When it comes to mountain bike tires, there are several factors to understand which will enable you to make an informed tire purchase. These include construction, tread design, compound.
TPI-The carcass or casing of a tire 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, measured in threads per inch, or TPI, 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. 67 to 127 TPI Lower thread count carcasses use coarser threads with more rubber surrounding them. This makes for a stiffer, but more durable tire. 50 to 67 TPI counts strike a good balance for heavy 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 must 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″ 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 built as cross-country or light trail tires.
Tread Design & Profile
The tread is where the rubber hits the trail, so understanding tread design is crucial to knowing how a tire will perform. There are three performance zones on a tire tread: the center, transition, and cornering zones. The center zone is where most of your time is spent, and is responsible for straight-line traction and rolling resistance. Tires with shorter, smaller, closely-spaced center knobs have minimal rolling resistance, are easier to pedal, and are better-suited for hardpack conditions. Taller, broader, widely-spaced knobs give more traction, but also more resistance, and are more effective in softer or looser conditions.
On either side of the center tread is the transition zone. This determines how your tire behaves when transitioning from vertical to angled in turns. Transition zones that are open, without tread blocks, are faster when the tire is transitioning, but tend to drift between the two knob sections. This is unsettling if you’re not ready for it, and it requires you to place the tire/bike right where you want it. A transition zone filled with knobs is very forgiving, and hooks up throughout the lean until it reaches the cornering knobs. This style of tire is slower through turns however. Cross-country tires with small, low-profile tread often have uniform distribution of knobs from the center, through the transition, and out to the cornering zones.
Finally, the cornering zone provides traction in the most extreme lean angles. Large, wide knobs will give more grip, although big knobs can “squirm” if the trail is too hard-packed. Lower profile knobs will be faster in hardpack, but will slide excessively in looser conditions.
Tread profile is also important. Round tread profiles tend to be more forgiving and versatile. Square profiles excel in loose dirt and tend to “carve” (until the breaking point is reached) compared to a round profile’s driftier feel. When mixing profiles, advanced riders should try a square front and round rear. Up front, once you have figured out how hard the square profile tire can be pushed, you’ll have an accurate and locked-in tire guiding you around turns. Since the rear wheel follows a wider arc than the front, it’s nature is to drift a bit more as it tries to follow the front tire around a turn, and a round profile will help maintain control during the drift.
The rubber compound used in the tread will influence tire longevity, traction, and rolling resistance.
Harder compounds last longer and roll faster, but softer compounds provide more traction. Often, several compounds will be used on one tire. The hardest compound provides a foundation for the tread blocks. Hard or medium rubber is used on the center tread for increased wear resistance and rolling speed. Softer compounds are often used for cornering knobs. This provides extra grip in turns.
Cross-country tires use harder rubber compounds, shorter, smaller, tread blocks, lighter casings, and folding beads to minimize rolling resistance.
Trail and all-mountain tires use medium to soft rubber compounds, wider, medium-to-tall tread blocks, slightly reinforced casings, and folding beads to provide more traction and longevity without adding too much weight.
Downhill tires use soft compound rubber, wide, tall, aggressive tread blocks, reinforced casings, and both steel and folding beads to maximize traction and flat resistance.
So there you have it, Art’s Cyclery’s quick Start Guide to Mountain Bike Tires. Now that you know what to look for, you’re sure to find the tire you need at artscyclery.com.