Stiffening An Expedition Camper Frame

[joeblack5]

Usually not habitat to frame but tires to habitat or axles to frame/cab.

Ok, I see, that is when the frame flex comes into till it lifts a tire.

To bad the video did not explain why the swivel is in the front, I made mine in the back of the unimog. Meaning the habitat is fixed to the center section of the frame. Any road unevenness or frame twist is half in the center compared to the rear or front.

Johan

 

[PathNotTaken]

Ok, I see, that is when the frame flex comes into till it lifts a tire.

To bad the video did not explain why the swivel is in the front, I made mine in the back of the unimog. Meaning the habitat is fixed to the center section of the frame. Any road unevenness or frame twist is half in the center compared to the rear or front.

Johan

Yes, right now unloaded the suspension in the rear does not flex. It’s as if the leaf springs aren’t springs but just rigid.

In an ideal world the frame would stay perfectly parallel to the ground and the axle would move underneath when a tire is lifted. in reality the frame does twist, but less because of axle articulation.


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[simple]

What about adding weight and seeing what it does. A pallet of concrete landscaping materials is around 2700lbs. 4 50 gallon barrels of water strapped to a pallet is 1668lbs.

 

[simple]

I think that adjusting suspension spring rates on the front and rear changes the amount of force required to pick a tire off the ground and thereby effects the amount of frame deflection. Someone on here could draw a free body diagram and prove it one way or the other.

 

[PathNotTaken]

What about adding weight and seeing what it does. A pallet of concrete landscaping materials is around 2700lbs. 4 50 gallon barrels of water strapped to a pallet is 1668lbs.

I have a tiny 1 car garage and not many means to buy/transport that much material. I’d be more inclined to replace the leaf springs with a much lighter pack if I was to test this. However, I’m not sure if it’s necessary since I think the results are pretty predictable.


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[simple]

Gotcha. Those were just generalized examples to get your wheels turning. I wouldn't buy a pallet of brick either but I'd give someone a 6pack of beer to forklift a pallet of something heavy onto my truck, strap it down and then pick a tire up with a forklift so I could look at / measure axle articulation and frame twist.

 

[PathNotTaken]

Gotcha. Those were just generalized examples to get your wheels turning. I wouldn't buy a pallet of brick either but I'd give someone a 6pack of beer to forklift a pallet of something heavy onto my truck, strap it down and then pick a tire up with a forklift so I could look at / measure axle articulation and frame twist.

Now that’s a good idea.


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[PathNotTaken]

What if you could remove the axle springs from the equation. Say support one side of the frame on blocks and put weight on the other frame rail. Could you disconnect a spring pack eye bolt or maintain the current axle to frame distance with straps.

How to weigh down the unsupported side tho ? Brick, water, fat friends ?

You'd then have two measurements for your M.E. friend. The first one tells you the frame can twist with the oe springs rate (on door sticker ?) and the second may quantify how much weight/force is needed to do move it so much.

For sure the eventual leaf pack spring rate will be crucial. You'll want the rate as soft as poss. aided perhaps by not having much variability in the fully loaded weight. I'm wondering if air bags might help in dialing it in ?

Those Euro camper examples are fun but the habitats are not attached to the vehicle cab.

On the sub frame, are you deciding how stiff to make it, balanced against it being as light as possible ?
Or is it ensuring that the design incorporates a system to accommodate the frame flex you calculate needing ?

Or both. LOL.

Airbags could definitely help. I'm planning a decent size water tank and aux gas tank so the weight variation will be unfortunately quite large. The final leaf spring is definitely important, and I agree it may be good to be on the softer side of things, but too soft of leaf springs can introduce a host of other problems.

I think a lot of the points you bring up explain why engineering has safety factors. there's just too many variables, and when you add in dynamic movement, the static measurement are pretty useless. This is why I (and most everyone) significantly overbuild things to be safe rather than sorry. I hope FEA will be helpful in just understanding how the stress transfers into the habitat.

 

[joeblack5]

Be carefully with removing rear springs, an e350 is rear wheel drive and you do not want to twist the axle and loose your drive shaft. Fea is really nice on paper but as with most modelling programs, garbage in gets garbage out.

 

[rruff]

Fea is really nice on paper but as with most modelling programs, garbage in gets garbage out.

I agree... too many unknowns in this case.

Your system of the centerline rear is interesting and definitely something I would consider... please go drive 200k miles and report back so I can have some more data on it
Jokes aside, my interior will place a large water tank on the back corner so I'm not sure how I feel about mounting them on centerline in the rear. Definitely worth considering though.

You are right to be skeptical! I don't even have a subframe... the base (bottom of camper) is a carbon fiber and foam panel. But the pivoting part using the body mounts doesn't seem risky at all. You might think a water tank is a lot of weight, but compare it to trying to resist the flex of your entire chassis.

The only other thing to add is the rear axle barely moved relative to frame. The leafs are not acting as springs and the axle is behaving as if it was rigidly mounted.

This is typical of a HD chassis with a torsionally flexible frame.