[Al Skierkiewicz]

To build on what you have already accomplished, you might want to look into shifting Andy Mark transmissions to give you optimized speed and/or pushing power. However, depending on the game you may not need one or the other. Many teams use aluminum box tube for chassis construction. Welding is the preferred method of assembly. Others use rectangular aluminum box tube for axle and drive train mounting while connecting the two tubes with cross bracing of other materials like round tube or structural extrusions.
Remember that the game should dictate function. If the game calls for a robot to precisely pick and place objects, a running speed of 20 feet per second is likely to prove frustrating to drive. A tank style drive is also not indicated if the game is going to require repeated high speed turns with small turning radii.