Axle reinforcement - R-pod Owners Forum

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Motor7

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Joined: 09 Jan 2019
Location: E. TN
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Posts: 196

Topic: Axle reinforcement
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.

2016 R-Pod 176T

offgrid

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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.

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

Motor7

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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...😎

2016 R-Pod 176T

Motor7

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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.

2016 R-Pod 176T

offgrid

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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.

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

StephenH

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Posted: 21 Feb 2019 at 1:17pm

Originally posted by Motor7

(text deleted)

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.

StephenH
Happy is the man that findeth wisdom,...

ouR escaPOD mods
Former RPod 179
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offgrid

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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.

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

Motor7

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Posted: 21 Feb 2019 at 10:20am

OG, I don't know what type of steel the Lippert is made of and I can't seem to find it on the web. I did find this:

"There are four types of carbon steel based on the amount of carbon present in the alloy. Lower carbon steels are softer and more easily formed, and steels with a higher carbon content are harder and stronger, but less ductile, and they become more difficult to machine and weld. Below are the properties of the grades of carbon steel we supply:

Low Carbon Steel – Composition of 0.05%-0.25% carbon and up to 0.4% manganese. Also known as mild steel, it is a low-cost material that is easy to shape. While not as hard as higher-carbon steels, carburizing can increase its surface hardness.
Medium Carbon Steel – Composition of 0.29%-0.54% carbon, with 0.60%-1.65% manganese. Medium carbon steel is ductile and strong, with long-wearing properties.
High Carbon Steel – Composition of 0.55%-0.95% carbon, with 0.30%-0.90% manganese. It is very strong and holds shape memory well, making it ideal for springs and wire.
Very High Carbon Steel - Composition of 0.96%-2.1% carbon. Its high carbon content makes it an extremely strong material. Due to its brittleness, this grade requires special handling."

2016 R-Pod 176T

offgrid

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Posted: 20 Feb 2019 at 5:15am

I actually like calculus, but then I fully admit to being a science nerd. Geek

. BTW calculus isn't required for the axle tube load and deflection calculations, those are simple formulas only.

Anyhow I think in practice the first consideration before doing this mod would be to measure your Lippert axle tube length and "crown". Use a string line from end to end along the bottom of the axle tube and measure the gap in the center. Do this with dry tanks and try to minimize load on the axle.

If you don't have around 1/4 to 3/8 inch or more upward bend then your axle has already flattened out some. StephenH was good enough to measure the crown in his brand new axle and it was 3/8 inch. Mine was 1/4 inch on my 2015. Our axle loads were roughly the same.

Much less crown than that and it should probably either be re-cambered or replaced, especially if you're seeing inboard tire wear. Ask a local shop that specializes in trailer work to see if they can re-camber it. If not or if you have a bent spindle or torque arm, and assuming you plan to keep your trailer, I'd suggest springing for a 5200 lb axle (along with the brakes, drums, and wheels to match).

If your axle tube is in good shape but you plan on running your pod heavy then I'd go ahead and get the reinforcing angle made up. Be sure the angle is at least 1/4 inch shorter than the axle tube and is positioned so it doesn't interfere with the torsion arms. Get the crown/camber put in at about 3/8 inch. We know that's about right for a brand new axle. If that's a little more camber than what you currently have on an older axle then that's fine, it won't hurt anything.

There are torque specs for the u bolts that you should follow, I think mine were 70 ft-lbs. They should be checked a few times to start out with until you know that they aren't changing.

Other than that there is the "water trapping" consideration we've been discussing. Your concern with/solution to that will depend on your local conditions. I'll post what I end up doing for that when I get to that point, but based on the discussion here I think after painting the parts I'm just going to flood the angle with Fluid Film, bolt it up and call it good.

1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold

crw8sr

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Posted: 19 Feb 2019 at 6:14pm

Very scholarly Offgrid! Thumbs Up

Reminds me of the first day of calculus class; stomach ache, nausea, blurred vision and a strong desire to sneak out of the classroom. Way over my feeble brain but I think I got the major point. I've put this mod on my to do list. Thanks

Chuck & Lyn
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Zoe Joy & Gracie, Yorkie
2018 R Pod 190
2019 Traverse

In moments of adversity;when life's a total wreck, I think of those worse off than me and really feel like heck.