Here is a quick look at our process for building a custom composite enclosure for a BMW 5-Series sedan. In this installation, we have made the most from wasted space, while still maintaining the original functionality of the car. The last part is very important to our process. What we do needs to sound good, and look good, but that is not enough. It still needs to be serviced by mechanics, and our enclosure covers the access to the car’s battery. When completed, just like the original trim, our enclosure is hinged at the bottom to allow full access to the battery and other electronics in the rear fender. The subwoofer never needs to be disconnected or removed to do it.

Let’s take a look at how we do it:

This is the passenger side of the trunk, as it came from BMW. This space is not particularly useful for storage, but will make a perfect enclosure for our Focal subwoofer.

We could have wasted a lot of time recreating this exact shape out of wood, plastics or fiberglass, but the original panel already fits perfectly. We do not always modify original car panels, but very often in an older car like this, there is no reason to put it back to stock later. First we built up the forward edge with MDF, and then coated the entire inside with several layers of fiberglass mat and resin. The original carpet on this panel is polyester based, so our fiberglass will chemically bond with it, and in a short time this piece is incredibly strong.

The way we layup the front is a bit of a trade secret. The technique is revolutionary, and yet so simple. If you want to learn it all, you will need to attend one of our boot camp classes at the store or through Mobile Solutions USA. I will give you a couple hints: 1. This is one piece 2. It comes right back off the enclosure 3. No fleece. I don’t keep a lot of secrets on how we build things, but the man who developed this technique makes his living by teaching others how to do it, and I respect his wishes not to give away the technique.

Once the fiberglass layer has cured, it is removed and trimmed with an air saw. This is probably the most dangerous part. Not the saw, it will barely cut skin, but the dust. Fiberglass dust from trimming these solid panels is like breathing in microscopic razor blades. We always wear face masks for this portion of the procedure. Next a series of 3 MDF speaker rings are laminated together and attached to the panel. First they are attached with CA glue just to keep it in place until the following layer cures. The green you see is a long strand fiberglass and body filler mix that dries extremely rigid. The top two MDF layers are enough to let the subwoofer pass all the way though, where it will sit and attach to the third. This will countersink the woofer 1.5″ inside the face of the enclosure, which will make more sense on the next step.

Alright, I didn’t get a picture of the next step. Here is what happens: We attach the two halves of the enclosure. This is done by covering the lip of the back half with a generous layer of the long strand filler. Then the two halves are clamped together until cured. Any material the oozed out the edges is sanded down and the enclosure is ready for upholstery.

Here is our enclosure installed in the car. The reason we recessed the woofer so far in was to make this grille insert. The grille is made from two MDF rings wrapped in black vinyl, with a piece of aluminum mesh sandwiched between them. By understanding the thickness of each upholstery material before we started, the rings are machined so well that the grille stays in place without fasteners. We will cover that topic in another post.

All of the raw materials used here to create this enclosure are available in our online store. Just click Shop at the top right of this page.