and inflation are properly maintained and operating temperature is kept below 225 degrees to 230 degrees, the tire will probably wear out without failing—barring other mechanical damage. If the load or speed increases the temperature to 250 degrees, the tire may blow out in a few hundred miles. Tires do not live long with a high fever! "Remember that when you get close to the critical temperature, an increase of 10 percent in the load, or an increase of five miles per hour in speed may reduce the tire life to 2 percent of normal.” The temperature of a tire is affected by the load carried, running speed, the temperature of the pavement, the atmospheric temperature and inflation pressure, especially if tire is under-inflated. Load or speed, or both, may have to be reduced to compensate for higher temperatures of pavement and atmosphere. Atmospheric temperatures have a direct effect upon the load and speed capacities of tires, because higher atmospheric temperatures raise the running temperatures of tires. Bright, direct sunlight may raise pavement temperatures to such a degree that the running temperatures of tires will be still further increased. Any dangerous increase in tire temperature calls for reduction in load or speed, or both, in order that the tire casing temperature may be kept below the casing failure point. When atmospheric temperatures are low (below 50° F.), synthetic rubber tires will carry loads above Tire and Rim Association ratings to about the same extent as natural rubber tires. However, overloading above these ratings is not recommended. When atmospheric temperatures reach 75° to 80° F., it is necessary to reduce both speeds and loads to Tire and Rim Association Ratings. At atmospheric temperatures of 105° F. and above, synthetic rubber tires will not carry even their rated capacity at usual speeds. Thus, if the tire casings fail before an average of at least one recap, it is direct evidence of overloading in relation to heat and speed, or evidence of other abuses. Cuts and Tears.—Synthetic rubber in tread stock is not as resistant to cuts and tears as natural rubber, especially when the tread is hot. For this reason synthetic tires require more frequent inspection for cuts so that repairs can be made before serious damage is done to the casing. Due to the lower resistance of hot synthetic rubber, running over curbs or encountering sharp obstacles may tear chunks out of the tread or may cause the tread and casing to separate at the point of impact. Figure 1. The adhesive properties of synthetic rubber when hot are below those of natural rubber, so that tread cracks or 5 ply separations may develop more easily. Running tires on and off the edge of a highway may also cause damage, as the load may be localized on one section of the tread and casing. Drivers should avoid sharp obstacles in the road or holes in the pavement because the resistance of synthetic rubber to such abuse is lower. Diagonal breaks and "X” breaks on the interior plies are definite evidence of impacts at too fast speeds or at excessive casing temperatures. When travel over stretches of rough road is necessary, vehicle speed should be greatly reduced so that the impact on the casing may be as slight as possible. The Relation of Load to Tire Heat Government Test Fleet.—In a release dated August H, 1944, the Office of Rubber Director, War Production Board, reported the results of tests made to establish the realtion of load to tire heat. The tests were made by the government tire test fleet operating at San Antonio, Texas. Three sizes of heavy-duty tires (7.00 x 20, 10 ply; 9.00 x 20, 10 ply; 11.00 x 20, 12 ply) mounted on rear drive wheels were tested at loads of 130% of Tire and Rim Association rated capacities. The road tests indicated that the average tire casing durability or life, with a 30% overload, was only 33.3% of the average durability obtained from tires loaded only 100% of Tire and Rim Association ratings. Casing durability was determined by blowouts, separations and other failures which ended the service life of the casing. Tire Manufacturer's Tests.—A large tire manufacturer reports the following findings based on tests made to determine the relation between load capacities and greater temperature build-up in synthetic rubber as compared with natural rubber. "A synthetic rubber truck tire of the S-6 rayon type operates at a higher temperature than the same size tire made of 100% natural rubber, under the same load, speed, pavement temperature and atmospheric temperature. The rated load of a 9.00-20 tire is 3450 pounds, and at this loading a tire made of synthetic rubber will operate ab 15-16° F., higher temperatures than a tire made of natural rubber. "In certain tests, under certain operating conditions, it has been established that the critical operating temperature of a 9.00-20 tire of rayon construction is reached at 230° F. With the 100% natural rubber, rayon tire, this temperature, 230° F., was reached by 9.00-20 tires traveling constantly at 40 miles an hour and carrying a load of 4200 pounds. With the S-6 rayon tire, under the same operating conditions, the temperature of 230° F., is reached with a load of 3600 pounds per tire.” These findings hold also for other tire sizes and indicate clearly that synthetic rubber tires build up a relatively greater temperature. Premature blowouts and many other casing failures can be traced directly to this greater internal heat build-up in the synthetic rubber casing. The only remedy at present available to the truck or bus operator is to reduce his loads or speeds to such a point that, under the actual temperatures of pavement and surrounding air, the temperature of the tire casing will not reach the temperature at which the casing will fail. Due to the fact that synthetic rubber tires run, hotter than natural rubber tires and because heat is more destructive to synthetic rubber tires ,the higher temperatures of spring, summer and fall call for greater vigilance, and reduced loads and speeds during the hot weather. Synthetic Rubber Tubes Two types of synthetic rubber are being used in the manufacture of inner tubes. The different synthetics are identified by a color band at least wide applied on the base section of the tube. The identification color code is: GR-S Synthetic (Buna S)—red GR-I Synthetic (Butyl)—light blue The characteristics of GR-3 rubber tubes are considerably different from those of natural rubber tubes. The GR-S rubber tubes are more susceptible to thinning and splitting and will tear and break at folds much more easily than tubes made of natural rubber. Simple cold patch and simple one side vulcanizing repair jobs are not satisfactory on GR-3 synthetic rubber tubes. Tears in a tube must be trimmed and "buttonholed” at the ends to stop further tearing. The details of methods of repairing synthetic rubber tubes have been outlined in tire manufacturers’ instructions and should be followed carefully in order to obtain safe and airtight repairs. Due to the weakness of synthetic rubber tubes, folds, sharp bends or sharp edges in the casing or flap tend to cause breaks quickly in GR-S rubber tubes. For this reason, extreme care must be taken in mounting a synthetic rubber tube to see that the tube has no folds and is very smoothly laid into the tire casing. Mounting procedures outlined under the sections on mounting on rims should be carefully followed. To facilitate the adjustment of a synthetic tube in the casing so that there will be a minimum of stress or irregular stretching in any part of the tube, the tube should be lubricated with a soap solution. Further information about this lubricant is given on page 6. The synthetic rubber tubes will take a "set” insofar as size is concerned very much more rapidly than do natural rubber tubes. Tubes are normally made somewhat smaller than the inside of the casing in order to make installation easier and to eliminate the possibilities of folds in the tube. Natural rubber tubes will retain their resilience and contract when deflated for a much longer time than will synthetic rubber tubes. All tire casings increase in size with use. When a synthetic tube has been used with an old or stretched casing, the synthetic tube will assume the oversize of the old tire. Then, if an attempt is made to use this old oversize tube in a casing with less "growth” it may be difficult to install the tube without folds. In fleet operation, tubes are frequently shifted from tire to tire and the "set” characteristics of the synthetic tubes may cause trouble as the tubes 6 grow older. To help solve this problem, it is suggested that the date upon which the tube is put into service be marked on the tube near the valve stem with an indelible pencil. In this way; some record can be kept of the age of the tube. At the time of mounting tires, the tire man may thus be able to make some selection so as to match old tubes with old tires. GR-S tubes tend to crack in the folds if stored in cartons for long periods in warm temperatures. It is recommended that the tube cartons be dated as received in order that the oldest tubes may be used first. For fleet operators it is recommended that new tubes be removed from the cartons and stored in new tires as soon as new tires are received. The tubes should be lightly inflated when placed in the tire. Used spare tubes should be lightly inflated and stored in dark places away from oily materials. GR-I tubes are for the most part being supplied for the larger truck and bus tires. Instructions for repairs of GR-I tubes are easier and may be obtained from manufacturers. Good Operating Practices Breaking in New Tires.—To obtain the best service results, tires, like many other mechanical devices, should be' "broken in” under light work. A light load permits the casing to stretch or "grow” slowly and allows the tread better to follow the growth of the casing without cracking, before the tread is hardened by the higher temperatures produced under full load. On many vehicles the most lightly loaded tire positions are on the front wheels. Where this is the case, new tires should be operated for at least 5,000 miles on the front wheels. On some vehicles in a fleet it may be found that all wheel positions are loaded to the maximum capacity of the tires; however, in these fleets there may be vehicles which by reason of their normal route and loads are always more lightly loaded. The new tires should always • be broken in on the most lightly loaded vehicles in the fleet and then should be rotated to the more heavily loaded vehicles after the break-in period. To determine the lightest loaded wheel position, the individual wheels of a number of loaded trucks should be weighed. This can be done either by using a "loadmeter”, which can be borrowed from a number of tire companies, or the wheels can be weighed on a conventional truck platform scale by running only one wheel of the loaded truck on the platform at a time. In heavy-duty operations where recapped tires have frequently failed within short mileages after recapping, it may be necessary to break in the new recaps on lightly loaded wheel positions. This practice will limber up the new recap tread and somewhat wear down the recap thickness before the recapped casing is placed on a heavily loaded wheel position. All tire casings "grow” with use. This growth depends on a number of variable conditions such as temperature, degree of flexing, rate of flexing inflation pressure, etc. The over-all size of a tire also depends on the degree of tread wear. After the initial “break-in” period, the chief reason for rotating truck and bus tires is the maintenance of proper mating between tires on dual wheels. On trucks and buses, therefore, tires need be rotated to and from various wheel positions only for the purpose of "breaking-in” -new tires and to mate dual tires properly on an actual size basis. 'If new tires are “broken-in” on the front wheels, they need be removed from these wheels only when it is necessary to replace tires on the more heavily loaded wheel positions. In the case of dual tires, it is generally found that the tread of the outside tire wears out faster than that of the inner tire, and that the inner tire usually carries more of the total load than the outer tire because of road crown. This means that on mated tires -the larger tire should always be in the outside position; tires should be rotated as necessary to maintain this condition. It is desirable to have the size of dual-mated tires as nearly identical as possible, but in truck operation this is seldom possible. However, if it is always arranged so that the smaller tire is in the inside position the optimum relation will be maintained. Mating of Dual Tires.—Several ways in which the actual size of the tire can be measured for mating purposes are given below: 1. The actual circumference of the center of «the tread of a fully inflated tire can be measured by a conventional measuring tape or by a tape which can be shop-made of half-inch banding steel. This is the most accurate method of measuring tires for mating. (See Figure 2.) Figure 2.—Home-made tape for measuring tread circumference. _ 2. A large pair of woo’den calipers which can be slipped over two dual-mounted tires can be used to show the differences in the diameters of the two mated tires. These calipers can also be used to measure the diameter of other tires for the purpose of selecting mates. (See Figure 3.) 3. Instead of the caliper, a carpenter’s square, large enough to extend across the side diameter of one tire and across the treads of both tires, can be used to compare the dual tires on one wheel. By keeping one leg of the 7 square tight against the sidewalls and bringing the other leg against the tread of the highest tire, the differences in size of the two tires and their relative position on the wheel can be observed. 4. A simple variation of the 3-point spherometer has recently been suggested for measuring the diameters of mated tires. (See Figure 4.). This gauge is not as accurate as the measuring tape described above, but if it is used with care on smooth, uniformly worn tires or if, on unevenly worn tires, three or four readings on different sec- Figure 4.—Tire-matching stick. tions of the center of the tread are made, the gauge is sufficiently accurate for mating purposes and is a convenient tool. Readings to the nearest %'inch diameter are possible on the calibrated scale. 5. A collapsible mating gauge can be made of a heavy cord furnished with a coil spring in the center and a wire hook on each end. In using this string gauge, the hook at one end is hooked around the spoke or lug of one wheel and the string is carried around all four tires on a dual-tire axle at center-of-hub height and the second hook fastened into a spoke on the opposite wheel. This string by its contact or clearance with the tire treads will indicate the differences in mating tires. (See Figure 5.). Figure 5.—String gauge for checking tire mating. 6. A wooden straight edge which will lie across the top of the treads of all four tires can be used to observe the differences in size of two mated tires and the position of the larger tires. The straight edge should be placed at center-of-hub height in use to eliminate effects of camber on some trailer axles. (See Figure 6.). New tires or remounted used tires should be measured with a tape after being mounted and inflated on rims or wheels for installation on a truck. The tape dimensions of the center of the tread should be marked on the tire with crayon so that by comparing taped dimensions tires can be easily and correctly mated. The caliper and spherometer type of mating gauge are convenient tools to use in measuring actual tread diameters of tires mounted on vehicles when tires are to be selected for rotation from one vehicle to another in order to obtain optimum mating. The string gauge, the straight edge and large square are useful as tools for quick inspection of mated tires to determine whether the inner tire is sufficiently smaller than the outer tire. The use of these inspection tools greatly assists proper mating of tires and eliminates such errors as occur from attempts to estimate tire sizes by looking at the depth of the skid tread or by sighting across the tops of two tires. Tire Mating Tolerances.—The inner of the two tires should not be larger than the outer one. The maximum differences which should be allowed between fully inflated inner and outer dual-mated tires are as follows: a. The center-of-tread circumference of the inner tire should not be more than iVs" less than the outer tire. b. The diameter at the center of the tread of the inner . tire should not be more than less than the diameter of the outer tire. 8 c. The radius at the center of the tread of the inner tire should not be more than 3/16" less than the radius of the outer tire. Whenever the size of the outer tire of dual tires is found to be less than the size of the inner tire, a re-mating of the tires should be made, even if it is only a reversal of the two tires on the same wheel. Mated tires should be checked and re-mated if necessary at least every 5,000 miles. Trailer Axles.—Trailer axles may bend or sag, and this sagging will place an excessive load on the inner tire. When tires are properly mated, greater wear on the inside shoulder of the tread of the inner tire usually indicates that the axle has sagged under excessive loading. Some trailer axles are manufactured with a camber to compensate for road crown. In the case of axles which have camber, the mating tolerances of the dual tires should be adjusted to compensate for the amount of built-in camber in order that the load between the inner and outer tires may be evenly balanced. Sagged axles should be straightened so as to balance the total wheel load between the mated tires. Rims.—Rims should not be undersize according to Tire and Rim Association standards. Oversize rims are being advocated by some authorities as it is believed that they give greater tire mileage and have apparent advantages; although if the rims are much oversize, the higher side rings may cause chafing just above the beads. Undersize rims are very harmful to synthetic tires as they cause flexing in the wrong place in the casing and increase the tendency towards tread cracks. Due to the deficiencies of synthetic rubber, the importance of correcting any bent or mutilated rims is much greater than in the case of natural rubber tires. Rims which cut into the sidewall of the tire are certain to develop failures above the beads. Rusty or badly pitted rims and side rings may cause chafing of synthetic tire beads. Rims which are so badly rusted in the bead base area as to be undersize may permit movement of the casing on the rim and result in chafing or other troubles. Some tires and rim manufacturers are now offering bead wedge rings which may assist in overcoming certain types of rimchafing. As protection against rusting, rims can be cleaned of rust and painted with a thin coating of a hard, quickdrying tough enamel, but the rims should not be coated with oily mixtures of graphite or other materials which are harmful to synthetic rubber. When rims are painted, the enamel must be thoroughly dried1 before tires are remounted. In large fleets, the loose side flanges may become interchanged on the rims. For example, a loose side flange for a 7" x 20" rim will apparently fit on a 9-101" x 20" rim. When such an interchange is made, the casing is supported differently on its two side walls by the two different size side flanges, and the deflection pattern cf the casing may be distorted. With synthetic tires, such distortion may reduce the casing life. Flange gauges are available for easy identification of flanges and rims. 9 Tire Inflation.—The recommended inflation pressures for synthetic rubber tires are, at the present time, the same as those prescribed for natural rubber tires by the Tire and Rim Association. For specific cases and special situations, tire manufacturers may make other inflation recommendations. The Tire and Rim Association’s inflation pressures are standard for "cool” tires, i.e., tires with temperatures which are not above atmospheric temperatures. These inflation pressure standards are based on the assumption that air pressures will increase somewhat as tire temperatures normally rise during a run. The standards for cool tires are set to produce optimum deflection of tire casings under their rated loads for normal spread of operating temperatures. In practical truck operation, air pressures of hot or warm tires may register higher than the Tire and Rim Association’s standards for cool tires because of tire heat. This pressure-temperature relation of air in a tire creates a real problem in the correct checking of tire pressures. If a truck or trailer has been used to make city deliveries for as short a distance as 5 or 10 miles just before being checked for a road run, the temperature of the tire may be ^any degrees above the "cool” standard. In such instances, checking the warm tires by "cool” standards will result in under-inflation. On the other hand, if the tire pressure is checked at a road stop and found to be at a tire temperature above "cool”, the checking pressure standard should be increased in order to compensate for the increase in tire temperature. Warm tires should never be "bled” down to the "cool” tire inflation standards. "Bleeding” the air pressure from warm tires results in the tires being under-inflated when they again become cool. It has been found that hot tires require about two hours to become thoroughly cool to atmospheric temperatures. From data based on tests made on heavy-duty truck tires, it has been found that for each 5.0° F. to 8.5° F. rise in tire tread temperatures, the inflation pressure of a tire increases 1.0 lb. per square inch. For example, a tread temperature rise of 70° F. to 170° F. will cause a pressure increase of between 12 and 20 lbs. per square inch. By using these pressure-temperature values, an approximate operating temperature of a tire on the road under load can be estimated by noting the amount of pressure increase above the initial cool inflation pressure. In operations where brake drum temperatures run high, temperatures may be developed in the base of the tire from heat radiation from the brake drum. Excessive heat from this source will also increase inflation pressures and will increase the possibilities of bead failure and tube failure. / Synthetic rubber casings may have diffe^nt growth characteristics than .natural rubber casings, so that any additional stretching caused by over-inflation of tires may aggravate ply separation, tread cracking and cord breakage. For these reasons, initial over-inflation of cool synthetic tires, in the mistaken idea that a greater than rated load can then be carried, may result only in severe damage. Over-inflation when cool results in the localization of heat "build-up” in the center of the tread during operation, whereas, best casing life is obtained when heat "build-up” is distributed across the entire tread and when the casing heat is kept below the damage point by proper loading and operation. Under-inflation below "cool” inflation pressures for given loads, on the other hand, permits excessive flexing of tire casings. This condition, combined with the greater heat build-up resulting from excessive, flexing, will cause premature blowouts or failures of synthetic tires. Consistent over-inflation or under-inflation can sometimes be judged by observing the type of tread wear. Figures 7 and 8 show extreme tread wear ‘ caused by incorrect inflation. Figure 7.—Underinflation. Figure 8.—Overinflation. Heavy-duty tire air pressures should be checked daily when tires are cool. Tire gauges should be checked frequently for accuracy against master air pressure gauges. Comparative Air-Loss System for Checking Tires for Punctures.—Much can be learned about the presence of slow leaks or punctures in tires by checking the air pressure of each tire before inflating the tire. Experimental work on which this statement is based has revealed the following facts: (1) There is a constant slow loss of air from tubes due to the natural characteristics of rubber tubes. This constant air loss is considerably less in synthetic rubber tubes; (2) in experimental tests, in which nails were deliberately driven into tires and allowed to remain in the casing, it was found that rapid air losses usually did not occur immediately as a result of such punctures; (3) the air loss from tires and tubes, which have been punctured by nails and objects, which remain in the hole which they have made in the tube, will be somewhat greater than thé constant small loss from un- 10 punctured tubes. The air loss from such small punctures will increase slowly until "fatigue” breaks down the rubber at the edge of the hole and permits rapid loss of air. When this rapid loss occurs the tire will go flat quickly. By recording the air pressures which are found in "cool” tires at regular inflation periods and comparing the observed pressures with the previous inflation pressures and by comparing the pressure in each tire with the pressure of its running mates, indications can be observed which point to slow leaks or punctured tubes. Any truck tire, in which the observed pressure is more than 5 lbs. below the lowest of its running mates, can be assumed to have a slow leak or puncture which should be corrected to prevent a flat tire on the road. The simple form shown in Figure 9 can be used to keep a record of the air loss from each tire on a vehicle in order that excessive air loss from any tire may be noted. The secret of using the comparative air-loss system for checking the air tightness of tires lies in the regular use of valve caps which have rubber sealing washers. It is impossible to maintain tire valves perfectly air-tight since, through wear and use, the tire valves may develop leaks. It should be remembered that a valve cap, which has its rubber, air-sealing washer in place, and which is screwed on tight, will absolutely prevent any loss of air at the valve. Therefore, if the valve cap is on tight and excessive air loss does not occur on any one tire, it is an immediate warning of the existence of a puncture in that tire. How to Check Your Tires by the Comparative AirLoss System.—1. Make certain that the valve caps have been screwed on finger-tight and that each cap has an airsealing rubber washer. 2. Before adding air, test and record the pressure in each tire. (See sample record card, Figure 9.) 3. After all inflation pressures in the tires on the vehicle have been tested and recorded, check the variations in the pressures. Slight differences in pressure will always be found, but, assuming that tires were correctly inflated at previous inflation, the following differences indicate the existence of puntures or other causes of excessive air loss: a. Any passenger-car tire that is 3 lbs. or more below the lowest of its running mate can be suspected of having a puncture. b. Any truck or bus tire that is 5 lbs. or more below the lowest of its running mate can be suspected of having a puncture. (Note: Check tires suspected of having punctures by dismounting the tire and testing the tube. Always check the inside of casing for foreign objects. 4. Inflate each tire to its specified air pressure. 5. Screw on valve caps finger-tight and make sure that the rubber washer in the cap is not damaged or missing. Running Gear Conditions. — Misalignment, loose ” wheel bearings, unbalanced wheels and all mechanical defects which cause tire wear need particularly careful attention when synthetic rubber tires are. used. Many running defects, especially misalignment, cause additional heating of the tire and such defects are, therefore, especially to be avoided. Recapping.—The Tire Inspector’s Manual No. R—57, issued by the Office of Price Administration, states: "The point at .which a tire can be most economically recapped is when the tire is worn smooth approximately % of the VEHICLE No. TIRE PRESSURE RECORD Comparative Air Loss System DATE CHECKED FRONT 1«t REAR AXLE 2nd REAR AXLE SPARK RIGHT FRONT LEFT FRONT RIGHT REAR LBPT REAR RIGHT REAR LEFT REAR OUT SIDE IN St GE OUT SIDE IN SIDE OUT SIDE IN SIDE OUT SIDE IN SIDE 3/^j/'44 6« 7° b9 ¿6 @ ‘7 70 6? ¿6 69 * Figure 9.—Sample record card. Note the variation of 4 pounds between all tires except one (circled) which is 5 pounds below the lowest of the others. This immediately warns of trouble in that tire. If a valve cap with a rubber sealing washer had been on tight, the leak will be found in the tube. Remove tube and tire for examination and repair. 11 Figure 10.—Time to Recap. total width of the tread, and not beyond the point where there still remains y^' of tread rubber above the breaker strip or outside ply of cord.” (See Figure 10.) Tires worn to an even greater extent are recappable and instructions covering the other wear limits are outlined in the O.P.A. Manual No. R—57 and supplements. (See Figure 11.) Tread Cut Repairs.—To assist in preserving tire casings for recapping, tread cuts and damage should be carefully watched and any cuts which permit water to enter the casing fabric should be repaired. At regular and fairly frequent intervals, small cuts or openings in the tire tread should be probed for foreign objects, which should be removed. A blunt icepick or a narrow screw driver can be used as a probe. Care should be used in probing to avoid further damage to the casing. This thread cut inspection can conveniently be done at the time the vehicle is greased. After any foreign objects have been removed from the tire treads, the tires should not be operated for at least an hour, because if an object has pierced the inner tube, its removal results in pulling the plug out of the hole, thereby changing a slow leak to a fast leak. A comparative air-loss check should be made just before the vehicle is returned to service to locate any such rapid air leaks. Mounting of Repaired Casings.—Repaired casings should be mounted so that the repaired section is located in as protected a position as possible. Whenever possible, they should be mounted on vehicles which carry lighter loads or operate at slower speeds. The most protected position for a repaired section of a casing is on the inner side of the outside tire of dual tires. Newly repaired tires should be mounted in this position when first returned to use. When necessary to rotate a repaired tire to the inner dual-tire position, the tire should be mounted so that the repaired section is located on the outer side of the inner dual tire. In other words, the tires should be mounted so that the repairs are to the center of the dual tires. Flaps.—Flaps can be used so long as they are in good condition. There should be no cracks in the flap or in the flap edges, nor should there be any folds or wrinkles in the edges. The edge and surface of the flap should be as smooth as possible in order to lie smoothly against the sidewall of the casing. Doubtful flaps should be discarded, as there is nothing to be gained from taking a chance with a flap and suffering a road failure which may result in the loss of both casing and tube. New tire flaps are being made of GR-S rubber and can be identified by a colored stripe at least wide on either the rim or tube side. The color code is the same as the one listed for synthetic rubbers. The resiliency and hardness of GR-S flaps is different from natural rubber flaps so that cracking may be experienced if the flap is of incorrect size or is mounted so that tension is exerted on the flap. The correct size and style of flap for each type, of rim must always be used. To permit flap readjustment in the casing during mounting and inflation, both sides of the flap should be thoroughly lubricated with a solution of vegetable oil soap. (See Section on "Tube and Flap Lubricants” for additional information.) If the flap sticks to the side walls of the •casing during the first inflation, the flap may be cracked by the resultant stretching. Such cracks are frequently the cause of early tube failure. Figure 11.—Past best recapping point. * 1 12 When casings are mounted on wide base rims, the correct flap for the wide rim must be used: Used flaps tend to "set” and crack if stored carelessly for long periods. When storing on a rack, place the flap in a straight, smooth position and change frequently to avoid "set” and cracks. Casing, Tube and Flap Lubricants.—Tire manufacturers have found that a thin coating of a vegetable oil soap solution will provide best lubrication for synthetic rubber parts. The soap solution can be painted or sponged on all areas of contact so that they are completely covered and lubricated with a thin film of soap. The soaps used must be alkali-free and free of fillers which might be abrasive. Most laundry soaps are not suitable; liquid vegetable oil soaps, or solid vegetable oil soaps, such as castile, coconut oil or similar face soaps, can be used. The soap solution should feel slippery and should be liquid enough so that it can be applied easily in a thin, uniform coating. The amount of soap required for a satisfactory solution will depend upon the type of soap used and the hardness of the water. The important point is to have a soap solution which gives a slippery film on the flap and inside of the casing above and near the tire beads. As an indication of the differences between types of soap and waters, one tire manufacturer suggests 3-4 ounces of soap per gallon of water, while a second tire manufacturer suggests a 10% solution or 13-14 ounces of soap per gallon of water. Depending upon the hardness of water, the amount of soap used should be somewhere between the amounts mentioned above. The soap should be thoroughly dissolved and mixed in the water so that there are no lumps. This mixing is especially necessary when solid soaps are used. The soap solutions will keep for several days after which they may become rancid and should be discarded. The soap mixture should be stirred each time before use in order to prevent settling. Recently, there have been offered on the market ready-mixed proprietary mixtures for use in lubricating synthetic rubber casings and tubes. These ready-mixed materials have some conveniences and advantages in actual use. In general, the satisfactory products have a stable vegetable oil soap base with an anti-freeze ingredient, a wetting agent and a rust inhibitor added. Each specific product, however, should be checked by the purchaser to determine that it contains no materials which are deleterious to synthetic rubber or steel rims. Powdered talc, soapstone or mica have been commonly used in tire work and can be obtained from the usual trade jobbers. These materials are not the best synthetic rubber lubricants and should be used only when soap solutions are not available. In using these dry lubricants, a uniform layer should be applied to the contact surfaces, but care should be used to avoid lump accumulations of the powders in other parts of the casing. Mounting Tires on Flat Base Rims.—1. Clean rust, dirt and oil from the rim, side flanges and locking rings. ' 2. Lubricate the inside of the casing at the beads and to about halfway up the side of the casing. It is preferable to do this by means of a thin coating of vegetable oil soap solution which can be applied with a brush or sponge. Just thoroughly coat the surfaces, but do not apply so much soap solution that the excess runs further into the casing. (See Section on "Casing, Tube and Flap Lubricants”). 3. Lubricate both sides of the flap with a thin layer of vegetable oil soap solution. 4. Inflate the tube until it is barely rounded out and lubricate the inner half of the tube with a soap solution. 5. Insert tube in casing and smooth out carefully to eliminate folds or wrinkles. (This may be difficult with old tubes which have stretched and "set”, but it is folly to mount a GR-S tube with a fold or wrinkle as rapid tube failure can then be expected.) 6. Insert the flap in the casing and smooth out carefully so that there are no folds or wrinkles which may damage either the flap or the tube. See that flap is properly centered between the tire beads. 7. Mount the casing, tube and flap on the rim and install side flanges and rings in the usual manner. 8. Center valve and pull it firmly against the rim. Hold valve in this position and inflate tire to recommended pressure to insure that tire beads and loose side flanges are firmly seated in place. 9- Next, deflate tube completely to permit any stretched portions of tube or flap to readjust themselves in the casing and relieve any strains in the tube or flap. Do not disturb tire beads or rim flanges during this deflation. (See Figure 12.) 10. Reinflate the tube and casing to specified operating pressure and screw on valve cap finger-tight. Mounting Tires on Drop Center Rims.—In preparing to mount tires on drop center rims, the tube and casing lubrications and installation methods outlined for flat base rims should be followed. After the casing assembly has been placed on the drop center rim, the casing assembly should be centered so that the tire bands are level with the bead ledges of the rims when the tube is first inflated. (See Figure 13.). This careful centering of the casing assembly will cause a minimum and evenly distributed amount of movement of the tube in the casing. Center the valve and pull it firmly against the rim. Hold valve in this postion and inflate tire to recommended pressure to insure that tire beads are firmly seated in place. Then deflate the tube completely to permit readjustment of the tube, keeping tire beads in place on rim ledges. Reinflate tire to operating pressure and screw on valve cap finger-tight. Valve Caps.—After a tire has been finally inflated, the valve cap should be installed finger-tight. Constant stress must be exerted in a campaign to keep tire valve caps tightly in place at all times. As a matter of fact, tests in- 13 TM£ WRONG WAV NOT LUBRICATED THE RIGHT WAY PROPERLY LUBRICATED NO AIR PRESSURE Tube inflated Just enough to shape it after insertion in casing and before mounting on rim. With proper lubrication, initial air pressure does not seal edges of flap against tire wall but causes flap to slip downward toward rim. INITIAL AIR PRESSURE B>2 With proper lubrication, flap moves into correct position without strain or distortion. Mounted in this manner, flap will not tear or split in service FULL AIR PRESSURE (IN SERVICE) NOT SPLIT B3 INFLATE - DEFLATE - RE-IN FLATE! Figure 12.—Mounting on flat base rim. 14 Figure 13.—Mounting on drop center rim. dicate that, after use, a certain percentage of valve cores leak; hence, the need for the sealing valve cap. (See Figure 14.) The valve caps should be of the type which have a rubber sealing washer in the top. (Some valve caps have been made and sold without this rubber sealing washer.) This rubber sealing washer may be found damaged in caps which have been in use for a considerable period of time. In any case, where the rubber sealing washer is missing or damaged, the valve cap should be discarded. Valve caps should be twisted on finger-tight. When this is done, the valve caps will not fall off in use nor will they leak. Tire Valve Stems.—Tire valve stems should always be bent so as to have ample clearance from the brake drum and the wheel spokes. When the valve stem rubs the brake drum, heat from the drum is carried along the valve stem directly into the tire and may cause a local heat failure of the tube. When a valve stem rests on the brake drum, it prevents the valve base from sitting squarely and firmly on the rim. This cocked position causes unusual stretches or strains in the tube rubber around the valve base which can develop slow leaks at this joint that are difficult to discover with conventional low-pressure testing. Also, the rubbing of the stem on the drum may wear a hole through the valve stem. Spare Tires on Vehicles.—Synthetic rubber tends to "sun check” badly and these "sun checks’’ will act as focal points for the start of later cracks. If spare tires are carried on vehicles, they should be protected against sun and weather. Regular black tire paint (asphaltic base type) can be painted on the tire and a canvas cover can be used to protect the rubber from exposure to sun and weather. Good spare tires should be rotated onto running positions frequently to reduce the effects of sun and weather checks. Tandem Rear-Drive Axles.—Trucks with tandem rear-drive axles may present still other tire-mating problems. Some tandem rear-axle assemblies are equipped with differential gearing between the axle differentials which permit each wheel to rotate at a speed dependent upon the actual rolling radii of its tires. Conventional mating of dual tires is satisfactory on such equipment. Figure 14.—This rubber washer is the sealing unit. Inspect the valve cap and be sure this rubber wash is not damaged or missing. Other tafidem rear-drive axle assemblies are not equipped with inter-axle differentials, and on such assemblies there exists a definite need for very accurate mating of the rolling radii of the tires on the tandem wheels. On this type of equipment, the truck manufacturers and the tire manufacturers can give special recommendations regarding methods of mating tires. General Operating Suggestions It is obvious that the driver’s actions have a great effect on the useful life of a tire casing. For this reason, drivers should be informed of the peculiarities and deficiencies of synthetic rubber tires so that they, too, may do their part in preventing abuse. The speed and temperature limitations of synthetic tires need to be constantly emphasized. The driver of a truck or bus has final control over tire damage from rough roads, running over curbs, running at speeds which develop dangerous tire heats and other abuses. In order to obtain the best possible heavy-duty service from synthetic tires, drivers must be encouraged by all possible means to accept their share of the responsibility. A few operating companies have instituted periodic tire cooling stops at mileages of between 45 and 75 miles. Each cooling stops need to be sufficiently long to permit a considerable degree of cooling if they are to be effective. One company finds that a 45-minute stop is necessary in order to obtain a noticeable amount of cooling. While such cooling stops may be worthwhile hot weather expedients, still the need for such cooling stops is more likely evidence that speed or load have been incorrect for the prevailing weather conditions. As long as casing temperatures are kept well below the danger point by correct loading, speed, mating and inflation, there is little to be gained in brief tire cooling stops. Even loading of a truck or trailer so that the weight is evenly distributed to all load-carrying wheels is a "must”. In doubtful cases, the individual wheel loads should be checked. Checking the total weight on an axle is not sufficient, as this will not check bad ioad distribution across an axle which may be causing a severe overload on one wheel and its tires. The rated carrying capacity of highway tires can be learned from the capacity tables prepared by the Tire and Rim Association and various tire manufacturers. 15 Loading of a truck is usually a responsibility of the dispatcher or dock foreman; hence, these representatives of management must also be included in any tire conservation program. Tire Fires.—Tire fires are always a serious threat to vehicle and cargo. Drivers should be thoroughly instructed concerning signs and causes of tire fires and proper methods of extinguishing them. Tire fires are definite evidence of bad operating practices and may be the result of individual wheel overloads, overspeeding, incorrect inflation, incorrect tire spacing, excessive use of brakes, continued running with one flat tire or other poor operating practices. After a tire catches fire, it has been found that the cooling effect of large quantities of water is more effective than the smothering effect of a small fire extinguisher. If water is not available, cover the flaming tire with a tarpau lin, blanket, coat, sand or dirt to put out the flame and remove tire from vehicle as soon as possible. Never leave a tire, which has been afire, until the tire has become cool; otherwise the fire may start again. Published Information All tire companies have published information regarding tire repair, and this material can be used in further educating the maintenance force and the drivers as to the care needed for the best use of synthetic rubber tires and tubes. ODT pamphlet “How to Prevent Roadside Flat Tires” describes the significance of pressure loss in tires and the need for valve caps in preventing flat tires on the road. SAE-ODT Maintenance Methods Booklet, "Steering Maintenance” describes the maintenance of vehicle parts which affect tire tread wear. Care and Use of Synthetic Rubber Tires, M.B. 41, Price 10 cents. Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C. ft U. S. GOVERNMENT PRINTING OFFICE: 1945-638461 16