Insulation

 

Insulation

Summary

This is a long post so here's a short summary of what I used to insulate:

• Primarily 1.5" Extruded Polystyrene (XPS) Foam Board (R value - 7.5) in the walls
• 1.5" XPS under the subfloor
• Tecvan "Duck Liner" 3/8" foil-backed non-combustible Neoprene foam sheet (R 0.89) layered with 3M Thinsulate SM600L (R 5.2) on the endcaps
• 1/16" thick "high density" Neoprene self adhesive foam tape applied to the ribs and crossmembers before the interior skin is attached

I used about 8 sheets of 4'x8' XPS on the interior walls. Getting the 1.5" thickness required a 90 mile drive (the 1.5" thickness isn't available at my local Home Depot) so I was very careful to layout the cuts to maximize yield and that took a lot of extra time. There was very little scrap left over so 8 sheets was the absolute minimum required.

I used about 80 square feet of the Duck Liner and Thinsulate on the endcaps. Duck Liner comes in 32.5'x3.25" (105 sqft) rolls and is available online from several retailers. Thinsulate is sold by lots of people online and typically comes in 5' wide rolls of standard lengths. I bought a 20' roll (100 sqft) and ended up with about 4' (20 sqft) left over.

Main Insulation

I didn't deliberate too much on the insulation material I planned to use. I'd done all the research when I renovated the '48 Boles Aero model 14 I own. I did check to see if there were any new options and there really weren't. So I planned to go with the same 1.5" thick extruded polystyrene foam board (XPS) I'd used on the Boles.

XPS is a closed-cell foam that barely absorbs water. Of all the major insulation types it is the least absorbent. Of the readily commercially available products is a close second to polyisocyanurate (Polyiso) foam board for insulation properties, but polyiso absorbs water a bit more readily than XPS. The price and workability of XPS and polyiso are very similar. Both are readily available in a variety of thicknesses at big box home stores. 

The biggest problem with foam board insulation in the Clipper is that foam board is very rigid: it doesn't want to bend and the Clipper is all curvy. Fortunately the curves are not compound so woodworking techniques for bending wood, principally "kerfing" can be used. Kerfing is just cutting deep parallel cuts in the material so that it can bend easier. The more cuts and the closer they are spaced the more the material will bend.

I found that the tightest curve I needed to insulate was the wall to roof transition area. It's something like a 48" radius (+/-, I'm not precisely sure). I used some scrap material to experiment and found that parallel kerfs spaced 2" apart with a depth of 1 1/8" worked really well to fit the curve when I bent the material with the kerf towards the inside. If you bend the material with the kerfs towards the outside it can split along the kerf, and even if it doesn't split you end up with a lot more volume that is uninsulated.

Kerfed XPS Foam Board 

Why 1 1/8" kerf cut depth? Well you'll see from the discussion below on insulating the endcaps (where I didn't use foamboard) that there's some technical rationale for choosing an insulation thickness between the exterior and the interior of at least 1/4". Basically it will help prevent condensation inside the trailer. I figured 3/8" of insulation gave me some margin for error.

Another problem associated with rigid foamboard is that you can't simply measure the space between the ribs and cross members and then cut a piece to fit. The ribs and cross members are aluminum formed into ~1.5" C-channel so the space inside the C is less than 1.5". And the foamboard doesn't compress so you can just wedge it in.

The solution is to relief cut a bit of material from the edges of the foamboard to create a "tenon" that can slide into the C-channel. It's really pretty easy but tedious.

Relief Cut "Tenon" in XPS Foam Board

The bigger problem associate with the C-channel is that depending on how it's oriented (which way the opening of the C is pointing) it requires from 0-4 tenons, and worse the piece can be "trapped" so that you can't slide it in.

For all but the simplest C-channel orientation the solution is to cut the board into smaller pieces so they can be "puzzled" into place. The number of pieces required and solution to the puzzle is related to the orientation of the C-channel. I found the worst case to be 5 pieces:

So how do you run electrical through the foamboard? The simple answer is to cut a channel with a hot knife designed for use with foamboard. There are a lot of different models available online but based on the reviews I chose one with a built-in fan that allows the knife to be used for extended periods. Originally I thought I might use the knife to do all the foamboard cutting but it became apparent that the table saw was the faster and more precise way to go. It's somewhat messy (all that foam "sawdust" sticks to everything) but I have decent a dust collection system so it wasn't too bad.

Channel cut in XPS foamboard for electrical cabling

Subfloor

I also used 1.5" XPS to insulate the subfloor where I was able. I did this while I was installing the subfloor on the frame. Here's the process I used:

• I cut out the subfloor and placed it on the frame. 
• Then crawled under the frame and used a Sharpie to trace the outline of the frame onto the subfloor pieces.
• I pulled up the subfloor and using the Sharpie-drawn outlines I cut XPS pieces to fit between the frame members.
• I attached the pieces using 3M 78 foamboard spray adhesive and stainless steel "foam board mounting gaskets" attached to the subfloor with stainless steel screws.

XPS Insulation Applied to Underside of Subfloor

Some installation notes: 1) the 3M 78 spray is costly and doesn't add much so I wouldn't do it again; 2) I used tongue & groove subfloor so installation required me to rotate the pieces down as I installed them. This required some relief cuts along the edges of the foamboard; 3) there is no foamboard where the fresh and gray water tanks are installed because it would take up too much vertical space; 4) there is no foamboard in the area where the shower sump is installed.

Endcaps

The endcaps presented a different challenge. I had no idea if there would be a constant 1.5" gap between the inner cap and the outer skin, in fact I doubted it. So using the rigid foam board might be problematic. So I chose a different path.

I decided to use a thin layer of closed cell neoprene sheet insulation adhered to the outer skin coupled with a layer of traditional batting insulation. The closed cell sheet insulation will, in theory prevent condensation against the outer skin and the batting insulation will provide additional insulation while also being able to adapt to a variable gap between the inner and outer skin. This is a technique many van conversion experts advocate and it made sense to me as a way to address my installation.

To prevent condensation the "dew line" needs to fall inside the closed cell sheet insulation so that no moisture can build in the batting insulation. There's a pretty good article that covers this at Faroutride.com. What they don't cover is how thick the insulation needs to be to ensure the dew line falls inside the closed cell insulation.

To find out thick the closed cell insulation needs to be I used a "Professional Insulation Thickness Calculator" provided by the Armacell insulation company. It's called Armawin and can be found at Armawin.

The tool is designed for use to determine insulation thickness needed to prevent condensation on cold pipes in an interior location, but the problem is the same as a cold exterior wall exposed to a higher-than-ambient relative humidity found inside a trailer (or van). There are a lot of variables you can plug in depending on your use case, cold temp, hot temp, relative humidity, material in use, insulation material, etc. 

I chose to evaluate external temperatures of 32F, internal temp of 75F and an internal relative humidity of 50%. I thought this would reflect my most likely and concerning likely use case: cooler spring and fall weather. Colder temps and higher relative humidity will increase the minimum thickness necessary.

In any case the tool indicated that when using a typical closed cell foam sheet neoprene insulation the required minimum thickness is 0.14" (3.6mm). I made multiple runs with different conditions and decided that the minimum thickness I'd consider was 0.25".

Sample Output from Armawin Tool

I ended up using a product from Tecvan they call "Duckliner". It's a 3/8" foil-backed neoprene closed cell insulation with an adhesive on one side (you peel off paper backing to reveal the adhesive). Neoprene foam sheet tears easily but the foil backing improves the handling qualities significantly. I suppose in some applications the foil backing might provide some radiant barrier properties but sandwiched between two sheets of aluminum in the Clipper walls it would be imperceptible.

As far as the batting insulation goes I read a lot of articles with pros and cons of all the different types that are readily available. They all have the downside that even if they are technically "waterproof" they can still trap moisture between the fibers. In the end I chose 3M Thinsulate SM600L insulation. It's widely used in the automotive and outdoor garment industry. From my perspective for a bat-type insulation the only only real negative I read was price - it's more expensive than most of the alternatives.

Installation was quite a process. First I made templates for the exterior roof segments. You only need one half of one end because they are replicated.

Template for Neoprene Sheet Insulation

A Bunch of Templates

Using the templates I'd made, I cut out the Duck Liner using an utility knife. Even though the adhesive is protected by paper it is REALLY sticky.

Using a Template to cut the Duck Liner

After the Duckliner was cut out I stuck it piece by piece onto the Clipper endcap ceiling. Some people recommended using 3M 90 adhesive spray on the ceiling but I did some tests that convinced me it wasn't necessary. That adhesive is really sticky.

Duck Liner Segment installed on Clipper Endcap

From there you just "rinse and repeat" until all the endcap segments are installed. Of course there was a lot of trimming as I went along because the templates weren't perfect and repeated transferring a trace and then cutting the trace leads to oversized pieces. Additionally the neoprene foam stretches a bit as you put it in place. But it wasn't too bad.

Duck Liner Fully Installed

After the Duck Liner was installed I set to work on the Thinsulate layer. In an attempt to make things go faster I made new templates that combined a few of the segment templates I'd made and used with the Duck Liner. I recommend getting some easy removal doublestick tape and testing the templates against the inside of the endcap. I didn't do this initially and they proved to be a little too large (which required additional trimming).

I laid out and traced the template onto the Thinsulate with a Sharpie, I oriented the black "scrim" side down mostly so the sharpie line would show. I could have also used something like a soap stone to mark on the black scrim side. Note that there is also a slightly more expensive "double scrim" version of Thinsulate that would be black on both sides.

Tracing the Pattern onto the Thinsulate

The Thinsulate can be cut with a sharp pair of reasonable quality scissors. For kicks I also tried using a fresh utility knife and it was hopeless.

After cutting out the Thinsulate, I sprayed the non-scrim (white) side of the insulation with 3M 90 adhesive spray and also the corresponding area inside area of the trailer. When they dry to a tacky feel (really quick in warm condition) you can carefully place the Thinsulate into place. It worked best for me to start at the highest corner and then work down the seam and then across. This allows gravity to keep the two glued surfaces separated until you press them into place.

It's important to note that you'll need to flip the template to trace the insulation for the opposite panel.

Thinsulate Installed on Half of the Inner Endcap

I've read that the 3M 90 adhesive may begin to fail if you leave the Thinsulate hanging "too long" on horizontal surfaces without some support. Some sources recommend that you be prepared to install the inside walls/skin shortly (days) after installing any hanging Thinsulate to prevent it from falling down. 

In light of this I delayed installing the Thinsulate until I'd removed the paint from the inner endcaps and then polished them (I'm tentatively planning to leave the inner skin bare) so that were ready to reinstall. Like everything else on this project, preparing the endcaps turned into a tedious, dirty and time consuming process so I'm glad I waited for this to be complete to install the Thinsulate.

Cleaned and Polished Inner Endcap Reinstalled

Rib to Skin Thermal Barrier

The Clipper originally had a thin felt-like material glued to the ribs and crossmembers where they contact the inner skin. I assume this was used to create a thermal barrier to slow thermal conduction from the outside to the inside (or vice versa when it's cold). This heat conduction is a problem for metal framed trailers as metal (and aluminum in particular) is a great heat conductor. If you touch the ribs when the sun is hitting the outside walls you might get burned. If you don't retard this heat transfer then it will simply conduct straight through to the inner walls and all the insulation will be a waste. So somehow you want to break the metal-to-metal contact to create a thermal barrier.

I spent a lot of time trying to figure out what Airstream currently uses as a thermal barrier in the new builds. There are a lot of forum threads on the subject but I could find no definitive information. So I found out what other people use, why they used it, how it worked out and then I selected a path.

In the end it seemed like a high density neoprene or EPDM foam tape would be the best bet. It comes in different thicknesses and thicker is better as a thermal barrier, but thicker foam would also lead to dimpling at the connection points. In the end I chose 1/16" thick tape. There are a lots of different material choices available for foam tape but not all foam is created equal. For example polyethylene (PE) foam has good thermal properties and is inexpensive but breaks down relatively quickly over time and has reduced thermal range when compared to neoprene.

Neoprene Tape Applied to Ribs Prior to Inner Endcap Installation

Neoprene Foam Tape

Bumper (or Brush Guard?)

 I haven't posted in a long while and I've accomplished a lot that I need to go back and cover, but I just finished this and I wanted to document it as reference for me in the future.

Installed "Bumper"

One of the prior owners (PO) of the Clipper removed the "bumper" and associated interior and exterior mounting brackets. These items were not provided in loose collection of parts that were strapped into the Clipper when I drove it home, so I needed to fabricate replacements.

I call it a "bumper" but it's really more of a brush guard. It's really not very sturdy and it's only connected to the frame at the rear center tube. Each forward end of the bumper is bolted through the skin onto a bracket that is bolted through the subfloor (originally plywood and in my Clipper Advantech OSB). And the midpoint mounts between the end and the rear center frame mount were just bolted through the skin. The design could lead to significant structural and body damage under any significant collision. So my conclusion is that the bumper is really just there to keep people or campground brush from rubbing the trailer skin (e.g. when backing into a campsite).

Interior Bumper L Bracket - Facebook Group

Flimsy Bumper - Vintage Trailer Boneyard

Based on many online images and some more detailed images and measurements from member of the Facebook Silver Streak Clipper group I set out to fabricate and install the bumper. This needed to be completed before the insulation could be installed because the interior mounting brackets actually reside inside the rear wall cavity.

I repaired a small damaged area with a patch curbside where the forward part of the bumper attaches.

Damaged Forward Curbside Bumper Attachment Point

I then fabricated some simple L brackets from 1/8" steel I had on hand, basically welding 2"x4" coupons into a L bracket. I drilled 1/4" holes (for 1/4"-20 bolts) and welded on 1/4"-20 nuts to receive bolts to mount the bumper from outside. This means the bumper can be removed for future service if necessary. I read a couple accounts that said that bolts were originally mounted from inside the walls to the outside rather than vice-versa, but that meant you needed to remove the interior panels to replace any damaged or rusted mounting bolts (or potentially bumper) in the future. I thought inserting them from outside in made a lot more sense.

Note that I also needed to drill a hole for the marker light wire to feed through since I'd already routed those through the subfloor. I also cut a rubber grommet to place between the bracket and the aluminum skin to provide a seal and protection against galvanic corrosion.

Fabricated Forward Bumper Attachment Bracket - Streetside

Fixing the brackets to the subfloor proved to be a challenge since at this point in the build process I didn't have access to the underside of the subfloor to attach nuts and washers (or better yet a backing plate below the subfloor). There is limited space behind the subfloor so the choices for blind bolt fasteners is limited. In the end I decided to use toggle bolts (Toggler Snaptoggle Anchor Bolts). I checked the product specs and they have decent retention strength, probably stronger than the subfloor itself.

Toggle Bolt Install - Curbside

As I mentioned previously, the midspan attachment appears to have been bolted directly to the skin with fender washers on either side of the skin. The skin was reinforced/doubled with an extra sheet of aluminum, but there was no bracket attachment to the subfloor. I'd seen vintage pictures of people standing on the bumper and even using the bumper as a jack point (yikes!) I thought it would be prudent to try to provide some added vertical support so I made some brackets similar to the ones described above.

There is virtually no clear space under the subfloor where these brackets would exist so I couldn't use the toggle bolts I used on the forward brackets (they need 1 7/8" clear space to insert). After considering the alternatives I ended up with a poor solution, but one that is better than nothing at all. I used wood insert nuts. These have relatively low retention strength in plywood and OSB (much better in solid wood). I watched several tests on YouTube where they managed ~300lb of lateral pull force and up to 1000lb of vertical load which isn't much, but any vertical load will be spread across the bumper mount points so they should provide some function.

Midspan Bumper Attachment Bracket Install

Note that I inadvertently cross-threaded one of the wood insert nuts so I had to drill out the hole and use a larger one (5/16") with associated larger mounting bolt. Hopefully no one ever has to examine that detail in the future!

On the exterior the midspan attachment points use a metal standoff to hold the bumper away from the frame. I used ~2" wide 1" flange C-channel cut to make two standoffs that are 2" wide. Base on pictures I believe the C-channel is what the factory used for standoffs but I don't know about the size or thickness. I decided to use 2" because it was close to the rear frame tube standoff distance and I thought it would work with the arch/bend of the bumper.

Midspan C-Channel Standoff as installed

As I mentioned previous I am nervous about any significant loads on the bumper. Just having one may invite people the stand on it or use it as a lift point, in fact I noticed that the Airstream "brethren" of the Clipper has no bumper. Walley Bynum, the designer of both the Clipper and early similar Airstream, seemed to improve the design as he went along and this might be one design feature he later deleted in Airstreams as an improvement (or maybe less problematic). 

In any case my wife Becky made some prominent "NO STEP" warning labels for the midspan standoffs.

The bumper itself was actually pretty straight forward. I purchased two 6' sections of 2" wide 3/16" mild steel bar and welded them together to create a 12' section. I had a welding shop bend the bar into a half hoop with diameter of 84". Originally I thought I might bend it myself manually but in the end I decided the welding shop would do a better job (although more costly).

I measured and drilled holes for the rear frame attachment and bolted on the half hoop bumper. I then measured to the midspan attachment, drilled holes and bolted on the bumper. Then I performed a similar operation on the forward attachment points. There is a standoff at the forward attachment point that is formed by two nuts used as spacer.

Bumper Attachment at Rear Frame

Forward Bumper Attachment (curbside) looking down from above

Each of the forward attachment bolts is basically a ~2" all-thread stud with two jam nuts that function both a standoff spacer and allow the stud to be screwed in or removed. On either side of the jam nuts are fender washers. There is a neoprene washer between the skin and fender washer that both seals the hole and prevents galvanic corrosion. The bumper is held in place with a washer, split lock washer and acorn nut. The acorn nut provides some protection from scaping against the end of the stud as well as being more decorative than a typical hex nut, but it does require more attention to installed dimensions since the acorn nuts have limited depth.

To finish the bumper, standoffs and interior mounting brackets I machined off the millscale, sanded, primed with self-etching primer, dry sanded, and sprayed on finish coats. The finish coats for the interior mounting brackets is some black Rustoleum hammer tone spray I had on hand. The finish on the bumper and midspan standoffs is a Rustoleum automotive gloss black enamel (3 coats).

All the bolts, nuts and washers (with the exception of the one 5/16" midspan bracket fastener mentioned above) are 1/4"-20 stainless steel. The wood insert nuts are brass so there is some chance of galvanic corrosion, however the blue Loctite thread locker I used should provide some additional protection.

A future upgrade I'm contemplating is to modify the forward mounting studs so that they more readily shear upon any impact. As discussed above any significant impact likely to damage the trailer body and subfloor. If these forward attachments would shear off easier it might provide some damage prevention while still providing the function of a brush guard. Making the stud shear more easily could be as simple as carving a notch around the shaft of the stud where the fender washer meets the bumper rail. I've read that this technique is used for devices like snow blowers and snow mobiles where high torque parts might suddenly transition from snow to hard ice.

Rivets

 All about rivets

Well not "ALL" about rivets but the rivets used on the Clipper. Here's a reference for most types of rivets.

Solid Rivets

I've found that the majority of the solid rivets used on the Clipper are of the type AN455AD-4-X. 

"AN455" is a specification for the size and shape of the rivet head and is commonly referred to as a "Brazier Head". For any give diameter of the rivet, the Brazier Head is slightly wider than a AN470 "Universal Head" rivet. This wider head helps distribute the load on the soft aluminum.

"AD" refers to the type of the material used. AD means 2117-T4 aluminum. These are sometimes referred to as "structural strength" rivets. They are made with 2117-T4 are identified by the a small visible dimple in the center of the head of the rivet. 

"A" type rivets (ie AN455A-4-X) are made with 1100 aluminum which is nearly pure and relatively soft. "A" type rivets have no dimple, the head is visibly smooth. An important physical property of rivets is "sheer strength"; AD rivets made from 2117-T4 have rough three times the sheer strength of A rivets made from 1100 aluminum.

Another item of note for "A" vs "AD" rivets is that "A" rivets used with 2024 aluminum can apparently lead to galvanic corrosion and some aircraft manufacturers (e.g. Boeing) recommend against using "A" rivets with 2024 aluminum (2117 aluminum rivets are fine). Looking at a galvanic series chart it appears that aluminum alloys have a broad range of potentials and this leads to the possibility of corrosion, but it also appears significantly less than other problem areas that we encounter in the vintage trailer world (e.g. steel vs aluminum).

"-4" is the diameter of the shank in multiple of 1/32", so a "-4" means 4/32" or 1/8".

"-X" is the length of the rivet shaft. This is dependent upon the thickness of the material stack you are trying to fasten together. The rule of thumb is that the length of the rivet should be the thickness of the material + 1.5 times the diameter of the rivet shaft (ie material thickness + 3/16" for a -4 rivet). This added length becomes the "work head" (the part of the rivet that gets smashed/mushroomed) when the rivet is installed. You can buy rivets of a standard length and cut them to the correct size before using them with a special tool.

When repairing the Clipper I'd like to use solid rivets of the type AN456AD-5 (note the 456 vs 455 and the -5 vs -4). These are known as "Modified Brazier Head" rivets. Typically when you remove and replace a solid rivet you need to drill out the hole to the next size up, in our case I'll be going from -4 (1/8") to -5 (5/32") diameter rivets. The function of the "modified" head is that a AN456-5 head is roughly the same size as AN455-4 head so that when you replace the rivet with the larger size the head will appear the same (ie you won't notice the newly replaced rivets). 

HOWEVER AN456AD rivets are hard to find so you're stuck using either AN456A (ie softer, non structural) rivets or AN470AD-5 structural universal rivets. The head size is roughly the same and it's difficult for the untrained eye to tell the difference (the head has a slightly taller crown). In my opinion its easier to spot where a repair has been done with 456 "A" rivets than 470 "AD" rivets because you typically view them head-on (the crown height is not apparent) and the "A" rivets won't have a dimple and that's fairly obvious.

Blind ("Pop") Rivets

Blind, or "pop" rivets are what most people think of when someone talks about using rivets (either that or images of 1920's hot riveting skyscraper ironwork). The are called "blind" because they are inserted from one side, hence the user doesn't need access or is "blind" to the other side. They are commonly called "pop" rivets because most are set by pulling a mandrel that separates from the rivet with a "pop" action and/or noise.

There are a huge number of blind rivets available. Most have "open" ends when the mandrel pops out so they will let air and water pass through. This makes them less useful for exterior applications. However there are some types that are "sealed" or have "closed" ends.

Another type of blind rivet that is popular for aluminum trailer repair is known as an "Olympic style" or shaved head rivet. These rivets work like a typical blind rivet but the mandrel breaks off proud of the head. The mandrel can then be "shaved" down to resemble a solid rivet head. This allows panels to be repaired or replaced with ready access to the back side of the panel.

Blinds rivets can fasten items very securely but they are not considered "structural".

Tubular Rivets

Tubular rivets look a lot like solid rivets but the end is hollowed out. They are set using a pinching mechanism (vs rivet gun) with a die (aka "set") on both ends vs a set on one end with a buck on the "work end". Tubular rivets are not structural but they do fasten metal pieces together well and can be set very thin. They can also be used to form a pivot point in light duty mechanical parts.

For the Clipper the tubular rivets are used in the Hehr "Standard" windows. They are used to fasten the frame together and also as a hinge pin in the window "link levers" that are used to open and prop open the windows.

"Tinners" Rivets

The wheel wells are constructed with galvanized steel not aluminum. The steel is galvanized, ie coated with zinc, to prevent corrosion of the steel. My research led me to the use of galvanized "Tinners" rivets to assemble the wheel wells.

A "Tinner" is a tinsmith. Tinsmiths  make and repair things made of light sheet metal, hence the name tinner's rivet. Tinners rivets come in a variety of materials and are sized based on the approximate weight of 1000 rivets. For example approximately 800 1.25 tinners rivets come in 1 lb of rivets and have a shank diameter that ranges slightly smaller than 1/8". These are known as 1.25 tinners rivets because 1000 would weigh about 1.25 lbs.

I used 1.25 tinners rivets from Hanson Rivets to fabricate the wheel wells. I actually purchased them through Grainger because it was cheapest. I used a flat set in my rivet gun and a standard bucking bar on the "work end". I found they install very much like typical solid rivets.

Subfloor Material

Advantech Subfloor

Just a short post to discuss my downselect of subfloor material to Advantech by Huber. I special ordered it via Home Depot for about $60/sheet (including CA fees and minimum order costs). This is actually much cheaper than my 2nd choice which was marine grade plywood (which I used in the Boles Aero), and I won't need to use penetrating epoxy as a sealer.

I was considering the following options:

1. Coosa Bluewater 26 - suuuppppeeerrr expensive composite that won't rot
2. Marine grade plywood - expensive but less prone to rot than normal plywood
3. Advantech or other engineered subfloor

After much deliberation I chose Advantech. It's an engineered subfloor specifically designed to withstand soaking moisture during the construction of a home. The first time I read about it I was repelled because OSB is notorious among the trailer restoration crowd and Advantech looks like typical OSB. But after some investigation I found that it's being adopted by vintage trailer restorers, particularly when they don't want to spend $300+ per sheet for Coosa board.

I had some initial difficulty finding it but contacted Huber Engineered Woods and they recommended that I special order through my local lumber store. It's not a normal stock item because local contractors won't spend the extra money to use it. Generally only large corporate builders use it because they recognize the cost tradeoffs associated with rework.

In any case it took about 3 weeks to get it delivered to the local Home Depot, but it's now in my garage waiting to be installed. It's the nicest and most uniform sheet good I've ever unloaded. Hopefully it lives up to it's promises.

Wheel Wells

Wheel Well Fit Check

The wheel wells need to be placed on the frame and under the subfloor to prevent water thrown up from the road from rotting the subfloor. I was excited about installing the subfloor when I re-remembered this step. So I stepped back and went about fabricating new wheel wells.

The wheel wells for my Clipper were missing (along with most of the interior walls, etc. Based on internet posts I believe the original wheel wells were constructed from simple plywood boxes. The only remnants of those boxes was the outer aluminum wall that is riveted to the inside of the exterior trailer skin.

Remnant of Original Wheel Well

I had decided a while ago that I'd fabricate some new wheel wells. I really didn't want to use plywood boxes that would rot out fairly quickly because replacing them would mean tearing down the entire trailer.

I considered several alternatives but ended up designing a simple wheel well and ordering the metal pieces from SendCutSend. I'd considered purchasing sheet metal and cutting it myself, but after pricing in shipping, handling and waste it was going to be about the same price.

I ended up using 0.036" G30 galvanized steel which falls somewhere between 22 and 20 gauge. That's pretty stout for wheel wells and "the internet" said it was a much better choice than using aluminum because it's stronger. The G30 designation indicates how much zinc is deposited for corrosion protection (G30 is on the low end). I would have preferred G90 grade which has more zinc deposited for rust prevention but it was not available when I ordered. Later I'll talk to some additional efforts I made to prevent corrosion.

Laser Cut Parts from SendCutSend

Each radius of the wheel well is made of 3 identical cut pieces. The sidewalls for each wheel well are 2 identical cut pieces (6 and 4 total for the 2 wheel wells). I've found that ordering a quantity of 4 or more from SendCutSend is the "knee in the curve" for pricing so when possible I make design choices that allow for part commonality. In this case the wheel well is slightly taller than required (3" above the max axle loading deflection per Dexter) so that I could use a common design for the radius pieces.

I used my 36" straight sheet metal bending brake to make the appropriate bends. Of course I misbent a couple bends on the first one and had to hammer them back (which is do-able with steel) and rebend it.

Parts Bent into Shape For Assembly

For assembly I wanted to make encourage corrosion prevention. I considered a few options but settled on using galvanized steel "Tinners" rivets. These are flat head rivets used by "Tinners", ie sheet metal fabricators. I'd never heard of them before but they are available from Hanson's rivets and their distributors. I could find very little on the internet about their use. The sizing is odd compared to other rivets: they are sized according to the weight of 1000 rivets. I used a 1 1/4 lb rivet which is just under 1/8" diameter (Hanson Rivet part number TRSL01.25) which gives about 800 rivets per pound.

Small number of 1 1/4lb (1.25) Tinners Rivets

Drilling and setting these rivets is almost identical to the aluminum ones except they have a flat head vs a "universal" or "Brazier" head. I used a flat rivet set on my rivet gun. The lack of a domed head to center the gun was a little challenging but not much of a problem - fortunately no one will be able to see the few occurrences where the gun skipped off the rivet head.

As is my practice with the aluminum skin panels, I used Sikaflex 221 between the panel seams. So the process is clamp; drill, cleco, drill, repeat; disassemble; spread Sikaflex 221; reassemble with clecos; rivet removing clecos as you go. The Sikaflex makes it a little messy where it squeezes out but it's easy to clean up.

Wheel Well Assembly (1 of 4)

Wheel Well Assembly (2 of 4)

Wheel Well Assembly (3 of 4)

Wheel Well Assembly (4 of 4)

Once assembly was complete I sealed the inside seams with a liberal application of Sikaflex 221. Then I sprayed the inside with Rustoleum Professional Grade Rubberized Undercoating Spray. It took a can for each wheel well to accomplish the 2 recommended coats (wear a respirator!) I chose the Rustoleum product based on some YouTube comparison videos. It's not as thick as I'd imagined and I'm a little concerned about durability. I was hoping for something similar to bed liner. 

I also sprayed the outside of one sidewall on each wheel well. The outside of the sidewall will sandwich up against the inside of the trailer's aluminum skin and be riveted to it (it replaces the aluminum box side in the image towards the top of this post). The zinc coating of the galvanized steel should prevent dissimilar metal corrosion between the wheel well and the trailer wall but I thought an insulating coat of rubber wouldn't hurt.

Wheel Wells Drying after Undercoating

One last note. One of the sidewalls of each wheel well is not flanged. That sidewall will be riveted to the inside wall of the trailer. I left the sidewall long and plan to trim it to the shape of the exterior wheel well after I wove the shell onto the new frame. I did trace the old wheel well onto the new sidewall, but I'm not 100% confident of the final fit of the shell on the trailer so I wanted to leave myself some wiggle room.

Dry fit of the new Wheel Wells

Overall I was very happy with the way this subproject worked out. It was fairly straight-forward, not too time consuming, and reasonably priced (about $250 for materials).

Outrigger Design

 

Finite Element Analysis of Perforated Outrigger at ~2.5x Nominal Load

I've been doing a little design work in a tool called FreeCAD. As the name implies it's a free computer aided design package that can be used to design and analyze solid objects. Originally I started using it because I needed to fabricate some Hehr Standard window link levers (the levers used to open windows) for my Clipper. I wanted to user a laser cutting service to make some stainless steel levers and they needed the information in a digital file format. FreeCAD (and many other programs) can generate this file and there was a large amount of training available on YouTube.

In any case I had some experience in FreeCAD before I started the trailer frame design process.

I did some rough paper sketches of the trailer frame I wanted to design. I then did a more refined drawing in PowerPoint (!). I then broke down the parts needed and did some iterations to modularize the design and create common components. I know from experience that the unit cost of parts drops pretty quickly if you order more than one so common components would significantly reduce my costs.

In the case of the outriggers I need 14 total all with the same basic "outer mold line". 4 are used to bookend the wheel wells. These need to be a "solid" design because they close off the belly of the trailer from the exterior. The other 10 can be solid or they can be perforated to save weight or serve as a pass-through for things like electrical conduit, plumbing or HVAC return air.

I made measurements of the original aluminum cross-members so that I could duplicate their profile. Using these measurements I created a sketch in FreeCAD.

FreeCAD 2D Sketch of Outrigger

From the sketch it's fairly easy to create a 3D solid model in FreeCAD. And when you have a 3D solid model you can use FreeCAD's Finite Element Analysis workbench to analyze the design. It's easy (but a little tedious) to change materials or material thickness to determine how they might respond to different "loads", ie the amount of force being applied.

I focused on using mild steel (vs something like aluminum) because it's relatively cheap, easy to weld and performs well in the real world. I iterated some material thicknesses with very demanding loads (up to 2.5x max design load applied to the outside end of the outrigger) to arrive at a conservative material selection.

FEM of Solid Web Outrigger for Wheelwells

Based on the design of the solid outrigger I then moved on to create what I'm calling a "perforated" outrigger, ie one with holes in it to remove excess weight. Weight is a key concern since I'm transitioning from a super lightweight frame design to a more study, but heavier design in steel.

I ended up iterating the basic design 3 times. First was just an oblong hole, next was a series of 3 holes, then the final design is simply a large cutaway. The outrigger is actually designed in 3 parts: a top flange; the center "web"; and the bottom flange. I ran FEM on just the web during the iterative process as I was really primarily interested in the stress and deformation at the outer end of the outrigger under load.

Initial Iteration of Perforated Outrigger

Iteration 2 of Perforated Outrigger

Iteration 3 of Perforated Outrigger - ~2.5x Max Load at Outside

After settling on a web perforation design I then moved on to model the entire outrigger with both top and bottom flanges.

Based on the analysis I semi-finalized the design and have ordered a set of laser cut parts to fabricate 4 perforated outriggers. Why not all 10? Well I'm using a "tab and slot" design approach that I've never used before so I thought it would make sense to do a "preproduction" run before I spent an even bigger hunk of money.

I'll follow up with a post on the tab and slot design approach as applied to the outriggers.

Laser Cut Templates Uploaded in SendCutSend.com