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Old 07-22-2015, 12:21 AM   #29
jcl
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I think the rear attachment point is very strong in compression, as would be experienced in a rear end collision. It is the place the collapsible bumper struts mounted on other models.

I think it is very strong in shear, as would be applied by the vertical load from the receiver.

I don't know that it was designed to be strong for a bending moment as applied by weight distributing equipment. I expect BMW are OK with the type of bending moment applied by a WD hitch with a maximum 600 lbs tongue load, which is their published spec. If you want to exceed that, you need to decide to use some of their safety factor up, or brace it. Just my $0.02

Jeff
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Old 07-22-2015, 12:24 AM   #30
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I can't access any actual records, but I remember several transmission and clutch failures. They also go through brakes very quickly. I think they all related to towing boats. The Porsche Cayenne is built like a truck and seems to take towing in stride. We had several clients that towed race car trailers all over the country with no apparent downside.
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Old 07-22-2015, 06:11 PM   #31
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Originally Posted by Bgibbs View Post
I can't access any actual records, but I remember several transmission and clutch failures. They also go through brakes very quickly. I think they all related to towing boats. The Porsche Cayenne is built like a truck and seems to take towing in stride. We had several clients that towed race car trailers all over the country with no apparent downside.
Thanks for posting.

The current F15 X5, and the previous generation E70 X5, have never been available with manual transmissions. The last X5 to have a clutch was the E53, built from 2000 - 2006. I have been following an X5 board since 2003, and haven't seen many clutch failures reported. If you are thinking of the E83 X3, it did have a manual transmission longer than the X5 did, but it was rated for a lower 3500 lbs towing. I tow a variety of trailers with a 2007 X3, with a manual transmission, and have not had any clutch issues.

There have been random transmission failures on the X5 automatics. These failures don't appear to have any particular relation to load, or vehicle mileage. They are often early in the vehicle life, and typically relate to sensors, actuators, solenoids and so on. They have not typically shown signs of overheating or stress on the friction plates, which are commonly reused.

Both the X3 and the X5 require trailer brakes for any trailer over 1600 lbs. I am on the original front pads at 110,000 km, and just replaced the rear pads before they were worn out.

Perhaps the failures you witnessed with boat trailers had something to do with the operators, and/or a lack of brakes on the associated boat trailers.

Jeff
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Old 07-22-2015, 06:25 PM   #32
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Jeff, would you happen to know if withidl who I believe also posted here about towing a 31 footer with his E53 has/had his hitch reinforced in any way?
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Old 07-22-2015, 06:41 PM   #33
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withidl's hitch is not reinforced
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Old 07-22-2015, 06:50 PM   #34
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withidl's hitch is not reinforced
Excellent!!!

Then here my thesis: bracing the E70 receiver like the E53 makes lots of sense given the signs of flexing I posted.

However if he was fine towing what he towed, and the E70 is if anything stiffer than the E53 (the torsional stiffness number should be insightful) I really don't see a case to interfere with the rear axle carrier.
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Old 07-22-2015, 08:08 PM   #35
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Excellent!!!

Then here my thesis: bracing the E70 receiver like the E53 makes lots of sense given the signs of flexing I posted.

However if he was fine towing what he towed, and the E70 is if anything stiffer than the E53 (the torsional stiffness number should be insightful) I really don't see a case to interfere with the rear axle carrier.
His E53 hitch includes the two stiffeners that yours doesn't.

I don't disagree with reinforcing your hitch where you see the flexing. I just don't think you can extrapolate that, if you do that, that no strengthening is required to the receiver/unibody interface. And if you are going to strengthen that point it would simultaneously improve the flex point you are seeing.

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Old 07-24-2015, 03:56 PM   #36
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I don't know that it was designed to be strong for a bending moment as applied by weight distributing equipment.
Jeff,

on the surface, I don't think this is accurate. I say on the surface because knowing you there's a deeper thought behind this statement that I just can't get to.

My understanding of the mechanics of the the WDH, if anything, it reduces the bending moment applied at the interface between the tow bar and the chassis.

In very simple terms the rear suspensions sags, but sag less so the direction of the moment is the same as in the weight bearing, but its magnitude must be less.

I don't want to reinvent the wheel and retype in my own word what someone else has explained well and accurately -> RV.Net Open Roads Forum: Towing: Weight Distribution (WD) Hitch --- How it Works.

What am I thinking wrong? Does your statement has to do to changes when we go from a static to a dynamic review of the system?

Thanks!!!
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Old 07-25-2015, 01:56 AM   #37
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My thought was simply that the E53 is different structurally than the E70, so it wasn't a guarantee of a problem, just that we had no info to know that we wouldn't have a problem.

Taking a quick look at your link (good description), I think you are saying that the original (no WD) moment applied to the receiver mounting point at the unibody (1000 lb x 10", just as an example) is reduced by the WD equipment. OK. But what about the opposing moment applied by the 2000 lb bars x 40"? There has to be a significant force applied to restore load to the front axle.

I haven't worked anything out here, so if I am missing something please let me know.

Jeff
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Old 07-26-2015, 02:17 PM   #38
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Originally Posted by jcl View Post
My thought was simply that the E53 is different structurally than the E70, so it wasn't a guarantee of a problem, just that we had no info to know that we wouldn't have a problem.

Taking a quick look at your link (good description), I think you are saying that the original (no WD) moment applied to the receiver mounting point at the unibody (1000 lb x 10", just as an example) is reduced by the WD equipment. OK. But what about the opposing moment applied by the 2000 lb bars x 40"? There has to be a significant force applied to restore load to the front axle.

I haven't worked anything out here, so if I am missing something please let me know.

Jeff
Jeff,

your question has been bugging me. I'd like to propose some calculations. In it's essence the chassis (whether is a frame or a unibody) can be sensitize in a beam with the two supports (the two axles).

There's several tools online that allows you to do the calculation of sheer forces and bending moment introduced in similar structures when load and moment are applied. I used the measurements in the the E70 X5 and assume a dead weight of 600lbs (that's 2.67kN).



The results are as following:



I then used a weight distribution system that reduces the load on the hitch by 1/3 (using the rule of thumb that it spreads the load 1/3-1/3-1/3). I then introduced a moment at the receiver that given the load on the receiver would result in a load at the wheels that is equal front and back (the assumption here is that since the two axle drop equally the load the equal. The X5 as a a natural 50-50 weight split).

Here's how it looks:



And the results are as follows:



I'd love your comment here before taking it any further and draw conclusions. In particular do you think that this is a accurate representation of the situation at hand?
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Old 07-27-2015, 12:52 AM   #39
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Jeff,

I'd love your comment here before taking it any further and draw conclusions. In particular do you think that this is a accurate representation of the situation at hand?
I think it seems like a very complicated analysis. I would have taken the drawing in your previous link, assumed a tongue weight of x, figured out what was happening at the receiver mount interface 10" in front of the ball (or whatever it actually is) without WD, and then figure out how that changed with 2000 lb WD bars torqued up to restore 100% of unhitched FAL.

Your original premise was that the bending moment at the receiver mount interface (of x) without WD, was reduced by the WD equipment. I suggested that the new bending moment at the mounting interface was in the opposite direction, and wasn't simply a fraction of x. You have to get some load back to the front axle.

To me, the risk in your analysis is that there are so many assumptions, I am not sure anything will be proven.

Jeff
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Old 07-28-2015, 03:04 PM   #40
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Hey Guys,

This is a great discussion! Thank you for your input.

I need to confess that I do not understand most of what zetatre and jcl are discussing about. However, I am convinced that I want to go ahead with the reinforcement. There are two questions I would like to ask again to understand the issues (or at least to try to understand):

- flat iron vs. square / rectangle tube: as jcl suggested (post #17), square / rectangle tube would give more stiffness. I do not require any particular ground clearance. Therefore, I would opt for rectangle tube, unless you would say that square tube is much better;

- reinforcement mounting location - as zetatre noticed (post #19), the Can-Am reinforcement is attached to the suspension carrier. Again, I do not understand all of the technical issues, but as far as I understood from this thread, the suspension carrier is not firmly connected to chassis and it would be better to attach it to the chassis (the orange member in the rear of the car, as mentioned in post #6).

I read about Can-Am Audi Q7 reinforcement – some were saying that the mounting points are less than perfect. I am just wondering whether is it good to follow Can-Am reinforcement of X5 or (if feasible) to attach the reinforcement to the chassis.

I would appreciate your thoughts on this.

Thanks!
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Old 07-28-2015, 04:18 PM   #41
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bono

I'll try to summarize my honest opinion and where I stand. Take it for what is worth.

I can see and understand how the E70 receiver is weak. It is a silly packaging: the receiver drops from tow bar and is supported in the back. It can flex left to right, but more importantly up and down.

The E53 is different as it has two side plates to resist side to side twisting and a front plate to resist up-down flexing.

I continue to be totally skeptical of any weakness in the chassis of the E70 or the interface between the hitch mounting locations and the chassis.
Using the withidl E53 as a reference:
1) The E70 has a higher torsional stiffness than the E53 (it "resists more bending")
2) The rear section where the hitch mounts is part of an undeformable barrier (which means limited crumple in the event of a collision at 80mph with significant overlap); when the E53 was design that was not a prerogative.
3) The rear section structure and metals in the E70 are in both yield and tensile strength superior to the E53.
Because of this I'm skeptical that any reinforcement involving the chassis is required. The logic is that if withidl chassis was fine towing what he towed for the many miles he did, the E70 chassis should be at least as fine.

Now, jcl points out the "reinforcement bars" that are part of the E53 hitch kit and are not present in the E70 hitch kit. Since the E70 lacks them the conclusion is that the area is weaker. My counter to that is that as far as we know those could simply be required because the E53 had shock absorbers in the rear (absent in the E70). Mounting the hitch to the shock absorber would be inadequate. Also, the E70 may not need them because that area is structurally stronger than the E53 (for the many reason listed above).

In conclusion, since I have seen little evidence of weaknesses in the chassis or the chassis/hitch interface I'm not inclined to do anything to it. In particular since the prevailing modification involves connecting the hitch to the rear suspension subframe. If in fact there is a weakness in the chassis or the chassis/hitch interface that seems the least optimal way to reinforce it (if it reinforces it at all...).

On the other hand I understand and see possible weakness in the design of the connection between the receiver and the two bar. My solution would be to address that weakness internally in the hitch system and not interfere with the chassis or the the rear suspension carrier.

I'm working on it...
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Old 07-28-2015, 05:13 PM   #42
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Having an X5 with aftermarket receiver which was later reinforced can-am style, my vote if I could do it again would be to run the reinforcement strut forward to somewhere on the unibody vs. the rear suspension carrier. My personal logic is that while I don't seem to be any negatives at the moment from the can-am style'd mod a direct attachment to the unibody is less intrusive to the design intent of the vehicle.

Possibly I could have lived with the receiver flex, but the WD system works much better now that it is eliminated.
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