pic: 2594 Chassis

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Unless that bellypan of yours can provide substantial rigidity to that frame, with warping and flexing you are not going to have a very fun time with those mecanum wheels when not all of them are in contact with the ground with the same amount of force. Just a thought.

I think what he is saying is: make sure you center the weight so that it is evenly divided between all four wheels. That way you will get the same force from all of them therefore making the mechanum wheels behave in a predictable manner.
Advise: put your battery in the center of the 4 wheels.

Great point about keeping the weight even. They stil need to account for the weight of the lift system and the minibot still.