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I personally wouldn't put much past 300lbs on the rear. The ass end would squat a lot if you had 500lbs on there.
I don't plan to put more than 230 lbs on the back - and only in town. I've carried the chair IN the back with two sets of batteries, a cooler, our bags, and the three of us and I didn't notice any difference in the ride or height of the vehicle.