I just purchased 26x2.15 Schwalbe Mondial tires for my Long Haul Trucker to replace my 26x1.5 Continental Comfort Contacts.
I tried to put the Mondial tires on tonight, but the schrader valve of my tube wouldn't fit through opening in the wheel. Do schrader valves vary in size at all, or is this a sign that I'll also need to purchase a new wheel set if I intend to ride with the Mondial tires?
The wheels I currently have are 559x18 Alexrims Racer's Edge Adventurers.
The tires shouldn't make any difference. Were you using Presta valves before? Presta valves are narrower than Schraeder valves. If you really want to use Schraeder valves, then you can get the rim drilled out. Or just get some Presta valve inner tubes. Maybe your old tubes will still work but I can imagine you'll want fatter tubes with such a big jump in tire size.
When I built up my 26 inch touring bike, I knew that I wanted to use Presta but I wanted to be prepared if I had to buy a tube in rural USA where I might only find a tube with Shrader valves. The rims I wanted to buy I could only find in Presta version. I drilled out the hole (do not recall which size bit) and put in the little Wheels Mfg adapter to take up the extra space. Since the alloy in the rim is soft, I used an old cordless drill, the kind of drill that has a crank to turn it, no battery required. Took less than three minutes to drill and debur each rim.