Mercedes R320 CDI towing 31' '79 Excella

[trekerboy]

Wow, this discussion has turned far more technical than I thought it would! Thanks everyone for your input.

We're going to give it a go with the R class and see what happens. This whole thing is a giant experiment anyway; can we actually live out of an AS with 2 kids under 3? Can an R class be modified to reliably tow 31' of shining aluminum awesomeness? If it doesn't work, y'all will be the 1st to know (with pictures, I promise). But if it does work and you see an R class parked next to you at the next place you camp, come say hello.

[dkottum]

You will do well to seek advice from the guy with 29 years and thousands of tow vehicle hitch setups similar to what you are doing. He's a second generation Airstream dealership owner and recognized hitching expert in North America. He has advised us personally and successfully in our last two Airstreams and hitch setups.

You are practically neighbors with Andrew Thompson at Can-Am Airstream in Ontario; that's your best bet to get your R class and Excella set up for best results.

As for your questions about traveling in your Airstream with the small family, the forum has many who are doing this. Post your specific questions and you will get help. We spend six months a year in our Airstream and enjoy it very much as retirees.

[Tuco]

Let’s take these one at a time, shall we?

“Because the force that matters at the front of the torque strut is vertical, not in other planes. There is no need to create a triangle or box.”

Any force applied to the receiver matters. In this thread it has already been determined that the force applied at the end of the reinforcement arm is at the very minimum, down. When engaging my WD hitch, the shank moves up and back toward the trailer (my personal observation). If this holds, then the reinforcement arm will move aft and down. This is when the rig is at rest and WD engaged. Underway all bets are off as the WD varies from positive to negative at any given time depending on road conditions. The reinforcement arm is seeing positive and negative loads at angles that are not just up or down. Unfortunately, the designers didn’t have the reinforcement arm in mind when they designed the sub frame mounts. Do you really think they designed in extra safety margins in the bushings for the odd angled loads imparted by the reinforcement arm? Any reinforcement should be designed as a triangle, not a box (which your current design is with the bushings acting as the fourth side).

“Because the rear subframe is designed to handle loads far in excess of the load applied by the torque strut.”

Since you don’t have any idea what load is being applied, I find your statement here just a little hard to swallow. Rostam doesn’t seem to agree with you on this. Now if you had some data…

“Because the subframe doesn't move freely, it is constrained by encapsulated mounts, at least on the models I have worked on. It is vibration isolated, not soft mounted.”

But it does…move. The more you work the bushing the looser it will become. This wear will accelerate bushing replacement intervals which unfortunately will be hampered because the reinforcement arm is welded to the sub frame (really?). Contrary to what you have stated, my research shows replacement schedules for these bushings at around 60k miles. With the number of vendors selling bushing for BMW’s at least, it sound reasonable. It is the internet though.

“Because the standard deflection of the receiver, as determined from reports of some owners who were not able to restore front axle loads without the torque strut, is greater than the amount of associated subframe vertical movement that takes place at the front of the torque strut.”

I’m at a loss on this one, can you explain this a little better?

“Because it is widely reported to work well.”

Because we have always done it this way…

"Because despite the potential for transmitting noise, it doesn't appear to do so."

Sigh…

“Because the strongest part of the rear unibody is where the rear subframe is attached. This approach takes advantage of that.”

Too bad the force applied needs to be transmitted through the bushings (which move).

<this space is reserved for some snarky comment about “Some People”>

[jcl]

Originally Posted by rostam

please Google "BMW rear subframe failure". It seems several BMW sedans have had rear subframe failures in the years past, and that BMW settled a class action lawsuit with the owners. It seems the rear subframe is not as stout as you may think. Fixing the problem costs thousands of dollars.

I was under the impression we were discussing, in terms of towing, BMW sport utilities. The X3 (E83) is based on the same platform as the E46 you googled. The X5 (E53, E70, F15, and X6 variants) are based on a five series platform, not a 3 series. I have had five BMWs, one was an E46, two were X models that towed. I have never replaced subframe bushings or had a subframe failure. They have never worn out. Have you had a subframe failure?

But the OP is interested in an R series Mercedes, not a BMW.

Fortunately, some forum members will be out towing and enjoying their new vehicles, while others are searching the internet to try and find evidence that supports their opinions.

[Knuff]

Guess, we are all in for an upgrade to a tank, or at least an F-350. It has to be a beast, otherwise there is no arguing with the experts...

Originally Posted by Tuco

Let’s take these one at a time, shall we?

“Because the force that matters at the front of the torque strut is vertical, not in other planes. There is no need to create a triangle or box.”

Any force applied to the receiver matters. In this thread it has already been determined that the force applied at the end of the reinforcement arm is at the very minimum, down. When engaging my WD hitch, the shank moves up and back toward the trailer (my personal observation). If this holds, then the reinforcement arm will move aft and down. This is when the rig is at rest and WD engaged. Underway all bets are off as the WD varies from positive to negative at any given time depending on road conditions. The reinforcement arm is seeing positive and negative loads at angles that are not just up or down. Unfortunately, the designers didn’t have the reinforcement arm in mind when they designed the sub frame mounts. Do you really think they designed in extra safety margins in the bushings for the odd angled loads imparted by the reinforcement arm? Any reinforcement should be designed as a triangle, not a box (which your current design is with the bushings acting as the fourth side).

“Because the rear subframe is designed to handle loads far in excess of the load applied by the torque strut.”

Since you don’t have any idea what load is being applied, I find your statement here just a little hard to swallow. Rostam doesn’t seem to agree with you on this. Now if you had some data…

“Because the subframe doesn't move freely, it is constrained by encapsulated mounts, at least on the models I have worked on. It is vibration isolated, not soft mounted.”

But it does…move. The more you work the bushing the looser it will become. This wear will accelerate bushing replacement intervals which unfortunately will be hampered because the reinforcement arm is welded to the sub frame (really?). Contrary to what you have stated, my research shows replacement schedules for these bushings at around 60k miles. With the number of vendors selling bushing for BMW’s at least, it sound reasonable. It is the internet though.

“Because the standard deflection of the receiver, as determined from reports of some owners who were not able to restore front axle loads without the torque strut, is greater than the amount of associated subframe vertical movement that takes place at the front of the torque strut.”

I’m at a loss on this one, can you explain this a little better?

“Because it is widely reported to work well.”

Because we have always done it this way…

"Because despite the potential for transmitting noise, it doesn't appear to do so."

Sigh…

“Because the strongest part of the rear unibody is where the rear subframe is attached. This approach takes advantage of that.”

Too bad the force applied needs to be transmitted through the bushings (which move).

<this space is reserved for some snarky comment about “Some People”>

[Tuco]

Originally Posted by Knuff

Guess, we are all in for an upgrade to a tank, or at least an F-350. It has to be a beast, otherwise there is no arguing with the experts...

1 Ton's are tanks or at least beasts. If you feel the need to upgrade then go for it! Honestly though I wouldn't push a Ford on anyone. I don't have a problem with folks doing more with less, it just bugs me when people try and justify bad engineering because of religious beliefs.

[jcl]

Originally Posted by Tuco

Let’s take these one at a time, shall we?

Sure.

Originally Posted by Tuco

any force applied to the receiver matters. In this thread it has already been determined that the force applied at the end of the reinforcement arm is at the very minimum, down. When engaging my WD hitch, the shank moves up and back toward the trailer (my personal observation). If this holds, then the reinforcement arm will move aft and down. This is when the rig is at rest and WD engaged. Underway all bets are off as the WD varies from positive to negative at any given time depending on road conditions. The reinforcement arm is seeing positive and negative loads at angles that are not just up or down. Unfortunately, the designers didn’t have the reinforcement arm in mind when they designed the sub frame mounts. Do you really think they designed in extra safety margins in the bushings for the odd angled loads imparted by the reinforcement arm?

If your receiver is flexing up and back, then it is pivoting. You may want to consider installing a brace.

The brace doesn't need to be designed to resist fore and aft forces, if the standard receiver is sufficiently strong for those forces. That may be why a simple tab mount at the front of the brace works well. We are speaking here of an OE receiver that is already solidly mounted, but requires bracing due to WD. If you are fabricating a hitch from scratch, then it would be a different story.

Originally Posted by Tuco

Since you don’t have any idea what load is being applied, I find your statement here just a little hard to swallow. Rostam doesn’t seem to agree with you on this. Now if you had some data…

Rostam is searching the internet for examples of E46 vehicles, which were a very different platform. The problems seen there were largely on modified and tracked vehicles, and higher power M3 variants, for a vehicle that went out of production around 12 years ago. But Google lives on. I don't see any references to towing use. Another poster related his concerns about the E46 suspension in a previous post, and suggested it was the same here. The service manual drawings showing how different the design was were posted in that thread. I can post them again if you like.

The data you don't want to acknowledge is the successful use of this type of brace over many years. If you have data on subframe failures related to towing, that would be good info to review.

Originally Posted by Tuco

But it does…move. The more you work the bushing the looser it will become. This wear will accelerate bushing replacement intervals which unfortunately will be hampered because the reinforcement arm is welded to the sub frame (really?). Contrary to what you have stated, my research shows replacement schedules for these bushings at around 60k miles. With the number of vendors selling bushing for BMW’s at least, it sound reasonable. It is the internet though.

Yes, it is the internet. Replacement bushings are commonly offered for higher performance applications, and are common for vehicles with larger tires than originally supplied. I've never worn out subframe bushings on my last five BMWs though. Two towed. Neither had a brace. More than 60,000 miles on all except the Z4.

Recommend you don't hamper service access, whether for bushings or anything else. Better to use a bolted connection, IMO.

Let's assume for a moment that there is no brace. OE hitch. Let's assume we are towing the 7700 lbs they rate the vehicle for in Europe, or the 6000 lbs it is rated for in North America. Let's assume the receiver is mounted perfectly, because the unibody is stiff enough without a brace. Play along for a minute. Now consider the forces on the subframe bushings. All fore and aft forces, all cornering forces, etc, are transmitted to the tow vehicle tire contact path through those bushings. By design. Put more weight in the vehicle, up to the GVWR and rated axle loads. The loads are carried by the springs, and those bushings. So it seems like they would be designed to handle the sort of loads we are talking about. The strut reduces rear axle loading, by allowing for effective WD. Any loads transmitted to the subframe by the strut are transmitted to the tire contact patches without going through the bushings. The bushings should do just fine with a strut.

Originally Posted by Tuco

I’m at a loss on this one, can you explain this a little better?

It would help if we separated the concepts of forces, and relative motion.

Don't install a brace. Apply WD. We can't get sufficient front axle weight restoration (see previous posts by owners). We can consider that is due to flex. Install the brace. See that you now get much better WD effect and are able to restore more front axle load. That is because it isn't flexing now. It isn't moving. Force is applied, little relative motion results. If it was moving as you claim, you wouldn't be able to restore front axle loads. QED.

Originally Posted by Tuco

Because we have always done it this way…

If we were doing as we had always done it, we would all be fabricating receivers. I think you are setting out to solve a problem that hasn't been shown to exist. Until we see evidence of a problem, doing it how it has been done successfully would be a solid choice.

Originally Posted by Tuco

"Because despite the potential for transmitting noise, it doesn't appear to do so."

Sigh…

We know the strut works, in terms of strengthening the hitch to resist WD forces. We don't have reports of fatigue failures, so the relative motion concern you raise doesn't appear to be valid. The concern with using the subframe as an attachment point isn't over it not working, it is over the potential for noise transmission to the cabin. It isn't a distraction, it is the reason one might not want to use the subframe as the attachment point.

Originally Posted by Tuco

Too bad the force applied needs to be transmitted through the bushings (which move).

If there was a problem, due to too much relative movement, you would see failures. As for the forces, see the comment above. Can we agree that all vertical, cornering, acceleration, and deceleration forces end up at the tire contact patch? If you transmit forces to the subframe, they get to the tire contact patches without loading up the subframe bushings. It is a win.

Jeff

[andreasduess]

Ah yes, the old Airforum discussion again.

Those with first hand experience: "works great, no issues to report"
Those with none: "it can't be so, you must be wrong"

[trekerboy]

Originally Posted by andreasduess

Ah yes, the old Airforum discussion again.

Those with first hand experience: "works great, no issues to report"
Those with none: "it can't be so, you must be wrong"

Yes, but as a non-engineering type I'm just trying to determine if there's merit to the naysayers concerns, or if it's just a question of differing philosophies (i.e. no "right" answer). Right now I'm leaning toward experience trumping theory, but there are obviously some very intelligent and passionate people on both sides of this conversation.

[rostam]

Originally Posted by andreasduess

Ah yes, the old Airforum discussion again.

Those with first hand experience: "works great, no issues to report"
Those with none: "it can't be so, you must be wrong"

Actually, if this forum is to be believed, most of the folks with modified underrated vehicles switch to a proper tow vehicle after a while. So perhaps the correct assessment is "it works, for a while..."

[dkottum]

Originally Posted by andreasduess

Ah yes, the old Airforum discussion again.

Those with first hand experience: "works great, no issues to report"
Those with none: "it can't be so, you must be wrong"

Google allows some to argue endlessly with those who actually tow travel trailers, even hitch experts, without ever having experienced it themselves.

[Tuco]

Originally Posted by dkottum

Google allows some to argue endlessly with those who actually tow travel trailers, even hitch experts, without ever having experienced it themselves.

That's right, 40 years worth of hands on engineering experience is worthless.

[jcl]

Originally Posted by trekerboy

Yes, but as a non-engineering type I'm just trying to determine if there's merit to the naysayers concerns, or if it's just a question of differing philosophies (i.e. no "right" answer). Right now I'm leaning toward experience trumping theory, but there are obviously some very intelligent and passionate people on both sides of this conversation.

A valid concern, well stated.

As an engineer (and an engineering type) some of the debate is around countering myths that are presented as facts. There is no single right answer.

When setting up a tow vehicle such as the R320, you should still pay attention to things such as tire loads, axle loads, and gross vehicle weight rating. Personally, I worry less about the manufacturer's recommended tow rating. That doesn't mean ignore it, to me personally, but rather evaluate what it signifies. The absence of a test at a higher towing load doesn't prove it is unsafe, just that the manufacturer hasn't tested it at that higher load. If the hitch can benefit from reinforcing in order to be able to withstand the forces of WD equipment, then I consider that a pretty straightforward modification. For decades, we didn't have vehicle manufacturer (original equipment, or OE) hitches, and so all hitches were fabricated. It isn't a new idea.

Just remember that bracing a hitch doesn't change the GVWR of the tow vehicle, or the tire and axle load ratings. What it does is allow effective weight distribution to be applied, therefore distributing the load better between axles, and thus allowing the operator to take full advantage of the published axle and tire ratings.

The main point being debated above is about using a torque strut, or brace, to improve the OE hitch's ability to withstand WD forces. It is a proven approach. Some are disagreeing with the specific mounting point chosen, because of the isolation mounts on the rear subframe.

Good luck in your decision. If you are talking to Can Am, you have a very strong resource there. Personally, I think an R would make an awesome tow vehicle, as long as I was able to manage payload by deciding how much to take along with me.

Jeff

[PKI]

Does the GL hitch that has a pair of bolted arm extensions fit the R? Seems like since the GL and R use the same chassis platform, it might. Pat

Addition: Ref Rostam's Post 52 of the Toureg 23D thread

[Tuco]

Originally Posted by trekerboy

Yes, but as a non-engineering type I'm just trying to determine if there's merit to the naysayers concerns, or if it's just a question of differing philosophies (i.e. no "right" answer). Right now I'm leaning toward experience trumping theory, but there are obviously some very intelligent and passionate people on both sides of this conversation.

Don’t let the discussion between an Engineer and a Fabricator worry you. These talks happen thousands of times a day across the country. Sometimes the fabricator needs to slap the engineer in the back of the head and say “hey this isn’t right”! There is a right and wrong answer here, but you need to understand what is really going on. Reputations are on the line and that is important to remember. The goal is to shut down anyone that disagrees with those who are looked upon as experts. No one knows what my credentials are and they haven’t even asked. It’s better to try to destroy someone than learn from them when the bottom line is threatened. This also applies to the “SUV” click. You’re the enemy if you don’t drive one and you will be DESTROYED! It doesn’t matter what my views might be about different TV. It always boils down to “your one of THEM”! Have you noticed how things are phrased as “myths are being presented as facts” or “google allows some to argue endlessly” or “those with no experience” or “what contributions to towing are you making”. The list goes on but you get the point. All of these things imply I’m clueless and can’t possibly know what I’m talking about. It doesn’t matter that I have maintained, modified or fabricated a wider variety of equipment than probably all of them combined. Can’t we all get along seems to be a resounding no with some people.

As far as your choice of TV, you should rethink this idea. It’s not because it’s an SUV, but rather that you plan on full timing. As a full timer myself you really need to know what you plan to bringing along and what you plan to do. If you are going to be boon docking, are you going to need a generator? Where are you going to store it? In the back of the car? Where will you store its fuel? Your enthusiasm for getting MB R320 is commendable, but you will need to make compromises. You’re looking at around 8k lbs when even lightly loaded. It can be done, but you will need to plan ahead. The problem is that with your trailer, the reinforcement will be necessary. I personally would not have “the RV dealer that shall not be named” do the reinforcement due to my objections stated in this thread. It will be your vehicle and your call though. As far as everything else that needs setting up, I wouldn’t hesitate to call “the RV dealer that shall not be named” to get that done. Follow the advice already given about payloads, hitches and all that goes with running past the limits. Educate yourself as much as possible, but above all be safe.
GL

[jcl]

Originally Posted by Tuco

Don’t let the discussion between an Engineer and a Fabricator worry you. These talks happen thousands of times a day across the country. Sometimes the fabricator needs to slap the engineer in the back of the head and say “hey this isn’t right”!

Dave (or MJ), I think it would be helpful if you looked at a simple force summation for the rear subframe. Consider the fore and aft forces due to acceleration and braking. Consider the lateral forces due to cornering. Consider the vertical forces that result in effective weight transfer to the front axle, and reduction in weight on the rear axle. When you attach the brace to the subframe (as compared to the unibody) all those forces are potentially reduced, not increased. That is why it doesn't make sense to think of increased forces on the bushings.

You express concern about the isolation mounts of the subframe, and proposed removing them in an earlier thread, but don't appear to be considering the isolation characteristics of the torque strut.

Sometimes engineers assist fabricators, especially if it is something that the fabricator hasn't built before. Since you mention credentials, out of interest, are you an engineer (Mech?), a fabricator, or both? You've got me beat on the 40 years, my engineering has been 30 years. I was pulling wrenches in a shop 40 years ago, though, and recall replacing rear subframe bushings about 36 years ago. It was a Jag with a V12, and it wasn't towing.

Cheers

Jeff

[Tuco]

Originally Posted by jcl

Dave (or MJ), I think it would be helpful if you looked at a simple force summation for the rear subframe. Consider the fore and aft forces due to acceleration and braking. Consider the lateral forces due to cornering. Consider the vertical forces that result in effective weight transfer to the front axle, and reduction in weight on the rear axle. When you attach the brace to the subframe (as compared to the unibody) all those forces are potentially reduced, not increased. That is why it doesn't make sense to think of increased forces on the bushings.

You express concern about the isolation mounts of the subframe, and proposed removing them in an earlier thread, but don't appear to be considering the isolation characteristics of the torque strut.

Sometimes engineers assist fabricators, especially if it is something that the fabricator hasn't built before. Since you mention credentials, out of interest, are you an engineer (Mech?), a fabricator, or both? You've got me beat on the 40 years, my engineering has been 30 years. I was pulling wrenches in a shop 40 years ago, though, and recall replacing rear subframe bushings about 36 years ago. It was a Jag with a V12, and it wasn't towing.

Cheers

Jeff

Sorry to get back to you so late, but today was jump day.

I understand what you are driving at with the force summation. My concern here isn't just the added forces of the reinforcement arm. If my goal was to keep the current configuration that might be worth while pursuing, but you know what I really want here. It would be nice to know (if the current configuration is kept) what the new forces applied were. In the past I have use linear motion potentiometers to measure a systems motion over time. I remember seeing a similar device for force, but can't remember its name off the top of my head. You had discussed destructive testing, why not some non destructive testing that will provide the data. This would decide the issue once and for all or at least give food for thought.

I think what you are trying to say is sometimes engineers need to slap fabricators on the back of the head. I won't disagree with that. If it is to work at all it needs to be a two way street. The only engineering course I have been through is the Navy's Nuclear Power program. So maybe a minor one in nuclear engineering with emphasis on mechanical systems since it only covered one type of reactor design (PWR). A good engineer learns from the designs of others, so your career started when you got away with tearing something apart and back together without your parents finding out. You knew you had a vocation when you were handed a box of parts with no instructions and it worked when you were done. For me it was a VW bus engine.

Dave

[andreasduess]

Just as a by the way as we're discussing each other's qualifications, you may wish to consider the qualifications of those who initially came up with these modifications. Andy isn't your average RV dealer.

If you want your qualifications to be respected you will have to extend that courtesy to others.

[Tuco]

Originally Posted by andreasduess

Just as a by the way as we're discussing each other's qualifications, you may wish to consider the qualifications of those who initially came up with these modifications. Andy isn't your average RV dealer.

If you want your qualifications to be respected you will have to extend that courtesy to others.

I have learned a lot from Andy over the last few years. His posts are insightful and knowledgeable. It would be silly for me not to listen to what he says and learn from it. The man has forgotten more about trailers, airstreams and hitch setups than I will ever know. What I am not going to do is place him on a pedestal and worship the ground at his feet. He puts his pants on the same way we do. We all make mistakes and only an idiot doesn't try and learn from them.

[trekerboy]

Well, I've made an appointment with Can Am RV in London, Ontario CA to get the hitch and related equipment installed. I'll post pictures and let y'all know how it goes!