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Old 06-02-2006, 05:00 PM
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JCL JCL is offline
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Location: Vancouver, Canada
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Differences with OEM hitch (long)

I haven't installed or seen a Putnam hitch, but there is a drawing of it available online. The one I lookedat is rated 3500 lbs, not the 6000 lbs of the BMW hitch.

While the receiver itself, and the crossbar, are very similar in concept, the difference is in how the crossbar attaches to the vehicle. Recall that the X5 does not have a frame, it is a unibody, so the attachment is more complicated, both from the standpoint of strength and what happens if you are ever in an accident (the area in question is part of the crush zone to absorb the impact of a collision).

At the back of the stock X5 are two 'shock absorbers' for lack of a better phrase. They run fore and aft, and transfer load from the bumper to the chassis in the event of impact. They are made out of expanded wire mesh, quite heavy, and designed for one-time use. They attach to the longitudinal sections in the unibody (running fore and aft, similar to chassis rails). At the back of the shock absorbers is a pentagon-shaped plate that the bumper attaches to with a carrier structure.

The BMW (OEM) hitch replaces the shock absorbers with solid hitch mounts, and then mounts the new drawbar to the back of the revised mounts. Loads are then carried by the chassis sections, not by the rear pan.

The Putnam hitch appears to attach to the back of the two shock absorbers. This has a few potential results. One, the hitch loads are transferred through these mounts. Second, the hitch loads are transferred to the rear pan, ie the sheetmetal behind the bumper. Third, the collapsible bumper can still collapse, but in this case it can no longer move into the space occupied by the drawbar.

Yes, the OEM hitch is more expensive. The reason that many owners decide to buy it has to do with not compromising the rear safety structure, and with wanting to ensure that the drawbar transfers load properly to the vehicle structure, so that it is designed into the vehicle.

There are various horror stories about aftermarket hitches bending parts of the X5, on various forums. I can see how it could happen. I haven't seen an X5 damaged by the hitch, but I have seen X5s after serious collisions. They are very safe vehicles, and I choose to maintain those safety features even though I have installed a hitch.

There is little difference between the actual drawbars, they are simply welded square tubes. If the welds and paint quality are good, no worries. There are some issues with various aftermarket drawbars not fitting the cutout in the plastic panel perfectly, but this can be checked easily. Finally, the BMW hitch has a bracket to mount the BMW wiring harness, which is the only way to go for electrics, IMO.

That is about it. I hear people suggest that they only need a light hitch for a bike rack, but I would ask whether anyone will someday use that hitch to tow with, and also, whether the bike rack puts any twisting moment on the receiver. The Putnam hitch is rated for 300 lbs applied at the hitch ball. With a bike rack, the bending arm (from the CoG of the rack and bikes, to the mounting point of the drawbar) is likely multiplied 3 or 4 times. Therefore, by calculating a similar bending moment the bike rack capacity is reduced by 3 or 4 times. It isn't hard to imagine putting 75 lbs of mountain bikes on a rack. More than that could potentially damage your vehicle, going by Putnam's design limits. Your mileage may vary, personally I carry my bikes on the roof ;-)

There are good photos of the OEM hitch on the home page of X5 World, in the articles.

2007 X3 3.0si, 6 MT, Premium, White

2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver

2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue
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