I'd agree the torque axis (for calculations relating to the mounting point strength) is at the front of the hitch crossbar.

There are other things to consider relating to why BMW rated it as they did. The limit could also relate to the rear axle loading (a multiplier over tongue weight due to the offset from the rear axle, with a corresponding lightening of the front axle). I would also consider the effects of a bouncing load, even on smooth roads, since a carrier acts differently than a trailer coupler (a trailer has a large rotational inertia, and can't apply a longitudinal twist).

My only point was to consider that the receiver wasn't designed for other than a point load at 8"; I have also seen many bent hitch receivers due to the use of cargo carriers (but specifically to the E53, only in photos).

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