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I have some new tubeless MTB rims and I'd like to avoid buying new tyres for them. The tyres are reasonably sturdy (Continental Race King Protection - 4 plies / total 240tpi) and I have sealant. As I understand it the sidewalls aren't as tough so you can't run on quite as low pressures as UST tyres but that's OK with me.
Any other potential problems?
I've been waiting until I had enough time to give this a proper answer, because the answer to the question in the title is "it depends" and it's a very important "it depends." It runs the gamut from yes to absolutely not. I want to cover road bike tires here as well, as I don't want folks making the jump from "it's ok for mountain bikes" to "it's ok for road bikes." Cyclocross bikes sit right about in the middle depending on the pressure you like to run
For mountain bikes you can run just about any tire you want on a tubeless rim as long as you use sealant. There are some exceptions, as I covered in my answer here. It's also important to note that when you go tubeless you cannot run the high end of your tires' pressure range anymore. The wider the tire, the more this is true. Most folks have no intention of switching to tubeless and pumping their tires up tight, but then again you'd be surprised.
For cyclocross tires, pretty much everything above holds true, especially the bit above pressure. If you're considering running anything above 50psi you need to get a dedicated tubeless cyclocross tire. It's just not worth the risk otherwise.
For road, the answer is an emphatic, unequivocal no. You cannot run road tires made for tubes on a tubeless setup. Ever. They will blow off the rim, hopefully before you even get them up to pressure, and you will get hurt or killed. You can convert a normal rim to a tubeless setup with a kit, but you cannot run a normal road tire on any tubeless road setup. If you want to know why, keep reading...
The whole reason pneumatic tires work is because they're pushing outward on the wall of the tube or tire. PSI stands for pounds per square inch. I assume everyone already knows that but what some haven't thought about is that the term is pretty literal. For every 1 PSI you pump up your tire the air pressure inside is literally exerting a pound of force per square inch on the inside relative to outside atmospheric pressure. That's why if you're a 150lb rider, bike and all, and you have your tires pumped up to 100psi, you've got a grand total of 1.5 square inches of rubber making contact with the road (there are other circumstances to take into account here like casing and compound, but we wont nitpick material physics here). That also means that you've got enormous amounts of pressure trying to tear your tire off the rim in every direction. The bead on wire (non-folding) and kevlar (folding) beaded tires is ever so slightly stretchy. Not very stretchy, but just enough that neither type of bead will stay seated on the rim under the amount of force which the air inside of a pumped up road tire is exerting outward on the tire. Think about what a tube does when you over-inflate it outside the tire- it expands to a much larger diameter than your wheel. Your tire is doing the same thing on a much smaller scale. It's doing this just enough that a standard bead without a tube in place can climb out of the channel where it sits and over the edge of your rim, causing catastrophic results if you're rolling.
With a tube in place inside a rim, the air pressure pushes against the tube first which in turn pushes against the tire which binds the bead in place against the rim. The interaction between the tube pushing against the tire vs just air pressure pushing against the tire is a little different. The tube helps reinforce the pressure against the bead and lock it in place whereas air pressure alone is not enough to fight against the tire's need to expand in diameter.
So how do tubeless road (and "official" tubeless cyclocross) tires fix this problem? Instead of a kevlar or wire bead- both of which have a little bit of stretch to them- tubeless road tires use a carbon fiber bead which has virtually no stretch to it at all. Since they don't stretch, carbon fiber beads are not susceptible to the pressure inside the tire tearing them off the rim like a standard bead (I'll mention that road tubeless tires typically have a little differently shaped bead as well, but this is more to prevent burping when matched with an official road tubeless rim). This is why you can convert a standard road rim to tubeless but you cannot cannot cannot run a standard road tire on a tubeless setup, official tubeless rim or not.
The same problem described for road tubeless can apply to cyclocross or mountain bike setups if the pressure is high enough. That's why carbon fiber beaded tires are available for cyclocross but you can run a standard cross tire tubeless at lower pressure. For mountain bikes non-elastic beads just aren't necessary because no one in their right mind pumps a mtb tire up that tight to begin with- tubeless or not.
Yes, you surely can use standard tyres with tubeless rims. I do so and it's ok.
But you can expirience some problems (from my own experience and what I saw):
Relatively high pressure (30+ psi) fixes most of this potential problems. And full suspension bikes have less possibility to get a puncture if you are riding trails, forest, etc. So you get more advantages, than disadvantages, from using tubeless rims with standard tyres.
And it's still a good idea to have a spare tube and pump in your backpack, because you can get a bigger puncture or even cut and sealant will be useless.
I've never played with tubeless bike tires, but I assume there are conceptual similarities to tubeless auto tires. First off, a tubeless tire isn't really tubeless -- there is a "tube" (thin layer of solid rubber) laminated to the inside of the tire to contain the air. Presumably the sealant you have would serve the same purpose, but it's not clear how well.
Second, the tubeless tire has a specially-designed bead area, smoother than a tube-type tire and designed to seal tightly to the rim. Your typical bike tire has a fairly rough bead area, and not one that would naturally make a tight seal. But again, the sealant may make up for this to a degree.
As to the sidewalls, the likely problem would be that the tube-type tire's sidewalls are not flexible enough (and the bead not stiff enough), and so, when flexed, might not allow the bead to remain in tight contact with the rim. This could result in seal failures when hitting a bump, etc.
Use tubeless or tubeless-ready tires. Racers often convert standard-tube-type tires to tubeless using sealant for an ultralight setup, but it's not always reliable. Expect more flats, trouble seating and sealing, and lots of trial and error with conversions.
I assume you actually want to run tubeless, since you mention sealant.
It may depend on which rim you have. Actual Tubless or tubeless-ready?
An example of tubeless-ready rims are the popular line of Stan's rims. They have a special design they call socket-beat tech, which will grip a non-UST tyre better than a standard rim, to resist burping etc.
I have good experience with Stan's rims and conti tyres all the way from the flimsy Twister 1.9" Supersonic, Race King 2.0 Supersonic, Race King 2.0 Race Sport and X-King 2.2" Protection. All worked well, not with extremely low pressures, but with much lower pressure than I needed to avoid pinchflats with tubes, and with much better grip as a result. I'm about 70Kg and run around 28-35PSI most of the time (although down to 25psi was possible with the 2.2"'s). Lower pressure than that would result in a wallowy feeling through fast corners, and the occasional burp.
If your rim is actual UST-spec tubeless, it may not grip the non-UST tyre as well, resulting in some issues. I'd say give it a try and see how it goes, since you have the tires and sealant already.
I use to run a Stan's BST rim with non-tubeless ready Maxxis High Roller 2 exo tire. They went tubeless easily worked great for almost all my biking, but if I jumped/ dropped over 4 ft my rear tire would blow off at the bead. I weigh right around 200 and run about 30 psi. I bought the non-tubeless ready tires because Stan's say you can use practically any tire, but after my experience I personally will never run anything other than a tubeless ready tire.
I'm using Michelin tires over Brazilian tubeless rims. I've got Stan's NoTubess kit for tape and seal.
My front wheel stays perfect, but for rear wheel my tire got slip off the rim once i was very fast on a hill with specifically stride, like @zetdotpi told.
In this case a good idea is to have a spare tube and pump in your backpack, together tools to remove the valve.
