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The best mountain bike rims are generally lightweight and spin well enough to grind up a hill, and are strong enough to handle speedy descents on uneven terrain.
Rims
The rim is usually made of metal. Mountain bike metallic wheel rims are now normally made of aluminium alloy. The rim is connected to the hub by several spokes under tension. Rims designed for use with rim brakes in order to provide a smooth braking surface, while rims meant for use with disc brakes or hub brakes sometimes lack this surface.
When it comes to rims, there are many considerations, that impact on performance such as the cross-section, aerodynamics design, mass, inertia, stiffness, durability, tubeless tire compatibility, brake compatibility, and cost.
Aluminum rims are often toughened with either single eyelets or double eyelets to dole out the stress of the spoke. Single eyelets buttress the spoke hole. A double eyelet is sort of like a cup that is bolted into the walls of a double-walled rim.
Clincher rims are rims for tubular tires. Most bike rims are clincher rims (standard) for use with clincher tires, where tubular tires are interlocked with an airtight inner tube.
Spokes
At the end of each spoke is a specialized nut, called a nipple, which is used to adjust the tension in the spoke. The nipple is usually located at the rim end of the spoke but on some wheels is at the hub end to move its weight closer to the axis of the wheel, reducing the moment of inertia.
The rim is linked to the hub by several spokes under tension. At the end of each spoke is a particular nut, called a nipple, which is used to fine-tune the tension in the spoke. Double-butted spokes reportedly condense thickness, are lighter; and more elastic, thus more aerodynamic. Single-butted spokes are thicker and a bit heavier. There are even Triple-butted spokes.
Cross section
Spokes are more often than not, circular in cross-section. However, some high-performance mountain bike wheels may use spokes of flat or oval cross-section- this is also known as bladed, to decrease aerodynamic yank.
Number of spokes
Mountain bike wheels normally have 28, 32 or 36 spokes. Although BMX bikes tend to have 36 or 48 spoke wheels. Wheels with less spokes have an aerodynamic benefit. Conversely, the reduced number of spokes results less support for the rim, requiring a sturdier and rim.
Lacing
Lacing is the pattern by which spokes connect the hub to the rim. Most manufacturers use the same lacing pattern on both sides of a wheel. However, nowadays some specialty wheels are being laced with different patterns on each side. The idea is create the lightest and most vertically stiff wheel.
Tubeless Rims
A tubeless tire system requires an air tight rim that is capable of being sealed at the valve stem and spoke holes, as well as the tire bead seat. You will also need a tubeless compatible tire-Universal System Tubeless (UST), initially developed by Mavic for mountain bikes is the most common combination of tubeless tires and tubeless rims.
The upside of using tubeless tires is the capability to use low air pressure for improved traction without getting flats because there is no tube to pinch between the rim. The downside to using tubeless tires, they are a bit more difficult to mount on the rim than clincher tires.