Filling your handlebars on your dirt bike with spray Styrofoam keeps rocks, dirt and water out of the handlebars. Some experts claim that it can even help reduce vibration through the bar, reducing arm pump. Try the canned expanding spray foam sealant used in construction and available from your local building supplies like Bunnings. Use the extension tube that comes with the can, and withdraw the tube as you spray. Trim off the excess after the foam has expanded and dried. This adds hardly any weight to your motorbike, and keeps all the dirt out of the handlebars.