I have a hybrid bike with 622 x 19 rims on it. I want to put mountain bike tires on so I can ride trails. But I can't find knobby tires to fit the wheels. So I want to get new a wheel set.

Is 622 equivalent to a 29" mountain bike rim?

Can I put 26" wheels on it without getting new brakes?

It looks like the frame will fit wider tires, so that's not what i'm worried about right now.

A 622mm rim (aka 700c) is commonly used on road bikes and their kin, most hybrids, and 29" mtbikes. Your hybrid won't be so good with 26" wheels/tires, due to (presumably) issues with brake reach,and other issues related to geometry. For instance, the BB will be considerably lower, and that would be bad in most cases. Worst case would be off-road.

You probably have a lot of clearance for tires, but perhaps not enough to run 29er mountain tires. Measure between the fork and stays, both laterally and vertically. If I were you, and i truly couldn't just buy a nice used mtb for about $100 on CL(And sink another $50 or $100 to fix/tune/replace tires), I'd get some 35mm or slightly wider cyclocross tires. Anything too gnarly for those tires will likely be too gnarly for other components on your bike.

just my $.02
-rob

Thanks for the reply. I live in a rural area so CL isn't a great option, plus I'm 6'4 so that makes it even harder to get used bikes.

I just want to to moderate trail riding for now, so I'm gonna go with the cyclocross option.

what kind of hybrid do you have? when you say trails.. do you mean follow your biddies with Mtn bikes, or just ride some fire and logging roads? if your hybrid has a ChroMoly frame and fork you might be able to do some moderate fire road riding. a nice fat 700x32 tire may do the trick for you.

Its a K2 Astral. Prob chromoly, no suspension. I just plan on riding logging roads / snowmobile trails type stuff.

You'll do fine. Prolly an aluminum frame, but you'll still do fine. A friend of mine road white clay in DE on his scandium singlespeed crossbike,with a 32mm knobby up front and a slick in the back with a 70" gear ratio and we couldn't keep up with him. (Yeah, he's a competitive rider.) White Clay is real singletrack, so...

You'll do fine, just don't push it. Your wheels will be first to go,so try to avoid drops log piles.

-rob