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.