GVWR>GAWR and Stabilizer Issues - R-pod Owners Forum

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StephenH

podders Helping podders - pHp

Joined: 29 Nov 2015
Location: Wake Forest, NC
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Topic: GVWR>GAWR and Stabilizer Issues
Posted: 06 Jan 2019 at 2:08pm

Originally posted by offgrid

2) the mounting points on the rPods are resulting in the derate from 4400lbs to 3500 lbs.

StephenH, I believe you have been given explanation 2 by FR? So it that is correct than we could potentially pick up 700lbs of load capacity.

Actually, I was considering that myself. I was not told that by FR. It makes sense that the additional distance between the mounting points and the wheels means more force able to be exerted on the axle beam than if the mounting point were closer. In addition, I saw this video that indicates the same thing:

https://www.youtube.com/watch?v=FtXRGOadTKU

Regardless, from an engineering perspective I think GlueGuy's proposal is a good one if we can make it work. The axle is a essentially a double ended cantilever beam with equal loads at the ends of the overhanging supports. The stress calcs for this load case are here:

https://www.engineersedge.com/beam_bending/beam_bending7.htm

What we see is that by reducing the overhang (c) we can make the axle bending stress lower, in fact with zero overhang the stress goes to zero. Interestingly this is true both for the axle sections outboard and inboard of the mounting points. So, this is one of the few design changes in engineering where you're not just moving the problem somewhere else, its all to the good, for the axle at least.

We do have to consider that we would be applying a new torsional stress to the rPod frame rails (because the new axle brackets would need to carry load back to the frame rails), but box tubes are very strong in torsion so that's probably not going to be much of an issue.

The video shows one solution, but it also included not only fabricating a new support bracket and welding it to the frame, but ordering a new axle with mounting brackets closer to the wheels.

[/QUOTE]

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

ouR escaPOD mods
Former RPod 179
Current Cherokee Grey Wolf 24 JS

David and Danette View Drop Down

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Joined: 24 Nov 2009
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Posts: 1222

Posted: 06 Jan 2019 at 3:37pm

Do you think Timbren Axle-Less Suspension would work on the a r-pod?

2018 Vista Cruiser 19BFD (2018-
2012 Vibe 6503 (2014-2019)
2009 r-pod 171 (2009-2014)
Middle Tn
2014 Ram 1500 Quad cab

StephenH

podders Helping podders - pHp

Joined: 29 Nov 2015
Location: Wake Forest, NC
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Posts: 6417

Posted: 06 Jan 2019 at 3:44pm

Originally posted by David and Danette

Do you think Timbren Axle-Less Suspension would work on the a r-pod?

No. The frame is not made to support the torsional forces that would be generated plus there is the difficulty of the wheels. With the Timbren suspension, there is no extension of the spindle. The wheels are next to the attachment point which would place the wheel under the body of the R-Pod instead of outside the body.

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

ouR escaPOD mods
Former RPod 179
Current Cherokee Grey Wolf 24 JS

offgrid

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Posts: 5290

Posted: 07 Jan 2019 at 3:53am

+1. The Timbren solution won't work on an rPod without adding a structural weldment to mount it to, which would need to extend laterally across the trailer to take the torsional loads. That would negate the claimed benefit of having an "axle-less" design.

StephenH, I saw that video too, but because it requires a new axle its only a solution for an rPod owner who already has a bent axle so has to buy a new one. GlueGuy's concept would allow us to reinforce our existing axles before they fail.

I'll try to take a couple measurements today and see how much bending load reduction we might expect to get with GlueGuy's approach. If anyone either knows or has access to an old axle they can cut open, the wall thickness of the axle tube would be useful for that calculation.

lostagain, using a Sherline tongue scale would work for what you're suggesting and also allow someone to measure the side to side weight imbalance of their trailer. mcarter suggested that several months ago in another thread. You would need to jack up one wheel at a time and lower the trailer onto the scale while raising the opposite wheel so the trailer winds up level side to side. Should work as long as you're careful.

In my case, I've obtained my tongue weight and axle weight the cheap and lazy man's way using a public scale, so its really only the side to side imbalance that's a question mark. I'm not sure I want to spend $125 just to know that. It's really only a one time measurement. But if any folks here want to split the cost of one I'd be game.

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

offgrid

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Posted: 07 Jan 2019 at 4:04am

The problem with going by the label on the side of the 171's/172's to determine axle rating is that the 3000 lbs listed there could easily just be derated from what's really the same 3500 lb axle used on the heavier rPods. There should be a sticker on the axle itself with the manufacturer, a weight rating, a part number, and a "variant number". If someone wants to take a photo of that and post it I can compare it to the data on my 179 axle.

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

offgrid

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Posted: 07 Jan 2019 at 8:37am

Well that was interesting.

I measured the overhang of the axle.spindle and the distance between axle attachments, and also took a look at how much it might be possible to reduce the overhang. I assumed that the wheel loads are applied at the midpoint between the inner and outer bearing races. I get an overhang of about 13.75 inches and a distance between supports of 60.5 inches.

I also was able to see the axle tube wall thickness at the ends. I couldn't get my micrometer in there but I estimated it to be 3 inch box tubing with a 3/16 wall thickness.

That gives an axle bending stress of about 12ksi (12000 psi) for a 1750 lb load on each wheel. Mild steel has a yield strength of about 35 ksi so that would imply the axle could take a bump load of just under 3g's. From what I've seen on vehicle design, a minimum load factor of 2.5-3 g's is pretty typical, so the current design is right in there, about as light as you'd ever want it to be for the loads it takes, but not severely under designed either.

Its worth noting that this stress is uniform over the whole axle tube if the same load is applied to both wheels, so if you were to generate a high bump load on both wheels (say by going over a speed bump too fast) you could end up bending the axle anywhere along its length. If you went over a bump on one wheel only then it would of course be likely to fail first on that overhang.

It also looks like by extending the axle attachment points outward we could get the overhang down to about 6.75-7 inches, which would reduce the axle bending stress by a factor of about 2! I think that's well worth exploring further.

One thought I had on an easy way to improve this would be to simply strap a 2.5-3 inch steel angle to the full length along the bottom of the axle. I'll take a look at the numbers on that and report back.

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

lostagain

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Posted: 07 Jan 2019 at 9:50am

offgrid, thanks for the info on weighing each bearing point with the Sherline scale. I was thinking of doing an initial tongue weight check with the trailer in it's mostly empty state state [just the normal junk we keep inside such as pots pans], then use it when loaded to get an idea where, relative to for and aft, the weight was shifting. It wouldn't take but a few minutes to check this with the trailer parked. Since we use a WDH the actual axle weight on our truck and trailer would change everything around, but if the rig is basically straight and the truck is balanced fore to aft, then we should be ok.

For us, going to a public scale is would take over an hour round trip and I figure I can get the basics using the little Sherline scale in about half that time. And since we travel with relatively light loads in the trailer, I hope [maybe more accurately, fantasize] that we will not be overweight. From experience, the heaviest loads we've carried have never presented any driving anomalies which would be of concern. Indeed, driving with the trailer attached is smoother than without, though we certainly won't win a drag race.

Never leave footprints behind.
Fred & Maria Kearney
Sonoma 167RB
Our Pod 172
2019 Ford F-150 4x4 2.7 EcoBoost

Vikingr

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Location: Maryland
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Posted: 07 Jan 2019 at 10:16am

Originally posted by offgrid

The problem with going by the label on the side of the 171's/172's to determine axle rating is that the 3000 lbs listed there could easily just be derated from what's really the same 3500 lb axle used on the heavier rPods. There should be a sticker on the axle itself with the manufacturer, a weight rating, a part number, and a "variant number". If someone wants to take a photo of that and post it I can compare it to the data on my 179 axle.

Below is axle sticker on my 172. Also, assuming it makes a difference, it's mounted in a diamond, not square, configuration.

< ="text/"> p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; color: #021eaa; -webkit-text-stroke: #021eaa} span.s1 {font-kerning: none; color: #000000; -webkit-text-stroke: 0px #000000} span.s2 {text-decoration: underline ; font-kerning: none; -webkit-text-stroke: 0px #021eaa}

offgrid

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Posted: 07 Jan 2019 at 2:34pm

If your axle is like mine it should be a 3 inch square box tube oriented at 45 degrees.

Looking at the part numbers I'm highly suspicious that there isn't any actual difference in load capacity. Both start with the same series of identifiers, which I think mean that the axles are 3500lb nominal Lippert torsion axles with 89.5 inches hub face to hub face and 59.25 inches inside attachment bracket to inside attachment bracket.

The only difference other than the suffix on yours that says Cap 3000# is that your 6th identifier says 220 while mine says Zero. I think that is specifying the starting angle of the swing arms which controls how much lift the axle is providing. It shouldn't effect the load capacity. I'll bet that both axles can carry the same loads.

Here's my label.

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

offgrid

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Posts: 5290

Posted: 07 Jan 2019 at 3:30pm

I took a look at the effect of just adding a 3 x 3 x 3/16 inch steel angle along the bottom of the existing 3 inch axle box tube. It got a bit beyond my ability to readily recalculate the combined section modulus of the two parts, but if I did it right I got about a 75% increase in load capacity.

This would be a pretty easy mod, just cut a piece of angle to the right length and clamp it to the bottom of the existing axle with say 3 or 4 heavy duty stainless hose clamps at each end. Those should be enough to handle the transferred load which is only going to be about 1000-1500 lbs at each end. I don't think it would be necessary to clamp it between the axle attachment points because the axle tube will press down into the angle in the middle under load anyway.

Added weight would be about 25 lbs. I might give it a try, couldn't hurt anything I don't think. To mitigate corrosion I'd drill some holes in the bottom of the angle so it doesn't collect water in there. Paint it and seal the edges with caulk.

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