New Build Begins- 2021 CV 515

This build wouldn't have been contemplated let alone started if it weren't for the knowledge and experience shared in this forum. Now that it is underway I'm going to do my best to share what we're doing in the hopes it can help another build along the way. We wouldn’t have made it this far without others having done the same so here goes….
Here's a list of what the build is or will become:

-2021 International CV515 Crew Cab
-Allison 2500 RDS 4x4
-Factory length: 297”. With brush guard, winches and rear spare ~26’-9”
-Wheelbase 199", Back of cab to end of frame 147"
-Rear airbags (Kelderman 4-Link), 3" front lift
-Continental MPT80-365/80/R20 on Hutchinson 20x11 (front and rear)
-Dual alternators
-65 gallons fuel
-Warn 16ti winches

front end.jpgfull length.jpg
-Tube steel 'skeleton' with loft over cab. Overall is 8’x8’x19'-5”
-Floor and roof deck are 2" 20g conform metal deck.
-Roof will have marine grade plywood above 2" deck to help with camber and ‘dishing’ of the aluminum cap.
-3 5/8" light gauge wall studs (desire thick closed cell spray foam)
-Exterior skin 0.063 aluminum panels adhered w/ VHB tape
-14" deep tanks under subfloor. 176g potable. 2 gray tanks, 60g and 43g= 103g total.
-Composting toilet
-3kw Victron Multi-plus Inverter
-1,200 Amp hour, 12V LiFePO4 battery bank (4x300 repurposing from existing)
-1,200 Watts roof top solar (repurposing)
-Autoterm 17kbtu diesel heater. 2 air plus 1 water heat exchanger with a 1.5kW backup heating element
-15kbtu RTU AC/heat pump w/ 5kW heating element
-AC/DC 73qt chest fridge/freezer
-Induction cooktop and a microwave
The habitat needed to be ~11’-0” long. Very conservatively it weighs in at ~12,500# wet and provisioned. Combine that with a need for a crew cab (dogs) and the chassis decision was pretty easy to narrow down. We were looking at either an F650, a Silverado 6500HD or its twin the International CV515.
I wanted to make sure the factory frame was longer than my build so I never got “sided eyed” by DOT, home or abroad. Stock Class 5’s don’t work. Even if we did cantilever the habitat off the back of any class 5 we overloaded the stock rear axle loads real quickly. The F650 doesn’t come with stock 4wd. Money could have “fixed” both class 5 and F650 ‘problems’ but it was looking like at least $20K. The CV515 could do the job, stock, for $63k. There are less Chevy dealers that will service a 6500 than ones that do and the 6500 was $2k more expensive. CV 515 decision made.
-Suspension modifications
Hands down, experience with off road maneuverability is my weakest link. My overland ‘driving resume’ caps out on logging roads behind the wheel of a TJ with a 2” body lift and a tent. To suggest it is above my pay grade to opine on how to best up fit the suspension of a 13 ton loaded CV is an understatement. I found an up-fitter that I felt did good work and didn’t look back. Capability is far more important to me than comfort or noise. A better driver would take a lesser vehicle much further but my plan is to have lot of room grow into it and not be found in a ditch along the way convicted of “more truck than sense”. I did put about 1,000 paved miles on the truck before the rear bags were put on. Not having bruised kidneys and a bleeding tongue is a pretty good perk. That I can say from experience ;)

-Layout/Floor Plan

Previously we spent about three years driving around in an RV part time. From those travels I wanted a habitat plan “downstairs” no shorter than 11’ and a queen bed loft. At 6’-4” of headroom the ceiling is as low as we want to go. With a 14” subfloor we are exceptionally tall at ~12’-8”. For perspective if I wasn’t excited about doing my own build, had double the budget and was an EU resident my turnkey vehicle would be a 16’ BlissMobile habitat on a 6-8 year old U500.
Habitat Floor Plan.jpgHABITAT ROOF PLAN.jpgSteel Elev-01.jpg
Generally speaking all the for-sale products and a lot of the custom builds I looked to for inspiration were a composite box sitting on a steel sub-frame atop a class 5 chassis. Here’s why we are doing something different (Why we’re Class 6 vs 5 discussed above).
I don’t have any experience in composite panels, design or assembly, and would be 100% reliant on others to deliver a strong dry box and sub-frame to the yard if we went that route. The best I could figure a composite box would reduce my gross weight by 2,000# and it would look incredible done right. It was ruled out because it wasn’t something I could call my own and was going to cost at least $20 k more to ‘hire out’. I have a background in structural analysis though and that did allow me to feel good about a steel design that was my own.
Torsional Separation
After reading about a lot of other builds the torsional separation between the chassis and habitat boiled down to 3 issues that I wanted to address as best I could:

It protects the habitat from chassis twist (walls don’t leak)
It minimizes the stress concentrations a ‘hard attached box’ would add to the chassis (truck doesn’t break before it should)
A ‘hard attached box’ would reduce the amount of flex the stock chassis has and lower the bar on terrain capability (truck gets stuck less)

Items #2 & #3 get addressed with a combination of springs and elastomeric pads between the habitat and frame rails. Item #1 is handled as a steel design problem. I made a lot of finite element models. A LOT. If anyone has interest in what went into that, maybe we have a side bar. Otherwise, 1 pictures=1,000 words. Some plans, a few details and some FEM images are here. Bullet points below will hit a few main points with words:

-Highest stresses in the box were in a cross members that was hard connected directly to the frame while only one tire was moved vertically. I ended up with 4” tube steel down low.
-Loft deflections were a concern. I’m starting with a 2 ½” gap and am anticipating 1/3” deflection above the cab before bumps.
-My walls were going to be thicker than 3” for R values so I wasn’t shy w/ tubes size for other members. Also makes for real easy lifting points and I can mount future equipment in a blink.
-With a roof and a floor in, but no walls, the steel box should weigh in at ~2,900 pounds.
-Aluminum frame would have been lighter. My guess was a 1,000# reduction was very generous. Abusive driving was not something I could use analysis for to feel good regarding lower ductility. Considering I have to outsource the steel welding let’s just say I feel way better about doing any future repairs or mods on something not aluminum
-The bottom of my steel box will connect to the chassis at six points. The two rear points are slotted connections that allow vertical movement but prevent movement towards or away from the cab.
-The other four points consist of elastomeric bearing pads and springs between the box and frame rails. Hopefully the sketches help.
I will definitely find some fork truck time to lift wheels before and after the mounting and see how the springs perform in a controlled environment. I can interchange springs to some degree. I can also scratch my bald spot and curse really loud. I fully intend to drive the empty steel frame around and try to take test measurements of what the springs and loft are doing while bouncing. I haven’t quite figured that test method yet. Suggestions would be very welcomed.
Cost is good part of the discussion. Right now it’s looking real close to $20k for the steel box, decking and fabrication including a stack of aluminum sheets for the skin.
corner defl dl_ll.jpgdeformation_stressconcen_dl_ll.jpghaunch spring bearing.jpgSpring Compression Calculator.jpg
I started researching other builds and their systems in January 2020. By December 2020 I felt good enough about the habitat layout and payload that I could commit to a chassis. The truck came off the line in late April '21. Suspension and other chassis up-fits were completed and the truck was home by early August. The steel was delivered a few weeks ago and the fabrication shop expects to be completed with the skeleton in January ‘22.
mock up-min.jpg
We've made some progress over the spring and it seemed like an update was in order so here's where what we've done.
The steel skeleton along with the floor and roof deck have been mounted. The spring mounts seem to be working great. The very front corner of the loft is just barely visible while driving and you can definitely see the habitat moving both side to side and up and down independent of the chassis.
The gap between the truck roof and loft floor was designed as 2.1" with just self weight. It ended up at 1.625". A little of that was steel fabrication tolerance and most was my level line across the mock up's domed cab roof must have been off a dead center bubble. Even though the math says it will not deflect enough to ever touch the cab I would have liked it to be closer to 3.5" after the fact. I feel like I've already exhausted an room for error not to mention it is damn tight in there if I need to get in that gap for anything moving forward.
After cutting in the roof penetrations for the fan, skylight and AC unit I went to town on insulating and waterproofing the roof. First I caulked all the deck seams with Sika 221 from inside and out. While the roof will have an aluminum skin which by all means should be water tight itself I wanted to maximize insulation and minimize thermal transfers. My solution was to to fill the 2" flutes of the metal deck with 2" XPS insulation cut to fit the flute as tight as possible. Hopefully this picture at a the fan penetration from the inside of the habitat gives you an idea of what is going on. With the XPS in those corrugations the top of the roof surface basically looks like alternating strips of metal deck and pink foam 'planks'. I then ran a 1/2" layer of XPS over all of that to keep the top of the corrugations away from the outer skin and smooth out the 'planks'. This layer of XPS flushes out with the top of the perimeter steel ring. I then rolled a layer of EPDM over this and figured with the rubber and steel both being air and water tight my roof was off to a great start and the aluminum was going to really be just puncture protection.
The installation was far more difficult than I could have imagined because of the compatibility of adhesives between all the differing material. I did make little test specimens for each layer of materials but that didn't reveal the depth of the difficulty.
First there is adhering the XPS to steel deck. That was darn tricky because the most readily available Liquid Nails is L-901 but is not compatible with XPS. There is a Liquid Nails product L-903 that is compatible but was too hard to source. The tubes look identical and you wont find the lack of compatibility of L-901 on the packaging so when the label says "all construction materials" it apparently doesn't consider XPS a construction product;).
I used Loctite PL3X as the manufacture is clear that it is compatible with XPS and getting it by the case wasn't too difficult. That was only start of the roof hurdle though. Because the steel corrugations have a lot of deformations pressed into the deck by design I had to use a lot of weight on the XPS strips to act as a clamp while the adhesive set up overnight. The 4 loose steel angles I threw on the first strip failed laughingly. Four 50# bag of sand every 2' worked for one pair of was a long two weeks.

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