Installed Seat & Harness Install (Seat / Harness install part 3)

It took a lot of trying-and-discovering-shit-it-doesn't-work to make the seat mounts. The harness install is much more straightforward and acceptable. Before installing, read the SFI and manufacturer warning on your harness. If you bought your harness used, which I don't recommend doing, go on the SFI foundation website and your manufacturer's website and read online versions of the guidelines. This is important and your life depends on it!

I used all stock mounting holes so there was no need to drill anything. This is possible on my year Miata because the right seat belt receiver bolts onto the transmission tunnel. This hole exists on all NA Miatas but on later models it might not be threaded.

I bolted in my sub and hip belts onto the stock mounting holes along with the stock seat belts so I can use the stock seat belt on the street. On the left side of the driver's side, loop the belt through the left harness slot so you can do this, or remove the stock seat belt altogether if you don't drive your car on the street.

No install is perfect, and here are my concerns of this install.

1. On my hard dog roll bar, the harness bar is a bit low. With the mount I made for the seat, the harness bar is about 1.5" lower than the shoulder harness slots on the Sparco sprint seat. This is barely acceptable. The harness mounting point should be on the same level or up to 10 degrees higher, and at most, -5 degrees lower. The solution would be to bypass the harness bar and bolt on to the rear deck using backing plates.

2. The sub strap is connected to the left seat belt mounting point. I hope the sub-strap hole on this seat is strong to keep the sub strap in line in the case of an impact. The sub strap, ideally would be mounted behind the sub strap hole on the floor under the seat or behind the seat. Instead, the strap on my setup goes to the side.

3. The hip belt angle works nice at my seating position. It looks like its at 45 - 50 degrees. Check out the picture with reference to the seat rails for reference.

On sedans race cars, some people mount all of the belts at the rearmost part of the race car to minimize any harsh angles and let the bolt-in tabs work at a more perfect angle. I don't recommend this because using really long belts or fully extended belts will result in too much stretching in the moment of an impact.















Final notes
I went on a test drive. This is my first experience in a race seat in a production car. Amazing! I felt very much connected to the car. The feedback received was very detailed. I won't talk about streetability here since that is not the focus of my blog, but it blocks rear view and hard to climb in and out of the car. Also, if you are road racing, you really should get a removable steering wheel.

I could not use any washers on the seat-tube mounting bolts. Also, the bolts were cut so that the thread length was only about 2.5 times the bolt head. I don't know if that is enough, but its slightly shorter than the stock bolts that screw into the stock seat. Also, the rectangular shape of the tube and the fact that the mount is four pieces makes me question how well it will take a side impact. For these reasons, it is my opinion that this is not safe enough for a road-racing application. Whether it is okay for track days, you decide.

Copyright 2009, 2010 (c) by Jerry Lee.

Fitment and Seat mount fabrication (Seat / Harness install part 2)

Disclaimer: I make no warranties on the safety of these mounts. Any claims I make here are unprofessional opinions. You must make your own independent judgment on whether to follow these instructions or not. I will not be liable for any injuries any persons or damage to your property resulting from anything you read from this blog.














Sub-disclaimer: personally, I would not use this mount for road racing. I'll state the reason why later.

The stock seat's mounting hole centers are 330mm apart. The sprint's mounting hole centers are 345mm apart. I started modifying the holes on the stock sliders/rail to make it work, but after a bit of drilling I realized it was not working. The 15mm difference in width does not allow for the bolt head to fit inside the slider properly. When the holes on the rail are widened enough to accept the Sparco seat, the bolt head will hit the slider. Others have narrowed the bolt heads to make it work, but the smaller bolt head, inability to use washers, and the widened holes on the already thin gauge steel on the sliders drove me away from using it. Therefore, I decided to make a simple adapter. Before I get started, measure twice and cut once! Saves a lot of time. Also, drilling by hand inevitably brings some error so be prepared to enlarge holes. Don't mess up too much. Things needed: -Two steel square or rectangular tubes. Make sure the steel is decent grade, ideally a cold-rolled steel like 4130/4140. The wall thickness should be pretty good, like 11 gauge (.125"). The tube must be at least 1 inch high. -four M8 bolts with a small head, shorter than 16mm. (I used a hex bolt) Use 10.9 gradr or higher. Thread pitch is 1.25mm -four M8 nuts with same thread pitch (10.9 or higher) -assortment of drill bits (small, medium bits leading up to 3/8") -dremel with grinder and cutting wheel -tape measure and straightedge

Preparation
Cut the tubes. Cut the tubes the same length as the sliders are. Should be around 14". Mark these tubes on all sides with a sharpie so you don't get confused. Arrows and writings will do: front, outside, inside, left and right. If you can't find a sharpie, masking tape and pen(cil) works too.

Tube - Slider mounting holes
Refer to this diagram if you are confused at any point.

Take a tape measure and measure the distance between each hole centers on the slider. Measure the distance to the ends of each hole and plot it out on the tube. To do this, measure the distance between A and D, then C and D. Add the two numbers and divide by two to find B and D. Using the distances, mark two dots for each distance on the tube. Draw a line using a straight edge and make lines like A, B and C. Stick one end of the slider and tube when finding an anchoring point for the tape measure, unless you can cut tubes perfectly. The picture is obviously not to scale, and keep in mind that for one hole C and D will be very close together. Mark the center point on the centerline of the tube along the longitudinal axis. Then, move the center points 2mm towards the center of the seat. This line is E2. The intersection of this line and the lines marking A, B, and C is where you will be drilling and where the sliders will bolt in. The hole sizes should be 8-9mm, or 3/8".


Tube - seat holes
Flip the tube over. Find and mark the longitudinal center axis. Draw another line parallel to the center 5.5mm apart, towards the outside of the center of the seat. At least 15mm behind the front hole you drilled on the other side, mark center points for two points that are 271mm apart. Find those points on the 5.5mm offset line and drill with the same size as the previous step.
Access Holes
You will need to find a way to put your hex key for the bolts that go into the seat. Using the holes you drilled on the seat side of the tube, drill straight down. Use the 3/8" bit so you can drill a small spot to find the center point. Then work your way up from a small bit to 1/4", or however big your hex key is.

Test fit and installation
After all these holes are drilled, test fit all the components and see if everything lines up. In my case, it fit at first try. Since rail-floor mounting holes are over-sized, if you followed proper directions this should work. Torque everything very well and install. If something doesn't line up and the error is minimal, dremel the holes a bit bigger. Use grade 8 washers whenever you can fit them.

Copyright 2009, 2010 (c) by Jerry Lee.