Frame rail thickness

It's all about section modulas. I did the calcs and with the stock 8" C-channel frame. To match it in strength with a 3"x4" I need 5/16" wall. That's what I'll be using. In the vertical direction 2"x4"x5/16" would be just as strong, just not in torsion.

I dont understand how 2x4 is to tall....my stock frame was 8 inches tall under the cab....I laid 2x4 tubing inside my stock frame and cut top of frame off 4 inches shorter.....which give me enough clearance to move my floor down 4 inches....at 4 inches of body drop my truck is a good bit into the rocker...it doesn't matter if you cut bottom of the frame off or the top regardless your frame went from 8 inches tall to 4 inches....which either way you still have the same amount of frame rail to cab floor clearance as stock....so explain to me why I would wanna use 3x3 instead of 2x4

if i would have used 3x3 on my truck and cut off 4 3/4" inches off the bottom of the frame... i would have to cut way past the pinch weld and get into the rocker to lay . and i wanted to keep my rockers..
also, my lower control arms would have hit the ground before the frame touched the ground. and you don't want to grind off the lower a-arm u-bolts and loose the control arm while driving down the road..

using 2x4 in the top of my frame and cutting off the bottom . i put some 2 inch wide 0.25" flat plate on the bottom of the 2x4. (this actually made my frame 4-1/2 inches tall)(including the thickness of the top of the original frame)
i cut my pinch clean off.. when i lay my truck out. the bottom of the rocker isn't touching the ground. (but its really, really close) it's about 1/4" from laying. this is because i put the 0.25" thick plate on the bottom of my frame. if i had not done this. the rocker would be perfectly on the ground and even with the bottom of the frame.
so, 3x3 wouldn't have worked on my truck unless i wanted to door it. and i aint doing that...lol

i agree. using 2x4 for the c-notch is ugly. not enough beef for me. 3x3 would work good. but 5x3 is beast!

I love this thread. Valueable info in here.

Sparky- you bring up a good point about the 2x4 notch looking thin...5x3 may be they way I go there, especially since I am now looking at doing bags between the axle and top notch beam.(more torsional support.

I also like the 1/4 flat plate on the bottom of the frame...to protect the 2x4.

Keep the good info coming guys!

its archived here somewhere. and i cant find it...

i do remember reading on here that some crewcab dually frames are different from each other. some will lay without a z. and the others wont.... are the 80's frames different than the 70's? or is it due to being built at different factories? im not 100% sure. but i do recall seeing a thread about it..

someotherguy , do you know anything about this?

I'm not sure which trucks got what frames,but I know there are different frames. I can tell that just from looking at your build thread Sparky. Your frame horns look nothing like mine. Mine is an 87' and I have no idea what years they used that frame for but it would be awesome to find out. I'm doing a 4.75" Stock floor no matter what, but it would be some great info to have on hand.