Wheels and Tires: Anatomy and Function in Vehicles
Wheels
Wheels are the fulcrum of a car on the ground, enabling propulsion and braking. They must withstand the vehicle’s weight, load, and suspension impacts while handling braking forces and determining the vehicle’s direction. These demands necessitate both strength and lightweight construction.
Cars utilize pneumatic tires mounted on metal rims. The tire, an inflatable ring, is attached to the rim, and pressurized air fills the space between them.
Wheels have holes for air circulation, which cools the brake drums or discs.
Wheel rims typically have a sunken base for easy tire mounting and dismounting. A tire’s profile, the shape of its cross-section, is crucial. It’s characterized by the base, the tab, and the heel seat. The tab’s height determines the tire section, while rim width and diameter are other key measurements.
Many vehicles feature light-alloy wheels (magnesium and aluminum) offering reduced inertia. Their lighter weight allows for increased thickness and rigidity. Wider designs increase the contact area with the ground, enhancing grip. The high thermal conductivity of light alloys improves brake and tire cooling.
A tire’s fundamental dimensions, usually engraved on its sidewall, are represented by two numerical values in inches and a letter. The first number indicates the internal width, the second the nominal diameter, and the letter denotes the rim’s edge profile.
Tires
Tires are inflatable rings fitted around wheels, maintaining an air cushion between the wheel and the ground. Their purpose is to provide optimal surface contact, ensuring grip while absorbing ground irregularities. This aids the suspension system, enhancing passenger comfort.
Tires consist of an inner tube and a casing, although tubeless tires are also common. The casing attaches to the rim through its beads. The inner tube, housed within the casing, holds the required air pressure. Tubeless tires use an inner liner instead of a separate tube.
Tires are made of synthetic rubber, often styrene-butadiene rubber (SBR), which offers superior grip and damping compared to natural rubber.
Inner tubes have a valve protruding through the rim for inflation. Tubeless tires have the valve mounted directly on the rim. This design minimizes air loss during a puncture and allows for repairs without removing the tire. The pneumatic valve is standard for these tires.
A drawback is potential air leakage due to tire deformation under acceleration and cornering forces.
Tire Components:
- Casing: The tire’s outer layer provides strength and consists of layers of fabric cords bonded together.
- Belts: Layers of fabric between the tread and casing prevent tread deformation and absorb shocks.
- Tread: The part in contact with the ground, featuring a pattern or sculpture responsible for:
a) Adhesion and grip.
b) Resistance to impacts, cuts, and external factors.
c) Water drainage.
d) Acoustic comfort.
Tread wear indicators signal tire replacement time.
- Shoulders: The tread’s edges, minimizing friction with the casing and protecting against side impacts.
- Sidewalls: Connect the tread to the bead, containing tire information and markings.
- Beads: Contact the rim, ensuring secure attachment, air retention, and force transmission.
Tire Construction Types
Tires can be classified as diagonal, radial, or mixed based on the arrangement of fabric layers within the casing.
Diagonal Tires:
Feature cords arranged at an angle of 30° to 42° for maximum casing circumference.
Radial Tires:
Have cords running radially from bead to bead at a 90° angle to the tire’s circumference.
Benefits of Radial Tires:
- Increased mileage due to reduced friction.
- Lower fuel consumption.
- Improved stability and handling.
- Enhanced comfort due to sidewall flexibility.
- Reduced heat generation.
- Lower operating temperature.
Mixed Tires:
Combine diagonal plies with a radial belt layer between the casing and tread, offering intermediate properties between diagonal and radial tires.