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Olddawgsrule View Drop Down
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Direct Link To This Post Topic: Floor failure!!!
    Posted: 08 Aug 2019 at 7:18am
Originally posted by Toyanvil

Is your frame rusted inside or out?

Not that I see!
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Direct Link To This Post Posted: 08 Aug 2019 at 7:22am
Originally posted by offgrid

The axle tube really shouldn’t bend under normal conditions so it’s fine to measure the crown with the weight on the wheels. I miked the frame tube thickness on mine at 0.1 inch. Pretty thin material.

The fact that you have been meticulous about weighing your rig should help with your FR warranty claim. I would emphasize that you know for sure that you were not overweight when you discuss it with them, and that you have not taken your rig on anything except normal roads. Let’s hope they take care of the problem for you.

Well, close to what I measured, but my caliper's are inexpensive. I'm at 1/8" (just <.125).

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Direct Link To This Post Posted: 08 Aug 2019 at 7:28am
Okay, I found a site with an example of our Tube steel strength with the formula and result. Now if only I was smart enough to understand the result.. Hopefully one of you can!

Quoted:
The strength of a beam in bending is proportional to the beam's section modulus. This value is calculated from the geometry of a beam's cross section. A google search will turn up tables of section moduli for common shapes like I-beams, circles, and rectangles.

In your case I will assume that the beam is being bent "the hard way." 

The section modulus for a rectangle is (b*h^2)/6, where b is the width of the rectangle (the base) and h is the height. 

A tube is actually composed of two nested rectangles, one defining the outer wall and one defining the inner wall. To calculate the modulus of the tube shape, we must subtract the inner rectangle's section modulus from the outer rectangle's section modulus.

For the 2"x4"x0.12" tube:
((2*4^2)/6)-(((2-.24)*(4-.24)^2)/6) = 1.18 [in^3]

1.18 pounds of force per inch to the 3rd power? 1.643 pounds of force per inch?
or: 192" to the 3 power x 1.18. This actually makes more sense to me..

It would be so nice to be smart!!!


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Direct Link To This Post Posted: 08 Aug 2019 at 9:08am
I did the frame load calc and posted it a few months ago, you probably can search for it. I assumed the frame rails were uniformly loaded beams cantilevered aft of the axle. But there is in addition the torque from the torsion axle that has to be taken by the beams at the same location, so it’s worse than you get just by assuming simple beam loads.

The US units of section modulus are inches cubed.
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Direct Link To This Post Posted: 08 Aug 2019 at 10:22am
Originally posted by offgrid

I did the frame load calc and posted it a few months ago, you probably can search for it. I assumed the frame rails were uniformly loaded beams cantilevered aft of the axle. But there is in addition the torque from the torsion axle that has to be taken by the beams at the same location, so it’s worse than you get just by assuming simple beam loads.

The US units of section modulus are inches cubed.

I did a bit of searching and don't see the Load calculations, found the thread I believe you are speaking of.. Do you recall the actual name of the thread with the calculations?

I thought so on the inches cubed.
My numbers went to 8400# with even load and center axle (which we don't have). My engineering friend called me and we went through several areas he required to even estimate the force required to fracture the beam. He came up with 6300#.

I see you folks have discussed bent axles before, so with my new numbers of 6300#, won't my axle have bent?

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Direct Link To This Post Posted: 08 Aug 2019 at 12:04pm
I’m on the road right now, I should be able to look it up later. IIRC, I think the axle has about a 2x safety factor at the 3500 lb max loading. That’s not a great safety margin for vehicle structural design. So about 7000 lbs would likely fail it. If you are down to 1/8 inch crown it’s probably been bent a little. It’s going to have to come off to fix the frame anyway so you might want to get it recambered while it’s off. Is your wheel camber ok?
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Direct Link To This Post Posted: 08 Aug 2019 at 12:29pm
Originally posted by offgrid

I’m on the road right now, I should be able to look it up later. IIRC, I think the axle has about a 2x safety factor at the 3500 lb max loading. That’s not a great safety margin for vehicle structural design. So about 7000 lbs would likely fail it. If you are down to 1/8 inch crown it’s probably been bent a little. It’s going to have to come off to fix the frame anyway so you might want to get it recambered while it’s off. Is your wheel camber ok?

I do appreciate your comments and efforts here! Safe travels!

Not sure how to check camber with a bent frame.. Another person mentioned that not all axles are set to a certain crown. The statement was the axle is set according to the unit.. I'd love to here from another 182g owner that is willing to check their axle to compare. 
I'd like to 'think' FR went to the extent of setting according to each unit they sell. Yet for some weird reason... I doubt it.

What I've read so far is you developed a spreadsheet to calculate the load. Is this something you would share?


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Direct Link To This Post Posted: 08 Aug 2019 at 5:18pm
I don't think a bent frame would effect whether you had too much negative camber or not, it would just make the trailer sit tilted on the axle. Might create an illusion though. What I get from most folks with reported axle failure is that the negative camber is pretty noticeable just by standing back and looking at the wheels. 

I don't believe that the axles are custom set by FR either. Several folks on the forum have gotten new axles and no one has mentioned any custom adjustments.

I have a spreadsheet that calculates changes in tongue and axle weight when you move load around in the trailer. If you want that PM me. I don't recall creating a spreadsheet for the axle and frame loads when I did those calcs  but I'll take a look.

One minor point on the section modulus calc, it is the moment of inertia divided by the distance from the neutral (center) axis to the most extreme fiber (2 inches in this case). Moment of inertia for a hollow rectangular shape is (bd^3-hk^3)/12 so Z= (bd^3-hk^3)/6d. This comes out slightly larger than what you calculated. But I measured my tube wall thickness at 0.1 inch rather than 0.12 so ended up almost the same as you for Z at 1.22 in^3. 


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Direct Link To This Post Posted: 09 Aug 2019 at 6:05am
Originally posted by offgrid

 

One minor point on the section modulus calc, it is the moment of inertia divided by the distance from the neutral (center) axis to the most extreme fiber (2 inches in this case). Moment of inertia for a hollow rectangular shape is (bd^3-hk^3)/12 so Z= (bd^3-hk^3)/6d. This comes out slightly larger than what you calculated. But I measured my tube wall thickness at 0.1 inch rather than 0.12 so ended up almost the same as you for Z at 1.22 in^3. 



I've really confused myself this morning, nothing new..

Using the 1.22 modulus, with 192" of length, how much force you come up with?

Here's what I have: 2x4x192x1.22 = 1,873.92 pounds of force to bend. 

Update: I ha a friend come over with his calipers and re-check. He got a reading of .09865.. I did it old-school and see that's much closer to what my cheapie calipers read.. The .1 Off-grid speaks of is what I will go with now. 
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Direct Link To This Post Posted: 10 Aug 2019 at 8:11am
Here's a screen shot of the spreadsheet I built (Mac) and the results at .10 steel.
I did run the examples to test the formula (shown on the left).



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