Material question

I have an 82 Chevrolet full size 1/2 ton in the shop right now that is in need of a frame replacement due to botched work from another shop. The owner is interested in doing a replacement chassis out of mandrel drawn 2x4 .125 wall tubing for a clean street rod kind of a look. I was curious what advise you could offer with regards to the strength of this thickness of material under a V8 fullsize truck. I am most concerned with the under cab portion having an undesirable amount of flex due to the wall thickness. There will  be some .120 wall round tube crossmembers through the underbelly in an X configuration with a trans mount/driveshaft hoop built in . Will this be enough in your opinion or would it be in my best interest to try and double up the rails for strength? I don't want to overbuild it unnecessary amount but I do want it done right. Normally I would do 2x4 x.188 wall with mitred cuts but that thickness is not an option with the pre bent tubing. Any insight you could offer would be greatly apreciated, thanx in advance.

   THIS IS WHAT I USED TO BUILD THE BACK HALF OF A FULLSIZE 2X4 14GAUGE IT REAL STRONG

Seems a little thin. Several places do a .188 wall. Wait for Max though.

You can get 3/16" wall mandrel rails through Air Zenith...

The .125 wall material is usually strong enough to do what we need, the problem is that when you lay the truck out the frame relaxes and you can see how much the frame actually flexes between the cab and the bed. The cab mounts add strength to the package so use thier mounting points on the frame to attach the cross members to. We try to eliminate as much frame flex between the bed and cab by building a truss of sort between the notch and the tranny crossmember through the driveshaft tunnel. The crossmember that runs just behind the cab loops over the driveshaft and then we use 1 1/4" tubing to go from the notch to the  highest point on the crossmember and from the crossmember to the trans crossmember at the frame. If you think through the truss it can be done in such a way to be no more intrusive than the driveshaft and will add a lot of strength.

---------------------------------------------Originally posted by fredstinting   THIS IS WHAT I USED TO BUILD THE BACK HALF OF A FULLSIZE 2X4 14GAUGE IT REAL STRONG --------------------------------------------- Thats not 14 gauge. I think you probably mean 1/4".   I think AutoWeld Chassis makes 3/16 mandrel rails. They are definitely available, and would probably be my choice for a fullsize.

Thanks for the info Max. I am going to check it out today when I'm at the shop to see what I can do in the way of a truss, I'm sure I can come up with something cool and functional. The truck is going to be bagged but will sit with a rake when aired out without the frame laying so it won't relax as much as a truck that lays frame, this should help with the bed/cab gap. My next question would be that I am pretty sure that there willl be a splice in the frame near the front clip because of some of the bends it will need, what do you recomend for re-enforcment of the joint? I would usually put a diamond shape gusset on the outside and call it good but I'm pretty sure this guy isn't going to like the look of that. Would a would a diamond gusset on the inside of the tubing plug welded and welded in with the butt joint be sufficient? The welds would be TIG for maximum penetration and I could offset the splices from one side to the other if it is necessary. I  could probably do the splice at the front body mount now that I think about it and use the body mount to help strengthen the joint, I'll look into that today as well. Any input you can offer would be great. Thanks for your advise, I really appreciate it. Keep up the good work!

On the dually I did, I didnt want to have any visible seams on the external rails. So wherever there was a joint, I cut 1" holes about an inch from the edges of the mating frame sections, and Rosette welded a gusset on the inside. Then you can just make a little filler plate to fill in the hole and metalfinish it smooth. You can see where the outer rail transition joins the front clip here: 

Both ideas will work well.  ^^^

Max, can you post pics of the rails that you run from the tranny crossmember to the notch?