Basic Guide To Mountain Bike Tires and Tubes
While most road bike tires look virtually identical, save for the occasional splash of color, mountain bike tires are far more distinctive. More importantly, fat tires perform a myriad of functions, depending on size, weight and tread pattern. There are skinny, ultra-light models designed specifically for high-speed cross country racing. And there are wide tires with massive tread designed for ultimate traction in burly rock gardens.
Add it all up, and the mountain bike tire industry is one of the most technologically advanced in the cycling industry. Unique rubber compounds, strategic tread patterns and the recent push toward tubeless technology are just part of the diverse and complex mountain bike tire story. But before we dive into the complex details, it helps to understand some of the basics.
First off, there are two basic tire types, road and mountain. Generally, road bike tires have smoother tread, a larger diameter and a narrower width. These factors conspire to make them more efficient on paved surfaces. Mountain bike tires, on the other hand, are generally wider with knobby tread, making them more sure-footed on dirt trails, but slower on smooth surfaces.
Within the mountain bike tire world, there are two primary designs, standard hook-bead clincher and the increasingly popular tubeless hook-bead clincher. More on what this all means later. First some tire basics.
In an ideal world, mountain bike tires would be light, puncture proof, durable, roll efficiently and maintain traction on all surfaces. Unfortunately that tire does not and cannot exist. But by tweaking certain tire characteristics, you can create tires that will do many things pretty well or some specific things very well.
For example, a super-knobby mountain tire will gobble up rough trail, but will be less efficient when things smooth out, while a lower-profile model will roll fast on smooth, loamy trails, but provide less traction over loose rocks and be more susceptible to puncture.
Nearly all modern mountain bike tires are made up of three main components, a casing, tread and a bead.
The fabric foundation of the tire is called the casing (No. 6 in Diagram 1). It is constructed to resist stretching while remaining flexibility. This allows the tire to hold its basic shape while conforming to trail surfaces. It is made of plies that are layered on top of each other. More layers mean more wear and puncture resistance, but heavier tires. A common way to boost puncture resistance without adding weight is to place a tough, cut resistance belt, usually Kevlar, under the tread. This helps prevent flats but also makes the tire more expensive and heavier.
Most modern tire casings are made of nylon, but polyamide, cotton and aramid (Kevlar) alone or in various combinations are also sometimes used. The size of the threads used in the plies affects the handling and durability of the tire. Thread size is called thread count and is noted as threads per inch or TPI.
High TPI Casing: A TPI of over 100 indicates very fine threads that make a thin but supple casing. Thin, supple casings conform better to the riding surface for improved traction. Racers will appreciate the improved handling. High TPI tires are also usually lighter, which improves efficiency and acceleration. The fine tight weaves of high TPI casings are more resistant to certain types of punctures. However the thinner casings are more prone to sidewall cuts, and high TPI tires are usually more expensive and less durable.
Low TPI Casing: Low TPI (60TPI and less) tires have thicker more durable casings that are ideal for all mountain trail riding or downhill racing. The thicker casings are more resistant to cuts, meaning low TPI tires are generally more durable and cheaper. But that thicker sidewall means they’ll be heavier and slower.
The part of the tire that contacts the ground is called the tread (No. 7 in Diagram 1). Depending on the tire and its intended purpose, the tread is designed to maximize traction, durability, puncture resistance, and/or rolling efficiency. Tire engineers wrestle everyday with balancing these requirements, knowing full well that no tire can do all things well. Tire engineers are concerned with three aspects of the tread: compound, tread pattern and shape.
Compound: Most tire treads are made of butyl or natural rubber that often includes additives such as carbon black, Kevlar or silicon. These additives improve specific tire characteristics. Some tires incorporate multiple tread compounds. For instance dual-compound tires have a tougher compound in the middle for straight-line durability and a stickier but less durable compound on the sides for enhanced cornering traction.
Carbon black compounds are typically more durable, longer wearing, and more accepting of abuse like poor storage and use on resistance trainers. But typically they are not as sticky and have poorer traction. Silica and other synthetic compounds are typically grippier and have better rolling resistance, but are less durable.
Tread Pattern: Treads range from smooth and slick to knobby. Many treads are multi-directional, meaning the tire can be installed in either orientation. Others are unidirectional and designed to be oriented in only one way. Some mountain bike tires have a tread that is intended either for the front wheel or the rear wheel.
Different tread patterns work better in different mountain bike riding conditions. For dry, rocky, hard pack, tires with many, small, closely-spaced knobs usually work best. For loose, sandy areas like many parts of the southwestern United States, wide tires with large paddle-shaped knobs are best. For muddy conditions, you want tires with fewer but tapered knobs that are spaced further apart to better shed mud. For winter riding on ice, you want studded tires. For moist, loamy and rooty trails, compromise tires with midsize knobs and spacing closer than mud tires usually works best.
The primary function of rear off-road tires is climbing. Rear tires use some arrangement of knobs or lugs that dig into the trail. Some have a V-shaped array of knobs oriented so the open end of the V bites the slope. Or some employ longer knobs that run side to side and that function like the paddles on a river boat. Front off-road tires are tasked with holding traction during hard cornering. Consequently their knobs tend to be oriented parallel to the plane of the tire. This puts more material resisting lateral drift.
The bead (No. 4 in Diagram 1) is one of the two steel or Kevlar cables that are folded into the inside edges of the tire casing of modern hook bead clinchers. In a practical, non-scientific sense, the beads do not stretch. This quality keeps the tire from blowing off the rim even at the highest air pressures. Beads can be made of steel or of flexible, foldable aramid cables. Both have advantages and disadvantages.
Kevlar Bead: Also known as folding bead, these make the tire lighter, saving 50-70 grams over the same tire with a steel cable bead. This also makes the tire foldable for easier transport and storage. The downside is that they are more expensive, and more difficult to install because of the floppy, flexible bead.
Wire Bead: Also known as steel beads, these hold their shape better for easier mounting, are less expensive, and make a good choice for use on trainers or as a training tire. The downside is that they are typically heavier.
Now that we have some of the basics covered, it’s time to delve into tire types. Here’s a breakdown of key categories.
Hook-Bead Clinchers: These are made to fit on modern rims that have inward projecting ridges at the top of the rim walls. Many older rims had smooth flanges. These hooked edges grab the tire just above the bulge of the bead. Standard clinchers are available in many different sizes, shapes, brands and tread patterns, and are generally lighter (even with the tube) than their tubeless cousins.
Other advantages of traditional clincher tires include the fact that they work with all appropriately sized rims, are usually cheaper for a given tire, and are easier to install. The primary detractor is that they are more susceptible to pinch-flat punctures, and typically require firmer inflation, which decreases traction
Tubeless Hook-Bead Clinchers: These are very similar to hook-bead clinchers, but they have a stiffer and more prominent bead that allows an air tight seal when used with tubeless specific rims or a standard rim adapted to be tubeless. Tubeless clinchers are designed to be used without tubes, but can be used on any appropriately sized rim with a tube.
Benefits include improved ride quality, increased ride efficiency because energy is not lost to tube creep (aka hysteresis), and because there is no tube, pinch flats are not an issue. Tubeless tires can also be run at lower pressures for better traction and comfort. The downside is that they can be more difficult and messy to install, are more expensive for the same tire, and are more difficult to repair on the road or trail.
Just like with car tires, bike tire sizing is a combination of diameter and width. Mountain bike tires are sized in inches. An inflated 26x1.95 mountain bike tire is about 26 inches in outside diameter and 1.95 inches wide. The most common diameters are 26-inch and 29-inch. The 29er size is based on the 700c road size and is essentially a wider, knobbier 700c tire fitted to a wider and stronger 700c rim.
ISO Tire measurement: The International Standards Organization (ISO) has adopted more precise metric standards for tires and rims. Unlike traditional sizing, e.g. 26x1.95, the ISO tire standard is based on accurate measurements of a tire’s Bead Seat Diameter and its inflated width. For instance, the ISO equivalent of a 26x1.95 mountain tire is 50-559 where 50 is the inflated width (50mm = 1.95”) and 559 (22”) is the Bead Seat Diameter. Here is a chart of common tire sizing equivalents:
Tubes are the butyl or latex rubber bladders that hold the air in standard hook-bead clincher tire and inside tubular tires. Latex tubes are considered a higher performance tube because they are more supple and lighter. Their downside is that they leak air more quickly so they need to be re-inflated more often, maybe after every ride. Butyl tubes lose air slower, so re-inflation may be necessary only once a week. Butyl tubes can be made lighter by reducing wall thickness. This makes them accelerate better but makes them more susceptible to punctures.
Unlike tires, tubes fit a range of tire sizes. For instance a 26 x 1.95-2.1 tube is compatible with any tire between 26x1.95 and 26x2.1. Tubes are commonly available with one of two valve stem types: Presta (skinny European-style stems) and Schrader (common U.S. car valve stem).
All Schrader valves have screw-in removal cores that allow tightening or replacement if they leak or to add tire sealant. Most Presta tubes do not have removable cores, and those that do are easily clogged by sealant. Presta tubes are available with different length stems to accommodate the depth of the rim used. The most common is 35mm, but you can also find 48mm and 60mm. These longer stems are handy for exceptionally deep rims.