The top bar shrinks too soon.

When we scroll down the page the top bar shrinks to a thin menu bar. This feature is good, but the problem is that it shrinks too soon i.e, the moment I rotate the mouse wheel the menu bar gets slim.

what I want actually is that the bar to go slim when the page is scrolled below the header image just like this website here.

Plz take a luk at my site and help me to resolve this issue…
http://hubbytes.com.cp-30.webhostbox.net/

Warm Regards