C5500 TopKick 4x4 Crew Cab Build

[Johnston]

" Have you run into situations, i.e. rural low overpasses, where >12 feet is a problem? My biggest past problem has been snagging tree branches, which have cost me a couple of A/C housings and vent caps. "

I didn't see if anyone answered this question if they did I'm sorry for the re-post.

But My work truck use to be 12' and had no problems with underpasses, power/phone lines, or fuel stations. Most of the Fire trucks now days are 12' high.

Hope to see it in Hood River this summer if your here windsurfing.

 

[DDW]

Hi NE,

I have also found the various panel suppliers not overjoyed to help with supplying panels for a camper box project . . .

I have been hankering to build a composite box with all the features laid out where we want them and as few compromises as possible. Going with a composite-only design is very attractive.

You might try one of custom or semi custom boat builders. They are used to doing things one at a time, many of them have the equipment to vacuum bag panels with whatever features you want. Business is VERY slow in boatbuilding, so they will talk to you about doing most anything.

I am thinking of having specific panels made (window and door blockouts, edge features, etc.), then having corner pieces molded in solid, and assembling by bonding. Its very easy to do tooling for a constant radius corner with jog overlaps or whatever you want, if you stick to the same radius or a couple of them have them run off X feet of it. A much stronger molding than the pultruded stuff. That would make building the box pretty quick and easy.

 

[NeverEnough]

Hope to see it in Hood River this summer if your here windsurfing.

It's been too long since I was dodging barges at the Gorge! I'll see if I can squeeze in a trip.

As for the 12', I'm trying to reduce the "snag" factor as much as possible.

 

[ersatzknarf]

Hi DDW,
Thanks, that's a very good suggestion about the boat builders.
Your ideas about having the features molded in solid seems it would add a lot of strength, also.

You might try one of custom or semi custom boat builders. They are used to doing things one at a time, many of them have the equipment to vacuum bag panels with whatever features you want. Business is VERY slow in boatbuilding, so they will talk to you about doing most anything.

I am thinking of having specific panels made (window and door blockouts, edge features, etc.), then having corner pieces molded in solid, and assembling by bonding. Its very easy to do tooling for a constant radius corner with jog overlaps or whatever you want, if you stick to the same radius or a couple of them have them run off X feet of it. A much stronger molding than the pultruded stuff. That would make building the box pretty quick and easy.

 

[NeverEnough]

A much stronger molding than the pultruded stuff. That would make building the box pretty quick and easy.

Depends on the pultrusion. A prefab panel built to suit would definitely make assembly quicker. I would take delivery of my windows and door FIRST, however, to make sure the blockouts are the right size. Don't ask me why I know that.....

 

[NeverEnough]

Stilll waiting on the panels, so keeping busy doing the rough cuts on the pultrusions, installling the leveling jacks on the cab, and making some mods to the trailer.

This is Sunjyo from Bosnia. He's been the principle fabricator on the trailer.

I decided to drop the bed to lower the cg so I had to add wheel wells. I explored more off-road oriented tires but decided on quality trailer radial for better towing. Its 32" diameter unladden which puts the bottom of the axles 13" off the ground.

 

[DDW]

Depends on the pultrusion. A prefab panel built to suit would definitely make assembly quicker. I would take delivery of my windows and door FIRST, however, to make sure the blockouts are the right size. Don't ask me why I know that.....

Most of the pultruded stuff (other than some round stock) is really made out of a slurry of glass fiber and resin without much continuous fiber. A proper layup with uni or bidirectional uni cloth will be stronger and tougher. Plus it can be epoxy rather than polyester.

You are right about the blockouts. I would probably NOT have the window and small cutouts blocked out, plenty of reasons for this. It is easily dealt with in a foam cored structure later, by rabbetting the core from the cut edge and then backfilling with thickened epoxy, wood, or for something really tough, G10 fiberglass. That is how it is done on cored boat hulls. A bit more difficult with a honeycomb core, but still doable.

 

[ersatzknarf]

Hi NE,
Any details on the pultrusions, please ? :sombrero:

Thanks !

Stilll waiting on the panels, so keeping busy doing the rough cuts on the pultrusions, installling the leveling jacks on the cab, and making some mods to the trailer.

 

[NeverEnough]

Most of the pultruded stuff (other than some round stock) is really made out of a slurry of glass fiber and resin without much continuous fiber. A proper layup with uni or bidirectional uni cloth will be stronger and tougher. Plus it can be epoxy rather than polyester.

Any details on the pultrusions, please ?

I'm using Strongwell's Extren structural pultrusions, all in stock configurations: equal leg angle, channel, and tube. It's my first experience with this type of product, so I'm going off of the manufacturer's documentation. From my conversations with a couple of their engineers and from what I've seen of the manufacturing process, Extren is all continuous fiber with multidirectional stranding. I know there's a lot of "architectural" (non-structural) stuff out there that is mostly resin. I got my hands on samples from several manufacturers for testing last fall and all the profiles exceeded expectations.

My use of the Extren is actually very limited and has very little to do with the "box", which is a monocoque stressed skin (for the same reasons DDW pointed out). I'm using it for floor joists (between the "basement" and living floor), rafters to supplement the strength of the roof (camper and trailer) to allow a LOT of people to stand in the observation deck areas, and "interface" areas that require a material suitable for mechanical connections (bolts and screws). For example, the openings for the slide-out and the trailer ramp-door require structural surrounds to allow bolting of the purchased actuator and door assemblies. The Extren will also serve as a corner/impact/wear guard for the composite skinned box.

Honeycomb is cool, but it's not like there's a ton of edge-banding, closeouts, fasterners, etc. available (except for 3/4" or thinner panels, which have much broader utilization). I debated long and hard about aluminum vs. Extren, but ultimately decided to spend the extra $$$ in order to eliminate the use of metal as much as possible, have a more compatible thermal expansion pairing, and better substrate pairing for bonding.

I did some corner joint tests early on using the 4x4x.25 Extren as an outside corner and the 2x2x.25 Extren as an inside corner and the panel failed before the corner joint. It would be plenty strong, and it's still the fall-back plan if we run into trouble.

As for polyester vs. epoxy, that's my brother's backyard, not mine. I know epoxy is a much stronger crosslinked bond, but it's also more brittle, and typically more expensive. We're using both, as well as Saba Sealtak for the PPE, depending on the joint, substrates, need for working time, and use.

Sorry about the ramble, I'm trying to delay going out for a workout as long as possible!

 

[DDW]

I have used a bit of the Stongwell product, it is certainly better than some of the others, but still not the equal of a real layup. According to their specs, it has some unidirectional properties: 30,000 psi claimed longitudinal strength, but 7,000 crosswise. Note that G10 which is isotropic is about 40,000 all directions, and a "real" pultruded rod is close to 80,000 psi. I can confirm this from experience, an "L" for example will split comparatively easily "across the grain" vs. lengthwise. Trouble is, for making a strong corner, this is backwards.

Now, I'm not saying this isn't strong enough or won't work for you - it probably is and will. I used a lot of it for ledges, cabinetry mounts, and such like and its a good product. Just not as strong/light as a layup.

Polyester resin is fine, but I don't want people to leave with the impression that it is more brittle. In fact a good epoxy will elongate 4-5% at fracture, vs. 2% or so for polyester. This (and its superior bonding strength) is why epoxy/glass laminate has significantly higher strength and many times the fatigue resistance of polyester/glass. Polyester is not often used in high performance boats anymore, and is never used in aircraft structures.

For bonding panels together there are many choices, but something with even higher flexibility is a good idea. Most glue joints are poorly designed from the point of view of the bonding agent - nothing to do about it due to the convenience of construction. But a flexible agent mitigates the stress risers in the joint. There are now some flexible epoxies available such as West G-Flex, also there are Methyl Acrylates like ITW Plexus. I know people are using Sika PU adhesives with success, but those are very weak compared to some other choices. Sikaflex and 3M 5200 are in the 500-700 psi range, while epoxy and MA are more like 1500 - 2500 psi. A flexible grade of MA will have >100% elongation to failure.

 

[NeverEnough]

Now, I'm not saying this isn't strong enough or won't work for you - it probably is and will. I used a lot of it for ledges, cabinetry mounts, and such like and its a good product. Just not as strong/light as a layup.

Cross your fingers! I'm only completely certain of one thing- it will be far stronger than the wet-noodle boxes I've enjoyed on our previous RVs. Of course, that's a low bar.....

 

[Ludedude]

Great build! Thanks for documenting and sharing your ideas with us. I can't wait to see the finished product

 

[DDW]

You may have already considered this, but one thing to watch out for on fiberglass extrusions: fiberglass is strong, but isn't very stiff. About the same stiffness as wood, considerably less than aluminum (about 1/5), and far less than steel (about 1/15). So a 2x2x0.125 fiberglass tube will be somewhat more flexible (about 2.5x) than a wooden 2x2 (which isn't hollow). If you are using it in floor joists or where stiffness is the controlling factor, you may want to do some calculations or experiments to make sure the section is large enough to do what you expect.

 

[NeverEnough]

You may have already considered this, but one thing to watch out for on fiberglass extrusions: fiberglass is strong, but isn't very stiff. About the same stiffness as wood, considerably less than aluminum (about 1/5), and far less than steel (about 1/15). So a 2x2x0.125 fiberglass tube will be somewhat more flexible (about 2.5x) than a wooden 2x2 (which isn't hollow). If you are using it in floor joists or where stiffness is the controlling factor, you may want to do some calculations or experiments to make sure the section is large enough to do what you expect.

The appropriate flexural modulus has been used where it matters. I could've used less material (i.e. smaller/thinner dimensions) for the required strength, but other design considerations make it attractive to incur the extra weight associated with larger pultrusions.

 

[ersatzknarf]

Hi NE,

Thanks ! :sombrero:

Glad to hear you went the non-metal route.

May I ask about the outside "corner caps" ? Are there composite pieces available for that application or is there another method planned ?

I'm using Strongwell's Extren structural pultrusions, all in stock configurations: equal leg angle, channel, and tube.

<snip>

I debated long and hard about aluminum vs. Extren, but ultimately decided to spend the extra $$$ in order to eliminate the use of metal as much as possible, have a more compatible thermal expansion pairing, and better substrate pairing for bonding.

I did some corner joint tests early on using the 4x4x.25 Extren as an outside corner and the 2x2x.25 Extren as an inside corner and the panel failed before the corner joint. It would be plenty strong, and it's still the fall-back plan if we run into trouble.

<snip>