T-Rex Arms

Sucks to have T-Rex Arms

When I removed the center body section it took a little while to undo the five screws/nylocs on each side that attaches it to the chassis. The reason for this is that the bolts are mounted from the bottom of the car and you need to get a wrench on the nuts in the interior to keep them from spinning. I don't have long arms (no, they aren't T-Rex arms) and I needed another person to remove several of the screws.

In any event, I'm in the process of filling up the side pods with cooling, heating and A/C lines which will make installing and removing these screws much more difficult in the future. I considered using a long piece of aluminum and tapping five holes into it. The primary issue with that approach is that each side would need a piece nearly four feet long which costs money and adds weight. Worst yet, there wouldn't be anything to prevent the screws from shaking lose.

To solve this problem I decided to take a page from aircraft construction and use self-locking, floating nut plates. A nut plate provides a way to add a captive nut behind a panel. In other words, you don't need to prevent the nut from spinning because the nut is riveted to the backside of the panel. In addition, the self-locking type have an asymmetrical thread which deforms and locks the screw into place. Unlike jet nuts, these nuts can be reused several times, but should be replaced when there isn't resistance when tightening. The floating version enables the nut to move forward/backwards and left/right allowing some misalignment during assembly which is very useful when mounting the body to the chassis.

Mounting plates cut from the fan cutout scrap

Since the nut plate needs to be fastened on the fiberglass side I was concerned with how close the rivet holes were to the fastener's hole. To resolve this, I designed a mounting plate and had a bunch water jetted out of 1/16" aluminum. This was essentially "free" because I cut them at the same time as fan shroud and they fit within what would have otherwise been scrap. I wanted the plate to sit flat on the fiberglass, so I used NAS1097AD3 rivets. They are 3/32" solid rivets that allow flush installation even in thin materials. The reduced head makes them inappropriate for use in a structural shear application, but I'm just using them to keep the nut plates from spinning.

In the picture below:

  • Solid rivet manual squeezer
  • 100-degree counter sink
  • Mounting plate with the inner two rivet holes countersunk (doesn't take much)
  • Two 3/32" solid rivets (NAS1097AD3); note how small the heads are
  • 6-32 floating, self-locking nut plate
  • Rivets held in place with painter's tape (made riveting much easier)
  • Bottom of mounting plate post riveting (note that the rivets are perfectly flush)
  • Nut plate riveted to mounting plate

I haven't figured out what types of rivets to use to attach the mounting plates to the fiberglass, but it will be a long time before I need to sort that out.