Axle reinforcement - R-pod Owners Forum

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offgrid

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Topic: Axle reinforcement
Posted: 19 Feb 2019 at 2:40pm

The modulus of elasticity (which defines deflection in materials) is pretty much the same for all steels. Meaning that for a given structural shape and loading the deflection will be about the same regardless of that kind of steel it is.

What changes is the yield strength (which defines at what stress level it will deform and fail to return to its original shape). The angle is regular A36 structural steel which has a minimum yield strength of 36ksi. Some high strength steels are much higher so can they can deflect more before failure. But they're also more expensive and harder to fabricate and weld to. Generally they are used where a lot of deflection is needed (like in springs).

I believe that the torsion axle tube is probably the same A36 material or very similar. Why? Because the torsion axle is designed to flex the rubber cords not the axle tube. If the axle tube was designed to flex more there would be issues with the tube to frame attachment and bracket welds, as well as unwanted changes in camber when the the trailer loads change.

I did the deflection and yield calcs on the axle tube and its a pretty stiff shape, like most tubes are, its deflection is quite small, as you would want and expect. On the yield side if I assume its A36 then its a little marginal for an over the road vehicle when the axle is fully loaded, also about what we'd expect based on the history of failures. When I did the calcs (roughly, its pretty complicated) for the combined angle and axle tube, still assuming A36, the loading at yield increased by about 50%, well above anything we might expect over the road, so I don't think there's not any reason to think that adding the angle is going to increase failures.

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

StephenH

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Posted: 19 Feb 2019 at 3:36pm

I'm looking forward to reading more and, when I get the opportunity, doing this mod myself. It certainly is less expensive than replacing an axle.

What would you think about cleaning both the angle and the axle and then using silicone RTV sealant in the angle so that it is compressed and fills the space between the angle and axle? It would likely take a couple of tubes of sealant to do so. It would also probably be best to paint the angle first and let that cure before doing this as RTV is usually not paintable.

StephenH
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offgrid

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Posted: 19 Feb 2019 at 4:07pm

Not sure I'd try to fill the whole void with silicone, it requires exposure to moisture in the air to cure so the interior area might never cure properly. Same with most polyurethane sealants. Maybe some type of 2 part urethane. I was thinking about just running a bead around the edges of the angle.

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

GlueGuy

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Posted: 19 Feb 2019 at 4:21pm

Originally posted by offgrid

Not sure I'd try to fill the whole void with silicone, it requires exposure to moisture in the air to cure so the interior area might never cure properly. Same with most polyurethane sealants. Maybe some type of 2 part urethane. I was thinking about just running a bead around the edges of the angle.

I wouldn't do that without some weep holes. No matter how well you seal it around the edges, moisture will still get in there. If not just condensation from temperature changes.

bp
2017 R-Pod 179 Hood River
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lostagain

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Posted: 19 Feb 2019 at 4:28pm

If a fully filled in watertight space is desired, then how about using something like 3M 5200 bedding compound. Aside from the cost, it may be the best choice. It cures in settings that are not exposed to the air so it may adequately harden in the interior yet still remain reasonably flexible to allow some movement without cracking or letting in moisture.

Seems to me another approach would be to paint the axle and the angle iron really well, then provide for some means, like a small tube with holes extending down the inside of the angle to allow water to be sprayed inside from time to time to flush out any accumulated salts. It seems like it'd be a losing battle to try to seal it. And the frame of the Pod is just as exposed to the rust, so flushing off the whole bottom and keeping it well painted may yield better rust protection over the long run.

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offgrid

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Posted: 19 Feb 2019 at 4:37pm

Yes, good point. In a more benign environment the weep holes would probably work.

But if I put weep holes in I'm likely to just get salt water flooding up in there and not be able to wash it out. The more I think about it for me the best bet would probably be to rust treat and paint the surfaces, bolt it up again, and then wash it out, let it dry, and coat everything with Fluid Film a couple of times a year as a water displacement material. I already do that with all the other exposed metal in my vehicles anyway.

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

mcarter

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Posted: 19 Feb 2019 at 4:54pm

Mike Carter
2015 178
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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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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

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."

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.

2016 R-Pod 176T