Reply
 
Thread Tools Search this Thread Display Modes
 
Old 07-19-2015, 04:46 PM   #15
jcl
Rivet Master
 
Currently Looking...
Vancouver , British Columbia
Join Date: Oct 2012
Posts: 1,046
Quote:
Originally Posted by zetatre View Post
But having seen and held the receiver, I'm unsure whether that thing can bent or would just transfer everything to the chassis anyway, which would make my point irrelevant...

Thoughts?
If you have no receiver installed, up to a few mph a rear end collision only hurts the bumper.

If you have a receiver installed, and you have a rear end collision over a few mph, the unibody will likely give before the receiver hitch. Over a certain speed, both will be damaged.

If you have a brace installed, I would think that that certain speed would be a few mph faster, but above that you are facing major damage in any case.

So I wouldn't worry about it at all.

I would consider future service access. A bolted connection makes sense.

Jeff
__________________

__________________
jcl is offline   Reply With Quote
Old 07-19-2015, 06:58 PM   #16
3 Rivet Member
 
Mountain View , California
Join Date: May 2015
Posts: 236
Thanks Jeff for your contribution to this thread! What are you thoughts in relation to using a flat iron vs. square tube?

thanks!
__________________

__________________
bono is offline   Reply With Quote
Old 07-20-2015, 12:02 AM   #17
jcl
Rivet Master
 
Currently Looking...
Vancouver , British Columbia
Join Date: Oct 2012
Posts: 1,046
I wouldn't use a flat bar myself, because I would rather have the stiffness of a box section. Doesn't have to be square, though. It would depend on the ground clearance required.

Jeff
__________________
jcl is offline   Reply With Quote
Old 07-20-2015, 11:04 PM   #18
2 Rivet Member
 
2017 30' Classic
Lexington , Kentucky
Join Date: Oct 2002
Posts: 97
Images: 2
Here's a photo of where the Can Am reinforcement attaches to the chassis on our 2015 X5 35d.
Attached Thumbnails
Click image for larger version

Name:	IMG_2905.jpg
Views:	97
Size:	264.9 KB
ID:	243556  
__________________
wlanford is offline   Reply With Quote
Old 07-21-2015, 11:35 AM   #19
2 Rivet Member
 
Oceanside, CA , California
Join Date: Jul 2015
Posts: 20
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 is offline   Reply With Quote
Old 07-21-2015, 12:26 PM   #20
worried...happy...wo...ha
 
Knuff's Avatar
 
2015 25' FB International
Menlo Park , California
Join Date: Oct 2013
Posts: 196
Images: 3
We went with the flat iron for reasons of ground clearance and also when thinking about the attachment point and where the weak spots are the flat iron seemed to be more than enough. Also to keep in mind that the reinforcement is mainly meant to redistribute force and not to bear the maximum load. It is an accessory piece. We also decided to use bolts for attaching the reinforcement bar to the crossbar on our Cayenne in case we have to replace the muffler at some point.
__________________
Knuff is offline   Reply With Quote
Old 07-21-2015, 02:55 PM   #21
3 Rivet Member
 
Mountain View , California
Join Date: May 2015
Posts: 236
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.

Quote:
Originally Posted by zetatre View Post
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...
__________________
bono is offline   Reply With Quote
Old 07-21-2015, 03:31 PM   #22
2 Rivet Member
 
Oceanside, CA , California
Join Date: Jul 2015
Posts: 20
Quote:
Originally Posted by bono View Post
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...
__________________
zetatre is offline   Reply With Quote
Old 07-21-2015, 04:11 PM   #23
3 Rivet Member
 
Mountain View , California
Join Date: May 2015
Posts: 236
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...
__________________
bono is offline   Reply With Quote
Old 07-21-2015, 04:38 PM   #24
jcl
Rivet Master
 
Currently Looking...
Vancouver , British Columbia
Join Date: Oct 2012
Posts: 1,046
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).
__________________
jcl is offline   Reply With Quote
Old 07-21-2015, 04:48 PM   #25
2 Rivet Member
 
Oceanside, CA , California
Join Date: Jul 2015
Posts: 20
Quote:
Originally Posted by bono View Post
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.
__________________
zetatre is offline   Reply With Quote
Old 07-21-2015, 05:49 PM   #26
jcl
Rivet Master
 
Currently Looking...
Vancouver , British Columbia
Join Date: Oct 2012
Posts: 1,046
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
__________________
jcl is offline   Reply With Quote
Old 07-21-2015, 06:07 PM   #27
Rivet Master
 
Vintage Kin Owner
N/A , N/A
Join Date: Sep 2012
Posts: 995
Images: 1
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.
__________________
rostam is offline   Reply With Quote
Old 07-21-2015, 07:13 PM   #28
2 Rivet Member
 
Oceanside, CA , California
Join Date: Jul 2015
Posts: 20
Quote:
Originally Posted by jcl View Post
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.
__________________

__________________
zetatre is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Question for owners of late model Ram 2500 6.7 Cummins Diesel Owners SteveH Tow Vehicles 110 03-09-2015 01:43 AM
BMW Tow Vehicle Question rdinflatrock Hitches, Couplers & Balls 7 08-04-2013 10:21 AM
BMW MOA International Rally July 20-23 Uberlanders Other Rallies & Events 21 04-24-2008 11:01 AM
Bmw Towing Capacity Please Help ACHALAT Tow Vehicles 4 02-03-2006 07:42 AM
BMW 740 Tow Vehicle JD1 Tow Vehicles 9 02-10-2003 08:28 AM


Virginia Campgrounds

Reviews provided by




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 10:30 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.

Airstream is a registered trademark of Airstream Inc. All rights reserved. Airstream trademark used under license to Social Knowledge LLC.