Bicycle frame rear triangle operate instruction

If your frame or fork is made of steel, you can change the spacing to accommodate different hub spacing. This is most commonly done to widen the spacing, to permit the use of a cluster with more sprockets than the frame was built for.

Note, if you're not careful, you can do serious damage to your frame or fork this way!

If your frame or fork is made of aluminum or carbon fiber, do not attempt to re-space it: these materials are not suitable for "cold setting."

If in doubt, try the magnet test: if a magnet won't stick to it, don't try to re-space it!

 From:http://www.sheldonbrown.com/frame-spacing.html