For a while, I have known that the Miata seats were pretty heavy. In fact, the pair of seats weigh a total of 62#. That is a rediculous amount of weight to be carrying around. If I was unrestriced by cost I would just purchase a set of lightweight racing seats for the car but that really wouldn't go along with my project goals. So, removal of the passenger seat and modifying the driver seat is the way I will have to go.
I approached this project in several steps.
1) Disassemble my passenger seat to see what was in there.
2) Weigh each part to see where the most weight could be removed.
3) cut out some of the structure but still keep the steat strong.
4) Build replacement parts for what I was taking out.
5) Replace the worn out fabric and foam in the driver seat for the good fabric/foam from the passenger seat.
After taking everything apart, I found the weight of the passenger seat was 32#. Although I didn't weigh it, I believe the driver seat would be 30# (more on that later). The weights of the parts were:
rails and bolts = 5.5#
bottom = 13#
back = 9.5#
recline adjuster = 3.5#
The first step was to disassemble the seat bottom. I cut all the hog rings off and removed the fabric. Under that was the foam seat bottom (3.75#) and the metal pan (8.5#). The metal pan had a large support "bump" towards the front. This got me excited because I knew I could remove this for some good weight savings. The bump turned up weighing 2# but sadly I found out later that this bump is not in the driver seat. I am not sure why the passenger seat has more structure to it. You can see the foam with a big hole in it for the bump here:
Here is the seat pan:
The seat pan has a multi-layer sheet metal for support on the inner hinge (closest to the transmission tunnel). I drilled out some of the spot welds and removed a couple small pieces that I didn't think were needed (0.5#):
The back of the seat was a big disapointment. It weighed 9.5# but after removing the vinyl I found that there really wasn't anyway to easily save some weight. It was just foam and some thick metal wire for support:
The recline adjuster was very heavy (3.5#) and very dangerous. The thick wound spring would have made an excellent toy. I reclined my seat to the place that I liked and made sure I left the recline mech in this position. Once I had it removed I traced it on a piece of scrap cardboard I had laying around:
I used the cardboard as a template for my new bracket that was made from 1/8" scrap aluminum that I found:
The seat rails (5.5# with bolts) were next. I adjusted the seat to where I liked it, marked a few points on the rails to keep the lengths correct and started cutting. The rail is made up of two pieces of metal with bearings in between them. The adjusting handle moves teeth into some little slots to keep the seat from sliding:
The handle, slots, teeth, bearings, and just about everything else was removed from the bottom of the rail and then the remaining ends were welded onto the rails. I was a little off on my measurements and had a heck of a time getting all 4 bolts to match up with the threads in the floor pan. I ended up having to wallow out the holes in the rails a bit to get everything to mount up. When I was finished I found that the seat was about 1/2" too far forward (my knees are bent a little bit more than I wish they would have been). I might try building some more rails (I have another set laying around) and fix this in the future. One interesting note, I will have to cut the rails off the seat if I ever want to build another set. They were welded after the top of the rail was bolted to the seat so I don't have any way to access the bolts:
You may have noticed that most of the pictures were from the passenger seat. I did all of my experimenting on that and then started the real work on the driver seat. In the end, I only ended up modifying the seat rails (2.5# savings) and replacing the recline mech with an aluminum bracket (3.5# savings). I spent two full days on the project but could have accomplished the same thing in 2 hours if I hadn't messed around with the seat back or cutting on the seat bottom on the passenger seat. I did get to swap the old fabric/foam and even though it wasn't a perfect fit it is much better than the ratty stuff I had on there before. I went to the local hardware store (not a big chain) and purchased a $3 box of hog rings and a $5 set of hog ring pliers to reassemble everything. Here is the 6# of material removed: