Tires are one of the least sexy parts of a bike, and as such, they don't get talked about much. People rarely rant and rave about tires (unless you are a tire whisperer like Mike Danish or Jon Posner), but really, they should.
When you're riding, your only connection to the ground is through your tires. And for a mountain biker, that connection is often in flux as the terrain varies drastically, you go over obstacles/rocks/logs etc. So having a good appreciation for tires is not only important, but one of the only ways you get faster and stay upright! Thankfully, you can learn that from a pro HERE.
So, tires are important. Finding which one to use then (once you know what you're looking for) can be a challenging and expensive ordeal. Chances are you're not going to go out and buy 10 different tires and give them all a whirl. There are tons of brands out there though, and I find that all of the top brands are typically making a tire for every situation. So for me, I like to stick with a particular brand for a bit and try out their offerings before moving on.
This year, I'm riding and racing Bontrager tires at the strong recommendation of a few friends. There were definitely a few naysayers to this plan initially, as Bontrager doesn't have the rich history as a tire maker as say Schwalbe, Maxxis or Continental, but I wanted to see what they were doing currently and took the plunge. I got three tires for various conditions and have been using them all year: the XR-1, XR-2 and the XR-3.
The XR-1 is exactly what you would expect from a race tire. Fast, light, low rolling resistance. It doesn't have huge knobs on the side, so its use is limited for only certain types of courses, but all in all, its an extremely fast tire that I really like. (There's a good reason Trek Factory Racing uses them front and rear). Looking for something fast for the rear, I have the XR-1 setup in back and I'd probably recommend going with a 2.20 unless the course is smooth with no puncture dangers.
The XR-2 looks almost identical to the XR-1 actually, but its got a bit more burl to it. With a more exaggerated side knob and slightly different shaped knobs, it can handle a bit more in the loose terrain category, while still rolling very fast. I have used it on super technical courses as a rear tire and been perfectly happy in the gnarly/chunky rocks and on smooth dirt road. It can go fast, provide a bit more traction and seems to be quite resilient to flats.
The XR-3 is the beef. While it isn't Bontrager's biggest tire, this guy is a confidence inspiring monster. Big knobs and plenty of traction give you nothing to worry about in loose over hard conditions. Having this tire on the front while doing fast descents on loose and crumbled rock is super fun! I've been pushing it to the limits for a few months on everything I can find and nothing seems to shake it! For gnarly racing or training, I have yet to find a better tire than the XR-3.
I've been impressed with the whole line up. With those three tire types in the quiver, I can't imagine what else I'd want for racing and training. For the faster and smoother races, an XR-1 front and rear would be perfect. For most courses, the XR-2 up front with the XR-1 in back would be a really nice combo with a bit extra traction for evolving conditions, and for the technical burl, the XR-3 up front and XR-2 or 3 in the rear is just solid.
Try them out. Bontrager has really upped their game with these modern knob shapes and tread patterns, you will not be disappointed by any of these tires.