i'm thinking about it. kinda wondering is theres any reason i shouldn't

You mean like fill all the cavities with that expanding liquid foam stuff? I suspect it would be quieter, and delightfully draft-free!

yeah, that and its supposed to stiffen things up

One of the big three is doing that from the factory, foam reinforced chassis...

it would only work if you could fill the cavity 100% so that it bonds to the frame, no different then foaming a boat.



if you did not achieve 100% then the foam would do nothing, I doubt using spray cans or pouring foam that you could get the correct results

What, then, would be the way to get the correct results?

a factory is doing it under ideal conditions, they obviously have specialized guns that inject the foam



the only way you could try to do it would be to close off every hole and close off the opposite end of where you were putting the foam in.

Then placing your container that would be pretty much the exact size of the frame rail, or making a adaptor for a container to be placed against the frame rail, mix your 2 part foam (because the spray foam from home depot doesnt do anything) and let it expand into the cavity while you held the container tightly against the frame until it pused itself back because it was full.



Then clear off your holes when it cured, which only takes a few minutes.



Someone mentioned how people re-weld the seams to get more rigidity.

Theres structural foam that comes in a kit of 2 bottles and a mixing tip that injects the foam into the cavities. You are suposed to tape off all the holes and put a pinhole in each one for excess foam to expand out of without making a huge mess.



Ive done it, and its not hard. Just take your time with prep work use due caution.

in your case the tip is just acting as something to direct the foam as it expands, so its pretty much the same thing I said.



if it does not bond to the frame rails 100% though your wasting your time

Ive been thinking about it. I thought it was for rollover protection to keep the a+b pillars from collapsing so easy. I didnt think it made the frame more rigid.