My version of axle support - R-pod Owners Forum

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lostagain

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Topic: My version of axle support
Posted: 30 Apr 2019 at 12:02pm

marwayne, as always your work is executed exceptionally well. Do you plan on coming up with something to support the part of the axle that extends from the frame/risers to the brake drum? How do you you plan on managing the stress put on that part of the axle now that the part inside the frame is reinforced?

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

marwayne

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Posted: 30 Apr 2019 at 12:50pm

All I'm trying to do is to prevent the axle to go into a negative camber

If you want something done right, do it yourself.
2011 RP172, 2016 Tundra 5.7 Litre, Ltd.

offgrid

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Posted: 30 Apr 2019 at 12:51pm

Originally posted by marwayne

I did the same as Toyanvil. Toyanvil stated that nothing is attached to the frame but to the risers, that's what I did.Toyanvil attached on the outside, I attached it on the inside.

i think there is a misunderstanding here of the forces involved. As GlueGuy says, the issue is that the cantilever created by the inboard axle mounting points to the frame is too long. Toyanvil's and my approaches adress this by supporting the axle tube outboard of those mounting points, reducing that cantilever load on the axle tube.

Unfortunately, Marwayne's approach does not provide this support.

Here is the load diagram and the stress equations (just think of it as flipped upside down).

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

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

marwayne

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Posted: 30 Apr 2019 at 1:16pm

Look I'm not an engineer I'm a finishing carpenter. Here is my point, since toyanvils design the down pressure is not vertical but in an angle therefore there is a lot more pressure against the riser than on my design. But as I stated before I'm not an engineer. I won't be able to try it fore a while, because it is snowing here and - 4 C.

If you want something done right, do it yourself.
2011 RP172, 2016 Tundra 5.7 Litre, Ltd.

marwayne

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Posted: 30 Apr 2019 at 1:39pm

It would be nice if I was able to add Toyanvils design to the outside of my axle, but that is beyond my capability.

If you want something done right, do it yourself.
2011 RP172, 2016 Tundra 5.7 Litre, Ltd.

podwerkz

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Posted: 30 Apr 2019 at 1:45pm

I'm no engineer but it seems to me that limiting the very slight downward movement of the center of the axle tube (during heavy loading, hard bumps and potholes) will INCREASE the forces on the axle tube just outboard of the frame attachment points.

If the middle tube section has no 'give', the deflection will be transferred outward, or so it seems to me.

r･pod 171 gone but not forgotten!

offgrid

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Posted: 30 Apr 2019 at 1:48pm

Originally posted by marwayne

Look I'm not an engineer I'm a finishing carpenter. Here is my point, since toyanvils design the down pressure is not vertical but in an angle therefore there is a lot more pressure against the riser than on my design. But as I stated before I'm not an engineer. I won't be able to try it fore a while, because it is snowing here and - 4 C.

Marwayne, I understand you're not an engineer. I am, and all I'm trying to do is help. Geek

Your point about Toyanvil's design is valid, it does create a moment (torque) on the riser. That's why I suggested he bring the diagonal load up as high as possible to keep the moment load on the riser down to minimum. He did that and I think he'll be OK, but I didn't run the numbers. In my design, all the forces are vertical so its not a problem.

What GlueGuy and I are pointing out is that your design doesn't adress the underlying problem which is a cantilever which is too long. The bending stress on the axle tube starts at zero out at the wheels and reaches a maximum at the axle supports. The longer the distance between the wheel and the support point (the cantilever length) the higher the bending stress is.

If the bump loads on both wheels are the same (say you go over a speed bump too fast) then the stress on the axle tube is the same between the supports as it is at the supports. Changing the distance between the supports (which is what your design does by adding an additional support point in the center) doesn't change the bending stress on the tube. So, you will still reach the yield stress point of the steel at the same bump load as if you didn't add your support. Once you hit yield the steel tube will not return to its original curvature and you'll have negative camber.

I hope that all makes sense.

Re testing to see if any one of these solutions solves the problem is not really practical or necessary. You'd have to set up a controlled experiment where you overloaded your trailer axle with the reinforcement by an known amount, and show that everything is OK. Then remove the reinforcement and load it to the same point and show that the axle fails. Oops, now you have a bent axle Disapprove

No one is going to actually do that and its not necessary. The engineering of cantilever beams is very well understood. Buildings are designed using these same formulae (no testing needed) and occupied by thousands of people safely.

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

lostagain

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Posted: 30 Apr 2019 at 2:34pm

From a legal perspective Confused

it seems that the bendy part of the axle is outside of the frame rails and that's what you need to ultimately reinforce with out moving the problem somewhere else. So, either offgrid's or Toyanvil approaches ought to help reduce the stress. I think Toyanvil's solution is much more appealing from a design aesthetic point of view, and like marwayne's work it was beautifully executed, but for simplicity and reducing the risk of moving stress points to uncontemplated places, the elegantly simple design of offgrid's angle iron support under the full length of the axle seems like it strengthens the bendy spot of the axle while not moving the force to create another weak spot. But, what the heck do I know, I'm a lawyer not an engineer.

So I vote that marwayne and Toyanvil get together and make a kit using offgrid's design and I'll be one of the first to buy one. [note to marwayne: My dad moved from northern Minn. to avoid the kind of cold you have right now and ended up being nice and warm in Bakersfield. You could head down there for a couple months in the winter as we go to Colombia, to escape the cold. Wink

]

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

Toyanvil

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Posted: 30 Apr 2019 at 10:48pm

Marwayne, I like it and may build one for my trailer. I looked at adding a crossbar to mine, but did not think about adding a support in the middle. I was just looking at supporting my lift blocks. If I were to do it again, I would weld a crossbar from one lift block to the other and make it all one piece. I think yours will work since a Torsion axle is almost solid the first 20 inches or so and hollow in the center. The problem with the axle warranty, is FR did not build the RPOD's with the right frame rail spacing. Axle companies recommend the frame rail to hub face not be over the 9 inch max, RPOD's are 15 inches. The sad thing is, it looks like FR is still doing it 10 years later.

offgrid

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Posted: 01 May 2019 at 5:09am

Originally posted by Toyanvil

Marwayne, I like it and may build one for my trailer. I looked at adding a crossbar to mine, but did not think about adding a support in the middle. I was just looking at supporting my lift blocks. If I were to do it again, I would weld a crossbar from one lift block to the other and make it all one piece. I think yours will work since a Torsion axle is almost solid the first 20 inches or so and hollow in the center. The problem with the axle warranty, is FR did not build the RPOD's with the right frame rail spacing. Axle companies recommend the frame rail to hub face not be over the 9 inch max, RPOD's are 15 inches. The sad thing is, it looks like FR is still doing it 10 years later.

In reality the axle section between the supports does nothing to help with axle bending, so supporting it in the center doesn't help. There really isn't a need for the axle tube between the supports at all, other than to reduce the risk of twisting the trailer frame. That's how these work:

https://timbren.com/blog/products-page/asr35hds11/

But even with outboard frame rails you can't get the wheel loads right under the frame members so with the Timbren axles you need a crossbar to carry the moment (torque) so you don't twist the trailer frame. You potentially have the same issue with your axle end supports but I think you're OK as is. A crossmember couldn't hurt though.

The rubber in the axle tube ends won't do anything to support the steel tube, its way too flexible. The tube is very stiff, so the deflection at which it fails is pretty small, only about 0.2 inches IIRC. That's why a flat tube with no crown is a clear indicator of a failed axle.

If someone really wanted to make up a welded part that would solve the axle bending problem without replacing the axle itself, I think the best approach would probably be to fabricate a tube the same length as the axle tube that attaches to the frame and axle at the existing locations and also extends outboard to provide a vertical downward support for the axle ends. It wouldn't need to be clamped to the axle tube at the ends (or the center) to work.

The part would also act as a replacement for the riser kit. Call it an axle reinforcement/riser kit combo. If you made it up from the same tube section as the axle it should roughly double the load capacity of the existing tube and provide about the same additional height as the existing riser kit.

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