Question for BMW X5 owners

[bono]

So, I understand that this is difference place they weld to than you suggested in post #6? This is to complicated for me to compare the diagrams you provided.

Originally Posted by zetatre

That is quite an interesting, and, I admit, a bit puzzling choice for a mounting location.

Specifically that is the rear suspension carrier



Primarily for reason of noise insulation, the suspension carrier is mounted to chassis via rubber bushings: their purpose is to isolate the road noise which doesn't get transferred to the chassis, thus "the car is more quite". And they achieve that, being rubber, but not being rigid.

Seems counterintuitive...

[zetatre]

Originally Posted by bono

So, I understand that this is difference place they weld to than you suggested in post #6? This is to complicated for me to compare the diagrams you provided.

In the post #6 I was simply speculating the welding point based on incomplete pictures I saw.

First, some background, just to level set. BMW and many other car maker do not attach the axles directly to the chassis. Instead for various reason, the axles are mounted to a axle carrier (basically another frame) and the axle carrier is attached to the chassis. One of the reason is to reduce road noise transfered to the chassis (and the cabin); to achieve this, there's rubber bushings between the chassis and the axle carrier: the axle carrier and the chassis are nor rigidly connected to each other.

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.

Now, to get back to us, we have the tow bar that is attached to the chassis. We recognized a weakness in the receiver that is allowed to flex. That's what we're addressing with the reinforcement and along the way we also feel that is a good idea to "spread the load on multiple points of the chassis".

Based on this situation and looking at the general position of the reinforcement bar, it was rational to me that if the purpose of this exercise is to transfer load to the chassis and reduce flexing of the receiver, you would attach the reinforcement to the chassis itself and I speculated it was welded to the rear cross member for the rear axle carrier.

Instead it appears that, at least in the F15 X5, it is welded to the rear axle carrier. The axle carrier is not attached rigidly to the chassis, but through rubber bushings (that's the item labeled as #2).

Given the objective, seems a rather odd choice of mounting location to me...

[bono]

Thank you for the explanation. Now even I understand I am wondering if Can-Am had any thoughts around it or they are just welding the reinforcement to the part which appears to be solid...

[jcl]

Good explanation above.

I think it would be optimal to attach the strut to the unibody, if there was a point with clear access and that part of the unibody was stiff enough to take the load.

I agree that attaching it to the axle carrier/subframe is a second choice, but in terms of resisting a vertical load at the attachment point of the strut that carrier doesn't seem like a bad option. The problem would be if a droning noise resulted in the cabin. If so, then time to reevaluate choice two. If not, then I would use the axle carrier.

Above concluded via interwebs engineering without physically inspecting an F15 (but it is my candidate vehicle so I am interested).

[zetatre]

Originally Posted by bono

Thank you for the explanation. Now even I understand I am wondering if Can-Am had any thoughts around it or they are just welding the reinforcement to the part which appears to be solid...

They have the reputation of knowing what they are doing.

What I started to notice in my receiver is an interesting pattern on the paint that is consistent with flexing. Considering the paint has some plasticity it looks as it it has ever so lightly stretched and this created ridges over it.

This is a picture of the plate that connects the receiver to the tow bar. As you notice the paint on the curvature peeled off; the "stretch marks" on the paint are on the side of the plates; they spider-web out from the curvature.



Based on everything I've observed in my car and read around the flex is really here: it's between the receiver and the tow bar. It isn't where the tow bar is bolted to the frame.

I believe the flex is induced by the German design of the receiver: if you look at BMW, MB, and Porsche all share the same concept:
1) The receiver drops from the tow bar
2) The receiver is supported on the back of the tube only

As I looked at the design used in most other cars I noticed that:
1) The receiver is built into the bar or welded directly under it
2) The front of the receiver is supported either by trapezoid plates, or in the case of most pickups and pickup-based SUVs has a U shaped bracket that bolts it to the bumper

Intuitively "German design" is more flexible.

As I dropped by a local fabricator who I've been working with on many project, we agreed that perhaps there's other more efficient way to resolve the flex of the receiver other than tie it back to the chassis itself. Specifically you would tie the receiver back to the tow bar in multiple locations.

Using sections of T steel bars you would connect the front of the receiver to the tow bar. This would significantly reduce the ability of the receiver to flex up and down (what caused the "stretch marks" in the paint).

Similarly using two T bars you would go from the side of the receiver back up to the tow bar at an angle. This would reduce the ability of the receiver to flex side to side.

This way you have the reinforcement self contained in the tow bar and unless you start showing signs of flex of fatigue elsewhere may were well have taken care the issue at hand in a very direct way.

[jcl]

Yes, you could reduce any flex in the drop bracket with bracing attached to the receiver and the cross bar, but that opens up the potential for shifting the weakest point in the chain one step back, to the attachment between the cross bar and the unibody. We have seen that in an earlier design, BMW reinforced the E53 unibody at that point. Those reinforcements are missing from the E70 and F15 receiver hitch kits. Either the new unibody is stronger there and they aren't necessary, or that point will become the flex point. Is it worth finding out the hard way?

I suspect the design of the brackets has to do with packaging limitations, and the need to drop the receiver to match the predefined opening in the rear valance.

Jeff

[rostam]

The older X5 hitches had two support arms, and were pretty substantial. The new hitches are not very stout. Is it possible to install an older hitch on the new X5? This will save you from user engineering a complex vehicle.

[zetatre]

Originally Posted by jcl

Yes, you could reduce any flex in the drop bracket with bracing attached to the receiver and the cross bar, but that opens up the potential for shifting the weakest point in the chain one step back, to the attachment between the cross bar and the unibody. We have seen that in an earlier design, BMW reinforced the E53 unibody at that point. Those reinforcements are missing from the E70 and F15 receiver hitch kits. Either the new unibody is stronger there and they aren't necessary, or that point will become the flex point. Is it worth finding out the hard way?

I suspect the design of the brackets has to do with packaging limitations, and the need to drop the receiver to match the predefined opening in the rear valance.

Jeff

I don't want to be pedantic, but right now the load of the trailer is already supported by the attachment between the tow bar and the unibody and there is absolutely no signs of weakness in that area other than the flex in the connection between the receiver and the tow bar.

Furthermore, the way the reinforcement are currently attached to the car are at a location that was never intended to have to have load applied the way the reinforcement applies it. Whether you attach it to the cross member or the rear axle carry, you're applying a perpendicular load to a structure that was intended to do a different job and that never receives similar perpendicular load under regular driving condition.

It is my presumption that while the engineering behind the cross member sizing and material and the rear axle carrier sizing and material never took under consideration perpendicular loads. In particular that area is part of a "torsion frame" and has different function and sees different loads.

The load applied by a trailer to the two longitudinal beam where the tow bar connects instead seems more consistent with the engineering of those cross member and it's a load absolutely reasonable to expect on that structure. And those two beams are part to and "undeformable barrier" for crash purposes which basically means some pretty strong stuff that does not deform. The only part of the car "stronger" than those two longitudinal beam (however you define it) is the B-pillar.

[jcl]

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

[Bgibbs]

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.

[jcl]

Originally Posted by Bgibbs

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

[zetatre]

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?

[bono]

withidl's hitch is not reinforced

[zetatre]

Originally Posted by bono

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.

[jcl]

Originally Posted by zetatre

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.

Jeff

[zetatre]

Originally Posted by jcl

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!!!

[jcl]

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

[zetatre]

Originally Posted by jcl

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?

[jcl]

Originally Posted by zetatre

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

[bono]

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!