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malkbean2
Groupie
Joined: 31 Jan 2017
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Posts: 80
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Topic: trailer tilt Posted: 24 Jan 2021 at 12:38pm |
Opinion requested from engineer type people: R-pod is parked on a 20
degree slope for 6 months. That is to say tilted side to side not front
to back. It is supported at 3 points , the 2 wheels and the front screw
jack. Is it likely that this would/could permanently bend or twist the
frame and/or bend the axle?
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rpod 178
Grand Caravan
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jato
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Joined: 23 Feb 2012
Location: Kewadin, MI
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Posted: 24 Jan 2021 at 6:52pm |
Noting that the 178 is 'static' not in motion, I personally see no way that from the description you give, that the frame would twist or axle would bend. The only cases I have heard where frame twists or axle bends is when torque or stress is applied to those areas, such as having a correctly loaded or more likely an overloaded situation and then traveling down the road and hitting a pothole, bump, or some large debris causing a situation where two opposing forces come to a point resulting in the weaker of the two to bend, or worse yet break under extreme stress or torque of the metal.
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God's pod
'11 model 177
'17 Ford F-150 4WD 3.5 Ecoboost
Jim and Diane by beautiful Torch Lake
"...and you will know the Truth and the Truth will set you free."
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offgrid
Senior Member
Joined: 23 Jul 2018
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Posted: 25 Jan 2021 at 8:34am |
Wow 20 degrees is a pretty steep slope. First question. Can you put leveling blocks on the low side wheel to level up the trailer some?
Here’s what I think the math says, someone correct me if I have it wrong. At a 20 degrees side slope, the center of gravity will shift towards the downhill side by the sine of 20 degrees multiplied by twice the height of the center of gravity above the ground. The factor of 2 is because the cg moves downhill as the trailer is tilted and then the point where a vertical line from the new cg location hits the ground is the new balance points the new cg location. The sine of 20 deg is 0.34. We don’t know how high the cg is but let’s be conservative and say it’s 3 feet. I can’t imaging it’s higher than that. That would shift the cg to the low side by about 2 feet.
The wheel spacing is about 7.5 feet so instead of the load on the wheels being balanced with 3.75 feet on either side we’re at 1.75 feet on the downhill side and 5.75 feet on the uphill side. So we end up with about 75% of the load on the low side and 25% on the high side. The 75% represents a 50% increase in load. BTW this calc would have the rpod rollover point at about 36 degrees which probably isn’t way far off. 36 degrees is crazy steep to get sideways on.
When I looked at axle safety factor before yield failure I found it to be around 100% (2g’s) so you should be ok on the axle tube.
That’s for the steel. Steel is very elastic and won’t retain a deflection as long as you don’t exceed it’s yield point. I’m not so sure about the rubber the torsion arm sits in. Unlike steel, rubber and other polymer materials can take a compression set and not come back all the way when the pressure is released. You could find that the low side ends up lower after long term storage because the torsion arm doesn’t rotate all the way back where it was.
If you can as a minimum I’d suggest parking the trailer with the slide side (which is heavier) uphill. And level up the trailer as much as possible so you don’t have to worry about it.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
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EchoGale
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Joined: 10 Mar 2019
Location: FL
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Posted: 25 Jan 2021 at 11:06am |
Unrelated to the frame/axle question but isn't also also a problem for the fridge?
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Julie
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offgrid
Senior Member
Joined: 23 Jul 2018
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Posted: 25 Jan 2021 at 11:28am |
If the fridge is left on yes.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
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malkbean2
Groupie
Joined: 31 Jan 2017
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Posts: 80
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Posted: 26 Jan 2021 at 11:42am |
Thank you offgrid for the detailed analysis.
So the axle tube is not likely to bend but the rubber inserts have a good chance of deforming. That makes a lot of sense given the long term of the unequal weight distribution, and it might even be detected by comparing the angle of the torsion arms.
And what of the frame? I read somewhere on this site that the frame thickness is 1/10". Would you expect the torque created from the unequal weight loads to impart a twisting force to the frame enough to exceed the yield strength of the frame and create a permanent bend?
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rpod 178
Grand Caravan
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offgrid
Senior Member
Joined: 23 Jul 2018
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Posted: 26 Jan 2021 at 6:35pm |
Good question on the frame. It is has roughly the same 2x safety factor (2 g load to read yield) as does the axle tube. So I think that will be ok.
The biggest consideration will be rubber possibly taking a compression set and effecting the torsion arm angle I think.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
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malkbean2
Groupie
Joined: 31 Jan 2017
Online Status: Offline
Posts: 80
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Posted: 05 Feb 2021 at 7:54am |
So takeaway from this is that it is poor practice to park a torsion axle trailer on a sideways slope for an extended length of time mainly due to the uneven weight distribution and possibly permanent deformation of the rubber inserts within the axle. Good to know and thanks.
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rpod 178
Grand Caravan
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offgrid
Senior Member
Joined: 23 Jul 2018
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Posted: 05 Feb 2021 at 9:06am |
It’s a possibility is all, I don’t know enough about the rubber compound in the torsion axle to say one way or the other. Not worth taking the risk for me, just to avoid putting some blocks under the low tire.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft
2015 Rpod 179 - sold
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