Automotive tyres—construction identification: In a modern radial tyre, how are the carcass plies oriented relative to the tyre beads and to each other?

Introduction / Context:

Tyre performance—ride comfort, rolling resistance, heat buildup, and handling—is strongly tied to carcass construction. Two classic constructions are bias-ply (cross-ply) and radial. Knowing the ply orientation allows technicians and engineers to infer expected performance and service behavior.

Given Data / Assumptions:

• Passenger/light-truck on-road tyres.
• Carcass plies (cords) and reinforcement belts under the tread are distinct elements.
• Terminology: ‘‘radial’’ refers to the cords pointing radially outward from the wheel center.

Concept / Approach:

In a radial tyre, the main carcass ply cords run radially from bead to bead, i.e., perpendicular to the bead and wheel rim. These plies are substantially parallel to one another. Over the carcass, circumferential belt layers (often steel) stabilize the tread. This differs from bias-ply construction, where multiple plies run diagonally at opposing angles (e.g., ±35°) and crisscross each other.

Step-by-Step Solution:

Verification / Alternative check:

Tyre sidewall markings (e.g., ‘‘RADIAL’’) and cutaway diagrams show radial cords plus circumferential belts; bias tyres list ply angles and do not include steel belts in the same way.

Why Other Options Are Wrong:

• Diagonal layers both ways: description of bias-ply (cross-ply) tyres, not radial.
• Inner tubes always used: many radial tyres are tubeless by design; inner tubes are optional depending on rim and application.
• Plies circumferential: circumferential layers are belts, not carcass plies.
• None of these: incorrect because the correct description is available.

Common Pitfalls:

• Confusing the tread belts (circumferential) with carcass plies (radial).
• Assuming all radials require tubes—modern passenger radials are typically tubeless.

Final Answer:

all plies run parallel to one another and vertical (radial) to the tyre bead