Quote Originally Posted by m_c View Post
Why?
Modern belts don't stretch.
Everything stretches - it's a matter of how much, which in this case depends on things such as the width of the belt and type of tension members in the belt.

Page 7 of this document has a graph to show how they stretch:

http://www.gates.com/facts/documents/Gf000289.pdf

If you're using the belt on a machine with low or zero cutting forces (e.g. laser cutter), then the gantry can be light so the force should be small enough for this not to be an issue. However if it's for a router as the title suggests I would do the calculation to find out how much the belt will stretch, or at the very least use the widest belt you can fit.