Long tried to create a responsive navigation. But still turned out. But I'm still not sure I did the right thing. Because I passed a value of 0 in the span. Or so it is still possible to do? It works :)
$big: (columns: 24, gutters: 1/2,math: fluid,gutter-position: split )
@include susy-breakpoint(700px, $big)
.nav-item // is ui>li.nav-item
@include span(0 border-box)