The cross bars are the weak point of the rack. I believe they are rated at only 125lbs of evenly distributed weight. Unfortunately, I don't think you will find any official statement specifically related to the weight capacity of the main bars. It's hard to say if Nissan is limiting weight due to structural strength of the rack, or due to the strength of the mounting points. In any case, I would not load more than the weight limit on the cross bars, for the main bars, you could probly get away with a little more and be safe, but you have to take dynamic loading into account. I've seen frontiers with RTT that seem to do fine.