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Topic ClosedAxle reinforcement

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Direct Link To This Post Topic: Axle reinforcement
    Posted: 21 Feb 2019 at 12:49pm
Motor7, yep that's exactly what I think, Lippert uses plain old structural steel tubing in these axles. Same with the trailer frame which is also made by Lippert. 

Look at it from the axle design engineer's perspective. He's probably chosen a 3x3 tube so that he has room in there for the rubber cords that allow the torsion arm to swing.  His remaining design choices for the tube are steel grade and wall thickness. He could go with a thinner tube wall and a higher strength steel, but the only thing that would do for him is allow for more flex in the tube before failure. He doesn't need or want more flex, so a thicker wall and a standard strength steel does the job he wants and is cheaper and much more readily available. 

I wish they had gone up to the next standard thickness in the tube wall but they didn't. Originally they probably didn't need to because they designed it for more outboard mounting points than we have in our rPods. The inboard mounting points reduces the safety factor in the design more than it should according to the loading calculations I did. 
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Direct Link To This Post Posted: 21 Feb 2019 at 1:17pm
Originally posted by Motor7

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While you may be right as to them being similar, all I know is that I have cut and welded quite a bit of angle iron in the last 11 years and the stuff I have used I would consider low to medium carbon steel steel. I just cant see them using low or medium steel in a axle tube, but if they do then I understand the infrequent failure. 

I wonder what the statistics are for axle failures. I don't know if we hear about all of them to be sure it is "infrequent failure."

Mine had the problem and the axle was recently replaced. The only miles on it are those from the dealer to my driveway. While our 179 was not fully unloaded when I measured the axle's bend, neither was it loaded to the extent it usually is when we travel. Before we get too many miles on it, I want to do this modification as it will be a whole lot less expensive than replacing the axle was.
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Direct Link To This Post Posted: 21 Feb 2019 at 1:54pm
I can't imagine there are large numbers of axles failing under warranty, it would have been in Lippert's  and FR's financial interest to change the design.  Most trailers don't get used that much nor are they that heavily loaded. Many owners with legitimate claims never file them.  And failures that happen after the warranty ends or are caused by excluded events aren't financially relevant so aren't counted. These are for profit businesses after all.

Manufacturers expect and generally take a reserve for warranty claims (they have to if they're publicly held, which Lippert is). If their actual payouts under warranty claims consistently exceed the planned amount their auditors will require them to take action to investigate why and make changes either in the design, the warranty language, or in the reserve amount. It would be different if the axles fail in a way that results in a life safety risk, but that doesn't appear to be the case. 


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Direct Link To This Post Posted: 22 Feb 2019 at 7:02am
SH, that sounds more like a factory defect, or a axle damaged in shipping. A completed trailer shipped on a trailer could have been improperly chained or strapped down so tight that the axle could easily be bent.

Completed axles shipped from Lippert to FR could also be damaged by tie down methods. Ever see how they ship then? Dozens stacked 10-12' high, then chained or strapped by any method the driver chooses.

As far as numbers, does anyone here have a total of the production run R-Pod since inception? I am just assuming, but I am guessing that number is quite high.
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Direct Link To This Post Posted: 22 Feb 2019 at 7:07am
Originally posted by offgrid

Motor7, yep that's exactly what I think, Lippert uses plain old structural steel tubing in these axles. Same with the trailer frame which is also made by Lippert.


I agree about the frame. If you are right, that's not good about the axle. How can our assumptions be confirmed or denied? We need to kidnap a Lippert engineer and metalurgist...šŸ˜Ž
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Direct Link To This Post Posted: 22 Feb 2019 at 8:50am
You would have to either kidnap them or at least catch one after work and buy him enough beers that he forgets about the nondisclosure agreement he signed as a condition of employment. Manufacturers won't let you talk to their design engineers. They aren't trained to deal with the public so they keep them in the back room and feed them peanuts Geek.

Why do you think the use of A36 steel for the frame is OK but not for the axle tube? Its a tube that is rigidly bolted to the frame, so you can consider it to be part of the frame structurally, it might as well be welded as part of the frame assembly. Its not like a spring axle which is directly taking the bump loads from the wheels and transmitting them to the springs, which in turn reduce the loads being transmitted to the frame. The torsion arms and torsion bars do that by transmitting those loads to the rubber cords, and the cords are the equivalent of the springs in reducing the bump loads being applied to the axle tube. 

So, its not like the axle tube "has" to be made of a particular material. It has to do a specific job in terms of its deflection and loading capabilities and a good design will accomplish that (and no more) at the lowest cost. They don't pay design engineers to over design stuff. I've run through the deflection and loading numbers on the tube and its not that bad. In our case the tube has wound up being somewhat shy of what it needs to be in the rPods because of the inboard mounting points.  if I was designing it I would be using A36 steel as well, just would have gone with the next higher wall thickness. Adding the angle under the axle tube is the equivalent of increasing the wall thickness.

So if you're really worried that adding reinforcement to the existing tube somehow makes things worse and you still want to upgrade the axle then I'd suggest going with a 5200 lb axle. That has a lot of other advantages in terms of a higher rated spindle, torsion arm, rubber cords, brakes, bearings, etc. I'd be interested in knowing what that axle tube size and wall thickness is for comparison.  

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Direct Link To This Post Posted: 22 Feb 2019 at 11:50am
For sure, if I ever have a failure, I would most certainly replace with a 5200 lb-er. One has to wonder then how the ride would be affected by that swap.

I rear somewhere that these 3500 lb Lippert axles are actually 5k ones and downgraded for the RPods. I have no idea of it's true, or just wishfull thinking.

As for your A36 question, the frame takes nowhere near the abuse the axle takes. Making it in a diamond profile is their solution to make it stronger than the square frame tube.
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Direct Link To This Post Posted: 22 Feb 2019 at 1:22pm
I think the ride would be fine with the 5200 lb axle. Iā€™d be concerned if it was a cargo trailer but travel trailers are never exactly lightly loaded. The Lippert axles in the pods are their 3500 lb series according to the part numbers on them. The lightest rpods appear to have 3500 lb ones derated to 3000.

Actually on a heavy rpod with a high tongue weight running a wdh the frame around the area where the a frame ties to the main frame rails is subject to load levels very similar to the axle tube. I would keep a careful eye on that area as well.



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Direct Link To This Post Posted: 22 Feb 2019 at 3:33pm
Originally posted by Motor7

SH, that sounds more like a factory defect, or a axle damaged in shipping. A completed trailer shipped on a trailer could have been improperly chained or strapped down so tight that the axle could easily be bent.

Completed axles shipped from Lippert to FR could also be damaged by tie down methods. Ever see how they ship then? Dozens stacked 10-12' high, then chained or strapped by any method the driver chooses.

As far as numbers, does anyone here have a total of the production run R-Pod since inception? I am just assuming, but I am guessing that number is quite high.

Ours was not bent (excess camber) until after last Spring's trip out west. I think bouncing around Chicago area roads with tanks that were getting full from use probably did not help. Those were some of the worst roads I have driven on since we purchased our 179. That includes returning from the factory at Goshen after it was repaired following our accident.

One other change on this trip was the change from the Equal-i-zer hitch to the Hensley Cub hitch. I'm not sure if the way it is set up if it is transferring more weight to the trailer axle compared to what the Equal-i-zer transferred.
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Direct Link To This Post Posted: 22 Feb 2019 at 3:58pm
Originally posted by StephenH

 
One other change on this trip was the change from the Equal-i-zer hitch to the Hensley Cub hitch. I'm not sure if the way it is set up if it is transferring more weight to the trailer axle compared to what the Equal-i-zer transferred.

Any differences in trailer axle load should be pretty small between different wdh's assuming the same TV and trailer loads and that you've got them set up the same way, bringing the load on the TV front axle back to where it was before connecting the trailer. To estimate how much load a wdh is transferring to the TV front and trailer axles you can use this calculator. Once you have your weights and dimensions set up you can adjust the wdh tension so that the front axle load is the same with the trailer as it is with the trailer weight set to 1 lb (the calculator blows up if you set the trailer weight to zero). For me it adds about 150 lbs. Not insignificant but also not a huge increase. 

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