An example of terrain that screams Tubeless. Rough, demanding, and tricky, with ...
An example of terrain that screams "Tubeless." Rough, demanding, and tricky, with sharp edges that are ready to pinch-flat your tube.
Tubeless is an advantage to hard-core riders who want maximal responsiveness and control, while avoiding pinch-flats in difficult terrain. "Going tubeless" is not worthwhile if you're basically a gutter-bunny who occasionally ventures onto smooth dirt trails.
The Good
  • Lower inflation pressures
  • Better bump absorption
  • Better control
  • Reduced rotating weight
  • No pinch-flats
The Bad
  • Costs more
  • Messy sealant
  • Backup tube required
  • Power inflator required
Pinch-flats occur when your rim pinches the inner tube against an edge, such as a rocky ledge or root. If there's no inner tube, you can't get a pinch flat! You can, however, cut a hole in the tire. (You're more likely to cut a hole through the tire casing while tubeless than you are with a tubed system, because you don't have an inner tube to add cushion between the rim and tire. Use a big tire, such as a 2.1, and keep your pressure above 30 psi.)
When there's no inner tube pushing against the tire casing, it will deform more easily when it rolls over a rock. That means sharp bumps become softer bumps. Tiny bumps such as small rocks don't cause as much vibration and shaking. The end result: a smoother ride, better response, better control.
A standard UST tubeless tire is heavier than the corresponding non-tubeless version. But once you allow for the heft of the inner tube, the weight is usually a bit less. (If you add sealant, your weight is about the same, or even slightly more.)Many tubeless riders actually use a standard tire on UST rims with tire sealant. This can shed over 100 grams per wheel. This reduction of "rotating weight" makes the bike "zippier." It accelerates better, corners faster, brakes faster.
In many circumstances (rough rock, snow, mud, slippery or rough roots) a lower inflation pressure gives you better control and a smoother ride. With a tube, pinch-flats occur as you try to find that ideal low-pressure ride. When you're tubeless, you can drop the pressure lower than you'd dare with a tubed system.
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Bad:  Expense.   Initial cost:   To go traditional tubeless, you need to buy special UST rims, which aren't cheap. You'll spend between $400 and $1000 to upgrade both wheels, depending on the quality of the rims you buy. A UST tubeless tire costs about twice as much as the same model in the standard variety.
The cheapest way to go tubeless is with a conversion kit. You add a rim liner to your standard rims, fill a standard tire with sealant, and you're there. A kit from Stan's No-Tubes will cost you about $55 after shipping. Sealant: You'll have to buy sealant periodically. But on the other hand, you won't be buying as many inner tubes. Tires:   If you're using tire sealant, your tires won't last as long -- especially true if you're using standard tires with sealant. The sealant eventually softens the rubber. After several months, you may get a puncture or cut that won't seal. And when you try to patch it, the rubber rubs right off the cords inside of the tire, so the patch won't hold. You may also have sudden sidewall blowout as the tire ages. Add this problem to the increased likelihood of cutting your tire on a root or rock (because there's no tube to offer shock absorption between tire and rim), and you're going to be buying more tires. CO2:   To these expenses, add cartridges for your power inflator. If you buy an inflator that uses cartridges with non-threaded necks (standard BB-gun CO2 cartridges), it's cheaper, but still a couple of bucks each time.
While you can run a UST tubeless without sealant, almost everybody uses sealer. The No-Tubes conversion requires it. The sealant makes changing a tire relatively painful. And where does that sealant go when you DO cut your tire on that broken glass bottle? It sprays on your expensive bike jersey, into your derailleur, all over your cables. When sealant gets into critical parts, you have a difficult cleanup problem -- when it's dry, it won't wash off!
You can repair small punctures in a dry tubeless tire with superglue. You can repair sealant-filled tires on the trail by patching it internally. But tubeless riders who don't pack a emergency inner tube (or two!) will, sooner or later, face a long hike. On the trail, you'll need a patch kit, spare tube, and maybe even a little bottle of sealant.
To seal a tubeless tire against the rim, you need a sudden burst of air pressure. Your mini-pump just can't do that. Most tubeless owners find they can't do it with their floor pump, either. That means a compressed-air source. So you need a compressed-air cylinder in your garage, or a gas station with real compressed air -- not just an insert-a-quarter type electric pump -- or, you burn up a CO2 cartridge every time you mount a tire. (That's why a higher percentage of guys at bike shops ride tubeless. They have a compressed-air hose ready and handy.)
If you stick a rim liner in your UST rim, you may be able to get the tire to seal with a standard floor pump. It will NEVER work with a mini-pump. But whether you use a tire liner or not, for on-trail inflation, you'll need a CO2-powered inflator. Power-inflators are great little gadgets. But many riders don't want to spend a few bucks on a CO2 cartridge every time they get a flat tire. (Yes, that's right. You can spend up to $4 for a CO2 cartridge that's good for ONE tire inflation.)
You can convert a standard rim to run tubeless by adding a rubber rim liner. The rim liner has a valve that sticks out through your valve hole just like an inner tube. The rubber strip provides an airtight (hopefully) seal against the rim, and against the bead of the tire. (You could experiment with making your own rim strip by cutting up a small -- child's size, 20 or 24-inch -- inner tube. This won't work on many deeper rims, and it's harder to cut a straight strip of tube than you might think.)
You can use regular, non-tubeless tires with the conversion kit. (You can also use the tire liner in the conversion kit on a UST tubeless rim, which makes it easier to seal a lightweight standard tire on your tubeless rim.) Not all tires, and not all rims, will hold in place when inflated without a tube. At higher pressures (40 to 50 psi) the tire may blow off the rim. If you have a chain-store bike with $4 tires, don't plan on converting those wheels to tubeless.
To provide a seal between the rubber rim liner and the tire's bead, a sealant is required. Although it looks like it adds lots of weight, it's only about 60 grams -- about 1/3 the weight of a lightweight inner tube. A removable valve core lets you squirt sealant directly through the valve stem without removing the tire.