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Tubing thickness for frame crossmembers?

dyeager535

dyeager535

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First, the obligatory "I searched!".

Boxing the K5's frame, want to replace/add crossmembers to try and tie it together better at the same time. No cage. Can't get a good idea of what thickness/diameter should be considered. Tubing cost is important, but truck usage is pretty light. I plan to run the tube completely through the boxed frame, which is how I see OEM's doing it.

I'm looking at 2" tube up front, which will span the gap between the front crossmember/spring mounts. From what I've seen these can tend to be a bit of a problem area, so I figured .120" would be a safe bet. I'd like larger diameter, but that would interfere with the mounts.

In the rear, I plan to replace the crossmember that is just in front of the rear axle, on the upward-angled portion of the frame. This location will clear up to a 6" tube, but I was thinking more along the lines of 3 or 4", which would still give clearance to run exhaust under it without some harsh bends. To give a better visual, this is the crossmember that is raised up to clear the muffler(s) with riveted on "legs" that tie the upper and lower frame channels together.

Undecided if I want to go back a bit further and add another one just forward of the front gas tank mounting crossmember. I believe that neither of the two rear crossmembers I'm contemplating is where they really need to be, the suspension mounting points, but for straight tubing, I am stuck due to driveshaft or fuel tank clearance.

I'd appreciate any input on tubing diameter/thickness. I haven't really researched pricing vs. thickness and diameter, but I am going to guess that the thinner the wall (and smaller diameter), the cheaper. I don't want to get stuff that is on the ragged edge, but if .095 or .120 is way overkill, then I'd step it down to whatever is recommended, assuming I can get it and it's cost effective.
 
if you're gonna replace factory crossemembers with tube I have learned from building crossbars in tractor loaders that diameter is way more important than thickness for torsional stability. FYI.
 
I have used .120 wall with good success. Its more about spreading the stresses than anything.

Gussets where the tube attaches to the frame. Your idea of running it through both walls is excellent.

When I do a crossmember I try to tie it into at least 3 legs of the frame, on our c channel frames that means top bottom and side. If you can tie it into all 4 walls on a boxed frame boo yah. All the more better.

Once again design is somewhat more important than material. When you are designing think "what can I do to spread the stresses out over a wider area?"

Then implement those ideas.

Don't limit yourself to round stuff or square stuff, think outside the box, use smaller tube if you can spread it out more, than join it with flat plate or more of the same type of tube.

Spread the load man.
 
http://coloradok5.com/forums/showthread.php?t=286260&page=21

Heres how I crosstied my boxed frame with HREW. I used 1-5/8ths cause I got it for free and it was larger than the stock crossmember. This is .120 wall and is more than enough.

As said use a piece of tube thats nearly as large as will work. Going through both sides of the frame is intensely strong. Material is not really important here, DOM, HREW, Structural Schd40 will all work. In fact structural schd40 is the only thing you are going to find for a reasonable price for a 2in or larger piece.
 
There is where the problem lies...haven't been successful finding anything that really gives me a place to start with in tubing diameter/thickness, so that I can make a better estimation of what would work. Of course without knowing the actual stress it's impossible to know what is technically "correct" for the application, but there are enough folks here that fabricate stuff, I figure I'd get some answers based on what has worked for others.

Any idea if .083" would work for the 3-4" crossmember(s)?

As to not limiting myself to straight, round tubing, I have a complete second frame which will donate the metal for boxing, but as well, the spare crossmembers off the donor frame (engine and transmission at a minimum) will be used to box the existing ones. All crossmembers will be welded in place except the transmission one.
 
Darn work, will have to take a look at that thread (pics) at home.

I had looked at pipe, was hoping ot keep weight down if possible. Didn't really spend the time to calculate what various diameters of pipe/tubing would add, but IIRC pipe is compartively quite heavy. Of course, I'm looking at maybe 10ft of material.

I just figured at $7/ft that was doable. That was for DOM, I am guessing it could be substantially less.

Edit: Found a pretty fun spreadsheet: http://members.shaw.ca/xoltri/forumpics/tubeweights.xls Might have come from Pirate4x4 originally. Can make OD and Wall Thickness whatever you want, it will calculate out weight, and the percent strength and weight difference for tubing, compared to each other. Change the highlighted row to whatever specs you want as a baseline, then adjust any of the other rows OD/wall thickness to whatever you want, and it will do the rest.
 
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.120 wall will be fine for most crossmembers but if you are going to wheel it in the rocks, anything that might come in contact with the rocks and carry a load use 1/4". I am redoing my trans and t case crossmembers now and they are both made of 1/4" and have been hit hard and both are bowed up an 1/8th to 1/4 inch in the center. :eek1:
 
Mine won't be contacting anything luckily, these shouldn't be doing anything other than resisting the frame attempting to move.
 
Have you considered just X membering your frame? Nothing stops a frame from shifting like some lateral triangulation!

You can also reference my build thread for that although Ive only done half of my X.

And good luck getting a square tube crossmember through both sides of the frame w/o a plasma cutter.

As far as the .083 stuff goes my trooper had a stock crossmember that thick in it. Was 3in OD about that thick and it had a gdamn upper link mount on it!
 
I have started considering it. :) The rear is a good area to go about triangulating, thats about the only spot with enough room to get anything in there. Straight crossmembers are just so much easier to run it seems, at least when considering having to cut through both sides of the frame. Or at least I'd be tempted to cut through both sides even on the "legs" off the crossmember. Kind of handicapped without having some good metal cutting tools where I can get some consistent cuts.

I've got the cutting thing handled. Was going to buy a plasma, Dad bought one of those oxy/acetylene setups that is supposed to work like a plasma. We'll see. I plan to use it to carve the donor frame up.
 
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Yeah if you are going to X the frame then I would start the legs at the transfercase and then go as far back as you can.

Oxy/acetylene is the old steadfast for cutting. Its definitely not as easy as a plasma but once you get the hang of it it works just as good for the thick stuff. Just be careful as it can warp stuff under 1/8in if you dont cut it fast enough.
 
Unfortunately I think with the way I plan on running everything (muffler under the floorboard just aft of the transfercase crossmember to clear what I plan to do further back) won't allow me to run the legs that far forward. However, worst case that would allow me to at least tie into the frame right where the springs bolt up, which is where I really wanted some support. That should also allow me to clear the driveshaft.

This is the thing my Dad bought: http://www.cobratorches.com/7.html Supposedly far from oxy/acetylene torch used to cut, even though that's apparently all it is as far as I can tell, just refined. He's seen it in action, and from what I have read and seen they do cut well, just haven't used it yet myself. Nice to have no electronics to fail (ala cheap Plasma) but then again, I don't like being tied to the tanks either. Was really nice to set the MIG up for gas welding for the first time, only to find that the "full" 80cf tank is apparently empty.
 
so have you thought about just using plate and making your own shapes?

Lots more work but you can put stuff where you couldn't put other stuff, put holes in it, bends tweaks etc.

One of the reasons I suggested gusseting to the frame it to mimic on a very small scale an X brace in the frame. Don't think the crossmember needs to be straight across, make the pass side a couple inches forward or backwards from the pass.

Lots of ways to go about it. A straight across crossmember is probably the least effective way to make a crossmember. At least on a frame such as ours is.

Even though you are going crooked ( if you choose to do it) still go through the frame like you had already planned
 
blazinzuk said:
so have you thought about just using plate and making your own shapes?

Lots more work but you can put stuff where you couldn't put other stuff, put holes in it, bends tweaks etc.

One of the reasons I suggested gusseting to the frame it to mimic on a very small scale an X brace in the frame. Don't think the crossmember needs to be straight across, make the pass side a couple inches forward or backwards from the pass.

Lots of ways to go about it. A straight across crossmember is probably the least effective way to make a crossmember. At least on a frame such as ours is.

Even though you are going crooked ( if you choose to do it) still go through the frame like you had already planned
Click to expand...

Yeah some 90* gussets from the frame to the crossmember on both sides will laterally support the rails some bit.
 
Trying to keep this as low budget as possible...have to be able to afford the shocks, the radiator, etc. Metal isn't cheap anymore, so I'm going to TRY and limit myself to as little extra needed as possible. I mean if there is no way around something, I'll have to bite the bullet I suppose. I figure I'll have plenty of material left for gussets.

I'll have to get a picture and have some fun with MSpaint, see what opinions are.

I'm thinking straight across tube to replace the existing crossmember just forward of the rear axle, like I had envisioned originally. But add legs coming off of that (forward) welded to the frame where the front spring hangars mount at approximately a 45* angle. Do the same thing off the rear of the crossmember, but tie the legs into the frame just forward of the gas tank.

If the idea is sound, would it be ok to tie the legs into the upper vertical portion of the frame (instead of centered) where the spring mounts are? That would give me clearance to tuck the exhaust tubing up, or at least make space if exhaust design has to change a bit and the muffler needs moved.
 
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