Roll Cage: Trans Tunnel Patch, Driver’s Seat Mount

Hmmm….I’m ready to be done with the cage and move on to something else…like driving it!  Oh well, can’t get done if you don’t get started.  Onward!

As frequent visitors will know, I had to cut a notch in my transmission tunnel to move the seat far enough away from the upper door bar.  Well, in reality it is to move my HEAD away from the door bar.  At any rate, its time to patch the holes.  The side and bottom of the notch both have a curve to them.  So, cutting a single piece and hammering it to “fit” would have been a pain, and probably not very successful.  I chose to cut the steel into three pieces: one for the side, one for the bottom, and one to fill the front triangle area.

I used poster board to trace out the patterns, then cut the steel to match.  First, I tacked the bottom piece into place.  I did this by tacking at one end, and then bending it into place, tacking as I went. 


Second, I put the side piece into place.  Similar to the bottom, I tacked it into place from the rear to the front.  I started with the floor seam, and used a screw driver to push it as far into the tunnel as I could…in order to maximize the clearance.  Then I went back and tacked the top edge to the tunnel.  Finally, I used another piece of poster-board to cut a pattern for the little triangle piece.  I tacked it to the bottom and side, then I used a hammer to mold it to the transmission tunnel…and blend it in.


I think it came out pretty well. 

On to patching the rear bulkhead.  The main thing here is that the bulkhead is in a “W” shape.  So, I cut a piece of 22AWG sheet and then bent it to shape.  Next, I tacked the top edge into place, and then pressed each edge into place, and tacked it down.  Finally, I decided to weld this myself.  A little practice on a non-critical part.  Finally, I recut the hole.  I still had to grind the hole out a little bit, but it came out with about 1/4” of clearance between the hole and the tube. 


So, that really completes the cage, proper.  All bars are tacked into place, and all related sheet metal is tacked, as well. The final task is to install the seats, and the seat back braces.  I’ve decided to install fixed seat braces, rather than buy a adjustable ones.  Its cheaper, and stronger. 

Anyway, I had to install the seat mount first to set the exact mounting location for the seat back brace.  Well, “install” might be the wrong verb..perhaps fabricate, make, figger out?  Anyway, I’d thought I had this figgerd Seat, and harness Test fit, I had to move the seat much further into the center to get the clearance away from the door bar.  In addition, I had to remove the rear humps to get the seat low enough for my head to clear the roof. 

That, of course, meant that my original plan wouldn’t work anymore.  I kept the plan of using the steel rails, and I used 2” x 3/16” steel plates to act as rear mounting points.  I bent these into an “L” shape, so that they would weld to the rocker panel and trans-tunnel.  Then I could weld the 1” rails to these.  Finally I fitted a crossbar of 1 1/2” x 3/16” strap for mounting the front of the seat.  At last, I sat in the seat and checked the final fore-aft positioning, and drilled the holes for the grade-5 hardware. 



After I finished mounting all the hardwar, I fit the seat and mount back into the car, and began measuring and installing the seat back brace.   Not much of a big deal, this.  The plate mounted to the back of the seat is 3/16” x 3” plate, 10” long.  I bent it to fit the contour of the seat.  That seems to be about as much steel as I can bend with a hammer and a vice.  My hands were sore after bending it.  I had to hit it so hard, that while making the second bend, the intertia from the hammer strike straightened the first bend back out.  Finally, cut the notch on the tubing, and then miter the other end to match the seat back.


I think it looks pretty good!


My last task for the day was to start on the passenger seat mount.  I had purchased a “Spec Miata” seat from Ultrashield.  This seat is about as minimal as they get.  Importantly, the seat pan is 16” deep instead of the 20” of my VS Halo.  When I sit in the seat it feels like my legs are unsupported.  This isn’t an issue in the drivers seat when the feet are working the pedals.  But, it is very obvious in the passenger seat when just “sitting”. 

Also, this seat has a 10deg layback.  This makes you feel like you are about to “launch” out the front.  Both of these issues can be improved by tilting the seat back some more.  This raises the seat’s front edge so it supports your thighs.  Second, it reclines the back so the passenger won’t feel like he/she is about to be ejected!  The trick here is “how much?”.  I could tell how much I wanted, but its hard to measure.  So, I placed a small piece of modeling clay on the mounts, and set the seat into place.  That squished the clay, which I could then measure.    Looks like I need some 3/4” or 1” square steel stock…since, I don’t have any. 

So, it was nearly the end of the day, and I didn’t have time to run to the store.  I fitted a back brace to the passenger seat as my last act of the day.


2 comments to Roll Cage: Trans Tunnel Patch, Driver’s Seat Mount

  • Hi
    We’re building a very similar cage and seating setup, so it’s really good to see your photos. I’m having trouble finishing the welds under the crossbrace and diagonal in the trunk. How did you get the mig gun under that tight area?

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