how do i design a tubular chassis in nx8?
Insert - Sweep, there are the tools you'll need. Use "Tube" for simple straight tubes. You'll need only a line of sketch for it. For bend tubes draw a sketch of the center line and sketch the profile in the end of the sketch line. Then you can create a tube out of it with "Sweep Along Guide". Rest of the Sweeping tools are more or less surface modeling oriented so forget about them with this task. If you are able to use Routing Mechanical then it's definitely an option too but requires more skills than sweep tools in modeling application.
Attached picture is of a tubular chassis I've done based on a real chassis. Tube center lines were created in Routing Mechanical and solids with sweep tools in Modeling side. First create uncut tubes and cut the ends properly afterwards as you wish, see picture 1.