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That's pretty cool man. The sides look solid and I like the turret and gunner window ideas. How far is the board you sit on above the ground? I think that I would be squatting inside there, but I'm 6'1, so I'm kinda tall. The wheels look kinda small too, I think you're right, adding an axle would help. If you had the money I'd suggest getting some bike wheels and making it pedal operated. That would be a bit more mobile and comfortable, well at least for tall people like me.
I am sure that it will still do well in combat, at least defensively. Have someone record a video of you guys using it in a battle.

I dont know about making it pedal operated but the im getting a job soon so then ill put on the axle. The smallest wheels work well and stay on but the side ones that i got from a shopping cart failed horribly. The board is about 5" of the ground. I'm 5'5 so The turret is okay for me but my homemades pressure chamber is awkward to fit. I will definitely make a video of it in action. (Battle of Hoth anyone?)

You're going to want nothing less than a WBL and/or Supercannon II for the gunner(s). With mobility like that, range is necessary.

Don't have much ideas for the wheels, but shopping cart wheels seem to be too small for something this big. Bike wheels wouldn't be a bad idea as M4 mentioned, but it'd be a bit more tricky to construct.

On a side note, I'd like to see what its like for us to fight your team and the tank. (Though actually organizing a war between such distant teams would be too difficult.) Get a good ranged weapon on there and that thing will be very strong defensively if anyone's dumb enough to get in range by themselves. However, once in range, they can charge in and stick their guns through a window or the top gunner's hatch.

Yeah I am planning on building a homemade gun for it. Too bad your teams too far away. Im waiting until i get hired to do any more work on it.

Update:
I am waiting on a friend to get an axel for my tank. After It is completed I will start my new design.