OEM Trailer Hitch

[JCL]

Originally Posted by TerminatorX5

I am trying to figure out, why the BMW engineers have come up with SO MANY parts for the something that could have been accomplished with only one piece... They have no brain and probably suck as engineers...

Hey, quit dissing the engineers! Those of us who are in the club will get defensive, lol.

It is really very simple. The X5 is a unibody. There are few places to attach external accessories, so BMW designed a way to transfer the load through to the formed box sections that stiffen the rear structure. Those inserts in your photo are what is key.

Other hitches sometimes use the same attachment point as the BMW hitch, but don't include the stiffeners. Some hitches, like the one you show, don't even do that. They bolt straight up to the sheetmetal that forms the trunk floor, or attach to the existing cross tie. It doesn't matter how strong the receiver is, or whether you use grade 8 bolts, it is about whether the floor pan can take that load without tearing at the seams (the failure mode that has been seen). And bouncing gives it a fatigue load different than a pure strength limit.

Anybody who has installed the expensive and complicated OE hitch gets it. Those who haven't, look at the price and get sticker shock. I can understand that.

[JCL]

Originally Posted by upallnight

His hitch has that kicker strut, so it not a pure cantilever. That bike wouldn't weight 600 lbs or even 330 lbs.

Fully agree that it isn't a straight cantilever. That said, the 600 lbs tongue weight limit has no relevance at anything more than 8" from the pin to the CoG of the load. It would be like saying that a bolt was overtorqued to double the specified torque, but it is OK because the tech used a longer wrench to accomplish it. Torque is torque.

I don't have the distances or weights for this carrier, but a sample calculation would be:

240 lbs bike, plus 60 lbs carrier, equals 300 lbs.

Load being applied at 24" vs 8" spec is a three time multiplier on the bending moment, so 3 x 300 = 900 lbs equivalent tongue load.

Using the OE hitch, it would likely survive. That assumes the load didn't bounch at all (which has a 1.8 multiplier effect, recalling limit of 330 vs 600)

Using an aftermarket hitch, no idea. But it appears to be surviving so far. I would still inspect the trunk seams from time to time.

[Dark5yde]

Originally Posted by TerminatorX5

I am trying to figure out, why the BMW engineers have come up with SO MANY parts for the something that could have been accomplished with only one piece... They have no brain and probably suck as engineers...

I sure hope your post was in jest. As the 2 hitches look like vastly diff beasts and I would be willing to bet the smaller 1 piece would fail far sooner than the BMW OE piece. Besides that I like the ECS kit as it comes with all the wiring and harness for a plug and play. Sometimes more money for OE isnt worth it, but in this case towing a trailer and being confident in how it is attached is worth the extra cpl hundo. For me at least.

[JCL]

Originally Posted by TerminatorX5

here is the physics 101 from the NASA website... in our case only the example 1 is applicable:....<snip>

It is perhaps easier to approach it by agreeing that the bending moment that BMW designed for can be easily compared, without worrying about the absolute value of that bending moment.

600 (or 330) lbs x 8 inches. Relate everything to that. If you have half the weight, at three times the distance, that is 50% beyond the design stress.

And all this is for a static load. The real issue is fatigue, as you inferred.

Anyway, I don't want to rain on anyone's parade. Just pointing out that there are more things to consider than the grade of attachment bolt. I fully agree with your comments on it not being about the receiver itself, but rather the integration of it into the vehicle, and the strength of the surrounding structure.

[Porschesolutions]

I installed the factory hitch/wiring as-recommended by this forum: very heavy-duty, no worries from now on. Wish my ball/carrier didn't bounce around so much inside the 2" square opening with light trailer loads on a light trailer.

[TerminatorX5]

Originally Posted by Dark5yde

I sure hope your post was in jest. As the 2 hitches look like vastly diff beasts and I would be willing to bet the smaller 1 piece would fail far sooner than the BMW OE piece. Besides that I like the ECS kit as it comes with all the wiring and harness for a plug and play. Sometimes more money for OE isnt worth it, but in this case towing a trailer and being confident in how it is attached is worth the extra cpl hundo. For me at least.

I have nothing but the OE tow hitch on my E53s, even though, the maximum that I am towing, is just a bicycle rack (OE rack, BTW), which is a heavy thingy to begin with (thingy = scientific term, people go to college to learn that!!!)... once installed onto the car, that thingy does not bounce around (the bikes have wheels spinning)...

[JCL]

Originally Posted by Porschesolutions

I installed the factory hitch/wiring as-recommended by this forum: very heavy-duty, no worries from now on. Wish my ball/carrier didn't bounce around so much inside the 2" square opening with light trailer loads on a light trailer.

In my experience the 2" square piece you insert in the receiver can vary in size depending on the supplier. You can also shim it. However, the first thing to check is whether you have sufficient tongue weight.

[xsx450]

They make stabilizers, look them up.

I get what you guys are saying, but you are just SAYING. I am doing. They are not bolted to sheet metal, or the floor pan. This is not one of those mounts that screws through your spare tire floor in the trunk.

All of our equations mean jack shit because for one they aren't accurate, and two I have loaded up over 900 lbs on this thing over 24" out of the hitch, jumping up and down. No stress fractures, no tearing subframe. Nothing.

The thing works perfectly, it is sturdy as shit for what i am using it for. and expressions like "Will fail much sooner" Sound retarded because its not going to fail. I am not an idiot that is going to try to pull a stump out of the ground with this thing and I am not an idiot that likes to spend $300 on nothing.

You guys on these forms make me laugh sometime (like right now). I think some among us need to spend money on anti anxiety medicine not throwing money down the drain to ease their conscious....

[TerminatorX5]

Originally Posted by xsx450

They make stabilizers, look them up.

I get what you guys are saying, but you are just SAYING. I am doing. They are not bolted to sheet metal, or the floor pan. This is not one of those mounts that screws through your spare tire floor in the trunk.

All of our equations mean jack shit because for one they aren't accurate, and two I have loaded up over 900 lbs on this thing over 24" out of the hitch, jumping up and down. No stress fractures, no tearing subframe. Nothing.

The thing works perfectly, it is sturdy as shit for what i am using it for. and expressions like "Will fail much sooner" Sound retarded because its not going to fail. I am not an idiot that is going to try to pull a stump out of the ground with this thing and I am not an idiot that likes to spend $300 on nothing.

You guys on these forms make me laugh sometime (like right now). I think some among us need to spend money on anti anxiety medicine not throwing money down the drain to ease their conscious....

where is the bottom arm bolted to? It is not clear from the pictures posted earlier on this thread.

[xsx450]

Originally Posted by TerminatorX5

where is the bottom arm bolted to? It is not clear from the pictures posted earlier on this thread.

It is bolted to the subframe of the x5 There is an extremely heavy duty box frame that runs right behind the rear differential which you drill a hole into and then use a special bolt which expands when you torque it the first time like a pop rivet and then you retorque it with a 1/2" Bolt.

Any downward force would be transfered very evenly with the mount. I believe the shear strength of the bolts does matter, unless you think the rear frame is going to rip off?