haha, so i just saw a picture of a rat with a tail all curled over his back from excessive wheel running. somehow it stuck me as hilarious. anyway, i figure that running in a wheel that's too small for them will definitely aggravate this condition, but i was then wondering what a "too small" wheel would be. i got picasso the biggest wheel i could find (9"?), but she still curls her tail when she runs in it, and she LOVES it.