All About Pneumatic Tires

8/11/2019 All About Pneumatic Tires

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Tire pressure monitoring systems (TPMS) areelectronic systems that monitor the tirepressures on individual wheels on a vehicle,and alert the driver when the pressure goes

below a warning limit. There are several typesof designs to monitor tire pressure. Someactually measure the air pressure, and somemake indirect measurements, such as

gauging when the relative size of the tirechanges due to lower air pressure.


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TPMS can be divided into two different types:

The dTPMS or Direct Tire Pressure Monitoring

System and the

iTPMS or Indirect Tire Pressure MonitoringSystem


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Indirect TPMS do not use physical pressuresensors but measure air pressures bymonitoring individual wheel rotational speedsand other signals available outside of the tire

itself. First generation indirect tire pressuremonitoring systems utilize the effect that anunder-inflated tire has a slightly smallerdiameter (and hence lower tangential velocity)

than a correctly inflated one. Thesedifferences are measurable through the wheelspeed sensors of ABS/ESC systems.


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Direct TPMS employ pressure sensors on eachtire, either internal or external. The sensors

physically measure the tire pressure in each tireand report it to the vehicle's instrument clusteror a corresponding monitor. Some units alsomeasure and alert temperatures of the tire aswell.

These systems can identify under-inflation in anycombination, be it one tire or all, simultaneously.Although the systems vary in transmittingoptions, many TPMS products (both OEM andaftermarket solutions) can display real time tirepressures at each location monitored whether thevehicle is moving or parked. There are manydifferent solutions but all of them have to facethe problems of limited battery lifetime andexposure to tough environments.


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If the sensors are mounted on the outside ofthe wheel, which is the case for someaftermarket systems, they are in danger ofmechanical damage, aggressive fluids andother substances as well as theft. If they aremounted on the inside of the rim, they are nolonger easily accessible for service likebattery change.


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A direct TPMS sensor consists of thefollowing:

Pressure SensorRadio Frequency TransmitterLow Frequency ReceiverVoltage Regulator (for battery management)Monitor


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Most originally fitted dTPMS have the sensors

mounted on the inside of the rims and thebatteries are not exchangeable. With a batterychange then meaning that the whole sensorwill have to be replaced and the exchangebeing possible only with the tiresdismounted. To save energy and prolongbattery life, many dTPMS sensors do nottransmit information when not rotatingwhich also keeps the spare tire from beingmonitored.


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Tires are specified by the vehiclemanufacturer with a recommended inflationpressure, which permits safe operation withinthe specified load rating and vehicle loading.Most tires are stamped with a maximum

pressure rating. For passenger vehicles andlight trucks, the tires should be inflated towhat the vehicle manufacturer recommends,which is usually located on a decal just inside

the driver's door or in the vehicle ownershandbook.


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Many pressure gauges available at fuel stationshave been de-calibrated by manhandling and theeffect of time, and it is for this reason thatvehicle owners should keep a personal pressuregauge with them to validate the correct tirepressure.Tires are not completely impermeable to air, andso lose pressure over time naturally. Somedrivers inflate tires with nitrogen, instead ofatmospheric air, which is already 78% nitrogen, in

an attempt to keep the tires at the properinflation pressure longer, though theeffectiveness of this is debatable.


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Most tires are filled with compressed air,which when dry consists of about 78 percentnitrogen, 21 percent oxygen, and 1 percent

other gases by volume. Water vapour(humidity) can make up as much as 5 percentof the volume of air under worst-caseconditions.


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High performance and dynamic drivers oftenincrease the tire pressure to near themaximum pressure as printed on thesidewall. This is done to sacrifice comfort forperformance and safety. A tire at higherpressure is more inclined to keep its shapeduring any encounter, and will thus transmitthe forces of the road to the suspension,

rather than being damaged itself.


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Tires are specified by the manufacturer with a

maximum load rating. Loads exceeding therating can result in unsafe conditions that canlead to steering instability and even rupture.


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St d d L d T bl


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Standard Load Table


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Speed Rating


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Speed Rating


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The Treadwear rade of a tire is the numericportion of the Uniform Tire Quality Grade(UTQG) Standards that are printed on thesidewall of a tire. These standards were

enacted by the National Highway TrafficSafety Administration (NHTSA), which is partof the United States Department ofTransportation. Higher treadwear numbers

indicate that the tread of a tire, and hence thetire itself, should last longer.


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The wear on tires that are being tested("candidate tires") is compared to the wear ofCourse Monitoring Tires (CMT), which aresold by the NHTSA at its UTQG test facility inTexas. Both types of tires are mounted onvehicles that will be driven in a convoy duringthe test, thus ensuring that the candidate

tires and the CMT tires experience the sameroad conditions. The convoy, typically one offour or fewer vehicles, will drive 7200 mileson public roads in West Texas. Candidate tire

wear will be checked during and after thetest, and compared to the wear on the CMTtires from the same convoy.


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Tire rotation or rotating tires is the practiceof moving automobile wheels and tires fromone position on the car, to another, to ensureeven tire wear. Tire wear is uneven for anynumber of reasons. Even tire wear is desirableto maintain consistent performance in thevehicle and to extend the overall life of a setof tires.


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Turning the vehicle will cause uneven tirewear. Car manufacturers will recommend tirerotation frequency and pattern. Depending onthe specifics of the vehicle, tire rotation maybe recommended every 8,000 km (5,000 mi).

The rotation pattern is typically moving theback wheels to the front, and the front to theback, but crossing them when moving to theback. If the tires are unidirectional, the

rotation can only be rotated front to back onthe same side of the vehicle to preserve therotational direction of the tires


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Tires that are fully worn can be re-manufactured to replace the worn tread. Thisis known as retreading or recapping, aprocess of buffing away the worn tread andapplying a new tread. Retreading is

economical for truck tires because the cost ofreplacing the tread is less than the price of anew tire. Retreading passenger tires is lesseconomical because the cost of retreading is

high compared to the price of new cheaptires, but favourable compared to high-endbrands.


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Worn tires can be retreaded by two methods,the mold or hot cure method and the pre-

cure or cold method. The mold cure methodinvolves the application of raw rubber on thepreviously buffed and prepared casing, whichis later cured in matrices. During the curingperiod, vulcanization takes place and the rawrubber bonds to the casing, taking the treadshape of the matrix. On the other hand, thepre-cure method involves the application of aready-made tread band on the buffed andprepared casing, which later is cured in anautoclave so that vulcanization can occur.


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In most situations, retread tires can be drivenunder the same conditions and at the same

speeds as new tires with no loss in safety orcomfort. The percentage of retread failuresshould be about the same as for new tirefailures, but many drivers, including truckers,

are guilty of not maintaining proper airpressure on a regular basis, and, if a tire isabused (overloaded, underinflated, ormismatched to the other tire), then that tire(new or recapped) will fail.

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All About Pneumatic Tires