Last blog was all about road tubeless, where this one will be more and less "controversial" than that one: tubeless for cross (more controversy) and mountain (no controversy).
Let's get the easy one out of the way - mountain bikes. To set up a mountain bike wheel tubeless, you need a mountain bike wheel, a mountain bike tire of the same size as your wheel, rim tape, a valve stem, and sealant. Tape the rim, put the valve stem in, mostly mount the tire, pour the sealant in, finish installing tire, inflate, ride. If the tire is super loose on the rim, add another layer or two of tape. You can use duct tape. Seriously, this one is easy, almost everyone does it, and it almost never causes problems. Why anyone would want to use tubes on a cross country or all-mountain mountain bike at this point is beyond me.
Cross tubeless is another bag of goldfish entirely. First let's start at the beginning and ask why you'd want to go tubeless for cross. In any dirt riding, lower tire pressure equals better traction. Period. That's why tubulars are still so popular in cross - you can go wicked low pressure with minimal risk of pinch flatting, even though it's not impossible (you guessed it - I've done it!). The other huge benefit to low pressure is suspension - it smooths out the ride. Which, in turn, increases traction both from keeping the tire on the ground and from allowing you to weight the bike properly.
The book on cross tubeless is still being written all the time. The leading proponents are the Stan's team, who use Stan's rims (primarily the Grail, currently) and Kenda tires. They use one wrap of Stan's tape and that's it - no strip, no Gorilla tape, nothing. Just one wrap of tape. I overheard Mical (in photo) say she'd used a Kwicker and 22psi in the front at the Night Weasels race, a super muddy, greasy, and always off camber race in which she'd gotten a close second. Jake Wells is pretty good, and he uses the same pressures he would otherwise use on tubulars with his Stan's/Kenda setup.
I've got a few different tubeless cross setups I'm feeling out. My favorite has been a pair of Stan's Iron Cross rims with a Stan's Raven tire front and Kenda Small Block Eight rear. I've raced these at 24 front/27 rear and felt like I was able to ride the bike better than on any other setup. Then I crashed the front into a hidden stump and ruined the rim, along with my shoulder. That setup was perfect for the drought conditions the season started in, but with any moisture those treads are useless. Totally unrelated to tire setups, I've been running into a lot of stuff this year, at great cost to my personal parts and pieces (knee and ribs at the moment). Currently, in addition to the Iron Cross/Small Block Eight, the Stan's Arch EX/Clement PDX combo is working like a freaking champ. All of these setups are disc rims, but can be closely replicated with rim brake setups.
The dreaded outcome of cross tubeless setups is of course the dreaded burp. The burp is a momentary break of the rim/tire seal, which allows some amount of air to escape. It's never good, but it can range between inconvenient and tragic in magnitude. Much like a rolled tubular, best case you are riding gingerly to the pits, worst case you are running to the pits. Most people who've used a tubeless setup have "found minimum pressure" with a burp. I have. Never in a race, touch wood. I choose to wreck my races in other ways!
So the natural question then is "why tubeless?" When set up well, tubeless cross allows the same low pressures and pinch flat protection as tubulars, with huge flexibility and cost advantages over tubulars. Even at its worst, installing a tubeless tire is way less involved than gluing a tubular, and requires no drying time. If it's Friday and you have a set of dry conditions tires on when it starts pouring at dinner, you have every opportunity to switch tires before tomorrow's race. Tubeless tires also cost about half as much as tubulars.
I'd rate the chances of disc brake ubiquity as perfect - that day will come. I make no such forecast for tubeless in cross. The very top players, with no real resource constraints and the ability to travel with dozens of sets of wheels, will suffer any inconvenience to enjoy whatever marginal benefit continues to exist. For the rest of us, tubeless is starting to look attractive. With some bugs left to iron out, it's no slam dunk, but there are some pretty resourceful and talented people fighting those bugs all the time.