May 31, 2006 update to the Mazda STS2 Miata project car

Sitting in the Cheap Seats - STS2

With the Super Challenge win in Atwater, our Pro Solo plans changed. Its not often that you win one of these things and it usually puts you in contention for the Overall title for the year. Not that we have any grand delusions of actually winning that ultimate championship, but it just makes sense to put yourself in the position to be there if the stars all line up for you. That’s why we took the Miata to Mineral Wells and Peru, instead of the STS Civic as originally planned. Sure enough, the Challenge points garnered at the former have placed us in the early lead for the Overall. Hopefully Peru will also be kind to us.

This also kick-started a couple of other development projects on the Miata to pluck some more low-hanging performance enhancement fruit. Losing the class by .006 seconds (Atwater Pro) will also do that to you. We had originally put off our final big diet items until later in the year, but decided instead to thrash a bit and move the timeline up. These included lightweight racing seats and an aluminum underdrive crank pulley.

Since we were pressed on time, the racing seats were stolen from our STS Civic and mounted in much the same way. These are a pair of Kirkey Economy Layback (10-degree) seats that are cheap ($100-$140 online), fit like a glove and weigh in right around the 15-lb minimum with the seat cover installed. Since they are hand made and come in a variety of sizes, there is some variance in weight, so each seat should be weighed to ensure legality. One of ours was a little light, so we shimmed the seating position up by laying a piece of flooring tile in between the seat cover and the aluminum shell. Doing so, it becomes part of the seat and is therefore legal. Mounting brackets and other bolt-ons don’t count towards that weight (watch for this rule to change for next year).

The passenger seat went in first and was the easiest. Four holes were drilled through the floorboards and the seat was bolted in place using fender washers to spread the load underneath. Carriage bolts (5/16”) were used at the top so as not to interfere with the passenger’s seating quality.

For the driver’s side, we chose to mount the seat on the original Miata seat rails to provide adjustability and make them easier to swap out (this is a street car after all). The biggest issue is that the base of the seat is narrower than the distance between the rails. This is overcome by fabricating a pair of L-brackets out of 1/16” steel sourced from the local Home Depot. A hack saw, vise, and angle grinder were used to make the necessary modifications. See the pictures for details.

We had to fab up some standoffs for the front of the seat to raise the base above the adjuster mechanism. This can also be used to fine tune the layback position of the seat to suit the driver. We used ½” square steel tube which nicely accepts the carriage bolt top. These took about four tries with the grinder to get the angles right on each end. Two original holes in the seat rails were used and two new ones were added, and the mounting bolts were cut to the proper length to keep from protruding inside the rail. Best of all, this whole project gave us the rationale for a new power tool acquisition as we finally replaced our antique single-speed drill with a modern variable-speed unit (also sourced from Home Depot)!

The results were great for two reasons. Since the original Miata seats weighed 25 lbs each, plus 5 lbs each for the rails, we saved 10 lbs on the driver’s side and 15 on the passenger side. For small, low-power cars our rule of thumb is that a 150 lb passenger slows us down by about .5 second. So each 30 lbs is about a tenth pickup.

Even better, though, was the newly lowered seating position. This helps gets the driver’s weight lower in the car, but in our case it also allows us to see the course without looking through the windshield header! The whole job took about half a day, including a trip to the hardware store, and the car is now faster and easier to drive. The cheap seats do take a toll on the streetability, though.

For Part 2 we lose weight and add power at the same time via the crank pulley.

Note: While typical of homebrew seat installs for autocross, this example is by no means recommended for track use or even safe street use.

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