Changing out the STD 5th wheel ????

[Lance A Lott]

I couldn't get Noteven's link to open I did find this.http://www.trucktrend.com/how-to/chassis-suspension/0904mt-maximized-air-bag-size/

[Star Dreamer]

On 11/9/2018 at 10:36 PM, phoenix2013 said:

Fascinating, thanks for the link DesertMiner. As an engineer I like to see innovating products. I only see in their specs the GVW numbers for the trailers they like to see behind these couplers and not the tongue weight limits. Torsion bars springs are pretty robust but not limitless and the rubber sections in this assembly would be pretty short further limiting the range of tongue weights you can put on them and not ruin them. Why is it that the hitch manufacturers and trailer manufacturers don't like to talk about pin weight or tongue weights. It shouldn't be a taboo and they shouldn't  claim ignorance about the subject.

I had a torsion bar suspension on my first fifth I bought new from Kropf manufacturing in the early 80's. This thing pulled like a dream and on long trips everyone, yours truly, wife and kids, loved riding (and sleeping) back there in the fifth. The only downside is that these are limited only to two axles and not three since there is no load sharing like there is with springs and you can overload them in three axle setups.

Dexter still makes them and for the curious here's the link 

I saw that Lippert offers these also and these jackasses can ruin anything they lay their hands on, but on closer examinations Lippert axles look very much like Dexter's so maybe these are one and same.

I did noticed that genyhitch  makes  gooseneck adapters with torsion bars springs.  I agree that there is no adjustability but they do offer several models and I'm assuming one can find one that will work for one's trailer. But that brings us back to my earlier point that knowing the range of tongue weights on different models is a critical bit of information which they should provide. It shouldn't be up to the customer to guess which one will work, buy it and see if it works. 

But, I love companies that engineer and innovate things, make them and sell them.

Our old trailer which was a 44' model had triple Dexter torsion axles which worked good but had limited ground clearance and was hard to keep aligned. Dexter use to have on their data that they are not recommended for triple axle trailers but a lot of car hauler trailers are made that way.  Our new 48' trailer has triple Lippert axles which I expect will have the same issues over time and maybe worse just because they are Lippert.  I like the torsion axles over Lippert leaf spring axles based on the number of issues people have been having with Lippert leaf spring suspensions.

[phoenix2013]

OK class, the next installment. The link above talks about some of that, here's more "meat on the bones".

All springs, steel or air bags used in suspensions have a region in which they work best (by design),  a "sweet spot", you might say . Manufacturers tell you what that region is so that you can pick the right spring for the weight you are going to "suspend".

OK I posted that earlier, but now you guys will have to pay attention. I'm not being a wise guy here , this is some esoteric crap that you really have to get into before it makes sense and it took me some time to figure it out. Incidentally, I did study the Firestone guide noteven mentions and the one from Goodyear too. So below you have a typical Goodyear bag and let’s look at some of its numbers.

First, the "girth", 7.0" at max inflation (100psi). then the 9.1" MAX EXTENDED HEIGHT and 2.8" COMPRESSED HEIGHT. So, in your application you have to make sure that you can fit that 7 inch diameter bag  (with some clearance probably 8") and that you have hard stop at 9.1 inches high and hard stops at 2.8 inches going down. Now let's look at these numbers.

Note that this airbag at max pressure (100 psi) can support 2200 pounds of weight or force. Note also that it has two different ratings one as an ISOLATOR and the other as an ACTUATOR. So what is the difference? Same device two different applications, as an ISOLATOR it will typically be operated close to maximum pressure and maximum load (100 psi and 2200 pounds) and it’s not expected to move much, 2 inches, from 5-7 inches in height. In this application it would be isolating vibrating or rotating machinery from concrete floor, it would be isolating electronic cabinets in combat information centers in the bellies of worships away from the hull, etc. Did you know that navy actually explodes charges very close to the hulls of new ship designs to find out if all that stuff keeps working after explosion? In those applications the airbags are working as vibration, shock and impact dampeners, sort of like “super shocks”. So why use these versus, rubber pucks or heavy steel springs? Wider operational range within the same device, better longevity over rubber pucks, you are compressing air molecules vs. rubber molecules.

It is in the ACTUATOR mode where the airbag shines as an air spring. Note that from compressed all the way down (2.8 inches) to fully extended (9.1 inches) the distance is 6.3 inches; they specify the usable stroke as 5.9 inches. The want you to design the hard travel limiters so that you stay below that max distance the airbag can tolerate.. OK, so I can bounce up and down 2200 pounds 5.9 inches, not so fast. Remember you are not using it as an ISOLATOR, you are trying to use it as a spring. Goodyear has a bunch of neat drawings showing you how to do it.

First they tell you that an airbag used as a spring is happiest when operated at half its maximum pressure (50 psi) and that the ideal (design) height for this particular airbag is 6 inches.

That height at 50 psi pressure are ideal but there is also a range which is shown in blue

See that little box I drew around 6 numbers, this is the range of forces, heights and pressures within which this particular air bag performs best as an air spring (ACTUATOR). If the force (pin weight in our application) falls outside that region use another airbag , there are plenty of others.

Here’s another chart showing relationships between heights, pressure and load, note how nice and flat (linear) the response of this “air spring” is in blue region at 40 and 60 psi. Can you run it at higher or lower pressure, yes you can. You can also run your car tires at 50 psi or run them flat at 10 psi, your car and your butt can tell the difference, kind of a crude parallel but it makes the point.

Here’s another chart

It talks about “Natural Frequency”. More on that in the “next installments” which will cover the history and details of the suspension technology, history of the air hitches and yes, I will answer the question why I added the polyurethane bumps to the ET hitches.

[dan412]

Thanks Henry great info can't wait for the next lesson.

[rickeieio]

28 minutes ago, dan412 said:

Thanks Henry great info can't wait for the next lesson.

X2

[Lance A Lott]

1 hour ago, dan412 said:

Thanks Henry great info can't wait for the next lesson.

I always knew my understanding of air bags was a crude one thanks to you it is getting smoother. I have no need to be an expert but I like to know that I have some knowledge and a source of reference behind what I say. Thank you very much. 

Do the bags with a girdle in them work the way I think they would? A little more like a progressive coil.

[phoenix2013]

Happy Thanksgiving to all. We got 22 pounder in the oven, waiting for the family and friends to arrive.Table is set,

and Florida weather is perfect, 77 degrees.

While waiting for events to unravel, I figured I spring another lesson on you guys.

On 11/20/2018 at 8:34 AM, Lance A Lott said:

I always knew my understanding of air bags was a crude one thanks to you it is getting smoother. I have no need to be an expert but I like to know that I have some knowledge and a source of reference behind what I say. Thank you very much. 

Do the bags with a girdle in them work the way I think they would? A little more like a progressive coil.

Lance A Lott, I wasn’t delve into these, because it’s a bit “outside where I am heading” but nonetheless an interesting question and a “re-route”.  The bands are not there to create a progressive response air spring, the airbag will still have a linear response.

The bands are there to contain the airbag expansion (bulging out) as the airbags grow in size volume wise and therefore force wise and to operate over longer distance from collapsed to fully extended.

Truly progressive springs like the one shown on the right,

are not very useful in suspensions because their response is exponential like the polyurethane bumps we discussed earlier. Probably what you are referring to are the dual rate progressive springs. Quite a few of these are used in suspensions these days in racing, automobiles, in front fork tubes in motorcycles, etc.

The same spring basically has two sections, both are linear but each operates at a different compression rate. These springs might have a rating such as 200/450/in, meaning that initially they will compress at a rate of 200 pounds per inch and once those coils are compressed the next section operates at 450 pounds per inch. Racing industry has done a lot of research into these to help vehicles with sway control and cornering. Remember the picture I showed earlier demonstrating the difference between linear response vs. exponential response (the linear response being about 500 pounds per inch).

Here’s how that spring would perform if it was a dual rate progressive

The line I drew would be for a spring where the “initial coils” would compress at a rate of 250 pounds per inch and once those coils were stacked at a rate of 750 pounds per inch. Not a very useful spring I just drew it to demonstrate the concept of linearity and dual linearity. Tuning those springs for racing or rallying is a true art besides a science and usually within very narrow parameters, weight of a car, weight of the fuel, weight of a driver (or two for rallying), etc., etc.. Imagine a challenge designing springs for a passenger car driven by a single skinny teenager, or four sumo wrestlers.

Now that you guys are “experts” in linearity, rate of change, etc. (you have been paying attention, right), I’m going to bring our old friend (drawing), it will make lot more sense and also show the other beautiful aspect of air spring, they can “self adjust” their performance with load

See that blue region of two inches of travel (from 7 inches down to 5 inches under load). At 20 psi it’s about 100 pounds or 50 pounds per inch (skinny teenager), at 100 psi it’s 700 pounds or 350 pounds per inch (four sumo wrestlers). Now you can see why air springs dominate application where loads can vary and vary over a wide range.

There is one major difference between steel springs and air springs. Within relatively the same space you can have springs that can carry few hundred pounds to tens of thousands, just design them with thicker leafs, or more leafs.

Well you can’t make air molecules thicker, only squeeze them closer together. So, more volume is the only way to deal with more weight using air spring. But the industry is trying, ever seen one of these. 3 feet round, 13 inches high, 40 mounting bolts on either side.

100,000 pounds of force at 100 psi, now that’s air molecules working for you.

I’m looking at its smaller brethren in that family for a “special” project.

Got to go and fortify myself on that 22 pounder before I get going on the "next lesson" 

 

 

 

 

[Lance A Lott]

Phoenix Thank you for the information. Hope you have a happy thanksgiving.

[phoenix2013]

Just now, Lance A Lott said:

Phoenix Thank you for the information. Hope you have a happy thanksgiving.

Lance A Lott, everything is good.

[Lance A Lott]

Ours is no longer looks like that, 11 people made a good dent in it. By the way 2 degrees this am last time Nadine looked it was up to 6, got up to 14 in Burlington, Vermont braking the record for the coldest thanksgiving. We have about a foot of snow, supposed to rain this weekend, should be fun.

[dan412]

Thanks Henry for explaining how air springs work . I need to replace mine but would like to try make it ride a little better if I can at the same time. 

[jenandjon]

This is good info. Unfortunately its going over my head. I know when I have a full legal load on my daily the air gauge reads 60 psi. That with 60,000 lbs in the trailer. When I am empty its 5 or 10 psi. So essentially I have the same bags on the HDT as I do my daily. HDT don't have a suspension pressure gauge so I'm just guessing I'm running around at 5 or 10 PSI on that also. Maybe 15 loaded down. I have a relatively light trailer compared to most of you. Around 10,000 lbs.  If i am understanding right these bags are designed to run at 40 -60 psi loaded. Like on my daily.  So we need bags that are designed to run at say 20 or so for a sweet spot on the HDT. So we still have the ride height AND smoother ride? If I am completely wrong here say so. You wont hurt my feelings. I'm just trying to wrap my brain around all this.

[phoenix2013]

16 hours ago, jenandjon said:

This is good info. Unfortunately its going over my head. I know when I have a full legal load on my daily the air gauge reads 60 psi. That with 60,000 lbs in the trailer. When I am empty its 5 or 10 psi. So essentially I have the same bags on the HDT as I do my daily. HDT don't have a suspension pressure gauge so I'm just guessing I'm running around at 5 or 10 PSI on that also. Maybe 15 loaded down. I have a relatively light trailer compared to most of you. Around 10,000 lbs.  If i am understanding right these bags are designed to run at 40 -60 psi loaded. Like on my daily.  So we need bags that are designed to run at say 20 or so for a sweet spot on the HDT. So we still have the ride height AND smoother ride? If I am completely wrong here say so. You wont hurt my feelings. I'm just trying to wrap my brain around all this.

jenandjon, there is the ideal technical solution and then there is the real world. Suspensions first and foremost have to be designed to do what the manufacturers tell DOT that this truck is capable of handling (GVW wise). That means: Class 3 (1 ton pickup) 10-14K GVW, Class 6 (MDT no CDL) 19.5-26K GVW, Class 7 (CDL MDT with air brakes) 26K-32K GVW and Class 8 (HDTs) 32-80K GVW.

Lot of people don't know what that REAL........LY means!!!!!!!    I certainly didn't when I bought my fist 1 ton pickup for RVing 40 years ago.

First of all DOT is government, don't BS government. bad things will happen if they find out. These are commercial TRANSPORTATION vehicles; manufacturers build-in significant margins into chassis and suspensions in case idiots overload them. Pickup would be a good example. Class 3 one ton pickup max Gross Vehicle Weight 14,000 pounds. Empty one tons depending on make and model weigh around 8,000 pounds, so the margin from 8,000 to 14,000 is 6,000 pounds, or 3 tons. Quite a bit more than 1 ton rating, that’s the “margin”. Now, I have driven my one ton once with 3 ton load, a bunch of sand for the cement floor I was pouring in my shop. That one ton drove like a pregnant whale ready to slip the cargo box off the truck in the corners (at 15 mph), most unpleasant 5 mile trip I ever took. So legally you can carry 2.000 pounds in your one ton. If you put a small 4,000 lbs Bobcat in the back you will be OK, but if you hit somebody and it’s bad, Ford will tell you, it’s between you and God, your lawyer and the insurance company, your insurance company will tell you it’s between you and the God, Ford people told us to tell you to talk to God.

So now that you guys understand the concept of DOT rating and what’s legal and what’s not I am going to bring up and interesting subject, people pulling big ass trailers with pickups and manufacturers selling people big ass trailer to be pulled with pickups. Ever notice why it is so hard to get info on pin weights on big ass trailers, it’s like they don’t like to talk about, avoid putting it in brochures, or if it’s in the brochure it says 3,500 lbs or some other BS, which we know is BS. A one ton will easily carry 3,000 pounds and with minor add-ons like helper springs 4,000 pounds will be OK, anything over that and things get squirrelly. So what do manufacturers do, they started building “floaters”, they move trailer axles forward, add more weight behind the axles, anything to keep that pin weight at no more than 4,000 pounds and then they thank the Lord for the manufacturers for putting in THAT MUCH MARGIN OVER THEIR DOT RATING.

Getting back to your original question, that class 3 pickup was optimized by design for about 10,000 pounds. (weight of the truck and weight of one ton of cargo), that was its “sweet spot”. If they did that DOT was happy, Ford was happy, everyone was happy and remember, that one ton was pretty harsh driving it empty, but the minute you put some decent load into the cargo box it would drive real nice. You got the suspension close to the “sweet spot” for which it was designed.

So all suspensions are optimized for the cargo capacity of its class within the DOT guidelines. They are also designed with a margin should they be overloaded. How they behave when they are overloaded or not loaded enough is not a primary consideration in the truck design. And here we come a full circle,

20-25 years ago MDT (Class 6 trucks) became the rage in RV pulling. RVers were dumping their Class 3 pickups and hooking their RVs to Class 6 MDTs rated for 26,000 max GVW. What’s a typical curb weight of a Class 6 Freightliner, 12-14,000 pounds. What’s the difference between that and GVW? 12,000 pounds of cargo or 6 tons. What was the typical pin weight of a fifth 25 years ago, 3,000 pounds, or 1½ ton, if that. What was the result? Braking was great, not so nice were busted frames in the RVs and a horrible ride, empty or pulling. That 1½ ton would not bring the suspension on that MDT anywhere near its “sweet spot”.

Now we come to the subject near and dear  to hearts of this crowd, HDTs, Class7 and Class 8. DOT says these suckers must be able to deal with 32,000 pounds and 80,000 pounds respectively. So where are the “sweet spots” on the suspension in these behemoths. Class 7 is 32,000 pounds, Class 7 is typically a factory single delivered with a 23,000 pound axle.  Its cub weight will be around 17K with about 11K of that weight on the front axle. Subtract 17K from 32K and you end up with 15K or 7½ tons. You have to be careful there, the rear end weighs around 6K and the axle is rated at 23K, add the rear end weight (6K) to the cargo weight (15K), total 21K, you are OK, you have 2K of “margin” on that axle. However, take a tandem truck and single it by moving the rear axle forward and you just overloaded that axle, tandem axles are only built and rated to carry 19K of load. So where is the “sweet spot” on Class 7 truck suspension, probably close to 23K.

Class 8 truck gets even more interesting. You have two axles each capable of carrying 19K load for a total of 36K. The curb weight of the truck will be around 20K with 11K on the fronts and 9K on the tandems, the difference being 27K for cargo. You say WAIT A MINUTE, WAIT A MINUTE, that’s way short of the 80,000K they are talking about for Class 8. Ah, the 80K GVW applies to the truck and trailer combination not just the truck. Then comes the very important DOT standard enforced by all those weigh stations you see all over US, the rule is: front axle no more than 12K, the tandem axles no more than 34K and the trailer axles no more than 34K. Add them all together, 12K + 34K + 34K = 80K, show up at any weigh station with more than that on any axle and they will park you and fine you. So what do you think the “sweet spot” is for the suspension on a Class 8, I’d say around 34K. Most trucks run much lighter than that but the manufacturers can’t design trucks for that, they have to design the trucks and design their suspensions to carry that maximum load for that class of a truck.

So that is why you are seeing these piddly low pressures in your suspension airbags, you are not loading these anywhere near their design weights and therefore pressures.

What are the practical implications of all of this for us, the RVers. The MDTs and HDTs were never designed for us, or our applications and never will be. That’s why we need an air hitch to act as a secondary suspension separating the trailer from the truck and its suspension which hardly even notices that pin weight you just put on it. However, when I design a suspension in the hitch, I know what I am dealing with and I can tailor its performance to be dab smack in the area of the “sweet spot” and I can take it a bit further with the use of different size air bags. Currently I utilize three different sizes of airbags, covering three different ranges of pin weights.

[Scrap]

Frame jacking is a pretty big consideration nowadays too.  Takes some stout components to combat that with a 1650+ engine.  It'll probably suck up a good chunk of custom shock valving as well.  Haven't been able to do a 21K rear with a 12-14L engine since the 90's.  

There's a tilt table mixed in with it all too.  You don't want to be the OE that flops off of that!

 

[jenandjon]

OK I got you now.