Gee, we liked your inventor drawing so much that we decided to build it last night
!
Actually we built this last season. I guess we think alike. Very similar to your plans using modified andymarks and extrusions. Each wheel can be individually adjusted for tension.
Several things to watch out for. Like Andy said, 25 chain stretches. Be prepared to adjust frequtently. Make sure you use more than just 2 T-nuts. The wheels will move. Don't worry too much about stiffness. By the time you add your arm, lift or whatever, it should be more than stiff enough. If you are using 6" wheels, please note that the bumpers must be installed above 2.5" which means that they won't be fully supported by the brackets as shown in your drawings.
This was our best base/drivetrain to date. Would like to give credit though to team 254 and 60 for inspiring some of the design elements and attributes such as the overall layout of the wheels and frame.
I think we might also be trying for PNW this year so we will definitely check you guys out.