Parts of a Tire: Tread, Belts, Plies, Bead and Sidewall

A tire is layers, not a lump of rubber

From the outside a tire looks like one piece of black rubber. Inside it's a stack of layers, each with one job. Knowing them explains a lot: why a bubble can't be patched, why repairs stop at the shoulder, and where a slow leak can hide.

Here they are from the road inward, using the definitions in the federal tire standard (FMVSS No. 139) and the tire makers' own descriptions.

Tread: the only part that touches the road

The tread is the patterned rubber on top. BFGoodrich says it provides traction and cornering grip and is designed to resist wear, abrasion and heat.

Continental names its pieces:

• Ribs, the raised sections running around the tire, made up of tread blocks.
• Tread blocks, the raised rubber segments that touch the road.
• Grooves, the deep channels between tread blocks, running around and across the tire. They move water out from under it.
• Sipes, small, thin slots molded into the tread blocks.

The federal standard adds treadwear indicators, the projections within the principal grooves that show how worn the tread is. When the tread is flush with them, you're at 2/32 inch. Our tread depth guide covers what that means.

Where the tread meets the sidewall is the shoulder. It matters for repairs, as you'll see below.

Belts: the stiff layer under the tread

Under the tread sit the belts, or crown plies. BFGoodrich says they provide the rigid base for the tread. USTMA describes the usual layout as typically, two belts with steel cords laid at opposing angles, which is where the term "steel-belted radial" comes from.

The belts are also why the tread area is the only place a tire can be repaired. USTMA limits repairs to punctures in the tread area, no larger than 1/4 inch (6 mm), and rules out the sidewall and shoulder, which flex the most and lack the tread area's reinforcement. Tap a zone below to see how the guidance treats each part of the tire:

Body plies: what actually carries the car

BFGoodrich describes the carcass ply as thin textile fiber cords bonded into the rubber, and says these cables are largely responsible for determining the strength of the tire. The federal standard defines the carcass as the tire structure, except tread and sidewall rubber which, when inflated, bears the load. USTMA adds that most tires have one or two body plies, each typically comprised of polyester, rayon, or nylon cords within a rubber layer.

Radial: how the plies are laid

The R in a size like 225/65R17 means radial. FMVSS 139 defines a radial ply tire as one in which the ply cords that extend to the beads are laid at substantially 90 degrees to the centerline of the tread. In other words, the cords run straight across from bead to bead, with the belts on top holding the tread flat. Our guide to reading tire size covers the rest of the size.

When the plies are damaged

This is what a sidewall bubble is. Michelin explains that a bulge or bubble generally indicates damaged cords caused by a severe impact, such as a pothole or curb. With the cords broken, air pressure pushes the rubber outward. No patch can rebuild cords, which is why our guide to a bubble in the tire sidewall ends in a replacement.

Michelin also warns that internal damage, such as broken cords or belt separation, is often invisible from the outside, which is why a hard impact is worth an inspection with the tire off the wheel.

Sidewall: protection and the label

The sidewall is everything between tread and bead. BFGoodrich says it protects the side of the tire from impacts with curbs and the road and is where the tire's details are printed.

It's also where the law puts the information. FMVSS 139 requires each tire to be marked with its size, maximum permissible inflation pressure and maximum load, and with the generic name of each cord material and the actual number of plies in the sidewall and in the tread area. Look closely and you'll find a short line listing the plies and cord materials under the tread and in the sidewall. That line is the tire's construction, in its own words. Our guide to tire sidewall markings covers the rest of the small print.

Bead: where the tire meets the wheel

The bead is the inner edge that sits on the wheel. FMVSS 139 defines it as the part made of steel wires, wrapped or reinforced by ply cords, shaped to fit the rim. BFGoodrich says the beads clamp firmly against the tire's rim to ensure an airtight fit.

This is a common place for slow leaks. Michelin notes pressure loss can come from a leaking valve, valve cap or even the wheel, and a damaged or corroded wheel lip where the bead seats is one way that happens. Our guide to pothole and curb damage covers what to look for on the wheel.

Inner liner: the air seal

Modern passenger tires are tubeless. The inner liner does the tube's job. BFGoodrich says today's tires use butyl rubber, which is virtually impenetrable to water and air.

The liner is why a proper repair has a patch on the inside. A plug fills the hole; only a patch seals the liner again. Our guide to tire plug vs patch explains the repair the industry accepts.

Putting it to use

You don't need to know any of this to buy tires, but it helps you read what you're buying and what a technician tells you. When you're ready, shop by size; every tire listed carries the same federally required construction line on its sidewall. Tires ship free to the 48 contiguous states and DC, and Extreme Tires installs them at its Sunrise shop or by mobile van across Miami-Dade, Broward and Palm Beach.