If you go too wide of a tire on a narrower rim it's going to flex the side wall and even the treads themselves thus creating a danger. You might get away with some extra width but too much can be detremental.
http://rimringz.com/product_images/u...o-tire-fit.jpg
Here is a site I use to calculate what tire width you'll want for the width x diameter.
Tyre Size for Rim Size/Width Calculator - What's the best tyre size for my rim?