Valid Cage design element? (Page 1 of 2)

Just build a plinth and be done with it.  In short, make vertical sides that create and area big enough for a legal spreader plate from 1/8" stock...even better if you TRULY make a box of 1/8" plate.

Seems like building a box would be a more complex design and therefore more room for error?

I sent him a link to this thread, will be interested to see what he says.

I can't imagine being the first person to have this question.

FWIW, most of the cages I put in my cars are on plinth boxes. I addition to them giving me the clearance to get radius welds up top without cutting the roof open, I prefer to put the forces on top of the sill or step. I usie a flat plate and then building a box that is of the same height as the sill and then put spreader plate on the vertical face of the sill and the horizontal face of the floor pan. When I've quizzed TEO about my cages, he said he likes that I tie them in to the sills/steps because that is the strongest part of the car. He said my large spreader plates on the floor (after tying in to the sill) does nothing. I still do it, though.

I try to have the tube sit halfway on the sill and halfway on my box mimicking what you were considering sans box.

Disclaimer-- I'm no pro at this and I'm always eager to learn better and safer ways of building the cages for my cars.

Normally it is a hollow box of the same material as the spreader plate.  All elements are fully welded to the same standards as the cage or spreader plate-to-body.

I vote plinth box. Easier to fabricate and to get the welds right and it's a standard construction method, which makes it less likely to fail tech. I personally like to plate the floor below the plinth box because Toyota floor metal is not always the thickest, and I like to pour heat into those welds. YMMV.

Going back to the "how not to fail Lemons tech" official publication, plinth boxes are not a featured topic.

I may end up building a plinth box after all, but I still want to know if the design in the original post is kosher to rulebook and a safe alternative.

The challenging part of the tube notch design would be notching the tube precisely, but I suspect with time, patience, a tender hand and some soft whispers, it can be done.

I also had concerns about spreading the load of vertical walls on plinth box and frankly with the sharp corners of those boxes at my ankles.  A spreader plate underneath the box as Spank & Mulry suggest would eliminate that shear concern... wondering out loud if the empty space a critical part of the plinth box design?

Spank obviously made the top plate extend beyond the wall plates for structural integrity but my ankles hurt just looking at that picture I guess race boots are high tops but still...

While Plinth boxes are standard and accepted, I do see the potential issue with them having welds in shear and whatnot.  I like the OP's suggestion, but of course it comes down to workmanship as much as design.

I made the plinth boxes for the cage Spank did for the Imp (which everyone says very nice things about).  The plinth boxes were made from thick square tubing.  I used CAD (cardboard aided design) to notch the box so that the box matched the contour of the sill and floor spreader plates.  It's basically the same shape as your solo cup stradding the sill and touching on the floor as well as the top and side of the sill.  It took a few tries to get it so that the plinth matches the plates along the top of the sill,on the sill slope and the floor. 

The top of the sill box overlaps the top edges of the box.  Not sure the box being hollow is worse than the tube landing on a spreader plate welded to thin metal like the floor.  A compression force would need to collapse the steel plate on top of a 4" box (while also pulling inwards the welds on the edge of the box) plus also shoving the 4" box through the spreader plates on the sill and floor.

The verticalish welds on the sill/plinth junction are not really in shear because the plinth is also sitting on top of the sill and the floor.  Both of those would have to fail before the welds on the side of the sill would be in shear.  I would think it would take a lot more fiddling to get the forward leg/cage A pillar notched to fit the sill.  Lots of angle of the dangle to deal with there and if you fail, you have to bend up another leg and get that correct again.. 

When it all went together I thought to myself  "my ankles are sooo hosed" but it has been zero issue so far.  I made sure there was very little overlap for the plinth box in that corner so there were no sharp edges.  If I do get into a wild flip I'm sure my ankles will hit it at some point (along with pretty much everything else in the foot well) but in normal use, the ankle/plinth interference fears were unfounded.  And the Imp is not a big car.

Another benefit to the plinth boxes is that it was straightforward to add tubing fore/aft and side/side between the boxes so that the seat is mounted to the cage and not the car.

THIS is TEO's pattern response for any/all tech questions. Followed by "Do a good job".

Either method is fine, It's all about good fabrication and quality 360 degree welds.

-John

FTFY.