Editor’s Note: This article is courtesy of the team at Art’s Cyclery. The original post can be found here.
When it comes to mountain bike tires, there are several factors to understand which will enable you to make an informed buying decision. These include construction, tread design, and compound. Here’s what you need to know about each one.
TPI (as in threads per inch) is the carcass or casing of a tire, and is made up of parallel threads, usually nylon, which are coated with rubber and oriented at a 45-degree angle from bead to bead. Higher densities of threads create a tire that is more supple with lower rolling resistance, but is less protected against punctures. Higher thread count tires, from 67 to 127 TPI, are used for cross-country and light trail riding.
Lower thread count carcasses use coarser threads with more rubber surrounding them. This makes for a stiffer, but more durable tire. These 50 to 67 TPI counts strike a good balance for heavier trail, all-mountain, and downhill tires.
A tire’s bead is the inner edge of the tire. Air pressure within the tire keeps the bead seated properly in the rim, and the tire on the wheel. Beads do not appreciably stretch.
For our purposes, beads can be thought of as folding and non-folding. Folding beads are made with a flexible material like nylon, Kevlar, or Aramid. Non-folding beads are made of steel and do not bend. Folding beads are much lighter than steel beads.
Sidewall construction influences a tire’s flat resistance, weight, and ride quality. Sidewall thickness is determined by how many layers of carcass, or plies, are wrapped around the tire bead, and by any inserts in between the plies. Inserts increase puncture protection, structural integrity, or both. Commonly used, lightweight, “breaker” layers are dense strips of nylon, Kevlar, or Aramid.
These materials are light and pliable enough to minimally affect ride quality but still provide protection. Cross-country tires will have little to no sidewall protection to save as much weight as possible. Thicker nylon or butyl inserts are used in all-mountain and downhill tires for added pinch flat resistance and stability, at the expense of weight. Thicker tires with more sidewall structure can be used with lower pressure. Thinner, lighter tires rely on higher air pressure for structure and pinch flat resistance.
Wider tires are heavier, but provide more traction and the ability to run lower pressure. Higher volume tires provide a bit of suspension also. For these reasons, tires from 2.25″ to 2.7″ and higher are mostly used on trail, all-mountain, and downhill bikes. Narrower tires are lighter, roll faster, and require higher air pressure. Tires from 1.9″ to 2.1″ are usually billed as cross-country or light trail tires.