Seat, and harness Test fit

Two weeks ago, I bought two seats, both Ultrashield.  For the driver (me), a 16”/20* Road Race VS Halo.  For the passenger, a 17”/20* Spec Miata seat.  The SM seat doesn’t have shoulder wings, or a halo.  But, its small enough to fit next to the VS Halo.  I bought it a little larger to accommodate a wider variety of passengers.  Track days are less risky than wheel-2-wheel racin’…so this seemed like a good compromise.  However, it means that I have two seats and harnesses to mount.  I also bought a Crown 5-pt latch-link harness for the passenger vs. the Crow 6pt F1 style harness for the driver (more on this later).

After test fitting the seat last week and some discussion on specmiata.com/mazdaracers.com, I decided to mount the seat on rails mounted to the stock humps.  The idea being to keep the seats off the floor while still maintaining some clearance between my helmet and the roof.  So, I bought 2 sets of 3/16” x 1” x 48” long mild steel to use to make the rails.

But, before I could make the rails I had to mount my bench vise.  I haven’t used my bench vise since I moved from Colorado….7 years ago!  I’ve been doing mostly woodworking and my wood vises mounted to my workbench are the right tool for that job.  Anyway, I made a plywood plate and screwed and glued it to a length of 2×4.  This allows me to clamp the bench vise into my woodworking vise.

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So, to make the rails I cut two 17 1/2” lengths of steel.  The stock humps aren’t flush, so the ends of the steel needs to be bent to fit.  I simply laid the steel in place and marked steel where the bends needed to go…about 1 1/4” from each end.  Then I put the rail-to-be in the vise, checked for square, and used a hammer to bend the angle.  For the first rail, I bent it slightly then test fit until I got the angle right.  Once I got the first bend, I marked the location for the second bend, and repeated the bending process for the back end.

To make the others to match I transferred the marks from the first rail to each of the remaining pieces of steel.  Then clamped the template and the new rail in the vise together bending until the matched.


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Once I had them all bent, it was time to drill some holes.  I’m reusing the stock seat mounting bolts, these are approximately 3/8” bolts, so I needed to drill 7/16” holes.  Since I can’t get to the underside of the mounting holes to mark the location, I marked a cross on the car intersecting with the center of the seat mounting holes.  Then laid each bracket over the mounting hole and transferred the marks to the steel.  Given the potential for error in each mark&drill process, its important not to mark&drill both holes at once.  Rather, I marked&drilled the first hole, then attach the bracket using the first hole, and then mark the second.

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To drill the holes, marked the desired location with a punch, then drilled an 1/8” pilot hole, then drilled the 7/16” hole.  Its nice to have two drills!  I used the cordless for the larger drill bit because it has a low-speed/high-torque setting (for screws).  This worked better than the 1/2” dewalt for the low-speed drilling needed to avoid burning the drill bit.  Add a little oil to lubricate the bit…voila!

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To deburr…a bastard file made quick work of it.

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Bolt the bracket in place, mark it, and repeat the process.  This way only this mark/drill step is dimensionally critical.

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Here are the rails for both sides.

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Next was to start test fitting the seat and harness.  I set the seat onto the rails and sat down into it carefully to find the right position for my legs.  I pushed in the clutch pressed in the brake, and slid the seat backwards until the clutch and brake felt comfortable and I still had a some flexion left in both knees.  After everything was just right, I used a Sharpie™ to mark the position of the front edge of the seat on the front-hump.  Once I had the distance right, I cut a 2×4 to brace the back of the seat so the bottom would stay flush with the rails.  Then I used bungy chords to hold the seat in place.

Once I had the seat all braced, it was time to fit the harness.   Here is a picture of the harness I selected.  It’s a formula style harness.  Notice that the sub belts don’t have latch hardware of their own.  Instead they pass through D-rings sewn into the lap-belt, and then loop around the shoulder belt hardware.  These sub-belts are extra-long because they are designed to be SAT on and run back to the lap-belt mounting location…rather than being bolted to the floor under the seat.  This type of harness is very comfortable to me….more so than the standard 6pt.   Another point is that the lap belt does not have standard adjusters.  The lap belt has to be adjusted during installation and then left “fixed”…similar to the way normal sub-belts are.

I used clip-in ends for the harness lap and sub belts.  The harness comes with bolt-ins, but they are both laces with 3-bar adjusters.  So, I bought clip-ins and eye-bolts.  I mounted the eye-bolts in the stock lap-belt locations.  NOTE: I used a C-clamp to temporarily attach the shoulder harnesses to the package tray…since they will eventually wrap around the roll-cage bar.

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Here is a close-up of the lap-belt and the sub-belt.  They both pass through the lap-belt openings in the seat.  With the harnesses installed I could sit in the seat and buckle everthing up, and tighten it down to make sure everything was going to fit okay.  I was concerned about the routing of the sub belt over the seat edge.  I’m going to call Crow (and maybe Ultrashield) next week to discuss this routing.  I also tried to find a route through the holes in the seat and down to the floor.  The F-type sub belts are supposed to route back to the rear edge of the seat.  An option is to mount to the seat mount bolts.  I don’t like this approach because it attaches the sub-belt to the rear hump which is only sheet metal that is spot welded to the floor pan.  Given that the F-type sub belts carry more load than the non-F-type sub belts, I don’t like the idea of putting 1700 lbs of sheer load on a sheet metal hump.  I’d much rather use a part of the car that I KNOW was designed to take the load.

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Anyway, once I finished with the driver’s side it was time to move on to the passenger.  As with the driver’s side I test fit the seat first.  It’s a SpecMiata seat which has a 16” deep seat bottom.  I’m not entirely happy with the mounting position, yet.  It feels too verticle.  I think I may need to remove the rear-humps and mount the rear directly to the floor.  This would give me a few more degrees of lay-back moving the head away from the from hoop, and putting a little more tilt to the seat-bottom.  The shorter seat bottom makes it feel like your legs are supported.  The additional tilt allows the thighs to rest on the seat bottom.

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Finally, here is a view of both seats and harnesses temporarily installed.


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Once I finished fiddling with the seats I still had some time left in the day…but, it was getting cold outside.  So, I looked for something to do in the shop.  First I cleaned up a bit.  I’ve got a few different projects going in there (building my kitchen cabinets, building a dining room table for my Mom, rebuilding my riding lawn-mower engine, etc).  Then I remembered that I had the rear-package tray covers that needed to be cleaned up.  As you can see below they were kinda rusty.  I screwed them down to my workbench top, and used a knotted wire attachment for my angle grinder.

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Here are the results.  Shiny and clean.  Ready for primer…as soon as we get a day above 50F.


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It was a good day.  Not a lot of MAJOR progress, but these things were fiddly and it takes time to do all these little things.

3 comments to Seat, and harness Test fit

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