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Old 11-10-2015, 12:28 AM   #71
jcl
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Originally Posted by Tuco View Post
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.

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Old 11-10-2015, 10:20 AM   #72
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Gl350

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
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Old 11-10-2015, 11:18 AM   #73
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Quote:
Originally Posted by jcl View Post
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?

Quote:
Originally Posted by jcl View Post
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.

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Originally Posted by jcl View Post
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.

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Originally Posted by jcl View Post
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.
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Old 11-10-2015, 12:06 PM   #74
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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.

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Old 11-10-2015, 12:18 PM   #75
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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.
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Old 11-10-2015, 12:58 PM   #76
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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
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Old 11-10-2015, 03:24 PM   #77
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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.
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Old 11-10-2015, 05:23 PM   #78
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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)

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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
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Old 11-13-2015, 06:20 PM   #79
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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
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Old 11-13-2015, 08:26 PM   #80
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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.

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Old 11-14-2015, 12:39 AM   #81
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The Invisihitch is reported by their reps to be a runner up in the BMW vendor competition. It is actually a well designed cross member and weldment. With a custom receiver, it could be the basis of a good bolt on hitch solution.

I have never inspected the E70 hitch, so no opinion. The Curt we installed seems to work at 6000/600 loading. We have about 15K on it so far with no problems.

The torque arm reinforcement is an improvement that would make the F15 a much more capable tow vehicle. Too bad there is not an engineering team with the capability to develop a kit that could be bolted in place to reinforced attachment points similar to the ones used to mount the rear axle carrier. There is, of course, a concern that modification could have the potential to degrade the rear crumple zone. However, an impact break away design for the reinforcement attachment, which would provide the transfer of vertical force, while not yielding to the forward horizontal forces of an excessive impact, seems possible. Lots to think about.
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Old 11-14-2015, 07:59 AM   #82
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Also as a side note Invisihitch is rated to 7,700 / 600 lbs, i.e. to BMW X5 full towing capacity. It is a shame that it was proven that it does not work with WD.
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Old 11-14-2015, 10:38 AM   #83
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Also as a side note Invisihitch is rated to 7,700 / 600 lbs, i.e. to BMW X5 full towing capacity. It is a shame that it was proven that it does not work with WD.
The rating of 7700lb tow capacity is not of much use with a 600lb tongue weight constraint. And that brings us back to reinforcement. The question remains how. Looking forward to hearing your experience with the upgrade. Pat
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Old 11-14-2015, 04:26 PM   #84
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... unless you tow a boat...

I am not an engineer, so I do not understand all the matters in relation to the reinforcement. My understanding is that it prevents the receiver from bending and that is the main purpose of the reinforcement. I do not know how much it would increase the tongue weight limit, if at all. I believe there is some safety margin built in. So, if BMW says that the car can handle 600 lbs, probably it can handle more. There are people who use X5 to tow the trailers with 800 lbs-1000 lbs tongue weight. There is one E53 without reinforcement and a couple of E70 with reinforcement.

My tongue weight with ProPride hitch will be in 800-900 lbs range.
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