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

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StephenH

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Topic: GVWR>GAWR and Stabilizer Issues
Posted: 07 Jan 2019 at 4:28pm

Originally posted by offgrid

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.

Would there be a difference if you clamped the angle to the top of the tube so any water would tend to drain out? Am I understanding that this is only done on the portion of the axle between the mounting brackets and the arm on each side?

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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Posted: 07 Jan 2019 at 5:05pm

When our axle or wheel was bent it didn't look like the axle itself was bent but I guess would be called the hub. Only one wheel was at a angle, my thinking strengthening the axle would not prevent the hub from bending. I am no mechanic or a mechanical engineer, I don't understand why only one wheel would be bent on a angle. If the axle bent wouldn't both wheels be bent on a angle. If I remember correctly I was told the axle was bent and the entire assembly was replaced under warranty with a stronger axle and a larger wheel size. This was on our Vibe with the same style axle and fenders on the outside of the camper as a r-pod.

2018 Vista Cruiser 19BFD (2018-
2012 Vibe 6503 (2014-2019)
2009 r-pod 171 (2009-2014)
Middle Tn
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GlueGuy

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Posted: 07 Jan 2019 at 5:08pm

That sounds like it could help. I would still worry about the large moment arm between the frame and the spindles.

If Stephen could get access to his old/bent axle, we would have a better idea where the bending is happening. I think there is about a 50/50 chance that the bend happens outboard of the frame.

bp
2017 R-Pod 179 Hood River
2015 Ford F150 SuperCrew 4WD 3.5L Ecoboost

offgrid

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Posted: 07 Jan 2019 at 7:04pm

If the force is applied to both wheels equally and simultaneously (like when going over a speed bump) the bending moment is the same from just outside one mounting bracket all the way to just outside the other bracket. So the failure ought to be equally likely anywhere along the axle tube between those points.

But if one wheel gets more load than the other (like going through a pot hole) then the worst case occurs just outside that mounting point. The bending force drops as you move outward from the brackets toward the ends of the axle, so its less likely that an axle failure would occur there. Its like if you have for example a piece of rebar sticking out of a wall and you push on it it will bend right at the wall.

So because potholes are more common than speed bumps I think its most likely that just outside the mounting brackets would be the point of failure, and most likely the slide side because we know that side of the larger trailers is heavier.

If the axle is indeed what is failing. If David and Danette are right and the failure could be outboard from the axle itself then reinforcing the axle wouldn't help. But the starting premise here is that the reason for the failures is because the axle attachments on the rPod are too far inboard. And it sure looks like that has a very large effect on axle capacity. Put another way, moving the attachment points doesn't change the loads on any parts outboard of the axle, so if the failures are somewhere other than the axle than they have nothing to do with the rPod axle mounting points.

StephenH, from a beam load perspective there is no difference if you clamp the angle on top of the tube or on the bottom, but practically speaking you can't because the axle mounting brackets would interfere with running a continuous angle. The angle needs to be continuous or it will leave a weak spot right at the mounting brackets. The bottom of the axle tube is clear from one end to the other.

GlueGuy, consider that adding brackets outboard to bring the loads back to the rPod frame rails will do the same thing as adding an angle under the axle, its really just two different ways to stiffen the axle in the critical area. Either way would work, its a matter of selecting bracket extensions or angles that are stiff enough to do the job. We know both by calculation and field experience (we don't have massive numbers of immediate axle failures) that the existing design is not that far off what's needed, so we don't have to go crazy I don't think. I'm more concerned that the hose clamps won't do the job at which point a weldment and bolted clamp arrangement that fit around the axle ends would probably be needed.

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

GlueGuy

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Posted: 07 Jan 2019 at 7:18pm

Consider also that FR (or Lippert) decided to de-rate the axle capacity because of the inboard attach points.

I would be concerned about the hose clamps being strong enough. Maybe if there were 6 of them or something....

bp
2017 R-Pod 179 Hood River
2015 Ford F150 SuperCrew 4WD 3.5L Ecoboost

offgrid

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Posted: 07 Jan 2019 at 8:18pm

Here's a link that lends some hope that heavy duty hose clamps could work.

http://www.jcshi-torque.co.uk/just-strong-jcs-clamps/

Another perhaps better approach might be to use two square U bolts at each end to clamp the axle and angle together, one placed on each bottom face.

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

lostagain

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Posted: 07 Jan 2019 at 8:40pm

So, let me see if I understand the concept of strengthening the existing axle. Given that the greatest point of stress is just outboard of the attachment point on the frame, one proposal is to run a piece of angle iron [3x3x3/16] for the entire length of the axle on the underside as that has a clear space from one end to the other. Then, the idea is that the angle iron will provide additional support at the stress point, thus increasing its resistance to breakage. How'm I doin' so far?

Since the angle iron, if its subject to the stress at the outboard axle connection point, will try to resist that stress, it will effectively get pushed away from the fracture or bend site unless it is securely held to the axle with some kind of fasteners. So far it's not clear how much stress those fasteners will subjected to, but it is clear that hose clamps will have to be numerous and sufficiently strong. Am I still there?

So, a question: Why don't you use some kind of U clamps appropriately sized to fit around the axle and the angle iron that wouldn't stick down far enough to interfere with ground clearance? Aren't they pretty strong? Just wondering from a lay person's point of view. Seems to me with 2 or 3 spaced on the inboard and outboard sides of the axle to frame attachment point, they'd fit the bill.

Ok, when you get up off the floor after falling down laughing, help me understand or I'll just be lost again.

Never leave footprints behind.
Fred & Maria Kearney
Sonoma 167RB
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StephenH

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Posted: 07 Jan 2019 at 8:55pm

What about these?

http://www.hoseclampkings.com/prod-21-1-476-74/breeze-hi-torque-htm-1800-17-1-4-18-1-8-inch-range-5-8-304-ss-band.htm

Although the U-bolt idea also might be good as those can definitely be very heavy duty.

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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Posted: 08 Jan 2019 at 6:35am

Ha Ha lostagain. Not laughing at your idea of using square u bolts, just laughing 'cause I beat you to it by 22 minutes Tongue

The reason I was mentally resisting them at first is because with the diamond orientation of the axle you need to have two of them at each point to symmetrically hold the angle to the axle, so its a little weird. But it should still work I think. The holding capacity of each u bolt in the direction of force is derated to cosine(45 deg)= 71% so two together would be about 1.4x the the holding force of a single u bolt used normally.

The only issues I'd take with your summary of the situation is first that there isn't just the one weak point, the stresses on the axle are continuous. The points just outboard of the axle attachment brackets are most likely to see the most stress over time but that doesn't mean they're the only points to worry about.

Second, I'm not sure what you mean by "get pushed away from the bend site"? The bending forces are upwards on the wheels so the axle ends are getting pushed upwards while the midsection of the axle is getting pushed downward. By clamping the to the axle at the ends the two parts should stay in intimate contact over their whole lengths under load. You could add more clamps inboard if you wanted to but I don't really think they're necessary.

StephenH, I think the hose clamps you found should work but I'm now leaning toward trying square U bolts if we can find the right size. They would probably need to be 3/8 inch to have enough thread area to carry the load and have a 3.5 inch opening to slide over the 3 inch axle plus the 3/16 in angle.

These would probably work. We'd need bolt plates, nuts and washers as well.

https://store.uboltsdirect.com/SecureCart/ViewCart.aspx?mid=0D3EC8DE-C0FA-487C-81A3-BB423F37A886&sctoken=164be636ce8a4f7da159cd2aa91502b4&optionId=1012189971,&bhjs=1&bhqs=1

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

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Posted: 08 Jan 2019 at 6:59am

This discussion brings me back to a question I asked a while back--does the axle bend up or down ? it would seem to me that you could weld the 3x3 angle to the axle and it would stop the bend regardless of the directin. Is it not possible to weld to the hollow axle instead of U bolts?

Vann

Vann & Laura 2015 RPod 179