Tubeless mountain bike tires. I've never run tubeless but have been told by many of my male racer friends the reason they run them is that they can run less air pressure which makes their bikes stick to the trail easier, without the fear of flatting as much as with tubed tires. You have to run more air pressure depending on heavier riders and type of rider if you are a basher and heavier, you need more air pressure, hence the reason they want to run tubeless. They still can flat and if you are by yourself, you should carry a tube in case you can't get the tire to seat again or it has a cut in it or something like that.

I never wanted to spend the money on them cause 1. I rarely flat cause I'm not too big; 2. I rarely flat cause I'm a cautious downhiller; 3. I only run 35-40 pounds in my mountain bike tires anyway. So, it is a personal choice I guess and there are a lot of variables into why you might want to run tubeless vs. tubed.

I raced several seasons in the Arizona desert and never flatted (even with cactus thorns everywhere) with tubed tires. The only time I even run slime is when I do 24 hour racing and that is because there are lots of thorns on the trail from people running into cactus at night! And, they are hard to miss when you are riding at night with lights.