Table of Contents
Tires (or tyres, different spelling for different countries) play a crucial role in vehicle performance, influencing braking, handling, and overall driving experience.
Tire Anatomy
Below are some of the main components of a tire’s anatomy.
- Tread: The outer layer of the tire and the main point of contact with the road. Tread design plays a vital role in providing grip for braking and acceleration.
- Sidewall: Provides structural support and protection, helping to maintain the tire’s shape and integrity. Additionally, it displays important information such as tire size, load index, and speed rating.
- Beads: This is a high-strength steel part packed in rubber to ensure the attachment of the tire to the rim.
- Inner Liner: The inner liner ensures consistent pressure inside the tire.
- Carcass: Ensures the tire can withstand the pressure of the road.
- Belt: Placed between carcass and tread to ensure uniform tire pressure distribution, improving overall durability.
Tire Function
Tires fulfill several crucial functions in vehicle operation.
- Load Bearing: Tires distribute the vehicle’s weight evenly across their surface.
- Friction Generation: Longitudinal friction between the tires and the road produces braking and traction forces, aiding in stopping and acceleration.
- Steering Support: Lateral friction enables the tires to generate steering forces, enhancing directional stability and control.
- Shock Absorption: Tires possess vertical elasticity, absorbing impacts from uneven road surfaces to provide a smoother ride and maintain vehicle stability.
For tires to perform, they require:
- Load Capacity: Tires must support heavy loads without deformation or failure.
- Wear Resistance: Tires should resist wear and tear, maintaining their tread pattern for an extended period to ensure longevity and safety.
- Heat Resistance: Tires must withstand heat generated during operation, preventing overheating and maintaining structural integrity.
- Traction: Tires should provide sufficient grip on various road surfaces to ensure effective braking, acceleration, and cornering manoeuvres.
- Durability: Tires must endure prolonged use without significant degradation, ensuring reliability and minimising replacement frequency.
Importance of Tires in Vehicle Performance & Safety
As a vehicle moves, it relies on the transmission of forces between itself and the road surface via the tires. This transmission is pivotal for maintaining control and stability, especially under varying road conditions.
The interaction between the vehicle’s powertrain system (all the components that work together to generate power and transmit it to the wheels to propel a car forward) and the road generates forces crucial for propulsion and manoeuvring. These forces are effectively managed by the tires’ contact with the road surface, ensuring that the vehicle remains responsive and stable during acceleration, braking, and steering.
Various factors influence the vehicle’s motion, including rolling resistance, wheel slip, and the wheels’ effective radius. Additionally, the transmission of lateral and longitudinal forces between the tires and the road surface profoundly impacts the vehicle’s overall behaviour and performance.
Tire Forces
To better understand the influence of tires on a vehicle’s performance, let’s look at the tire forces at play:
- Longitudinal Tire Force: This force occurs due to the friction between the tire and the road surface in the direction of the vehicle’s motion. It assists in the acceleration and deceleration of the vehicle and occurs when the wheel’s rotational speed differs from its free rolling speed.
- Lateral Tire Force: This force occurs due to friction between the tire and the road surface when cornering. It helps in maintaining the vehicle’s stability and steering control during turns and occurs when the direction in which the tire is moving differs from the direction it is pointing.
- Combined Slip: This condition occurs when both longitudinal and lateral slips happen simultaneously. It can occur during manoeuvres such as aggressive cornering or sudden braking.
- Rolling Resistance: This is the resistive force the tire rolling on the road surface generates. Rolling resistance opposes the vehicle’s motion and consumes energy, affecting fuel efficiency. The magnitude of rolling resistance depends on factors such as tire construction, inflation pressure, road surface, and vehicle weight.
- Impact Force: Tires can experience impact forces when encountering bumps, potholes, or other irregularities on the road surface. These forces can cause deformation of the tire and suspension components and can potentially lead to damage if severe.
- Shear Forces: Shear forces occur within the tire structure as it deforms under various loads and conditions. These forces contribute to the tire’s ability to maintain its shape and integrity while undergoing stress during acceleration, braking, and cornering.
Tire Ratings
Tire ratings are the standard indicators for performance aspects that help consumers select the correct tyre for the required use. There are two significant ratings: load index and speed ratings.
Load Rating/index
This numeric value on the sidewall of a tire identifies the maximum allowable load (kgs) carried by a single tire. Load ratings exist to prevent tire overload and ensure safe operating conditions as per the manufacturer’s recommendation. Overloading can result in premature tire wear, not to mention severe heat generation, leading to tire failure.
The following is a breakdown of each of the digits/symbols displayed on the above tire code example.
Width
Height
Diameter
Load Rating Value
Speed Rating
Width
Height
Diameter
Load rating value
Speed Rating
- Width: measured in millimetres (mm) from one side wall to the other.
- Height: measured as an aspect ratio percentage. The aspect ratio is determined by dividing the tire’s height off the rim by its width. (55 indicates the height is 55% of its width).
- Diameter: measured in inches. The letter preceding the number indicates the sidewall construction of the tire. It is either ‘R’ for Radial, ‘D’ for Diagonal or ‘B’ for Bias Ply.
The table below shows a single tire’s standard maximum allowable load with a corresponding load rating value. In the given example, the load rating value for the tire is 91. Looking at table below, we can see that the corresponding maximum allowable load in kilograms is 615kgs. Generally, the higher the load index or rating, the stronger the tire.
Load Index
Max. Load (KG)
Load Index
Max. Load (KG)
* This table is not exhaustive and includes only a range of Load Index values that exist for tires.
Speed Rating
The maximum speed at which a tire can operate smoothly is known as speed rating and is commonly denoted by the letters T, H, V, W or Z on the tire sidewall. The ability to manage driving speeds depends entirely on the tire’s speed rating – higher speed ratings will allow handling of high-speed driving.
Here is a general guide to tire speed ratings:
Speed Rating
Maximum Speed
Relationship Between Tire Pressure, Load Index & Speed Rating
The term “tire pressure” describes the pressure of the air inside a tire measured in kilopascals (kPa) or pounds per square inch (psi). Correct tire pressure ensures optimal tire performance, an even distribution of loads, and smooth contact with the road surface.
Tire pressure, speed rating, and load rating are interrelated factors that collectively determine a tire’s performance and suitability for specific vehicles.
A tire can only safely carry the desired load if correctly pressurized. Subsequently, your tires will fail to meet their designed speed rating.
Load index aside, reaching the desired speed rating requires the correct tire pressure. The recommended tire pressure ensures sidewall stiffness and proper shape, providing stability and traction.
Tire Placards
Tire placards contain helpful information to help users select the correct tire for their vehicle, including the tire size, the inflation pressure of rear and front tires, tire construction, speed rating, and the manufacture date.
In Australia, tire placards are legally mandated and have been required in all new passenger, 4×4, and light truck vehicles since 1973. Regarding 4×4 vehicles, two placards are commonly displayed depending on the vehicle’s intended use – one to cover a passenger load and one for light-truck fitment. In Australia, each tire has a tire identification number on the sidewall comprising four digits. For example, if the digits are 0218, the tire was manufactured in the second week of 2018. Figure 4 below indicates the different terminologies associated with the tires manufactured in Australia, along with their identification placards.
In the USA, the DOT tire identification number shows compliance with the standards of safety established by the US Department of Transportation. It provides additional information about the manufacturing of tires. It is usually displayed as 12 numbers and letters on the tire’s sidewall. The first two letters are the plant code where the tire was manufactured, and the last three to four digits indicate the week and year of manufacturing. Figure 5 shows the breakdown of valuable information provided on tire placards.
Recently, in the US, providing a partial DOT serial number on tires has been declared essential to indicate the manufacturer, plant, and tire size information. The partial DOT number may not help provide the manufacturing date, but it can still assist customers. The aim of using partial DOT instead of complete DOT is to ease tire identification without putting the vehicle on a lift – complete DOT numbers are mounted towards the inboard side of the vehicle.
Using suitable tires according to your vehicle type should be a priority. For example, using car tires instead of trailer tires can cause severe problems because trailer tires are designed to carry heavy loads and provide stability to the vehicle. Due to these higher loads and higher friction, there is a large amount of heat generation, dissipated by the optimised design of trailer tire tread and sidewalls – something your typical car tire can’t handle.
Conclusion
In conclusion, understanding tires, their ratings, labels, and applications is crucial for ensuring optimal vehicle performance and safety. Tires serve as the sole point of contact between a vehicle and the road, influencing braking, handling, and overall driving experience.
By exploring tire anatomy, function, and the various forces they encounter, we gain insight into their critical role in vehicle dynamics. Tire ratings, including load index and speed ratings, provide essential guidelines for selecting the right tires for specific vehicles and driving conditions. Additionally, tire placards offer valuable information to assist users in making informed decisions about tire selection and maintenance.
Ultimately, choosing the appropriate tires for a vehicle’s needs is essential for ensuring efficiency, stability, and safety on the road.
Frequently Asked Questions
Hopefully, after reading this article, you now know tires are made to carry specific loads. Car tires generally have lower load and higher speed ratings than trailer tires.
The correct tire pressure is on the tyre placard or manufactures plate, it can also be found on the tyre side wall.
Select the correct tyre for your application. Regularly check for proper tyre inflation, rotate and balance wheels if required.
