Stiffening An Expedition Camper Frame

[PathNotTaken]

If you are referring to that massive "pivoting frames" thread, that made my head spin, too... People like to over complicate it seems.

You can probably use all the mounting points on your frame and still have a "pivoting system". My frame has 8 and I used them all. The rear ones just need to be on the centerline so they can tilt.

I think you will be able to make it lower with springs, but if you are planning to forego wheelweels, that will define your minimum height I think, and you wouldn't gain anything.

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.

As for the wheel wells, you're absolutely right. I will have wheel wells to insure the minimum height possible. the main difficulty of minimum height in my build is the 3.2in frame hump over the axle. I plan to build the subframe around this hump and have the floor sit just above it, with a bit of clearance.

 

[PathNotTaken]

So I jacked up the rear tire as high as I could (~24in) and did by best to measure this flex. Essentially, the two frame members move independently of each other, and will therefore be at different angles, which is what I measured. The channel itself does not bend: ie even in this flexed scenario it stays true/straight relative to itself. I determined this because the angle is identical right behind the cab as it is in the rear for the same frame channel.

As for opposing frame channels, I measured a 3 degree difference. Using basic trigonometry, this is a vertical displacement of 6.43in over 123in of frame, which is pretty significant. With that said, the most amount of travel I could reasonable get out of spring that would fit this application is ~1.4in, or about 21% reduction in stress applied to the habitat by the frame flex vs rigidly mounted. I understand none of this math is very exact with varying vehicle conditions, but it's definitely concerning. I'm not great at math so maybe I am making false assumptions here. This is without the rear bumper so it may be a bit less, but still significant.

 

[PathNotTaken]

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. With the vehicle being naked and a 5,000lb+ payload this is expected. I'm not entirely sure how this changes things once the leaf springs are the proper stiffness for the final camper weight, but I assume the frame flex will be quite a bit less when the suspension is doing its job?

 

[joeblack5]

Good work, my take on this is that you want to do your measurements at full load , simulated with weight or with your habitat.
Then a rear wheel should be lifted as high as another wheel comes of the ground, from my or back.
This is a situation that is easily archieved in the real world and will occur in medium of road situations..

For instance Mengel pass , South of Death Valley and the road from the sand dunes to the hot springs in death valley. Both good adventures.

Whatever construction you do the above is the most forgiving test that your habitat box will have to survive since it is static or at very slow speed. Now add a bit of speed and some inertia to it and pray that the mounts in your habitat are not being ripped out or that walls come apart.

The small bus I showed before was at a medium speed accident ,black ice, 30-35 mph, slid to the opposite lane, impact with bank on the other side of the road, plowed up the bank about 6ft with the front wheels, got stuck then the rear end came around , pulled the nose of the bank and then rolled. Came to standstill in opposite lane, pointing opposite direction from original travel. The whole body got racked by about two inches.. it was a bus so it stayed intact. No separation between cab and body, all cab mounts still good and frame straight. Front windshield broken, bus entrance beyond repair. Rear door racked and could not close, driver door bend 2". It look was a lot of work to undo the damage. I would love to see pics of other habitats that had a similar situation and see how their damage is.

Needles to say I am in favor of bus bodies and ambulances.

Johan

 

[Victorian]

A crash like this really sucks. Glad you are ok!
Ambulance bodies are extremely sturdy and by law need to be roll over tested. Therefore I believe those are the strongest boxes out there as they should be.

As for any other composite habitat:
In my 35 year carrier in the expedition truck industry I have seen and worked on several Expo trucks that went through accidents/roll overs etc. In most cases, the chassis (the actual truck) was totalled. The composite boxes where all structurally intact and repairable. The interior, windows/doors where a different story. Ideally, a camper body should have interior cabinets/ divider walls structurally bonded with the box (not screwed!). These should ad a massive amount of structural strength. Without those or insufficient mounting strength it can turn into a much weaker overall structure.
There is also a big difference in impact distribution between a steel/aluminum framed habitat and one without any internal structure! Even a small impact can transfer the energy through the whole habitat via studs and other structure in the walls. Without, you most likely only have a localized impact area that should be easy to repair.

In any case, if done correct, any enclosure (schoolbuses, ambulances, Expo trucks etc) can be insanely strong and therefore protect the occupants.

I want to do a roll over test with one of our truck campers so badly, but haven't found an institute willing to work with us on that. Mostly because there are no guidelines/regulations for campers to follow.

 

[PathNotTaken]

Needles to say I am in favor of bus bodies and ambulances.

Pretty nuts to see one survive an accident like that without much trouble. Shows they do what they were designed to do.

For instance Mengel pass , South of Death Valley and the road from the sand dunes to the hot springs in death valley. Both good adventures.

I did Mengel and Steel Pass earlier this year, fun roads as always.

Good work, my take on this is that you want to do your measurements at full load , simulated with weight or with your habitat.

I 100% agree, but sadly this isn't super feasible. I am trying to do it without the habitat to understand how much force the habitat is going to need to resist. And I can't get the axle to articulate without the weight of the habitat. I have a M.E. friend who could hopefully run some FEA given the flex measurements I already measured (and I can always adjust the parameters) to see how the subframe handles it.

 

[PathNotTaken]

I want to do a roll over test with one of our truck campers so badly, but haven't found an institute willing to work with us on that. Mostly because there are no guidelines/regulations for campers to follow.

This would be pretty cool to see. I'm sure with your box and an interior structure you would put the greater RV industry to shame. I've seen those crash and it sure isn't pretty.

While rollover is a concern, I want to bring the thread back to all other driving conditions that aren't the vehicle tipping. @Victorian obviously you have extensive experience with these campers, do you think measurement of bare chassis frame flex without a habitat are valid indicators of how much frame flex a spring mounted habitat needs to support? With my measurement of ~6.5in of vertical displacement from the behind the cab to opposite rear of frame, and the estimate 1.5in of spring travel I can achieve doesn't seem very significant in reducing this force. While this should be considerably less with the axle/suspension articulation, it's still concerning. Would love for others with knowledge in this area to weight in as well!

Looking at the EarthCruiser core from Fuso (which is probably even more flexible of a frame), they are only getting 2in max out of their spring setup:

 

[Victorian]

This would be pretty cool to see. I'm sure with your box and an interior structure you would put the greater RV industry to shame. I've seen those crash and it sure isn't pretty.

While rollover is a concern, I want to bring the thread back to all other driving conditions that aren't the vehicle tipping. @Victorian obviously you have extensive experience with these campers, do you think measurement of bare chassis frame flex without a habitat are valid indicators of how much frame flex a spring mounted habitat needs to support? With my measurement of ~6.5in of vertical displacement from the behind the cab to opposite rear of frame, and the estimate 1.5in of spring travel I can achieve doesn't seem very significant in reducing this force. While this should be considerably less with the axle/suspension articulation, it's still concerning. Would love for others with knowledge in this area to weight in as well!

Looking at the EarthCruiser core from Fuso (which is probably even more flexible of a frame), they are only getting 2in max out of their spring setup:

View attachment 801872

Earth Cruiser has been doing this for a long time and I'm sure it's fine on their trucks.

Your method of measuring flex could be just fine. Most builders are doing it that way.
Watch this:

 

[rruff]

This one is always fun... as I recall looking at their website, they used springs initially but decided that it imparted too much stress, so went to a pivoting system.

 

[PathNotTaken]

Pivot sounds like a no brainer on those massive expedition trucks. At the rate my frame flexed, it would be well over 1ft of vertical displacement in a frame that long.

 

[rruff]

Here are a couple pics of mine at near max flex... not quite teetering, but close. You can see how much the axle is articulated, vs the frame (bumper and taillights), and the camper. The poly mounts seem to act well as pivots, offering very little resistance, even though vertical displacement is nil. I could tilt the beam that far with my hand. This is the soft compound.

 

[IdaSHO]

Main difference between a captive spring mounted subframe and a pivoting mount subframe is stresses applied to the camper itself.
Spring mounted, the camper body itself is essentially always being stressed when not on flat ground.
Being spring mounted, the camper must overcome the spring load in order to allow flex.

Not the case with a pivoting mount. Pivoting mount allows the camper to float in its own plane, allowing the chassis to flex as it wants/needs.
Assuming you allow for enough pivot angle, the camper body seems none of the stresses a spring mount does.

Both work, when designed and implemented correctly.
Though Id say a pivoting mount is a bit more forgiving as it requires zero math to sort out spring rates, and it protects otherwise "delicate" campers from chassis flex better.

 

[Alloy]

Pivot sounds like a no brainer on those massive expedition trucks. At the rate my frame flexed, it would be well over 1ft of vertical displacement in a frame that long.

These Class 7/8 trucks are not the same as Class 3,4,5 trucks. On these trucks there isn't enought load to compress the suspension/tires.

For vehicles that are closer to the GVWR testing loaded will show there isn't enough free movement to allow for much twist in the frame.

Torsion will also change +/- along the frame which is caused by the constuction/drive train/crossmembers/bumper/winch mount.

 

[joeblack5]

These Class 7/8 trucks are not the same as Class 3,4,5 trucks. On these trucks there isn't enought load to compress the suspension/tires.

This i understand and agree on.

For vehicles that are closer to the GVWR testing loaded will show there isn't enough free movement to allow for much twist in the frame.

This I do not understand , you mean there is not enough space between the habitat and frame to allow much twist before it interferes with the habitat?

Torsion will also change +/- along the frame which is caused by the constuction/drive train/crossmembers/bumper/winch mount.

I agree.

 

[Alloy]

This I do not understand , you mean there is not enough space between the habitat and frame to allow much twist before it interferes with the habitat?

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