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phoenix2013

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Knee surgery was postponed to April 19th, gimping using the walker, feeling useless, figured will get this done. Put the initial parts in CAD.

 

DSC_0037.JPG

 

Let the guessing begin. Mark and Gregg you have an unfair advantage, let others have fun.

No, it is not an implement one obtains in certain establishments with opaque windows and semi parkings in the back.

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Let the guessing begin. Mark and Gregg you have an unfair advantage, let others have fun.

No, it is not an implement one obtains in certain establishments with opaque windows and semi parkings in the back.

Well, I guess I'm eliminated too.....What would I have won???

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I think those are new style 5th wheel pins to work with a new design super et hitch head in liue of the Brinkley version. The different lengthes allow for adjustable heights so you do not need to change your pin box height. The ridges give a better gripping surface and avoids the dreaded "is it latched correctly issue".

 

They are made of a new high strength polymer that also absorbs the shock when hitching up but will handle pin weights of up to 10k and trailer weights up to 80k. :)

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Jim, the RV industry continues to amaze when it comes to stupidity, greed and buyer beware. This project is in part addressing one of these and at the same time it's another piece of continuing improvements in the ET line.

Last year we addressed the fact that the custom manufacturers "happily" crossed way beyond 22K GVWR and 6K in pin weights. An ET Super Sr. was designed to handle these rigs.

This project is an attempt to handle a new phenomenon, "the floaters". DRV has designed and is selling about half a dozen fifths that exceed 40 feet and 20K GVWR. Nothing wrong with this, congratulations, atta boy.

Do you know what the pin weights are on these rigs? Maybe you guys can point me to the right place, I've been all over their site and it's like looking for the Holy Grail. Almost like they don't want you to know what it is, or eliminate it as a point of "discussion" between you and some hapless DRV salesman. The 40 feet plus models are indeed nice and no doubt objects of desire to many a buyer who shows up at the dealership with his pickup. At which point the salesman can alleviate the buying subject's trepidation about the adequacy of the puller by whispering into subject's ear, "but the pin weight is only 4K".

So what happens when the length goes up, the weight goes up, but the pin weight stays the same (to make the pickup happy), you get yourself "a floater". Incidentally, I claim a copyright on that term, it's a new one.

An ET is a long excursion, long time stamp hitch, with sufficiently proportional pin weight, it goes down and goes up. with "a floater, it goes down, it goes up, it goes down, it goes up.

 

Yes, there will be an interaction between the bags, bags inflation and the snubbers. How much, at this stage I don't know. we'll have to do some testing to see if there will be a single, or uniform solution, or whether there will be some "tuning" required, depending on the fifth.

Some of your old timers might remember the McCall hitch.

2081567880096176628cPTZPt_ph.jpg

 

Great hitch, designed and manufactured, way ahead of its time. The two horizontal airbags were implemented to reduce chucking forces. They required "tuning" (air pressure wise) to maximize their response to the fifth being pulled.

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Phoenix, So do you think it will be a factor of too light of a pin weight in comparison to overall trailer weight or will length and how much weight is behind the axles on the trailer going to factor into it (which I think it will)?

 

I guess for example our 44' trailer weighs 18k fully loaded but we only have a 3k pin weight which is only 17% of trailer weight in lieu of 25% I have heard that 5th wheels could be. A lot of our weight is near the rear of the trailer when we have the van loaded up as it weighs almost 7k itself. Luckily a van is front heavy and the front axles on the van sit right above the trailer axles. We haven't noticed too much bounce but we also do not have a camera mounted to watch the hitch going down the road although our son has ran alongside us to watch what it was doing. I imagine as the weight of the trailer increases and further reducing the percentage of pin weight, it will make a difference. Although in your example 20k trailer with a 4k pin weight is at 20% which is still better than ours!

 

Dave

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Phoenix, ok I'm getting what you're doing and why. Fifth wheel axle placement is being moved forward by manufacturers to carry larger percentage of the load to keep pin weight within pickup limits . . . and that's false sense of safety. When hitched up, ET air pressure is adjusted for the light pin weight vertical component but still has to deal with horizontal fore-aft inertia. Guessing both of those snubbers function in compression; the longer one on the down stroke and the shorter one on the up stroke. Tuning accomplished using shims under the snubber(s) and air bag pressure?

 

Am I close on all this?

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Seems like a snubber would help, but will only be tuned perfectly for a particular pin weight and rotational inertia (rotation about the trailer axle center line), road condition and road speed. Just dreaming up a potential solution. How about a closed loop system that would try and keep the pin in the center of the hitch travel. It would need a position sensor (many COTS LVDTs or RVDTs out there) and a high band width (two or three times the resonant frequency of the "floater") air servo valve. Then a micro controller to take care of business. A lookup table or polynomial solution could be applied via the controller. An algorithm could be developed that would start from a "one size fits all" and would learn as the bumps and floating started. As it learned, the polynomial coefficients could be revised for the specific road conditions and trailer response. The design could be fail safe with the existing height sensing valve staying in the system, since its bandwidth and gain is so much lower than the proposed control systems influence. Just dreaming.

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Phoenix, ok I'm getting what you're doing and why. Fifth wheel axle placement is being moved forward by manufacturers to carry larger percentage of the load to keep pin weight within pickup limits . . . and that's false sense of safety. When hitched up, ET air pressure is adjusted for the light pin weight vertical component but still has to deal with horizontal fore-aft inertia. Guessing both of those snubbers function in compression; the longer one on the down stroke and the shorter one on the up stroke. Tuning accomplished using shims under the snubber(s) and air bag pressure?

 

Am I close on all this?

Jim, you are close, except that the snubbers would be utilized in the upper direction only (sort of like super duper shock absorbers). The ET handles downward forces pretty well, thanks to the leveling valve, it also transitions the forward and back forces (chucking) downward, because of its geometry (independent, air bag only suspension and parallelogram).

Super%252520ET%252520Sr%252520cutaway.jp

 

super%252520et%252520sr%252520cutaway%25

 

super%252520et%252520sr%252520cutaway%25

 

Jack observed how well this works, when he watched me at slow speed backing into a slot with lots of forward and back jerking and crossing drainage depression in the street. I'm sure a mathematical model and algorithm could be developed to calculate the forces that the snubber is likely to encounter. Knowing what I know about the complexity of the system and the interaction between various force vectors it would be quite an undertaking, certainly more than I want to undertake in my seventh decade on this earth, although it would make a hell of a theses for somebody on a PhD track at MIT. I am approaching this in a more empirical fashion working with "volunteers", including one who owns a "floater".

 

Dave, as you might have gathered from my sarcastic description of DRV offerings and their marketing methods, the reason I speak so ill of it, is because I find it hard to believe that they don't know better. Sort of like General Motors engineers knowing about their killer ignition switches but continuing to sell these for a decade, or Takata with their airbags, or Volkswagen with their software, etc. Greed is a powerful motivator. Pushing pin weights below 20% of GVWR, sooner or later is a recipe for disaster. I know personally an individual who almost lost the rig and the trailer (and possibly his life) when it went into sway oscillation going down hill and braking. Luckily, he was able to to bring it under control. And we are not talking about a guy with a pickup, this was a 42 footer, almost 30K GVWR, pulled by a Volvo. Guess what his pin weight was with the fifth fully loaded, 16.7%!

Here's a page from a manual for those studying for their CDL.

 

Seminar%2525205.png

 

Take a look at the bottom at the bullet point number six. Again, being sarcastic, having CDL apparently is not a requirement to be an RV design engineer.

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Henry and I - and others - have had a myrid of conversations over the years attempting to help individuals with "challenging" situations relating to pin weight on both lighter and heavy rigs. The events he spoke to above were just one instance. It can be terrifying - believe me. I've seen a rearward load shift in a freight van result in a tractor on its side.after some "maneuvering". The same applies to our trailers. A light pin (percent wise) is nothing to ignore. You can get away with it (perhaps) indefinitely on a well balanced trailer....but any imbalance exasperates the situation And can make for "interesting" behavior. People get away with it because of the light loads, IMO.

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I would think, a dangerous proposition, that more trailer axles would somewhat negate a light hitch.

 

For instance, I've had experience with a balanced (no hitch weight) single axle trailer. I thought I had plenty of truck. I was wrong.

 

We recently traded from a tandem 5er to a tri-axle. Granted, the whole trailer is different, but the tri seems to ride much smoother, less "pitching" when crossing road undulations.

 

Of course, most of our trailers are tandems, and most have pretty limited room for load shifting, unless you have a toy hauler.

 

As a point to ponder, how much hitch weight does the second trailer in a commercial "doubles" configuration have?

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Jim, you are close, except that the snubbers would be utilized in the upper direction only (sort of like super duper shock absorbers). The ET handles downward forces pretty well, thanks to the leveling valve, it also transitions the forward and back forces (chucking) downward, because of its geometry (independent, air bag only suspension and parallelogram).

Super%252520ET%252520Sr%252520cutaway.jp

 

super%252520et%252520sr%252520cutaway%25

 

super%252520et%252520sr%252520cutaway%25

 

Jack observed how well this works, when he watched me at slow speed backing into a slot with lots of forward and back jerking and crossing drainage depression in the street. I'm sure a mathematical model and algorithm could be developed to calculate the forces that the snubber is likely to encounter. Knowing what I know about the complexity of the system and the interaction between various force vectors it would be quite an undertaking, certainly more than I want to undertake in my seventh decade on this earth, although it would make a hell of a theses for somebody on a PhD track at MIT. I am approaching this in a more empirical fashion working with "volunteers", including one who owns a "floater".

 

Dave, as you might have gathered from my sarcastic description of DRV offerings and their marketing methods, the reason I speak so ill of it, is because I find it hard to believe that they don't know better. Sort of like General Motors engineers knowing about their killer ignition switches but continuing to sell these for a decade, or Takata with their airbags, or Volkswagen with their software, etc. Greed is a powerful motivator. Pushing pin weights below 20% of GVWR, sooner or later is a recipe for disaster. I know personally an individual who almost lost the rig and the trailer (and possibly his life) when it went into sway oscillation going down hill and braking. Luckily, he was able to to bring it under control. And we are not talking about a guy with a pickup, this was a 42 footer, almost 30K GVWR, pulled by a Volvo. Guess what his pin weight was with the fifth fully loaded, 16.7%!

Here's a page from a manual for those studying for their CDL.

 

Seminar%2525205.png

 

Take a look at the bottom at the bullet point number six. Again, being sarcastic, having CDL apparently is not a requirement to be an RV design engineer.

 

Henry & Jack I hear your concerns.......

 

As some folks may recall .......once in a while I tend to get on my soapbox and grumble about Weight & Balance management but for the most part most forum folks tend to concentrate on the Weight issues and tend to ignore the Balance portion of the equation......this is sad because the most crucial vehicle dynamics are largely driven by the coupling factors of Weight AND Balance.........

 

In my mid-life I was surrounded by big-brain folks that were multi-taskers of aero-elasticity, structures, aerodynamics, vehicle-dynamics, etc. etc............being a low-life pilot I was in charge of.........being to go-fer for chink-food for lunch while the "Big-Brains" would be flinging chalk scribbled math equations all over the three huge chalk boards at the old wind tunnel.......most days the math-challenge-of-the-day involved vehicle-dynamics.........

 

Now my math skills were so primitive that I was lucky to figure out a well published weight and balance with cargo and fuel for a giver aircraft......the "Big-Brain-Geeks" were the folks that actually computed the actual limits of various vehicle-dynamics........now of course the pilot did play a small part in "proving" the final equations.........the pilot had to go out and "verify" that the calculated limits were valid.....GULP!!

 

Anyhow most days the calculated limits were pretty darn close.........on a rare occasion various "cross-coupling" might occur and things would not be ...........as predicted..........bad juju, in formal engineering terms.

 

Fast forward to this "challenge" and I tend to be grumpy like Henry regarding the "Thin-to-nonexistant-margins-of-design" many products exhibit these days.........Of course being grumpy for older geezers is expected and once perfected is very useful for discussing politics and the latest medical devices.........

 

Vehicle-dynamics seems not to respond to "grumpy" very much so it seems that we are forced to utilize the tried and true three real-world choices of returning to "Normal-Trailer-Mass-Distribution" (25% to 35%) pin-mass........or........get the "Big-Brain" geeks to the calk boards to calculate the solution........or my all time favorite.......try changing 300 to 600 various ......"Things" and see if you get ....."lucky"......

 

Prior the sending me out to "verify-the-Calculated-vehicle-dynamics" the Big-Brain-Geeks would often lecture me for a bit about the basics of vehicle "stability"........it went something like this.

 

1. Stable - Oscillatory (waves diminish to null )

 

2. Neutral - Non- Oscillatory ( once the vehicle is displaced it tends to remain displaced)

 

3. Unstable- Divergent ( Oscillatory mode to vehicle destruction unless dampened by other forces)

 

The majority of vehicle uses benefit from stable handling.

 

A few specialized vehicles may benefit from Neutral stability (Eg, some forklifts)

 

A very few specialized vehicles may benefit from unstable handling (extreme-tech-fighter-aircraft........maybe trials motorcycles) for the most part the more unstable the vehicle is the shorter the vehicle and operators life's tend to be.

 

IMO almost all RV suspension systems tend to be fairly low quality and tend to trend to potential unstable operations if not highly dampened.

 

While it is commendable to improve the damping of the Z axis of the ET hitch, I wonder just how much cross-coupling of the Y Truck and trailer axis could be a factor in the unstable nature of the problem trailer.....I would imagine the current ET hitch is likely already the most stable factor in the truck / trailer combo.......

 

It's a ugly "patch" but 2,000 lbs of hitch ballast might be a hillbilly "patch"

 

 

 

Much of my previous life was cursed by weight and balance limitations and once you start operating past the "practicable-limits" life tends to get pretty ugly.....and maybe short.........

 

Now that I am retired I just run the numbers on my hillbilly spreadsheet and watch the truck trailer weight and balance change until my loadings start to make sense......

 

Drive on............(Divergent is for........Top Gun)

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Dolly,

That spreadsheet you built is pretty darn good, though I haven't had much time with it as they've had me in the desert and Alabama for the last month. I think there's a lot to be said for an anti-roll bar in a 5er as well as consideration of the polar moment where sway is an issue. Up front, you've got tire, air suspension and hitch air in the "Z". No roll damping up front.

Ford I see has semi-active suspension now. Hmmm, cheap active suspension... can you smell the wood burning?

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