Tire Definition & How Tires are Measured? Parts, Measurement of Tire Sizes, Protection

In this article, we will learn what is tire, tire definition, how tire sizes are measured, and different parts, let’s explore!

What is Tire? Definition

Tire Definition

In automobiles, the tire is a ring-shaped component that wraps around the rim of a wheel and is responsible for transferring the weight of the vehicle from the axle to the ground. It helps the wheels maintain traction on the surface on which they are driving.

What is a tire

TIRE is a radial component that transfers weight from an application to the ground and provides traction on the surface over which the wheel moves. Tires are most often used in vehicles such as automobiles and bicycles. They’re pneumatically inflated structures that act as a flexible cushion to absorb stress when the tire travels over uneven terrain. It achieves this because of its low bearing pressure, which does not cause the surface to deform significantly.

Construction of Tires

A tire is a rounded construction made up of stacks of rubber. Each of the piles’ sides is wrapped around a wire bead, which binds the tire to the wheel rim, and the tire is constructed of elastomeric fabric piles on top of a rubber liner to provide traction.

A Tread rubber compound is applied to the fabric piles, and a second rubber compound is applied to the sidewalls of the piles. The tire is vulcanized in a mold in order to shape the tread pattern and vulcanize the individual components together to form a single piece of rubber.

Also Read: Types of Cars

Parts of Tire

A tire cover’s main parts are the carcass, beads, sidewalls, and tread.

Carcass. The inner lining of the tire is formed like a horseshoe and is made up of many layers of textile cord stacks.

The tread, bead, and walls are all molded onto these cord piles, which comprise the backbone of the tire structure.

Bead. The inner edge of the tire is formed by this, and it locates and centers the cover on the wheel rim. It’s stiff and strong enough to hold the carcass in place. An infinite wire core is formed circumferentially through the bead to achieve this, the wall on the other side of the room.

This is the external rubber covering of the carcass that is located between the bead and the tire tread. The thickness of the sidewall influences the amount of protection provided to the carcass as well as the stiffness of the tire when the tire is flexed during use. When the wheel is rolling, this is the area of the tire that makes contact with the road surface, which is called the contact patch. Rubber compounds are used in its construction, and the pattern design has a substantial influence on the tire’s grip, road holding ability, and service life.

Radial and cross-ply  

Radial and cross-ply are two types of tires. Cross-ply tires are made out of nylon strands that are crossed diagonally over each other. After that, thick rubber heaps are put on top to create robust, stiff sidewalls. While cross-ply tires are durable and resistant to sidewall damage, they also have a higher rolling resistance, which can cause the tires to heat up rapidly, causing the air pressure to rise.

Also read: Types of bikes

Purpose of a Tire

To offer strength

Several textile strands are crisscrossed and implanted in the rubber of the cross-ply tire to provide strength and durability. These strands are built in layers, which are referred to as plies, and serve two functions. Beginning with the construction of walls that are strong enough to maintain air pressure while still being flexible enough to allow for deflection. The second need is that they offer support for the tread.

Both of these needs are incompatible since, in order to achieve enough tread bracing, the ply must be sufficiently stiff, which nesses the inclusion of stiffness in the walls. Radial plies have only one purpose: to make the tire’s wall strong enough to withstand the pressure exerted by the air pressure. They are unable to withstand the pressure of the tread. They do not need to be crisscrossed and may be set out easily following the natural shape of the tire’s tread. This results in a wall that is both flexible and robust, which is excellent.

Become a Six Sigma Green Belt Expert by mastering concepts like Fishbone/Ishikawa diagram, Root Cause analysis, Co-relation & Statistical analysis of data while working on industry based Use-cases and Projects.

To provide support

It is necessary to use a layer of rayon or steel cords to form a belt beneath the tread in order to offer stability. The major goal of this belt is to ensure that the whole tread is securely flat and open to the road at all times. The substance of the cable and the manner in which it is put into the tire are the two most important differences across tires.

The angle formed by the cross-bias cord is between 30 and 40 degrees in angle. This results in the formation of a cross-cord sidewall, which provides the necessary strength for the transfer of both acceleration and braking torque, respectively. There are now only two-ply tires in place of the previous four-ply tires.

To absorb stress

In addition to being more flexible to absorb stress from road irregularities, two-ply tires provide greater self-aligning torque to the steering system after a turn, allowing them to operate at a lower temperature. The tire might wiggle as it goes across the tire footprint or contact patch thanks to bias-ply. Rubber retains energy in this way. The tire rapidly expands as it exits the root print, moving past the neutral point and into a stretched state.

XPH is your home for BMW, Audi, Ford Mustang, VW, Porsche and Nissan GTR aftermarket parts.

Opening and closing of tread

One of the main causes of regular tire wear is the tread closing and opening as it goes across the contact patches. When compared to bias-ply tires, the tread stability and elimination of wiggle in belted tires result in a 100 percent improvement in tire performance.

When compared to bias-ply tires, belted tires run cooler, consume less fuel, have better grip, and have double the blow-out resistance Belted tires do not flex as easily as bias-cord tires, resulting in more road shock being carried to the wheels and suspension system. As a result, wheel spindles, knuckles, and suspension systems must be stiffer to compensate for the increased road stress.

Ply angle

These radial-ply belted tires feature radial cord angles that range from 88 to 90 degrees, while the belt cord angles that range from 12 to 20 degrees have been developed. In comparison to belted bias tires, the radial cord produces a softer sidewall, which results in a more comfortable ride. The tread form is maintained through the contact patch or footprints by a belt (steel wires, Fiberglass, or rayon) wrapped around the radial cords.

Radial-ply belted tires have a lower slip angle than Radial-ply belted tires, resulting in increased cornering power and less wear. The Radial-ply belted tires provide a harsh ride at low speeds, require a significant amount of steering effort, particularly while parking, and are expensive to purchase. It rolls more easily and burns less gasoline because the steel-belted radial tire has less tread bending than other types of tires.

Also Read: What is Governer

Measurement of Tire Sizes

All tires have their specific size. There is a way to measure a tire size for an unprofessional person. Tire size matters a lot as a slight change of width or length can make the tire useless. So there is a way to measure tire size. This process has certain steps

Metric sizes for different types of tires

P-Metric or Euro-Metric tires are standard on most passenger vehicles that display the actual tire size, SUVs, and light pickup trucks (with a towing capacity of less than half a ton). On P-Metric tires, you’ll note that the letter P appears before the number sequence. Considering a P-metric classification for a tire, the Tires and Rim organization has taken into consideration the “Type of tire for “passenger vehicles.”

The tire size will be the first number in the series, and there will be no preceding letter for Euro-Metric before that. For the purpose of referring to a “passenger car” tire, the European Tire and Rim Technical Organization (ETRTO) has standardized the term “Euro-Metric.” Passenger vehicles such as sedans, minivans, SUVs, and other light-duty pickup trucks are the primary applications for tires in the P-Metric and Euro-Metric dimensions.

Also Read: What is Engine

For SUV pick up van

If you have an SUV, pickup truck, or van, your placard may have a different size designation for heavy-duty light pickup trucks and vehicles vans, which is notably frequent on 34 tone and bigger pickup trucks and vans. LT-Metric and Euro-Metric Commercial are the two most common sizes in this category, with LT-Metric being the most popular (aka C-type).

Due to the fact that they both have metric measurements, both differential tire size-types have the same structural features as P-Metric and Euro-Metric tires. However, they have many characteristics that separate them from their passenger car equivalents. In the case of LT-Metric tires, the letters “LT” will appear before the size number sequence.

Spring is here! Save big on our best-selling WAGNER TUNING BMW F CHASSIS N55 CATTED DOWNPIPE

For light trucks

The organization for tire and rim studies researched a lot and then standardized the LT-Metric designation for a tire of a light truck. The European Tire and Rim Technical Organization have defined the designation Euro-Metric Commercial, or C-Type, for a light truck type tire.

For other types of tires

Temporary Spares, which begin with the letter “T,” are regarded to be a different sort of tire that falls under the Metric size group classification system. If a size begins with the letters PP,” it indicates that it is for a “special trailer,” and it may only be used on a trailer. Regardless of whether you’re looking at clothing or shoes, the numbers in the size indicate the same thing.


The width of the proper tires for your vehicle, measured in millimeters, is the first figure in your tire size information, and it is exactly what it says on the tin.

If you’re talking about the width of a tire, you’re always talking about the distance from one sidewall to the next. Consider, for example, a tire with a different size designation that is suited for passenger automobiles and has a nominal width of 228 millimeters, as shown in the illustration.

Also Read: Fuel Pump

Aspect ratio

The number that comes before the slash mark is referred to as the aspect ratio of the tire, and it basically informs you how tall your tire’s profile is by comparison. The aspect ratio of a tire is derived by dividing the height of the tire off the rim by the width of the tire. When the particular aspect ratio of a tire is close to 70, it indicates that the tire’s height is around 70 percent of its width.

Even though ride roughness is a frequent trade-off, smaller aspect ratio tires, such as those in the 60 series, generally outperform larger aspect ratio tires, such as those in the 75 series, when it comes to car handling performance.


 When seen in relation to the aspect ratio, this reveals the sort of internal structure that contributes to the stability of your tire. In the United States, radial tires are the most often seen type of tire on the road today, and as a result, the letter “R” will typically appear in the tire size designation.

When you look at a radial tire, you’ll see that the internal ply cords are aligned radially from one bead to the next, which is almost perpendicular to the direction in which the tire is rotating. You may also notice a run-flat tire, which is potentially dangerous due to the fact that it is not correctly inflated.


The width of the rim

A number follows it, which gives the rim size and also indicates that the diameter code is in inches, which may be used to attach a tire to it. For tire sizes, a 16-inch diameter rim would be a good match for a tire size of the same diameter as the rim diameter. When it comes to determining the size of automotive tires, the rim size is quite important.

The index of loading

The load index of your vehicle tire, which comes in the order after the rim size, informs us how much weight, in pounds, the tire can sustain when it is completely inflated. The load index of your automobile tire appears in the order following the rim size. Because it does not give us a specific number of pounds that the tire can support, this figure is referred to as the load “index.” Increasing the number of vehicle tire load indexes equates to increasing the amount of load capacity specified in the index. Depending on the value of the load index, it is feasible to transport 95 to 785 lbs. of cargo. The load index has values ranging from 1 to 150, with 1 being the starting point.

The passenger car load index is tenth on the list. When a single load index number is used on passenger vehicle tires, such as the P-Metric and Euro-Metric kinds, the tire will have just one load index number; however, when two digits are used on commercial (C-Type) tires, the tire will have two load index numbers.

If the tire is used in a single application, the load index is represented by the first number; if the tire is used in two applications, the load index is represented by the second number. Passenger tires are not appropriate for dual-purpose use. The Load Range of light truck tires is frequently denoted by a letter, such as Load Range E, which stands for Extra Load Range. It is possible that your tire dealer will mention the term “load range” because it is an older term that is still extensively used in the tire business.

Also Read: Gear Box

Facts regarding load index

One of the most important but usually misunderstood aspects of the load index is that the statistics produced by different standards bodies are not necessarily on the same scale. With this respect, it can be said that load capacities may be different with the same load index. As a result, it is vital to verify not only the load index number but also the actual load capacity of the system.

Rating speed

When a tire size sequence is completed with a letter, it is designated as P225/70R16 91S, which stands for “speed rating.” It is the same way that your load index number refers to a specific load that your speed rating letter refers to a certain speed capability that was determined by a verified laboratory test. An S-rated tire has the ability to travel at speeds of up to 112 miles per hour, while an R-rated tire has the ability to travel at speeds up to 106 miles per hour.

Keep in mind that this is not a safe cruising speed for a vehicle of this size. Of course, you should always adhere to the set speed limits when driving on public highways or roads. Replacement tires must have a speed rating that is equal to or greater than the vehicle’s Original Equipment in order to maintain the vehicle’s speed capability.

Would you like to test?

Floatation sizing

You should be aware of one additional size type, particularly if you are shopping for off-road tires for a light car or a sport utility vehicle. Flotation sizes are referred to as such, and the values used in this sizing scheme are different from those used in Metric forms. With the exception of a few critical variations, Flotation-sized tires and LT-Metric tires are nearly identical in terms of application. For one thing, they cannot be utilized in two separate applications at the same time, and for another, the same size tire may have a different load capacity than its LT-Metric equivalent.

Also Read: Diesel Engine

Protection of Tires

  • Keep the tires out from sunlight as best as possible because mostly in the summers season the tires become very hot and there is a chance of tire burst. This can result in an accident but one should park the car in a cool dry place to avoid this situation
  •  Wash the tires regularly and do check for stacked stones in the threading gaps of tires sometimes a stone gets stuck which results from the tire wearing off
  •  If you are parking a car for more than three months remove tires from your car that will be very effective.


What is TPMS?

It is a pressure monitoring system in your car that monitors your car’s tires’ pressure.

What is speed rating?

The velocity group (or range of speeds) at which a tire can carry a load under defined service circumstances is indicated by its speed rating. The current speed rating system was created in Europe in response to the necessity to regulate tire performance at uniform speeds.

Why do tires wear so quickly?

Tire rotation isn’t the only element that might cause premature tire wear. The following are some examples: incorrect inflation, driving conditions, misaligned cars, old vehicle parts, and a variety of other factors can all contribute to poor tire performance.


In short tire, measurement is a very important process in the formation, assembling, and fixation of tires on the rim of a car. The car tire size is very important as it helps a person differentiates between the different categories of tires. It also helps the person to choose a better tire for his car according to his requirements.

High Rated Automotive Engineering Course

Automotive 101: A Beginners Guide To Automotive Repair

Automotive Engineering: Automobile Fundamentals and Advanced

Automotive Engineering; Hybrid Electric Vehicles

Do it Yourself – Automotive Electrical Diagnosis – Beginner

Do it Yourself-Automotive Electrical Diagnosis-Intermediate

Automotive Engineering; Common Rail Direct Injection(CRDI)

Fundamentals of Internal Combustion Engines – IC Engines

Hybrid and Electric Vehicle for Beginners FULL Course 2021

Automobile Safety: Understanding Car Crashes for beginners

Hybrid Vehicles The Fundamentals & Operating Principles

Introduction to Automotive engineering – Performance

Automotive 102: Battery, Charging System & Starting System

If you are interested to have certificate programs

Get started in your dream career today with 60% off registration at IAP Career College.

Our Videos

Refer YouTube

Our Apps

Check out our ‘MechStudies – The Learning App’ in iOS & Android

Check out our most viewed articles,

What is thermodynamics

Intensitve & Extensive Properties

What is Pressure

Bernoulli’s theorem

Venturi meter

Pump basics

Globe valve


Reference Articles


Hello Everyone! We are the MechStudies team and really delighted to present our articles. We will capture all types of Mechanical Engineering articles in the simplest way with various diagrams, practical examples, etc. Cheers!

Recent Posts

You cannot copy content of this page