Quote:
Originally Posted by e30cabrio
(Post 963935)
This thread's excellent info got me worried (my X came with a hitch with spliced into the tail light harnesses electricals) I assume is Uhaul. I just looked at the hitch which is hard to see as it is pretty well covered and there is no Uhaul sticker and it appears to be mounted under the bumper struts. What do I need to look for to determine what hitch I have?
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It may not be U Haul, as the Curt, Hidden Hitch, Reese, and others all mounted the same way. The easiest way to see if it is OE is to see if the two strengtheners are installed (apart from finding a Westfalia label).
Look at the two screen shots of the OE hitch install manual, below. Those inserts replace the two one-time collapsible shock absorbers the bumper was originally mounted on. The faces they bolt to (the pentagon shapes) are designed to be strong and resist impact loads pushing forward, because that is the only way the bumper shocks are loaded, from impact (other than the weight of the plastic bumper). When the bumper shocks are removed for an OE hitch install, and those strengtheners are added in their place, they are bolted up from below, and down from inside the tool area in the trunk, tying it all together with four additional bolts per side. That ties the hitch receiver into the longitudinal box sections of the rear unibody. That is why the OE hitch is stronger in bending, ie vertical forces on the hitch ball. You can see the new bolts in your trunk area or underneath. If they aren't there, the hitch is just attached to the bumper shock absorber mounting faces, which are sheet metal. If you have a longitudinal strut running forward from the hitch for 2 feet or so, then that is the brace that several aftermarket manufacturers added in lieu of using the stiffeners, after they had some bent rear pans earlier on.
The picture of the Curt hitch shows that this particular one is attached to the same locations as the OE hitch (the pentagons) but it doesn't include the strengtheners, so it can twist the rear pan (unless it has a longitudinal brace underneath).
The picture of the Reese hitch shows that this particular one doesn't even use the pentagon locations, but rather just the rear sheet metal between them. It does include the longitudinal brace, but I am not sure where this particular one bolts to at the fore end.
It is interesting that many consider the aftermarket hitches suitable for load carriers and bike racks, as opposed to towing. The failure mode of the early hitches was always in bending. The receiver square under the bumper ended up pointing down at the ground from a vertical load applied further rearward than the hitch was designed for. BMW lists a spec limit of 600 lbs tongue weight, applied at a restricted maximum distance from the receiver pin, for their OE hitch. (I think it is 8 inches from memory). Applying the same 600 lbs further rearward applied a proportionally larger bending or twisting moment to the receiver, and has resulted in damage to the rear pan of the vehicle with aftermarket hitches. An additional longitudinal brace addresses that weakness of the aftermarket hitches. I think luggage racks are actually harder on hitches than towing is, as a trailer isn't likely to pull the hitch off during acceleration, and the hitch is very strong in resisting braking forces. It is the bending moment caused by the tongue weight that causes the damage. This doesn't matter with a 15 lb road bike on a rack, for example, but can matter with a motorcycle carrier, or a luggage carrier that has a CoG 24" rearward of the design point for the hitch ball.
