Welding plates to floor

weld all the way around

I had this same question, seems like it would be stronger and less likely to punch through if it were in a stitch weld instead of 100% around.

Replace your whole floor with 3/8" stock sheet. That way it is plenty strong and you'll be able to handle small arms fire from below.

ROFLMAOPIMP!!! That coul have gone wrong in som many ways but the balance was perfect... Alomst looks like something from a video game.

For bolt in cages, there should be a equal sized pad(recommend 1/4" thick at least) used on the underside of the floor pan to "sandwich" it.  By design, this method spreads the load equally around the entire pad.  Since you are welding, do the entire pad....welding wire is cheap and your life isn't.

not being much of an engine guy, i do know a little about fabbing metals together, shadetree style. as long as the plates are sitting on the pinch welds in the floorboards, the sandwich idea isn't needed.
no matter how you do it, tho, be sure to get good penetration without blowing out the floorboard.

Agreed. The main point of sandwiching the chassis between the plates if to make it a uniform part of the cage/chassis.

The main function of the cage is to prevent you from being crushed/decapitated in a rollover event. So the brunt of the force is going to be placed on the interior feet of the cage; therefor a lager plate on the OUTSIDE will do relatively nothing. What you want to prevent is THIS:

http://jalopnik.com/5390934/mustang-cov … -tire-wall

Seems to be a common thing... Mustangs commiting suicide

Driver and Passenger actually walked away, amazingly enough.

Remember, the floor pans are usually weak, especially in the crapcans we race!  Adding a thick plate on top of it can help, welded and/or bolted. 

Bolting a backing plate will only help if somehow the cage is "pulled" away from the floor, where the pad on top helps keep it from "punching" through the floor(like on the blue car pictured).   In this photo, you can see the bar that punched through the floorpan:

On our car, we built a platform that rested on our door sills, these are much stronger than the single sheet floorpans as they are thicker, and in most places will be doubled up or even triple layered sheetmetal.  1/8" thick plate welded on top, and this is the least of our worries.

For the front windshield bars, they land on 1/8" thick pads as well, but again we chose to tie to the door sill once again, with the pad being stitched, bend 90*, and re-welded.  From here the vertical is welded to the sill, and the horizontal again welded to the floorpan.

There are many other ways to do it, but spreading the load to more than just the floorpans will help immensely.  Any route you take should include thick mounting "pads", I'd never weld a tube directly to the floorpan.

It can still happen if the mounting points are not properly reinforced.

An even better addition with cumulative effects is doing this: