I own a Trek 3700 Mountain bike and I got a dilemna. I want to do a mini triathlon at the beginning of september. My bike has the knobby tires. I was wondering if I can put skinnier tires( i think the term is "slicks" on my mountain bike? What is a good size to get? I want to be able to change wheels when i'm training for my triathlon and then change them when i'm ready to hit the trails again. I would buy a road bike if I can afford it but i can't until next summer. Are wheelset expensive?