The hitch was designed for that particular limitation of drop and extension. Anything more than that would apply a higher bending moment that the unibody attachment points are designed for. You can take that chance if you want, but I wouldn't recommend it. The limitation hasn't changed with the new hitch manufacturer, those are the same dimensions in my 2003 installation instructions, and the same label that was supplied by BMW with my hitch with instructions to attach it to the vehicle.

I bought my ball mount from my dealer when I bought my hitch. It was within the specs shown on the schematic. There are threads about posters who have purchased non-OEM ball mounts and had rattles. You can always shim them, but I figured it was easier to just buy the ($$$) BMW part as it fit snugly.

I don't agree that you necessarily want a Class IV mount. Most will be labelled Class III at 6000 lbs/600 lbs tongue weight. Class III isn't a precise specification, there are 5000 lb hitches and 6000 lb hitches, all labelled Class III. Some Class IV hitches are labelled 6000 lbs, others 7500 lbs, and others 10,000 lbs. If a Class III ball mount is labelled 6000 lbs, that would be sufficient.

It is a side point, but the spec doesn't cover bike racks. The hitch is designed for the load to be applied at the hitch ball. If you mount a bike rack, you need to figure out the new CoG, and weight. Then figure out if you are applying a greater bending moment than the hitch was designed for. Most bike racks are very light and it won't be an issue, but it is worth checking.

Good luck.

Jeff

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