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1991 Miata Foamectomy

Joe - October 29, 2015

The Unexpected Compromise

During the heydays of British roadster nostalgia that is the early 90s, the bargain corner was (and still is) dominated by the small RWD roadster named Mazda MX-5 Miata. It offers 50-50 weight distribution, responsive steering, sharp throttle responses and impeccable “tossability”. It was the perfect formula of the era: agile, retro, fun and terribly seated – come again?

That’s correct, our beloved roadster suffer from the “Cross-marketing Syndrome” where a performance oriented vehicle’s primary audiences are those in or near their retirement age - whom would love to tell you about how their stock picks in the 80s over a nice cup of tea. Not unreasonably then, that certain design criteria such as the gentlemanly need for comfort were prioritized over a bucket seat. To achieve this, it was decided that a literal ton of firm foams were to be stuffed in the seat, resulting in a cushy and uncomfortably high seating position. This feature was carried into the 3rd generation (NC) of the mid 2000s, and was especially noticeable in the early generation (NA).

Why Is It Such a Problem?

If I were to nitpick and be “that guy”, I’d no doubly tell you that a higher seating position mean higher center of gravity, resulting in more body rolls and overall will cost you exactly 0.00004s on your next autocross. However, the bottom line is it is uncomfortably to drive in. Especially for the NA (90 – 97) module, the heightened body position combining with a relatively large and unadjustable steering wheel (see below) meant that in more than one occasion, driving experience is affected.


90 – 93 Canadian 3-Spoke Non-airbag Steering Wheel (~365mm), picture source.


NA (90 – 97) 4-Spoke Airbag Steering Wheel (~365mm), picture source.

The issue is more pronounced when you are over 5” tall, specifically:

  1. When shift your right foot over from gas to brake, the very top of your leg will be banging against the steering wheel. Overtime a purple or red patch will appear in that area, signally your imminent doom. This also means that your next properly executed heel-and-toe shift could very well be your last.
  2. When steering, since there is no excess space between your leg and steering wheel, it is a challenge to maintain both hands in contact with the circle past the 6’o clock position. This is noticeable whenever you need to make a turn – which I’ve been told that it was indeed a common thing to do with a Miata.
  3. Side thigh bolsters are relatively low comparing to the center boosted foams, making the bare minimal contact. During a track session as you pull lateral G left and right in a corner, you may begin to notice that unlike the car you’re driving, your own bottom seems to not wanting to settle down.
  4. In SCCA (Sports Car Club of America) sanctioned events (and those who follow SCCA rules), in an open top car (convertible or Targa top) the top of driver’s head with helmet must be below your roll bar. This is difficult to achieve with stock seats while you still have a neck.

I have observed these problems with my own 91 Miata as well. However given that the steering rack location is not perfectly centered, I’m not certain if item 1 and 2 is an issue with our wrong-hand-drive friends over the pond. Nevertheless, after a little over 5,000km daily driving, 2 autocross sessions and a track day, I’ve decided that I do in fact need to do something about it.

The solution to this problem is fairly straight forward: just cut it. I’ve mostly followed the excellent guide at Miata.net, but for the purposes of this article I will reiterate some of their steps with my own experience. I’ve found the end result to be a satisfactory improvement over OEM, while it is no replacement for an actual racing seat setup, it does improve the day-to-day livability of the car by a mile.

All in all an essentially free upgrade, what’ve you got to lose?

Is There a Doctor In Here?

Since I originally wasn't planning on writing this, I did not take photos of the disassembly process. However I will present to you the next best thing: the installation photos in reverse order, please use your imaginations as you see fit.

Tools:

As they say, the efficiency and quality of a job done is a combination of the performer skill and his/her tools. Since I am a no good shade tree mechanic, I’ve enlisted the help of my trusty tool box:

  • 14mm socket
  • 14mm wrench
  • 12mm socket
  • 3/8” socket extension
  • 1/2" to 3/8” socket adapter
  • Needle nose pliers
  • Philip screwdriver
  • 1/2" torque wrench
  • Knife (bread knife works very well as I found out later, in general they should have smalls serrations.)
  • Zip ties (or new hog rings, personally zip ties are much faster)

Objective:

The goal of the mod is to sit myself lower and further into the seat, simulating the bucket seat position while maintaining OEM looks. The best way to achieve this is to cut away the bottom foam bulge so the center will “collapse” down where your butt lies and spring back up when you are not. Some have opted to cut the top, the benefit of doing so is the foam itself will not be under so much stress as it will still have the bottom supports. On the other hands it is considerably more work dealing with hog rings, so I have decided to take the common approach for sanity sakes.

In addition, I have also decided to trim away the front to give myself more room at steering wheel and trim the back foam for more side supports. Note that the seat design has changed a bit from NA to NB (99 – 05), while this guide is strictly for NA Miatas the principle remain similar.

Step 1:

Fold the seat and pull it all the way forward to reveal plastic covers for the mounting nut (may be missing depending on your car’s history), simply pull the end up with some force and it will come out. The two 14mm bolts are factory torque to 29 ~ 37 ft.lb, but you may never know if Superman was working at the time. Apply penetrating oil (such as PB Blaster) if necessary then break them loose, mine was in fairly good condition so out they come.

Note that to access to these bolts with adequate socket wrench will require socket extension to clear door sills, the bolt head itself is fairly thin so take care not to round them out.

Step 2:

Pull the seat back all the way to reveal the front, uncover the bolts and break them. If your seat has speakers make sure to disconnect them, press the side of the connector and pull gently (sorry no pictures, but it should not take much force).

After all bolts are loose and wire disconnected, undo all the bolts and the seat itself is free. To remove the seat itself, it is strongly recommended to have the top and windows down to make the job as easy as possible.

Step 3:

With the seat out, flip it over to remove the rail so we can access the side hog rings (represented in photos with zip ties). Similarly to the floor mounts, pull the adjustment handle to slide the rail down to access front 12mm bolts, and slide up to access the back bolts. Set them aside, you do not need to disconnect the wire connection.

Because the left side (right side in the photos) rail is actuated with wire it is best to remove that side first, otherwise you will have to use pliers to hold the “jaw” open while you slide it. Since I didn’t find any torque information for these, during reassembly I’ve simply tighten them snug.

Step 4:

Find a flat place to set the seat down on its side, turn it over to the right side where the pivot is held together by a Philip screw and remove it. The seat is still connected as the screw is only there to cap the pivot (see photo).

Step 5:

Flip the seat to the other side to remove the latch mechanism. Free the plastic cover on the latch by undo the two Philips screws, this will allow you to wiggle the piece a bit but it will not come off.

There are two 14mm bolts connecting the lower portion of the seat to the mechanism. Since space is limited, I’ve simply lifted the cover up to undo the back bolt and shifted the front to undo the front. Once the bolts are removed, you can separate the upper and lower half by lifting lower half out of the pivot shown in step 4.

Note that if your seat has speakers, they will still be connected by the wires. These wires have clips that go into the metal bracket and are best removed (optional) once the foam is out. For now simply lay the portions side by side as you work on the lower half, take care to support both sides as to not stretch them.

Step 6:

With the lower half bottom up, you will be able to see all the hog rings keeping the leather attached to the frame (here once again represented by zip ties). If you decide to keep these rings (you shouldn’t) as souvenir, I found it’s easiest to remove them using needle nose pliers. Rotate the ring until the ends are accessible, and then angle the pliers such that when you close it the rings will be pushed apart (see artist’s impression above). Alternatively just cut them off with a cutter and replace with zip ties, they are much less of a hassle to install.

Step 7:

Once all the rings are loose you can start peeling the leather around the seat like an orange. Take special care of the stitches on the corners as they are not very strong and can break if too much force is used. In the end you should have a nice blocky piece of foam to trim away with and the leather cover is still attached on top with more hog rings on wire frame – it is not necessary to remove it further unless you want to work on the top side foams.

The metal bracket it sits on has no fasteners and simply pops out with some encouragement. On the back of it you can now see the clips for the headrest speaker wires. The trim piece for the pivot flange is not important and you can simply leave it on there. However if you must clean every piece of item, then pop out the center cap (red arrow) and the whole thing will slide right out.

Step 8:

Consulting with your original diagram, outline where the cuts will be using a marker. Here you can see why the seat is so high – the bulge is well over an inch thick!

At first I used a sharp fruit knife, it did not work out very well as the cutting edge tends to deform the foam rather than cutting it. Then I tried to use a serrated meat knife, which all it did was having its rather large serrated teeth getting caught on the foam, spewing burrs everywhere on the kitchen floor.

Infuriated, I got up and left. At the nearest Canadian Tire however, I managed to find a bread knife on sale for $6.99. You can see it in the comparison photo that compare to my previous attempts, this has small and shallow serrated edge which worked wonders with foam. If you could not afford a knife, electronic turkey cutter is also a very viable option.

With the right tool in hand, cutting begins on the bulge. Make an incision with the knife at the front of the foam, working slowly and one strike at a time until the whole piece of cut off. Do not use a lot of force as this will easily disorient your cutting angle, check your progress against your Sharpie markers often and correct mistakes as you go.

From my photo it’s evident that some water got into the car at some point and gave my seat some nice rusty sideburns that would make any Irishman proud. Evidently my cut was so perfect that it JUST cleared the hog ring securing the top leather cover.

Step 9:

To address the thigh support, a crescent shape cut is made to remove a trapezoid shaped piece on the bottom. The idea is that this piece will taper up and less material will be removed as you work your way towards the back, the extra removal in front will in theory place your leg lower than before. Technique used is similar to before with many more paranoia checks to top the proverbial cake.

Start by scoring the lines you draw by lightly cutting along it. Then from each corner in 45°, lightly start the cutting process without applying a lot of force. For this part I let the blade gliding on the peeled back section, removing at most 1cm of material every stroke. Once both corners had a triangle piece of material removed, use the same procedure as before to cut the whole piece loose.

And now that’s what I can a properly shaved bottom, save the pieces you cut out in case you don’t like the new seating position.

Step 10:

With the seat foam done, time to move onto the backrest portion of the mod that was far simpler. Lift up the sleeve on the back, there should be three hog rings holding the back support foam cover to the frame – get rid of ‘em. The whole piece can then be lifted out like you see here, there may be a bit resistance as the foam is deformed so just give yourself enough room to work with. Once opened, there should be six rings holding the foam to the cover by means of an integral wire loop in the foam itself. Foam will be freed once you remove them, but I decided to reuse the rings and simply pried them open.

Step 11:

Mark up your cuts and carve away, while not pretty it is quite functional with essentially half the thickness reduced. It is worth noting however, that some opt to simply remove this piece altogether. I tried this approach but found the side boaster was hitting my lower back with the now recessed back support.

Step 12:

Reassembly is simply the reverse, where hog rings are replaced with zip ties as necessary. One thing I’ve decided to do was to put a plastic bag over the freshly cut foam to protect it against dews and other nasty stuff near the floor. With the metal bracket on you can see just how much space is now available for cushioning.

Once everything is back together, make sure to torque the floor bolts to 29-37ft-lb and give yourself a beer for a job done.

When the (Brake) Dust Settles...

After driving around with the new seats for a while, I can tell you this has definitely helped with overall comfort and much more secure on track. There are now more room for my legs and hands as I steer and brake, my helmet will finally sit below my roll bar, and heel-and-toe was made infinitely easier. Cosmetically speaking, you do notice a slightly sag in the material that’s evident below, but the tradeoff is immense and it is well worth the time to do so.

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