Question for BMW X5 owners

[Road Ruler]

Bono, the redo looks good. I'm envious of you towing with "the ultimate towing machine.

[bono]

Thank you guys! The only thing I am worrying about it the harassment from the “truck crowd”, when I am on the road. I have a thick skin, so I should survive.

[Tuco]

So let me get this straight. You are saying that you have connected a brace from the uni-body of the vehicle to a vibration dampened differential carrier as a reinforcement? So when a force is applied to the carrier it just moves and never transfers that force to the uni-body? Worse yet you state that Can-Am does this also? Then you have the gall to talk about the "Truck Crowd"? Unbelievable...

[bono]

I would like to ask people who are not interested in towing with BMW X5 (or other German SUVs) to refrain from comment in this thread.

[Tuco]

Great, since I am interested in you succeeding in towing with your "German SUV" I can post all I would like then!

[Easyrider06]

Safety should be the concern of all, its not whether you tow with a X5, Dodge Ram or a Tiger Tank, its just that you have to have a safe towing set up as we will be the ones on the road with you. No wants to see anyone hurt and I apologize if I hurt your feelings but I'd rather do that than have something worse happen because you don't want to hear what other RV'ers have to say. Good Luck...

[jcl]

Originally Posted by Tuco

So let me get this straight. You are saying that you have connected a brace from the uni-body of the vehicle to a vibration dampened differential carrier as a reinforcement? So when a force is applied to the carrier it just moves and never transfers that force to the uni-body? Worse yet you state that Can-Am does this also? Then you have the gall to talk about the "Truck Crowd"? Unbelievable...

Yes, the brace goes to the differential carrier. Not the differential, which is soft mounted and can move relatively easily. The carrier is quite firmly mounted. It would be worth inspecting one to see how they do it.

When the vertical force from the brace (which is critical in restoring front axle loading) is applied to the carrier, it does not move appreciably. It is in fact designed to transmit vertical forces to the vehicle structure. If you look at the service manual, it advises lifting the rear of the vehicle using a trolley jack under the differential (just not the differential cover), and that action transmits the weight of the rear of the vehicle through the soft mounts, and then through the carrier mounts and rear suspension.

Yes, this is routinely done on many similar vehicles. We know how Can Am do it, for one, because there are lots of photos available from satisfied owners, on BMW X5 models, as well as on MB, VW, Audi, and so on. We aren't talking a few vehicles, we are talking many hundreds.

The potential downside of using this mounting point is if there is a resulting drone or noise in the cabin from the new transmission path for road noise. If there isn't (and there haven't been reports of such noise) then there isn't much downside to worry about IMO.

The issue with the "truck crowd" is usually a claim that such use of an SUV is somehow unsafe, or impossible. These concerns are not usually backed up with data. They often include the phrase "you can't tow with that...". It is just that people get frustrated and then it escalates.

I once rented a Genie hydraulic bucket lift (41 ft telescopic boom lift, IIRC) mounted on a trailer. It was a great way to install Christmas lights on a large house. I picked it up at the rental yard with my X5. The yard crew said something like "you can't tow with that....". It towed beautifully. It isn't just an Airstream thing, apparently.

Jeff

[jcl]

Originally Posted by Easyrider06

I use my "Beemer" for what it is intended for, a fine driving machine...

My Beemer was an R50/5, from the late sixties. My Bimmers have all been fine driving machines, and two of them have done yeoman service while towing

[Tuco]

Originally Posted by jcl

Yes, the brace goes to the differential carrier. Not the differential, which is soft mounted and can move relatively easily. The carrier is quite firmly mounted. It would be worth inspecting one to see how they do it.

When the vertical force from the brace (which is critical in restoring front axle loading) is applied to the carrier, it does not move appreciably. It is in fact designed to transmit vertical forces to the vehicle structure. If you look at the service manual, it advises lifting the rear of the vehicle using a trolley jack under the differential (just not the differential cover), and that action transmits the weight of the rear of the vehicle through the soft mounts, and then through the carrier mounts and rear suspension.

Yes, this is routinely done on many similar vehicles. We know how Can Am do it, for one, because there are lots of photos available from satisfied owners, on BMW X5 models, as well as on MB, VW, Audi, and so on. We aren't talking a few vehicles, we are talking many hundreds.

The potential downside of using this mounting point is if there is a resulting drone or noise in the cabin from the new transmission path for road noise. If there isn't (and there haven't been reports of such noise) then there isn't much downside to worry about IMO.

The issue with the "truck crowd" is usually a claim that such use of an SUV is somehow unsafe, or impossible. These concerns are not usually backed up with data. They often include the phrase "you can't tow with that...". It is just that people get frustrated and then it escalates.

I once rented a Genie hydraulic bucket lift (41 ft telescopic boom lift, IIRC) mounted on a trailer. It was a great way to install Christmas lights on a large house. I picked it up at the rental yard with my X5. The yard crew said something like "you can't tow with that....". It towed beautifully. It isn't just an Airstream thing, apparently.

Jeff

Jeff,

I was going to respond much differently in my post until the "Truck Crowd" comment. Some people on here always try and make the issue "Us vs. Them" instead of learning from everyone regardless of the type of TV they have.

As far as mounting the brace to the differential carrier, this defeats the purpose of isolating the carrier from the uni-body. Just because it has always been done this way doesn't make it the correct solution (but maybe the most cost effective). That said, here is what I would suggest as a solution. If you add a 3/8-1/2" reinforced plate between the carrier and the uni-body that extended toward the rear of of the vehicle, it would provide a mounting point for the brace. This would allow the carrier to perform as designed without adding stress to the carrier. Without seeing the way the carrier mount area is shaped I can't tell you how much work this might be. After the install a 4 wheel alignment would be required, but the suspension should perform normally. With this design the forces are applied to the uni-body directly and not through the carrier (and its isolation mounts) to the uni-body.

One other thing I noticed is that this brace only helps in the vertical axis. Horizontal stress is not addressed. In one of the posts a question was asked about adding gussets onto the side of the receiver. If exceeding design specs is the goal, then adding structural strength in the horizontal axis would be desired. Not being able to see the strength of the uni-body in this area makes it hard to come up with a solution for this. What I do know is that any solution will have to distribute the stress across a section of the uni-body since no one point will be up to the task (maybe a plate that connects between the bumper shock towers).

I apologize for being snarky, but the "us vs. them" is beyond old.

[cameront120]

Originally Posted by Tuco

...but the "us vs. them" is beyond old.

It certainly is. So for those who have no interest (or experience) in towing with anything other than a truck perhaps the best course of action would be to not comment at all. Leave these discussions for those who are interested in towing with something other than a truck. You don't see the import SUV crowd trolling the truck threads, do you? The warnings that it can't be done are boring and predictable, not to mention unfounded. As it is being done with great success.

[jcl]

Originally Posted by Tuco

As far as mounting the brace to the differential carrier, this defeats the purpose of isolating the carrier from the uni-body.

True, assuming that there is significant compliance in the mounts for the carrier. But note that as you move to the higher performance models of the same vehicle, typically the rear suspension mounts are firmer so as to provide more precise handling at the limit. Compliance is greatly reduced, by design. This suggests that compliance at this interface isn't always the primary design consideration.

I also think that in one sense we are really just trading one isolation path off against another. If you want to isolate the passenger cabin from road noise, then adding a strut from the trailer attachment point to a point on the unibody right under the rear seats would not help as it would provide a direct path for noise from that source.

If there was clear access to a convenient point I would go to the unibody. However, it is pretty tight under there. I go back to asking whether there is a droning noise in the cabin with the strut attached to the carrier. If not, then this sounds like a solution in search of a problem.

I haven't installed a hitch on the new F15 X5, but I am familiar with the other X3 and X5 models. The fore/aft and vertical strength at the receiver seemed pretty good to me. The issue is applying significant weight distribution and flexing the receiver up. The only place I would be trying to strengthen the receiver is from bending up due to WD bars, particularly where the receiver is mounted on the drop plates.

Jeff

[jennian]

Have similar question on reinforcement. Looking a Curt class lll which has a flat bar running from receiver that bolts to frame over th rear axle. Does this accomplish the same reinforcement that can-am does?
Thanks

[Tuco]

Originally Posted by jcl

True, assuming that there is significant compliance in the mounts for the carrier. But note that as you move to the higher performance models of the same vehicle, typically the rear suspension mounts are firmer so as to provide more precise handling at the limit. Compliance is greatly reduced, by design. This suggests that compliance at this interface isn't always the primary design consideration.

This brings up a second solution then. Since the differential is double isolated from the uni-body and the stiffness of the bushings increases with performance level. Then eliminating the bushings in the carrier (at the uni-body attachment points) and replacing them with steel bushings would give us what we are looking for. In this way the receiver brace will have a direct path to the uni-body (through the carrier). I like this solution because it doesn't change the suspension geometry or add much weight. The only down side is how will noise levels in the cab be effected without the added isolation?

Originally Posted by jcl

I also think that in one sense we are really just trading one isolation path off against another. If you want to isolate the passenger cabin from road noise, then adding a strut from the trailer attachment point to a point on the unibody right under the rear seats would not help as it would provide a direct path for noise from that source.

I had thought the goal here is to strengthen the receiver mount so that more load can be added. Whether that is the WD hitch, a larger trailer or both. I also may be missing how this ties in.

Originally Posted by jcl

If there was clear access to a convenient point I would go to the unibody. However, it is pretty tight under there. I go back to asking whether there is a droning noise in the cabin with the strut attached to the carrier. If not, then this sounds like a solution in search of a problem.

Again, the problem is that in it's current form the brace is being attached to the uni-body through the carrier and its 4 bushings. I am assuming noise is secondary.

Originally Posted by jcl

I haven't installed a hitch on the new F15 X5, but I am familiar with the other X3 and X5 models. The fore/aft and vertical strength at the receiver seemed pretty good to me. The issue is applying significant weight distribution and flexing the receiver up. The only place I would be trying to strengthen the receiver is from bending up due to WD bars, particularly where the receiver is mounted on the drop plates.

This configuration of TV/Trailer are normally paired with a rigid style WD hitch. I am assuming from other posts that most are in a locked position while moving to reduce sway. This creates a large fulcrum from the trailer tires to the vehicle attachment point. My concern here is work hardening of the attachment point. If you think in most instances this won't be a problem then ok, but I would suggest an increased inspection cycle.

[jcl]

Originally Posted by jennian

Have similar question on reinforcement. Looking a Curt class lll which has a flat bar running from receiver that bolts to frame over th rear axle. Does this accomplish the same reinforcement that can-am does?
Thanks

WRT the GL350 and Curt hitch, it is designed to accomplish the same thing, which is to spread out the load and not just lever on the rear attachment point of the receiver to the unibody. I can't comment on the effectiveness of it in that particular case, as I haven't installed that hitch on that vehicle.

Andy at Can Am would be a good person to check with for this particular application.

Jeff

[bono]

jennian, are you referring to BMW X5 hitch or Mercedes GL hitch? Definitely Curt hitch for X5 is worse then OEM, since the receiver drops lower from the tube. This means that you do not have to cut the bumper (at least that much), but my understanding is that the long drop plates are not the best idea for WD system.

[Easyrider06]

I don't think its really an "us vs. them", the question was put out there and open to all (I thought) for our input. I apologized for my apparently poor response that no one wanted to hear. I'm new here, so is "learning by fire" the standard course of action? I'll pass on that as I've fought my wars and survived. I do want close with a salute to our Veterans who have preserved the right for travelers both foreign and domestic who use U.S. roadways to enhance their "Life's" experience!!!
Happy Birthday USMC!!!
James-MAJ/IN/USA(OIF&OEF) ***Gold Star Parent/USMC(OIF)***
Have a Nice Day

[jcl]

Originally Posted by Tuco

Again, the problem is that in it's current form the brace is being attached to the uni-body through the carrier and its 4 bushings. .

But we don't know that that is actually a problem. It may be a philosophical problem. Empirically, it doesn't appear to be a problem at all.

This is not just a differential carrier, although that is what we have been referring to it as. BMW calls it both a rear axle carrier, and a rear subframe, in the service manuals. Those four bushings transmit lateral and longitudinal forces from cornering, accelerating, and braking, as well as supporting the vehicle together with the rear springs. The whole structure looks pretty solid.

If the four bushings actually moved as much as is being implied, then the vehicle would not have the precise handling that it does. The rear axle would self steer.

For my part, I wouldn't want to replace those bushings with anything solid in pursuit of resolving a problem that hasn't been shown to exist.

[jcl]

I was interested in an earlier comment in this thread about BMW attaching the differential to both the rubber-mounted carrier, and to the unibody. I didn't recall this, so went back and looked at the drawings for the model being discussed, a Z3 (which, admittedly, will rarely be used for towing)

Originally Posted by zetatre

There's a certain level of compliance between the chassis and the axle carrier which can actually be nontrivial. Just as an example, in the old BMW Z3 the rear differential was bolted to the rear axle subframe AS WELL AS to the chassis. Since the subframe was mounted to the chassis using similar rubber bushings, the axle carrier would move relative to the chassis when, for example, cornering. This movement cased the differential to pull on the chassis and along with weaknesses in the mounting point of the differential to the chassis caused the trunk floor to get ripped off the car.

The Z3 used a trailing arm rear suspension adapted from a smaller vehicle. It had only two vertical mounts from the carrier to the unibody, similar to the two forward mounts in the photo above of the X5 carrier. There was a third mount, at the rear in the centre, which was attached to the differential. However, it utilized a rubber bushing just like the front mounts. Photo below. The rear rubber bushing attached to a bracket on the unibody, and that is the bracket that failed. It was more common in cars that were tracked, apparently. That rear mount took all of the twisting force of the rear axle, and as installed power was increased over the years, those forces climbed. It still shouldn't have broken, but it wasn't due to the carrier being mounted with rubber mounts and also solidly.

Further trivia: there are solid subframe mounts available for many BMW models from the aftermarket, often made of Delrin (plastic) which doesn't deform. These are common on smaller BMW cars, but these mounts are now available for some X5 models if one was concerned about the subframe moving.

Jeff

[PKI]

Note, be aware that the E53 and E70 factory hitches are very different from the 2014/15 (F15) X5 factory hitch. The F15 class III tow bar is only available with a 2" ball. That requires owners who want to tow an Airstream to acquire one of the aftermarket hitch solutions. Consequently, the experience and logic used to develop a hitch installation is of significant value to folks considering the X5 as a tow vehicle. Thanks for your contributions. Pat

[jcl]

Good point. The OE F15 hitch is similar in concept to the Invisihitch. Good for lighter trailers with no WD, nicely hidden when not in use. Would not be my choice if using WD.

The F15 is built on the E70 platform, and so experience from the E70 should be valuable. Not sure if it makes more sense to start with an E70 hitch or a Curt, and then modify as necessary with a longitudinal strut.

Jeff