Construction of a monster truck

I found these tutorials: http://www.ourfamilyunit.com/diy/monster-truck-cake-tutorial
http://freelifeglutenfree.blogspot.com/2012/01/gravedigger-monster-truck-birthday-cake.html
Hope it helps.

Cut a Styrofoam 2 inches shorter lengthwise and 2 inches less wide than the cake you want. It should be as high as the wheel you want. Cover this in black fondant. Attach to the cake board. Now cut another cake board the same size as your cake and decorate the cake on it. After you are done decorating attach the cake to this black covered Styrofoam using some royal icing. Make sure to balance it. Stick the wheels onto the Styrofoam and finish decorating.