flexin frame...need some fab advice plz

I posted awhile back about the bed on my 34 hitten the cab during normal driving. I cant remember who said there was some frame flex going on.

Yesterday afternoon I got the urge to move the cab forward and take a better look at the whole thing. So I got my harbor freight 6ton stands and went to work.

I went slow, jotted down some ideas on how I wanted to put it back together and got it up off the frame. Well I think I found out why the frame has so much flex. From what little I know of fabrication this surly cant be the way the to attach a back half to the main frame rails....

I need to have some experience here so PLz help me out a bit...

What I am thinking is to grind down the old welds, box the crossmember then fab triangles for the top and bottom out of quarter inch plate. Then finish of with more plate that would cover the end of the newly boxed crossmemeber and the rear frame rail.

Is there a better way?

 

It would seem to me that the tubing thats inboard is where your flex is, I would try and plate the area your talking about as well as the other tubing. providing that I'm seeing this pic correctly. is the open end tubing the "middle" of your frame, so the cab sits to the left and bed on the right where its narrow?

 

Lets see... the whole joint sits under the back 6 inches of the cab. The round tubing is mounted to the front of the ladder bars. Would that still cause flex?

 

I thought why I was down there I would build some gussits from the round tubing to the crossmember as well...

By the way my sorry as aint gunna weild it...ill get a professional.

 

the flex is happening in the 2x3?4? that runs across the back of the cab.

you need to work in a piece of 2x3 on a diagonal from the inside of the front rails to the inside of the cross piece where the rear rails meet it... unfortunatly that is where that round bar is that you say mounts the front ladderbars

the next best is probably to do the plate as you were planning then run a piece of 2x3 from that corner to the outside of narrower rails.this should take some of the torsional forces off that cross piece, which is most likely the main source of flex.

 

Awesome thanks. I have a gap of about 5 to 6 inchs between the motor and firewall. I might just take 3 to 4 inches out of the frame before it connects to the rear cross member.

 

Weld a 2" x 3" x 3/16" end cap on the open end of the box tubing. It probably isn't your only structural problem, but it is one of them.

 

"the next best is probably to do the plate as you were planning then run a piece of 2x3 from that corner to the outside of narrower rails.this should take some of the torsional forces off that cross piece, which is most likely the main source of flex."

I should add that the piece you add should be on an angle, creating a triangle, not parallel with the cross piece.

 

Got another question....the frame rails aer 2x3....should i try to firm them up along the side as long as i got it apart?

 

Stock Ford frame rails were about 5 inches tall, for a reason. I don't think you adding more steel in the horizontal plane is the correct way to solve this problem. You need more height.

Maybe building a truss type frame is the way to go in your situation. Or glob another piece of 2x2 under you existing rail.

I heard of a local guy who was building the frame for his car from 4x2 tube, but he was gonna turn it sideways so he wouldn't lose as much room when he channels the body. I'd lay money he's gonna have the same problems as you do.

 

That is WAY ****ed-up...

Sorry but it is.

I think you're lucky the cab was there to stop the flexing or it would have ripped right off by now.

Let's see some better pictures... LOTS of 'em. It's hard to see the whole physics and geometry from these, but I can see from the first pic that the crossmember is already bent out of square.

What are the clearance issues, also, so we don't all come up with ideas that can't work?

...and point out to us what's front, back, up and down...

Boy-o-boy...
I'm glad, for your sake, that you decided to pull it apart to address the issue.


JOE

 

Ummm, wow! Where to start?

First, does your ladder bar mount crossmember go all the way to the other side? In the pic on the left it looks like it just ends in midair toward the top of the picture but it might dip under the trans tailshaft, I can't tell. If it just ends you need to brace it against the rectangular crossmember. Wouldn't hurt to do it anyway. Then it needs triangular gussets between the crossmember and the lengthwise rails. Should have one at the top and one at the bottom at each location so you're putting force into the strongest part of the tube. It's going to be hard to put a gusset on the bottom where your ladder bar mounts are but you could probably piece together something that will work. Next a vertical plate between top and bottom gussets. Again, that's going to be difficult or impossible where the ladder bar mounts are located, but some is better than none. What you've done is build torque boxes onto the front and rear of that crossmember so that there is a substantial structure to resist the forces that are trying to tear the frame in half. Ideally the torque boxes on the "wide side" should extend all the way to the inside edge of the "narrow" frame rails. The "narrow side" torque boxes should go all the way to the outside of the "wide" frame rails. The pieces should be as close to a 90*/45*/45* triangle as possible but you can compromise that some if necessary for clearance. Like making the face curved instead of straight for tire clearance or cutting a chunk out for ladder bar clearance. And like bobw said, cap the end of your crossmember. As is that's nothing but a hinge in the middle of your ch***is. Which side is the front, the wide side or the narrow?

Oh, and there's no need to use 1/4", the same thickness as your frame members should be fine.

 

That's like saying 55 ain't fast enough to kill ya.

Trying to re-engineer this guy's frame from those two pictures is silly. (No disrespect)

Let's have more to go on, before we start thinking.

I'm not even sure what I'm looking at.

Unless your pinion angle is screwed way up the green arrow points to the rear, correct?
Unless my eyes are blurry, it looks like the frame is bent in the direction of the red arrows, the crossmember has a curve, and the tube is actually shearing. That means we have to stop stuff from moving an alot of different directions. The blue line is where I would have extended the side rails and welded the crossmember inside (and likely of heavier wall tubing). The pretty purple arrow is where I see the steel compressing, and the ugly dotted line is where you get to learn where "OH ****!!! " comes from. The yellow "X" is me wanting to know why that frame rail is positioned so far inboard....???



...and what's around it? That's the big thing.
Can you post some pix from further away, and maybe with the body set?

Hate to waste time on a fix and not be able to do it for clearance reasons.

JOE

 

You're right Dog, I'm making ***umptions about this that may be premature. If the crossmember is both bowed and twisted then the damage is already well along.

 

I'd dis***emble the whole thing, and then s**** the frame for cash by the pound.

 

Got it torn down last weekend and went to welding. I am going to post some pics and try to explain what I have done and where I want to go with this...first off thanks so much for all your input...its extremely helpful. here we go....

After I stripped everything down I recessed quarter inch plate plugs into the crossemembers....Yes both the rectangle and circle are crossemembers. the ladder bar mounts were mounted to the round one. After plugging the wholes I capped each side as shown. I put in the square gusset (top of frame) from the ladder mounts to the rectangle crossmember and side of the frame. I put triangle gussets on the bottom of the crossmember frame joint as well.

Now that its all apart I am going to put the power master cylinder under the car along with moving the body three inches forward...so to close up the gap I want to shorten the frame 3 inches. There was a 5 inch gap between the firewall and the engine..yuk

Also, I am thinking I should add another rectangle gusset from the where the rear frame and crossmember meet forward. Then add some "heigth" on the bottom of the frame perhaps with some thick square tubbing....

 

oops almost forgot...i have to box that rectangle crossmember as well...damn i got alotof work to do...

 

I am no frame builder, but the weak link looks to me like the rectangular tubing that runs across the frame that the front and back are tied to. You basically have the front and back attached to pieces of sheet metal formed in a rectangle. Put in all the gussets you want to, but unless something is done about the open ended and seriously notched piece of cross tubing, I think the frame will flex. That drive shaft notch looks deadly and I don't think the round tube will help it that much with your problem. Boxing the rectangular cross tubing on the ends and at the drive shaft notch should help stabilize it. That is my entirely unprofessional opinion.

 

I believe the area you are looking at is taking the hit but the epicenter is further back under the bed, is there sufficient bracing in the rear rails? This is my unprofessional opinion also

 

You've been working hard on this one and have stated there are several significant problems with the original design as far as the flexing of the ch***is, the positioning of the body and bed on the frame and the overall length of the unit. I would suggest spending some time developing a well thought out plan and a dimensional sketch of what the ch***is should be before any further work is invested. The original design was flawed and your reinforcing the flawed design not correcting it. Your mention of the cab and engine not being correctly layed out reinforces the need for this planning. Its easier and less expensive to change something on paper than in metal. You need to consider many issues when constructing or modifying a frame. What type of front suspension are you useing or going to use ? How much engine are you useing? Do you want a good handling streetable ride or are you going to race a little? All of this needs to be factored into the work your doing now. Maybe you have that drawing already and we dont know about it. From the looks of your latest pics you have redirected all of the twisting forces into outer sections of the box crossmember and that has only redirected those forces into a smaller area which may only aggravate a stress cracking potential in the box tube. I would suggest removeing the entire center and starting over with a better design or even possibly following" Lolife's " suggestion of replacing the frame. Instead of pictures can you draw up and post a sketch of the complete ch***is . This way theres a better understanding of where everything is at and what other issues may be present.

Larry

 

What Jim says is right, you still have a design flaw in that the front rails and rear rails do not have a direct load path, they all go through that middle crossmember. The main problem your current frame has is that the middle crossmember is being twisted, and rectangular section tubing is not the best for torsional twist. I would add some triangulation 2x3 tubing from the ends of the rear frame rails and then forward, as far as practical, on the front frame rails. Kind of opposite of the pieces youhave on the inside right now, but forward farther if possible.

 

I cant draw **** on the computer...but I have completed one step of many. here is the objective....

I would like to strengthen the frame that is under the car so it is safe enough to cruise...go down the drag strip and enjoy. Right now the car has an ifs in the front...its an allison clip along with a allison rear ladder bar clip.

There are several weak points to this one and I am having a good time sorting through the issues.

Yes I do want to goto a dropped axle when i get the $...they are pricey. I will keep the power disc brakes...cause i like to stop good.

This will really get you...I have the original frame sitting on the other side of the garage...right now I doubt my skill into zing it and laying it out proper. My plan is to make what i have work, use that practice to make the OG frame correctly. Before I start the new frame I would like to get a plasma cutter to make that project a little bit easier.

I picked this half ***ed project up for ab out 5000 in colorado and have put considerable time just to get it up, moving, stopping and now i would like to get the wheel base right along with hiding the booster and making it safe. i realize it will never look right fenderless and IFS...but I must have patience.

Again any advice and input is greatly appreciated. Today I will cutt the thing off at the crossmember that i just boxed in and braced. What is the best way to attach those frame rails back to the crossmember?

 

if there is room you should ad to sections from the ends of the rails to rear section. the ones on the inside do little to help the flex.

 

OK, here is a real ****ty pic, best i can do with paint. This is what i mean about making the load path so the middle crossmember does not have a torsional (twist) load on it.

I added a couple extra pieces going on the inside of the rear frame, in addition to the longer pieces on the outside of the front rails.

 

That should be pretty easy to do...also i was thinking on the rectangle crossmember, after i box that peice in I should run plate or some smaller square tubing across the top....???

 

also I was thinking about our flexing issue...I never remeber feeling the frame twist much....just bounce back to front. In fact if I put it on stands by all four wheels I could bounce on the bed and the whole frame would woble up and down...

here is some pix of the the rear of the frame...

 

I do not understand what you mean by adding a plate or tubing across the top of the rectangle cross member. I do not see where this would add any real value, you need to keep the crossmember form being in twist from the two rails not being in-line.

 

This model I just created will get you the additional frame stiffness that you require. The pink represents the frame as it currently exists. The blue peices are rectangular tubing, ----make them as long as you possibly can. I am ***uming they will be under the cab section of the truck. If possible run them right up to the firewall of the truck.

 

Thanks to everyone for the time on this...i spent a few hours cutting it apart...so its now in two halves...I will box in the crossmemember then add the frame supports to the outside frame rails just as many of you suggested and Brian did the model on.

thanks again...

 

In my opinion, 2x3 is simply not strong enough for a frame. The design allows for far too much flex in all planes (ladder frames are particularly bad for horizontal flex) and the usage of inferior material just exacerbates the problem.
If you can, weld another 2x3 onto the underside from the rear wheels to as far forward as humanly possible. Make it one piece!! I know that this will look bad, but not as bad as a broken frame at 70 mph.
Consider a 6-8 point roll cage for the added strength.
The real problem is that the frame design from the beginning (material, layout, and execution) was, and is, poor.
Cosmo
(no expert, but that frame scares me)