Flexible filament is expensive and doesn't work in every printer. I wanted very flexible tires and I wanted to be able to cast them. I give you the truggy tire mold. I made my tires from Oogoo you can find the recipe here: http://www.instructables.com/id/How-To-Make-Your-Own-Sugru-Substitute/ Oogoo sets in pretty fast (I de-molded in half an hour) so you can print one mold and make as many tires as you want. You can also make it any color you want by adding a bit of oil based paint, I went with traditional black for my tires but I could have just as easily made them red. WARNING you need to mix up a fair amount of oogoo to make this mold (like over a half a tube of caulking. Oogoo is stiff enough that when mixing this much it will break a Popsicle stick sop use a metal stir stick and work fast. Also you might want to try printing and casting some of my smaller molds first as you have to work pretty fast to get a good casting (my first had some missing bits around the edges you can see in some of my pictures. My second casting went much smoother as I knew how much oogoo I needed to prepare.