VA WRX seat modification: foamectomy

[H8DeadW8]

The following is a description of how I modified the seat on a base model VA WRX for better support during spirited driving. This is for information only, and there has been no work done to demonstrate the safety of this modification.

The why: Support of the driver’s body is critical to motorsport driving. If you make steering inputs that result in your body being physically thrown around in the car, you will have to make adjustments to hold yourself up, which detracts from your ability to drive the car as intended. On my Miata, I have gone down the path of improving the stock seat and later replacing it with a racing seat, custom formed 2-part polyurethane foam insert and racing harnesses. My experience is that neither a racing seat nor harnesses give you that ultimate connection with your car, and the hard foam insert is a game-changer. That said, there are many reasons not to fit the whole racing seat/insert/harnesses to your car. There are issues with hassle, expense and especially safety. On top of that, the VA WRX has airbags in the side of the seat, and aftermarket seats do not have provision to retain them. Modifying a stock seat is an option that gets you part way there. These stock seats are designed to be usable by the majority of the public, which helps to sell cars. Modifying a seat to fit your particular body can make it work better for you. In my case, I gained about ¾” of head room and improved the bolstering.

The how: Subaru manuals can be found for free online, and they are much more detailed on disassembly and reassembly than I intend to be here. As I mentioned earlier, there is an airbag in the side of the seat. These things can be dangerous, so be I was careful to follow the manual’s procedure for disconnecting the battery and discharging residual voltage that may be in the system before I took the seat out. The pressed sheet metal under the seat can be sharp.

Once I had the halves of the seat apart, I removed the covers from the foam inserts. In many places, the cover is held onto wires that are embedded into the seat foam by ~1/4” hog rings. I marked the locations of the hog rings with a sharpie before cutting them with a diagonal cutter.

With the foam inserts removed from the covers, I started to modify the foam. On the lower seat insert, there is ~1” of foam that projects above one of the hog ring wires that is molded in place. I trimmed roughly half of this foam away on both sides of the wire using a bread knife. I found a sawing motion much more effective than pushing hard on the knife. The result was 2 wedge shaped sections of foam removed from either side of the wire. I wanted to sculpt the foam more than was possible with the bread knife and settled on using a wire wheel mounted in a drill. A wire wheel in an angle grinder is way too much power. This process makes a ton of dust. When finished, there was a gentle curve where my posterior sits, but no removal of foam under my thighs. I moved on to modifications of the seat frame. These things are sharp, so I deburred the frame with a file and deburring tool. I then removed the spring assembly from the seat. I used a vise and pliers to effectively lengthen the 4 springs by about ½”, and then bent the springs into a gentle downward curve so the springs would snap into place on the frame nicely.

The seat back was similar to the base. There is also a hog ring wire dividing the rough halves of this insert. I reassembled the seat without the covers and sat in it so I could mark where I would remove foam so my shoulders could sink back further into the frame. I also marked where I wanted the lumbar support to end. Modification was similar to the base, starting with the knife for bulk removal and progressing to the wire wheel for sculpting. As for the spring assembly, it is held in place with 4 wire clips. It is common for enthusiasts to remove the upper clips to improve bolstering, and a previous owner of my car had done just that.

I reassembled the seat using small zip-ties in place of the hog rings. A pair of needle nose pliers was useful for getting the zip-ties under the hog ring wires. I pulled them tight and trimmed the ends. As I reassembled the seat, there were times at which it looked a little baggy and stretched in weird ways, but this largely fixed itself as reassembly progressed.

The results: The final result is a nice increase in bolstering and around 3/4” increase in head room. Comparing the modified seat to the unmodified seat, it is definitely better. That said, it isn’t a full race seat with insert. I might go back in and work on the foam again. All it takes is a couple hours and a hand full of zip-ties. I would still like to sit lower in the car. Looking at the seat mounts though, I did not see a good way to get the seat much lower. There is a part that looks like a section of a gear. At first it appeared that cutting another tooth or 2 into the assembly could lower the seat by another ~3/8”, but further inspection revealed that the gear was not the limiting factor. I think mounting the seat lower would require a custom mount. I did weigh the seat, and it was 45lbs. This has me thinking about replacing the seats, but there are a lot of considerations when doing that.

H8DeadW8

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[Charlie-III]

Nice writeup.
Cutting foam, any serrated edge blade tends to work better. Some use an electric carving knife, steak knife or bread knife. A straight edge (utility knife) tends to not cut but tears.
Instead of a wire wheel, an angle grinder with a 3-5" plastic disc and 80 grit self adhesive sanding discs works well.
Ty-wraps, I would use 1/8" nylon with a metal lock tab, they work better than cheaper versions that can get brittle quickly.

Note, serrated blades also work on foam like "Great Stuff" foam in a can or home insulation sheets.