Tech Today: Mobsteel Suicide Slab Gets Audio Makeover, Part 2

WORDS BY JOEY KNAPP

During the first part of this Lincoln build log, I was in Lake City, Florida at my shop, Pinnacle Autosound. I designed and fabricated grille inserts for door panels that Bing Xu had shipped to me from Simplicity in Sound in Milpitas, California. We left off with the panels being packaged and shipped back to Simplicity in Sound. The second part in this series finds me in Milpitas, at Simplicity in Sound, ready to pull back the cover on this big old slab.

I had a meeting planned with the client for my first full day in California. He had some specific requests on space that would be available in the trunk. He planned to use the car for a bit of travel and wanted to make sure some luggage would fit along with the audio gear. I had stopped by Simplicity in Sound the night I got in to California to plan what I thought would be a good arrangement of the six amplifiers and four subwoofers. The meeting with the client went well. He liked the idea I had come up with, so the fun began!

Installing the Subwoofers

Upon taking apart the pieces from the old audio installation, I learned the Mobsteel guys had built a one-inch square tubing frame into the bottom of the trunk floor. This worked to my favor, because

it gave me a flat and level surface to build from. This subfloor would serve as a structure to secure my first project into the trunk—the subwoofer enclosure.

The floor not only had a nice metal structure, but it also had a deep center area that had been added for the previous subwoofer enclosure. This gave me plenty of room to get the airspace I would need, and it also left room for some layers of trim. I was happy to learn that the Focal Utopia M 10-inch subwoofers did not require much airspace. That meant I could have a bit extra space for cosmetic enhancement.

Starting from the face of the enclosure, I planned a ½-inch layer with some lighting along the perimeter, a ¼-inch layer to offer a smaller opening to cover the lighting, a ¾-inch layer that would be white—for a bit of pop—and a ¾-inch final layer to serve as the lip for the grille to sit on when the floor trim was added.

This 2 ¼-inch depth the box needed to sit into the floor was easily set by not mounting the enclosure to the floor, but to the metal subframe of the floor. In the first of many metal projects for this car, I made a pair of brackets to suspend the enclosure 2 ¼-inch below the surface of the trunk floor. I left the top baffle off the enclosure so it would make getting the enclosure in and out of the trunk easier, and it would also give me access to drilling through the sides for mounting the brackets.

Speaking of getting the enclosure in and out: This car is a back breaker! All access to the trunk is either from leaning over the side, or being in the trunk. I made sure to clean and tape up all of the painted surfaces around the trunk opening, because I knew I would be in constant contact with the paint.

The bracket design would use a combination of one-inch angle iron, ¾-inch angle iron and some flat bar stock. The bottom of the bracket was one-inch angle iron with the bottom lip extending under the enclosure to support it. The top of the bracket was ¾-inch angle iron that overlapped the one-inch square tubing of the subfloor.

I connected the angled pieces together with two vertical pieces of 1/8-inch x 2-inch flat bar and two angled pieces of 1/8-inch x 1-inch flat bar. These pieces were all welded up using the SiS Build Pro welding table to make sure everything was square. A quick test fit of the brackets and enclosures revealed that I needed to add some 1/8-inch spacers on each side to have everything fit snuggly.

With the spacers in place, I drilled the holes through the sides of the enclosure for the bolts that would secure the brackets to the box. Then I added three holes through the top angle iron into the one-inch square subframe. These would serve as the pilot holes for the rivet nuts I would add. I removed the enclosure and sized all the holes appropriately and then added the rivet nuts. Another trip inside the trunk to bolt it all together and I had my first little project complete.

The bracket was designed to hold the enclosure securely and level with the trunk floor.

The back sides of the baffles were treated with a layer of sound deadener.

The baffles were wrapped in black vinyl, so the parts visible through the grille would be attractive

Mounting the Amplifiers

Things were busy at Simplicity in Sound. Both Bing and Jesse had projects going on that required them to be in the woodshop. To avoid the table saw traffic jam, I moved my focus to making the metal amplifier mounting racks for the six amplifiers. My design had two amps on each side and two in the front of the trunk. The passenger side of the trunk had the electronics for the trunk lid motorization and the front of the trunk had two batteries, so the plan was for those areas to have four subwoofer amplifiers, while the driver’s side would house two amplifiers for the speakers.

Both of the side amplifier racks were fabricated from different width 1/8-inch flat bar stock. The mounting points for the amplifiers were drilled and tapped. The front amplifier rack was fabricated from ¾-inch square tubing and some two-inch flat stock. The tubing was used on this rack because it was a wider span so it needed additional strength to keep from sagging. The amplifier mounting points on this piece were added via rivet nuts, because the tubing metal is really too thin to drill and tap.

My final metalworking project was to make a simple rack to mount the Mosconi Aerospace DSP below the subfloor of the trunk. Placing the DSP in this location left plenty of room for wiring,

while making it easy to get to the DSP when the trunk was assembled. All of the metal mounting frames bolt to the car very solidly. All the mounting points are also easily accessible for future servicing.

The rear door was able to use a smaller baffle.

The grilles were designed to coordinate with the existing trim on the doors.

The grilles were composed of a layer of painted acrylic, a layer of polished aluminum and a mesh grille layer.

Door Panel Assembly

The box I shipped from Florida had finally arrived, so I began work on the door panel installation. I wanted easy access to the grille opening on the door, which is part of the reason why I didn’t install the grilles permanently in the panels in Florida. I knew I needed to make baffles in the doors to hold the speakers, and leaving the openings empty would allow me to precisely place the speakers to make sure both the tweeter and the woofer were centered on the grille opening.

The front door had quite a bit of metal cutout, along with the normal factory openings in the inner skin. It was so open that I decided to create a baffle to cover most of the door. This would give the speaker a very solid mounting foundation as well as help to seal the door and add mass to it. I added a number of mounting holes along the perimeter of the baffle and transferred those holes to the door for the addition of rivet nuts.

Once Jesse Lucero had finished running the wires into the front doors, it was time to address the sound proofing and then put them together. The outer door skin had been previously sound deadened, so I focused my attention to the inner layer. I covered the small remaining holes with deadener and then added strips around the perimeter of the baffle to seal it to the door. A layer of deadener added to the back of the baffle helped seal it further.

Jesse had the back doors wired up at this point, so I moved on to those. The metal on the back doors had fewer openings than the front doors, so the baffles could be much smaller. The back baffles were just big enough to hold the woofer and the tweeter. The rear doors were sound deadened as well, and the bolted- in baffles were also sealed to the door with strips of sound deadener. I installed all the speakers in the proper locations and now could finally finish the assembly of the door panels.

Everything had arrived safely from Florida. I got all the pieces separated and then began the assembly of the grilles.

The grilles would be held on the door by a combination of three things. The metal mesh is partially held on by the magnets I installed in the acrylic pocket. The grille assemblies stay aligned with 1/8-inch acrylic rod segments that I installed into the top, painted black, with trim rings.

The rod segments line up with holes that extend through each layer of the grille and into the acrylic pocket. The tolerances between the black top layer and the aluminum layer are fairly precise. I used the pins to ensure the layers wouldn’t shift when gluing them together. I wanted to make sure the lines stayed clean and true.

This brings me to the final thing used to hold the grilles together: E6000 glue. I used E600 on the layers to hold them together. I like E6000 because it adheres well to a wide variety of surfaces, and it remains flexible. I let the glue set overnight, and then finally got to pop on the door panels.

Completing the Trunk The doors were done. Jesse had installed the radio and run the wiring back, so now that the front of the car was completed, it was time to finish the trunk. Since it would be the easiest part to get out of the way, I started by fabricating the cover for the front amps. I had spaced these amps further apart than the side amps, to leave room for inserting one of the larger star logos I made. I always take into consideration venting the trunk to the cabin, so I planned for the sides of the piece to have two large grilles.

The curve on the top lip of the trunk had an angle that didn’t match any of the Mobile Solutions curves SiS had, so I had to go old school when I made it. I traced the curve on the piece of wood and then used a thin strip of wood taped to the piece to form the curve. Using a piece of wood like this, secured at each end, provides a smooth shape. I went back and taped down a few extra pieces to keep the curve secure while I used one of our 12vTools ¼-inch spiral flush trim bits to cut the shape. With the top curve cut, I put the trim piece in place to mark the amplifier location.

The front of the trunk opening had a trim piece that normally would have hinged for the folding top storage. This piece was now just bolted in, and removing it gave me easy access to the backside of the amplifiers for marking their location on the trim board. An added bonus: The batteries were much more accessible with that piece off, too.

With the amplifier locations marked on the trim panel, I used a Mosconi Pro template to cut the amp holes. The final cutouts on this piece were for the grilles. The grille shape was just a simple shape that fit the area. I rabbeted the back side of it to make a recess for the grille to fit into. The front side was chamfered slightly to make upholstery easier and to also soften the shape.

The final task to finish off this trim piece was to devise a way for it to mount to the car. I used a combination of magnets and bolts to attach the piece. I added two spacers in the center to stabilize it and allow it to magnetically secure to the metal amp rack behind it. I had originally attached two magnets to each spacer, but after some test fitting, I determined one magnet on each would provide a very secure mount that was more easily removable. To secure the ends, I added threaded inserts, so bolts could go through a factory metal brace and tighten into the trim piece.

Next I moved on to the trim pieces for the amps on the sides of the trunk. Because the passenger side panel had the trunk hydraulics mounted in it, the amplifiers had to be shifted more toward the center of the trunk. So that both sides were symmetrical, I mounted the driver’s side the same distance outward. This offset posed a bit of a challenge when it came time to design the side trim panels.

Normally, I would have made a trim piece that flush-mounted the amplifiers. This would have required a thick trim piece protruding into the trunk. It would look a little awkward. I came up with a plan that would allow the top half of the trim piece to be flush with the trunk opening. The bottom half of the trim would angle down from the top to flush-mount the bottom half of the amplifiers. To add an interesting element to the lower trim panel, I designed them with a recessed area and decided to add grille mesh. Once upholstered, I planned on pressing the vinyl into the pockets of the mesh, to somewhat mimic the style of the perforated metal on the amplifiers. The lower portion of the trim piece was designed to attach with magnets.

I also planned on the mounting bolts for the upper trim piece to be hidden behind the lower trim piece. Any future wiring servicing would be easily addressed by just pulling off the trim piece. If the area behind the amplifiers needed to be accessed, it would just be a matter of unbolting the top trim piece, then unbolting the amplifier mounting rack.

The floor was the next step. Since the enclosure mounting had been finalized, the top could be installed. I had Jesse cut out the speaker holes and then glue and nail on the top baffle. This gave me the bottom layer of the stack of pieces that would trim out the space between the enclosure and the top of the floor.

I cut out the remaining layers and selectively chamfered some of the inner edges. The very top layer, which is the new floor of the trunk, was made from ½-inch finish plywood, and it was secured in place by centering dowels and magnets. The two side floor pieces that flank the center were created by trimming cardboard pieces to make a template and then copying that template to ½ finish plywood.

These pieces would be secured by friction and magnets. While I worked on the rear side trim panels, Jesse was building the center trim panel. These panels would help make the transition from the floor to the back of the trunk and help hide some of the plumbing for the hydraulic trunk cylinders.

The thin strip of wood was bent, secured, and reinforced in preparation for routering.

The final trim pieces completed the remainder of the floor.

The tool I made helped to press the vinyl into the mesh holes.

The completed trim piece was ready for installation.

Metal mesh was glued to the trim to serve as the template for the embossing.

Upholstery Pieces-

I timed my work well, and had one more day left in California. I spent this day adding the star logos in the top of the enclosure and middle of the front amp rack and then working on upholstering all the pieces. I also primed and painted all of the metal parts I had fabricated. One of the most time consuming parts of this process was pressing in all the holes in the grille material for the lower side amplifier covers.

To help make that easier, I ended up taking a Harbor Freight pick tool and cutting off the point on it. I then rounded the end, and sanded it smooth. This gave me a tool I could use to push the vinyl into the individual pockets. It wasn’t overly fun, but the result was pretty cool and something you don’t often see.

I left California with the car in good hands. Jesse and Bing would finish the wiring and final assembly of the trunk. Then, after a tuning session with our good friend Mike, the car would be ready for return to the client.

There were many parts of this project that stretched me and pushed my growth a bit. I am slowly starting to mold the way I approach designing builds with the CNC router capabilities in mind. I have become more comfortable with its use and the mechanics of its operation, so I will continue to incorporate more and more of it into my work. Finally, I enjoyed the collaboration between Pinnacle Autosound and Simplicity in Sound, and I look forward to more collaborations in the future!