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Old 11-25-2019, 06:58 PM   #41
jcl
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So you simply choose to ignore the hitch rating. So my questions is how do you decide what ratings you can ignore and by how much?
I donít ignore ratings, I seek to understand what they are based on, using engineering principles. I also consider how they are related. Eg, you canít tow a 27 with a Fiat 500 even if you install a Class V hitch, since you run into the axle weight rating limits.

Tire and axle ratings are coded in your FMVSS, so they have a regulatory/legal basis. Good idea to follow them. The ways in which they are calculated are prescribed. All manufacturers use the same procedure.

The GVWR is enforced as part of the US Federal Motor Carrier regulations, so follow them if you are under their jurisdiction, ie a commercial carrier.

Tow capacity ratings are not in the same category. They are a starting point. Case in point. My X5 did not come with a tow rating in North America. Zero mention on the door jamb or in the manual. The same vehicle was rated to 7700 lb in other markets, unless one ordered the no-cost upgrade in Europe (it was just a decal, but had tax implications so wasnít included as standard equipment). The NA dealer-supplied hitch had a 6000 lb rating. The decal for the ďvehicleĒ tow rating came in the hitch receiver box. What was my rating, if I used other than the dealer hitch?

Many vehicles have different tow ratings in different markets. My daughterís Toyota had a relatively high tow rating in Europe, a lower rating in Canada, and the US manual said donít tow, the vehicle isnít designed for it. Further warnings sounded suspiciously like the bus full of orphans warnings. Same vehicle in each market, from the same factory. Different appetites for risk by different marketing departments. The US model included blind nuts in the chassis for mounting a hitch, but only discoverable if you read the non-US manual. They lied. The non US manual included drawings for how to design and mount a hitch receiver. Which manual you gonna trust?
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Old 11-25-2019, 07:01 PM   #42
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I thought that the Porsche had a hitch weight of 700 or so. I also didnít think that Porsche allowed the use of a WD.
Sometimes it seems like all the stuff cryptic.
Hitch weight rating with no WD is 770 lbs. The no WD thing for the Cayenne is a European issue. WD is prohibited in Europe, hence no WD rating is given on any European built vehicle and the manuals say to not use WD when towing with them.

The other thing to bear in mind is that it is not like the hitch would fail catastrophically when it does fail, at least none that I have heard of or seen for myself on failing hitch receivers. If the stresses get to big for the hitch to withstand you will start to see the hitch bend or distort, which is easily detected when looking at the 2" receiver tube. They don't just suddenly break and fall off the vehicle. That is why I keep a close eye on mine.

The other thing to keep in mind is that these, like many other German car components, are seriously over engineered so they will take much more than what is advertised. They don't test them to the breaking point and say, "it broke at 770 lbs so that is the max we will rate it for."
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Old 11-25-2019, 07:09 PM   #43
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Interesting that it also says do not cut weld or modify. I guess they forgot to add “unless you know what your doing”. Or was that intentional?
It means that if you cut or weld it without knowing what you are doing, you could compromise the strength of the assembly. So they can’t stand behind the rating. Standard manufacturer practice. In my company, we applied these warnings to all sorts of safety-related structures when we designed, built, and sold them, as did all other manufacturers.
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Old 11-25-2019, 07:11 PM   #44
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Why do you think WDs are prohibited in Europe?
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Old 11-25-2019, 07:18 PM   #45
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FYI. Here is the towing placard on the GL/ML hitch. Note that a WD hitch is excluded from use and that the tongue load is limited to 600 lbs. So, if you tow a 27 with it, you will be exceeding the manufacturer's recommendations.
As has been noted previously when you have claimed this, a WD hitch is not excluded from use. The manufacturer is silent on the subject. Absence of proof is not proof of absence.

You have previously posted that the maximum offsets shown for weight carrying hitches preclude the use of WD equipment. This isnít true either. They preclude the use of a WD hitch without the WD bars connected. As you know, WD changes the characteristics of the load applied.

Interesting that the label you show has two ratings, depending on which vehicle it is installed on, despite having a common part number. So it appears that the rating is considering axle loads limits and tow vehicle dynamics, which vary by vehicle, and which is very logical. So, how does WD use impact those factors? Potentially in a way that would change the rating, if in fact the manufacturer had a capability to test a WD hitch during product development?
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Old 11-25-2019, 07:20 PM   #46
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Why do you think WDs are prohibited in Europe?
Because they are incompatible with surge brakes, which are a Euro standard. Lots of published history on how that all came about. Essentially, vested interests lobbied successfully.

Should we discuss the limitations of surge brakes, which can’t be applied independently of the tow vehicle brakes?
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Old 11-25-2019, 07:42 PM   #47
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Because they are incompatible with surge brakes, which are a Euro standard. Lots of published history on how that all came about. Essentially, vested interests lobbied successfully.

Should we discuss the limitations of surge brakes, which can’t be applied independently of the tow vehicle brakes?
How would they effect surge brakes?
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Old 11-25-2019, 07:58 PM   #48
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How would they effect surge brakes?
WD hitches may reduce trailer brake application upon deceleration, which with surge brakes relies on compression of the trailer tongue. Apart from the inability to apply them independently. Which makes them illegal in many North American jurisdictions for trailers over a specified trailer weight.
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Old 11-25-2019, 08:02 PM   #49
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The WD bars will not allow the trailer to move forward when the tow vehicle brakes are applied and actuating the surge brake master cylinder on the trailer tongue which applies hydraulic brake pressure to the brake cylinders on the wheels.
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Old 11-25-2019, 08:03 PM   #50
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I never really looked at surge brakes and always thought it was all housed in the hub. That all makes sense now.
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Old 11-25-2019, 08:32 PM   #51
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Towing 27FB Globetrotter with Mercedes GL450

Best example in the US of surge brakes is the larger U-Haul dual axle trailers. Big hydraulic master cylinder and fluid reservoir just behind the hitch and hydraulic lines and hoses to the drum brakes on the wheels.

Springs and linkage to actuate the brakes when you slow down or go downhill, and a big spring and chain linkage to fire it for breakaway setup. Thereís no way WD bars would even fit, much less function with that setup.
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Old 11-26-2019, 04:36 AM   #52
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It is not just the issue of hitch and axle loading that has to be considered. There is also understeer/oversteer. If the trailer is too big for the tow vehicle it will cause the rig to jacknife in a hard turn.
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Old 11-26-2019, 06:26 AM   #53
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It is not just the issue of hitch and axle loading that has to be considered. There is also understeer/oversteer. If the trailer is too big for the tow vehicle it will cause the rig to jacknife in a hard turn.
Actually a rig may jackknife in that situation because of lack of driver skill and/or driving too fast for the road conditions. I have seen many solo pick up trucks spun out on curves in poor driving conditions.

Of course WD equipment, properly set up, will help to keep downforce on the steering axle and allow the tow vehicle to have close to the correct amount of under/over steer.
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Old 11-26-2019, 06:31 AM   #54
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Actually taking weight off the rear axle will exasperate the oversteer situation. It is best to not have to use a WD hitch.
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Old 11-26-2019, 07:26 AM   #55
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I am not exceeding any of the Cayenne's weight limits. If I was simply towing on the ball (without WD), I would be exceeding the rear axle and non WD capacity of the receiver. Receiver capacities are always higher when using WD and, when properly adjusted, all my weights are within load ratings, including front and rear axle ratings, cargo capacity of the Cayenne and the AS, GVWR of the Cayenne and AS, GTWR and the trailer towing capacity of the Cayenne.
If you are relying on the WD hitch moving a few lbs off of your rear axle to be within your limits the TV is inadequate to do the job safely. You can hype your Cayan all you want and modify this an that it simply wasn't built to do the job .
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Old 11-26-2019, 07:31 AM   #56
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Actually taking weight off the rear axle will exasperate the oversteer situation. It is best to not have to use a WD hitch.
What you are talking about isn't understeer, but rather you have identified one of the main disadvantages of a pickup trucks. If lightly loaded, the rear end tends to loose grip on the road, break loose and spin out. With the MB there won't be this problem and with WD both the front and rear tires will have the best possible grip of any road surface.
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Old 11-26-2019, 07:55 AM   #57
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I'm talking about oversteer. That's when the trailer pushes the back of the truck sideways and causes the front to turn more than you intended. In order to ameliorate this condition it is important to have as much weight on the rear wheels as possible.
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Old 11-26-2019, 08:00 AM   #58
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I am always amazed by these discussions; most posters seem to say there is only one correct solution. Maybe so, I have never seen the math that proves that. Our club has over 100 trailers in it. I have seen people pulling with pickup trucks, SUV's, Old SUV's(Suburbans), and cars. They all seem to get to their destination and get home. There has been one case of a member encountering sway and destroying both the truck and trailer, with a small pickup and a 23. It seems like almost anything will get the job done unless or until you encounter the one situation that signals "the end". My decision was to give myself plenty of margin from nameplate ratings for what I tow and what I want to haul in the truck bed. I really don't care to ask anyone whether they agree or disagree with my choices of TV and hitch.
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Old 11-26-2019, 08:17 AM   #59
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If you are relying on the WD hitch moving a few lbs off of your rear axle to be within your limits the TV is inadequate to do the job safely. You can hype your Cayan all you want and modify this an that it simply wasn't built to do the job .
Ah, the tow on the ball only crowd has joined the party. I have not modified my TV at all. If it wasn't built to tow can you please explain why, given it came from the factory with a tow package, pretty stout to boot, and provided tow ratings for it, which I happen to be well under.

These types of comments only come from people who have little or no knowledge of physics or engineering.

Perhaps we should all get back to the OPs question, is the GL 450 Mercedes a suitable tow vehicle for a 27 GT.
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Old 11-26-2019, 08:24 AM   #60
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My decision was to give myself plenty of margin from nameplate ratings for what I tow and what I want to haul in the truck bed.
Larry
I like that. "When in doubt, make it stout". This adage is what got us to the moon when the Russians failed.
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