Top 5 Mountain Bike Upgrades: #4 Tubeless Tires
Benefits of tubeless tires:
Ability to run with lighter air pressure
When riding with lower pressure in your tires, more rubber makes contact with the ground which allows for more stability and traction. This becomes especially noticeable in tight turns as the tire compress and flattens somewhat. Riding with lower air pressure also makes for a generally softer and more comfortable ride. Since the tires are somewhat more supple with lower pressure, the tires will absorb more of the impact of obstacles like roots and rocks.
No Pinch Flats
The most common flat with a traditional tube and tire is a pinch flat, which occurs when the tire compresses against the rim far enough for the tube to get pinched between the rim and tire. This usually occurs when rolling onto the edge of a rock, a tree root, or crossing over some other trail obstacle. The resulting tear in the tube looks somewhat like a snakebite, which is a common name for a pinch flat.
Fewer flats, period
Most sealants used to mount tubeless tires also work very well at sealing small punctures. In some cases, small thorn-sized punctures that would normally require removing and patching a tube would not even be noticed. In other cases, simply adding more sealant can quickly fix the issue.
I've even fixed many small thorn-sized flats trailside without a patch with specially made rubber glue for tires (you could even use super glue in a pinch). Just squeeze open the hole in the tire as much as possible, put in a few drops of glue, wait a few minutes for it to dry, pump up the tire and go.
Improved sealants make mounting easier
Back in the day getting tubeless tires to mount correctly was sometimes a Herculean task. With today's improved rims and sealants, it's really pretty simple. Tubeless ready rims have valve stems that screw directly into the rim. These valve stems also have removable cores. Install the valve, mount the tire, remove the valve core, apply sealant through the now open valve stem, re-screw in the valve core, spin the tire to spread out the sealant, pump it up with air, go ride. Pretty simple, really.
Disadvantages of tubeless tires:
Generally higher cost of tubeless ready rims
Tubeless ready rims are generally more expensive than a rim for a traditional tube and tire. Since a good set of rims is one of the more expensive parts of a mountain bike, upgrading to tubeless tires can be a pricey upgrade.
There are sealants that claim to be able to mount a tire tubeless on just about any rim, but I would not normally recommend their use. I've seen situations where guys lost air pressure on a ride and the tire unseated from the rim. Flats happen. But in this situation, it's not just a flat, since you would need more sealant to get the tire to seal back on the rim.
High air volume is required to seat the tire on the rim.
Needing high air volume to seat the tire is not a problem if you have a high volume pump or access to an air compressor. Might be a little inconvenient to get an air compressor to mount a tire, but it's not that big of a hassle. However, if there's a major problem on the trail and the tire becomes unseated against the rim, it is highly unlikely that a hand pump would ever be successful in re-seating the tire trailside. It's usually a good idea to carry a spare tube for emergencies (yes, you can put a traditional tube inside a tubeless tire) and/or a small air canister.
Mounting can be a challenge
While improved sealants and rims make mounting easier, getting a proper seal can still be difficult. With some rim and tire combinations, this can actually be quite a hassle. I've personally had some tires that just never sealed adequately even after multiple applications of sealant.
Since tubeless tires sit tighter on the rim, getting tubeless tires on and off the rim can be a challenge. You really shouldn't use tire levers either to help mount/unmount the tubeless tire, since the levers can damage the bead of the tire that seats against the rim. Strong hands and fingers, along with appropriate cursing, is usually required.