Bead
The bead is the edge of your tire, the part you tuck into the rim when changing a flat. There are two beads per tire, one on each side, holding the tire inside the rim, providing a backbone of support. Traditionally made from a strong steel wire, the bead is what makes the tire difficult to fold.

When manufacturers became obsessed with component weight in the 1970s, steel beads were replaced with Kevlar, a strong, light synthetic fiber invented by DuPont in 1965. This change cut two ounces per tire. Kevlar is more flexible than steel, which makes for easier packaging, storing, and transporting.

Fabric
Woven between the two beads is a cloth fabric that provides the body of your tire, also known as the casing. When your parents were kids, the fabric used was a cotton canvas, for it’s strength and durability. Nowadays, some manufacturers still use cotton, but most use nylon, a synthetic fabric created during WWII as a replacement for silk. Nylon has superior tensile strength to cotton, with a breaking point of about 15-pounds, as compared to cotton’s 10-pounds, for a midweight thread. At the very high end, we’re seeing hybrid fabrics using Kevlar, which are lighter, and more durable, but also pricier.

Unlike normal cloth, where threads of fabric are interwoven, bike tires use layers of threads placed side by side called plies. Each layer of plies is placed perpendicular to the one beneath it. The term “TPI” refers to how many threads exist per inch of tire fabric. A higher TPI has come to signify higher quality. The thought is that more threads means that thinner threads are being used. Thinner threads create tighter plies, which require less rubber to fill in the gaps between threads. This all adds up to lighter tires with less rolling resistance.

The downside to using lighter tires with a higher TPI is that the tires are thinner and thus less puncture resistant. But manufacturers recognize that more threads per inch also creates a more flexible tire, which is more naturally puncture resistant, and thus solves its own problem. Some manufacturers take it a step further and add durability back in by putting a thin layer of Kevlar in between the fabric and the tire tread. Tough and light, what the Kevlar adds in weight, it more than makes up for in puncture resistance.

Italy-based manufacturer Vittoria, with their substantial R&D department, has the current claim to the highest TPI in the industry, with a chart-topping 320. Given a tensile strength of 15-pounds (per thread), you’re looking at fabric that can withstand 4,800-pounds (warning: don’t try this at home). As a comparison, they make road tires all the way down to 26 TPI, with an average of about 120 TPI. The price differential is about $50 per tire between the highest TPI tire and the lowest. As usual in road bike components, the mantra “you get what you pay for” sticks.