Car Tyre (tire) Terms & Specifications Explained
Vintage Car cross ply tyres
Tire pressure monitoring system
Tire pressure monitoring systems (TPMS) are electronic systems that monitor the tire pressures on individual wheels on a vehicle, and alert the driver when the pressure goes below a warning limit. There are several types of designs to monitor tire pressure. Some actually measure the air pressure, and some make indirect measurements, such as gauging when the relative size of the tire changes due to lower air pressure.
Tires are specified by the vehicle manufacturer with a recommended inflation pressure, which permits safe operation within the specified load rating and vehicle loading. Most tires are stamped with a maximum pressure rating. For passenger vehicles and light trucks, the tires should be inflated to what the vehicle manufacturer recommends, which is usually located on a decal just inside the driver's door or in the vehicle owners handbook. Tires should not generally be inflated to the pressure on the sidewall; this is the maximum pressure, rather than the recommended pressure.
Many pressure gauges available at fuel stations have been de-calibrated by manhandling and the effect of time, and it is for this reason that vehicle owners should keep a personal pressure gauge with them to validate the correct tire pressure.
Tires are not completely impermeable to air, and so lose pressure over time naturally. Some drivers inflate tires with nitrogen, instead of simple air, which is already 78% nitrogen, in an attempt to keep the tires at the proper inflation pressure longer, though the effectiveness of this is debatable.
The tire contact patch is readily reduced by both over-and-under inflation. Over-inflation may increase the wear on the center contact patch, and under-inflation will cause a concave tread, resulting in less center contact. Most modern tires will wear evenly at very high tire pressures, but will degrade prematurely due to low (or even standard) pressures. An increased tire pressure has many benefits, including decreased rolling resistance. It has been found, that an increased tire pressure almost exclusively results in shorter stopping distances, except in some circumstances that may be attributed to the low sample size. If tire pressure is too low, the tire contact patch is changed more than if it were over-inflated. This increases rolling resistance, tire flexing, and friction between the road and tire. Under-inflation can lead to tire overheating, premature tread wear, and tread separation in severe cases.
High performance and dynamic drivers often increase the tire pressure to near the maximum pressure as printed on the sidewall. This is done to sacrifice comfort for performance and safety. A tire at higher pressure is more inclined to keep its shape during any encounter, and will thus transmit the forces of the road to the suspension, rather than being damaged itself. This allows for an increased reaction speed, and "feel" the driver perceives of the road. Modern tire designs allow for minimal tire contact surface deformity during high pressures, and as a result the traditional wear on the center of the tire due to reasonably high pressures is only known to very old or poorly designed tires.
It may be, that very high tire pressures have only two downsides: The sacrifice in comfort; and the increased chance of obtaining a puncture when driving over sharp objects, such as on a newly scraped gravel road. Many individuals have maintained their tire pressures at the maximum side wall printed value (inflated when cold) for the entire lifetime of the tire, with perfect wear until the end. This may be of negative economic value to the rubber and tire companies, as high tire pressures decrease wear, and minimize side wall blow outs.
It is dangerous to allow tire pressure to drop below the specification recommended on the vehicle placard. Low pressure increases the amount of tire wall movement resulting from cornering forces. Should a low-pressure tire be forced to perform an evasive maneuver, the tire wall will be more pliable than it would have been at normal pressure and thus it will "roll" under the wheel. This increases the entire roll movement of the car, and diminishes tire contact area on the negative side of the vector. Thus only half the tire is in contact with the road, and the tire may deform to such an extent that the side wall on the positive vector side becomes in contact with the road. The probability of failing in the emergency maneuver is thus increased.
When driving on sand, tire pressure is sometimes lowered to reduce chance of bogging.
Further, the tire will absorb more of the irregular forces from normal driving. With this constant bending of the side wall as it absorbs the contours of the road, it heats up the tire wall to possibly dangerous temperatures. Additionally, this flexing degrades the steel wire reinforcement; this often leads to side wall blow-outs.
Low pressure tires can be subject to pinching. If the vehicle drives into a pot-hole, the side wall can temporarily collapse, thereby pinching the tire between the steel wheel and road. The can result in a tire laceration and blow-out as well as a damaged wheel.
Feathering occurs on the junction between the tire tread and side wall, as a result of too low tire pressures. This is as a result of the inability of the tire to perform appropriately during cornering forces, leading to aberrant and shearing forces on the feathering area. This is due to the tire moving sideways underneath the wheel as the tire pressures are insufficient to transmit the forces to the wheel and suspension.
Tires are specified by the manufacturer with a maximum load rating. Loads exceeding the rating can result in unsafe conditions that can lead to steering instability and even rupture. For a table of load ratings, see tire code.
The speed rating denotes the maximum speed at which a tire is designed to be operated. For passenger vehicles these ratings range from 99 to 186 miles per hour (159 to 299 km/h). For a table of speed ratings, see tire code.
Tires (especially in the U.S.) are often given service ratings, mainly used on bus and truck tires. Some ratings are for long haul, and some for stop-start multi-drop type work. Tires designed to run 500 miles (800 km) or more per day carrying heavy loads require special specifications.
The treadwear rating or treadwear grade is how long the tire manufacturers expect the tire to last. A Course Monitoring Tire (the standard tire that a test tire will be compared to) has a rating of "100". If a manufacturer assigns a treadwear rating of 200 to a new tire, they are indicating that they expect the new tire to have a useful lifespan that is 200% of the life of a Course Monitoring Tire. The "test tires" are all manufacturer-dependent. Brand A's rating of 500 is not necessarily going to give you the same mileage rating as Brand B's tire of the same rating. The testing is non-regulated and can vary greatly. Treadwear ratings are only useful for comparing Brand A's entire lineup against itself. Tread wear, also known as tire wear, is caused by friction between the tire and the road surface. Government legal standards prescribe the minimum allowable tread depth for safe operation.
Tires may exhibit irregular wear patterns once installed on a vehicle and partially worn. Furthermore, front-wheel drive vehicles tend to wear the front tires at a greater rate compared to the rear tires. Tire rotation is the procedure of moving tires to different car positions, such as front-to-rear, in order to even out the wear, thereby extending the life of the tire. However care must be taken with unidirectional tires (tires that are designed to rotate in one direction only, for a vehicle that is going forward) so that the correct rotational direction - indicated on the side wall with an arrow-like symbol - is maintained after the swap.
When mounted on the vehicle, the wheel and tire may not be perfectly aligned to the direction of travel, and therefore may exhibit irregular wear. If the discrepancy in alignment is large, then the irregular wear will become substantial if left uncorrected.
Wheel alignment is the procedure for checking and correcting this condition through adjustment of camber, caster and toe angles. These settings also affect the handling characteristics of the vehicle.
Tires that are fully worn can be re-manufactured to replace the worn tread. This is known as retreading or recapping, a process of buffing away the worn tread and applying a new tread. Retreading is economical for truck tires because the cost of replacing the tread is less than the price of a new tire. Retreading passenger tires is less economical because the cost of retreading is high compared to the price of new cheap tires, but favorable compared to high-end brands.
Worn tires can be retreaded by two methods, the mold or hot cure method and the pre-cure or cold one. The mold cure method involves the application of raw rubber on the previously buffed and prepared casing, which is later cured in matrices. During the curing period, vulcanization takes place and the raw rubber bonds to the casing, taking the tread shape of the matrix. On the other hand, the pre-cure method involves the application of a ready-made tread band on the buffed and prepared casing, which later is cured in an autoclave so that vulcanization can occur.
During the retreading process, retread technicians must ensure the casing is in the best condition possible to minimize the possibility of a casing failure. Casings with problems such as capped tread, tread separation, unrepairable cuts, corroded belts or sidewall damage, or any run-flat or skidded tires, will be rejected.
In most situations, retread tires can be driven under the same conditions and at the same speeds as new tires with no loss in safety or comfort. The percentage of retread failures should be about the same as for new tire failures, but many drivers, including truckers, are guilty of not maintaining proper air pressure on a regular basis, and, if a tire is abused (overloaded, underinflated, or mismatched to the other tire on a set of duals), then that tire (new or recapped) will fail.
Many commercial trucking companies put retreads only on trailers, using only new tires on their steering and drive wheels. This procedure increases the driver's chance of maintaining control in case of problems with a retreaded tire.
The interaction of a tire with the pavement is a very complex phenomenon. Many of the details are modeled in Pacejka's Magic Formula. Some are explained below.
When a wheel and tire rotate, they exert a centrifugal force on the axle that depends on the location of their center of mass and the orientation of their moment of inertia. This is referred to as balance, imbalance, or unbalance. Tires are checked at the point of manufacture for excessive static imbalance and dynamic imbalance using automatic tire balance machines. Tires are checked again in the auto assembly plant or tire retail shop after mounting the tire to the wheel. Assemblies that exhibit excessive imbalance are corrected by applying balance weights to the wheels to counteract the tire/wheel imbalance.
To facilitate proper balancing, most high performance tire manufacturers place red and yellow marks on the sidewalls to enable the best possible match-mounting of the tire/wheel assembly. There are two methods of match-mounting high performance tire to wheel assemblies using these red (uniformity) or yellow (weight) marks.
Camber thrust and camber force are the force generated perpendicular to the direction of travel of a rolling tire due to its Camber angle and finite contact patch.
A tire rotating at higher speeds tends to develop a larger diameter, due to centrifugal forces that force the tread rubber away from the axis of rotation. This may cause speedometer error. As the tire diameter grows, the tire width decreases. This centrifugal growth can cause rubbing of the tire against the vehicle at high speeds. Motorcycle tires are often designed with reinforcements aimed at minimizing centrifugal growth.
Circle of forces
The circle of forces, traction circle, friction circle, or friction ellipse is a useful way to think about the dynamic interaction between a vehicle's tire and the road surface.
The contact patch, or footprint, of the tire, is the area of the tread that is in contact with the road surface. This area transmits forces between the tire and the road via friction. The length-to-width ratio of the contact patch affects steering and cornering behavior.
Cornering force or side force is the lateral (i.e. parallel to the road surface) force produced by a vehicle tire during cornering.
Dry traction is measure of the tire's ability to deliver traction, or grip, under dry conditions. Dry traction is a function of the tackiness of the rubber compound.
The tire tread and sidewall elements undergo deformation and recovery as they enter and exit the footprint. Since the rubber is elastomeric, it is deformed during this cycle. As the rubber deforms and recovers, it imparts cyclical forces into the vehicle. These variations are collectively referred to as tire uniformity. Tire uniformity is characterized by radial force variation (RFV), lateral force variation (LFV) and tangential force variation. Radial and lateral force variation is measured on a force variation machine at the end of the manufacturing process. Tires outside the specified limits for RFV and LFV are rejected. Geometric parameters, including radial runout, lateral runout, and sidewall bulge, are measured using a tire uniformity machine at the tire factory at the end of the manufacturing process as a quality check. In the late 1990s, Hunter Engineering introduced the GSP9700 Road Force balancer, which is equipped with a load roller similar to the force variation machine used at the factory to grade tire uniformity. This machine can find the best position for the tire on a given wheel so that the over-all assembly is as round as possible.
Load sensitivity is the behaviour of tires under load. Conventional pneumatic tires do not behave as classical friction theory would suggest. Namely, the load sensitivity of most real tires in their typical operating range is such that the coefficient of friction decreases as the vertical load, Fz, increases.
Pneumatic trail of a tire is the trail-like effect generated by compliant tires rolling on a hard surface and subject to side loads, as in a turn. More technically, it is the distance that the resultant force of side-slip occurs behind the geometric center of the contact patch.
Relaxation length is the delay between when a slip angle is introduced and when the cornering force reaches its steady-state value.
Rolling resistance is the resistance to rolling caused by deformation of the tire in contact with the road surface. As the tire rolls, tread enters the contact area and is deformed flat to conform to the roadway. The energy required to make the deformation depends on the inflation pressure, rotating speed, and numerous physical properties of the tire structure, such as spring force and stiffness. Tire makers seek lower rolling resistance tire constructions to improve fuel economy in cars and especially trucks, where rolling resistance accounts for a high proportion of fuel consumption.
Pneumatic tires also have a much lower rolling resistance than solid tires. Because the internal air pressure acts in all directions, a pneumatic tire is able to "absorb" bumps in the road as it rolls over them without experiencing a reaction force opposite to the direction of travel, as is the case with a solid (or foam-filled) tire. The difference between the rolling resistance of a pneumatic and solid tire is easily felt when propelling wheelchairs or baby buggies fitted with either type so long as the terrain has a significant roughness in relation to the wheel diameter.
Self aligning torque
Self-aligning torque, also known as the aligning torque, SAT or Mz, is the torque that a tire creates as it rolls along that tends to steer it, i.e. rotate it around its vertical axis.
Slip angle or sideslip angle is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing (i.e., the angle of the vector sum of wheel translational velocity.
Performance-oriented tires have a tread pattern and rubber compounds designed to grip the road surface, and so usually have a slightly shorter stopping distance. However, specific braking tests are necessary for data beyond generalizations.
The work load of a tire is monitored so that it is not put under undue stress, which may lead to its premature failure. Work load is measured in ton kilometre per hour (TKPH). The measurement's appellation and units are the same. The recent shortage and increasing cost of tires for heavy equipment has made TKPH an important parameter in tire selection and equipment maintenance for the mining industry. For this reason, manufacturers of tires for large earth-moving and mining vehicles assign TKPH ratings to their tires based on their size, construction, tread type, and rubber compound. The rating is based on the weight and speed that the tire can handle without overheating and causing it to deteriorate prematurely. The equivalent measure used in the United States is ton mile per hour (TMPH).
There are several types of abnormal tread wear. Poor wheel alignment can cause excessive wear of the innermost or outermost ribs. Gravel roads, rocky terrain, and other rough terrain causes accelerated wear. Over-inflation above the sidewall maximum can cause excessive wear to the center of the tread. Modern tires have steel belts built in to prevent this. Under-inflation causes excessive wear to the outer ribs. Often, the placard pressure is too low and most tires are under-inflated as a result. Unbalanced wheels can cause uneven tire wear, as the rotation may not be perfectly circular. Tire manufacturers and car companies have mutually established standards for tread wear testing that include measurement parameters for tread loss profile, lug count, and heel-toe wear. See also Work load above.
Wet traction is the tire's traction, or grip, under wet conditions. Wet traction is improved by the tread design's ability to channel water out of the tire footprint and reduce hydroplaning. However, tires with a circular cross-section, such as those found on racing bicycles, when properly inflated have a sufficiently small footprint to not be susceptible to hydroplaning. For such tires, it is observed that fully slick tires will give superior traction on both wet and dry pavement.
A flat tire occurs when a tire deflates. This can occur as a result of normal wear-and-tear, a leak, or more serious damage. A tire that has lost sufficient pressure will impair the stability of the vehicle and may damage the tire further if it is driven in this condition. The tire should be changed and/or repaired before it becomes completely flat. Continuing to drive a vehicle with a flat tire will damage the tire beyond repair, possibly damage the rim and vehicle, and put the occupants and other vehicles in danger. A flat tire or low-pressure tire should be considered an emergency situation, requiring immediate attention. Some tires, known as "run-flat tires", have either extremely stiff sidewalls or a resilient filler to allow driving a limited distance while flat, usually at reduced speed, without permanent damage or hazard.
A modern radial tire may not be visibly distorted even with dangerously low inflation pressure. (This is especially true of tires with a low aspect ratio, sometimes known as "low profile" tires.) Thus maintenance of adequate tire pressure can have important safety implications despite the fact that most car owners neglect it. Tire designers have tried to make new tires fail-safe so that the failure of the operator to maintain the tire pressure won't cause a major safety concern, but there are limitations on this.
Tire bubbles, also referred to as bulges/bumps/protrusions, occur when the sidewall of the tire has failed, resulting in a protrusion. Causes of bubbles include having an impact at high speed, over inflation, or poor tire construction/manufacturing. It is generally recommended to replace the tire since the probability of tire failure has increased. They can occur on the inner or outer sidewall.
Hydroplaning (or aquaplaning)
Hydroplaning, also known as aquaplaning, is the condition where a layer of water builds up between the tire and road surface. Hydroplaning occurs when the tread pattern cannot channel away enough water at an adequate rate to ensure a semi-dry footprint area. When hydroplaning occurs, the tire effectively "floats" above the road surface on a cushion of water – and loses traction, braking and steering, creating a very unsafe driving condition. When hydroplaning occurs, there is considerably less responsiveness of the steering wheel. The correction of this unsafe condition is to gradually reduce speed, by merely lifting off the accelerator/gas pedal.
Hydroplaning becomes more prevalent with wider tires (because of the lower weight per contact area) and especially at higher speeds; it is of virtually no concern to bicycle tires under normal riding conditions largely because of the lower speeds. The chance of car hydroplaning is also minimal at bicycle speeds as the weight per contact area of car tires is not much lower if any than bicycle tires.
Dangers of aged tires
Research and tests show that as tires age, they begin to dry out and become potentially dangerous, even if unused. Aged tires may appear to have similar properties to newly manufactured tires, but rubber degrades over time, and once the vehicle is traveling at high speeds (i.e. on a freeway) the tread could peel off, leading to severe loss of control. In tropical climates, such as Singapore, tires degrade sooner than in temperate climates, and more care should be taken in these climates to ensure that tires do not fail. Also, tires on seldom-used trailers are at the greatest risk of age-failure, but some tires are built to withstand idleness, usually with nylon reinforcement.
Many automakers recommend replacing tires after six years, and several tire manufacturers (Bridgestone, Michelin) have called for tires to be removed from service 10 years after the date of manufacture. However, an investigative report by Brian Ross on ABC's 20/20 news magazine found that many major retailers such as Goodyear, Wal-Mart, and Sears were selling tires that had been produced six or more years ago. Currently, no law for aged tires exists in the United States.
An asymmetric tire may refer to a tire whose tread pattern does not form in line symmetry or point symmetry vis-à-vis its central line, thus having a distinct inside and outside edge. They may be mounted on either side of the vehicle. Since the tread pattern of many ordinary tires do not form symmetry in relation to design or pattern noise, the method of mounting tires is specially prescribed. This type of tires is used in many cases to promote tire performance, braking performance, and turning performance, since tread contact changes according to the change in alignment during travel.
Tires may also be directional, where the tread pattern favors operation in one direction. This usually takes the form of v-shaped grooves that help to disperse water from the center to the edge of the tread. Symmetric directional tires can be used on both sides, but once mounted on a rim cannot be moved to the other side, since the tread pattern will be in the wrong direction. This restricts tire rotation. Some directional tires are also asymmetric, in which case there will be specific left and right-handed versions.
An asymmetric tire may refer to a passenger car radial tire in which asymmetric structure stabilizing belts are built. Generally the stabilizing belts give a self-aligning torque when a motor vehicle is running straight ahead as well as when it is cornering. However, the sidewalls of the radial tire are so flexible that there will be a delay in the lateral reaction between the tread of the tire and the rim of its wheel as the vehicle is being steered positively. The lateral force will be transmitted from the front wheel to the rear of the vehicle, which will tend to be steered off course. Whereas the asymmetric belts bring a gradual change in the lateral displacement of the tire tread corresponding to the rim while the cornering load grows. The progressive change will harden the sidewalls to produce an immediate response to steering, which results in safer driving.