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Fabricating A Cross-member

Discussion in 'Builds and Fabricators Forum' started by FinoCJ, Jul 8, 2021.

  1. Jul 8, 2021
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
    Joined:
    Jul 18, 2013
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    5,593
    Looking at building a cross-member, and wondering what steel would be best (e.g. 4x2" rectangular channel, 3/16 thick etc). I am going to need a 'drop' section by a couple inches and something like these might be doable - more the general design and construction than the details of the trans or end mounts:
    [​IMG]
    [​IMG]

    The upper one looks a bit easier to fabricate without the miter cuts? In both of these cases, I like having the top flat on the passenger side to mount the D18 side support, but wondering about the wall thickness of the tubing, and having to bolt through the tubing for the trans mount isolator...would it need to be sleeved. And sort of the same issues with the end mounts along the frame rails.

    I got a suggestion of using channel of some type - that would eliminate the bolting 'though' the tubing issues. would that work? I don't love the idea of the open side down and having the ends ready to catch on everything that goes under there....The tubing would hang just as low, but at least its smooth on the bottom and would slide over stuff a bit better.

    Basically, this fab is a bit over my head - I was looking at the posts in Scotts thread from a bit ago, but its not helping much....The OEM cross-member bolts under the frame rails, and thus hangs down 1.5" below the frame rails. I was planning to copy that set-up, albeit with 2" tubing so it would be a bit lower still, and the center drop section will hang down another inch or so. Does it make sense to bolt it to the bottom flange of the frame rail, or should it 'wrap' around and up the outside of the frame rail with some flat steel plates to bolt through the frame rail from the outside?

    thanks
     
  2. Jul 8, 2021
    Mr Vaughan

    Mr Vaughan

    try @ITLKSEZ, he's great with fabrication. he might have some useful stuff in the varg thread
     
  3. Jul 9, 2021
    73 cj5

    73 cj5 Not ready for the junkyard yet

    Clinton, Mississippi
    Joined:
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    2,137
    You could probably come straight down from the frame rails so you don't have to make miter cuts if you want something quick and easy. Once you sit down and stare at it for a while it'll just fall into place.

    [​IMG]
     
  4. Jul 9, 2021
    Muzikp

    Muzikp Active Member

    Sacramento Ca.
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    I have been coveting the one on Seabea @tymbom for several years now, it's really nice and "probably doable" for a guy with my skill level. The build of it starts here Seabee M38A1 Build
     
  5. Jul 9, 2021
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
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    In the upper picture, the 90º angles at the bolted connection look like a weak point to me. The very rigid square tube would focus stress and fatigue at those places.
     
    fhoehle and Glenn like this.
  6. Jul 9, 2021
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
    Joined:
    Aug 13, 2015
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    6,656
    The way I build anything, simplified:
    1. Figure out what you want as far as mounts at the frame and install them.
    2. Install the mount on the thing you are hanging and put it where you want it.
    3. Connect the dots with a structure that is strong enough to support abuse, but not stronger than its surrounding structures. If it is stronger, use rubber/poly bushings at the frame to allow for flex.

    The “connect the dots” method allows you to stay as simple or get as wild with the design as you want. Mild or wild, the 3 points of contact are doing the same job.

    If you go with square tubing, I’d connect it to the frame with something like this to allow some flex.
    Roll Cage Tie-In with Poly Bushing Kit
    It also makes removal a breeze by only having to pull 2 pins to drop your crossmember.

    At this point, you might be adding more work by trying to use the factory crossmember mounts. You’ll end up with a zigzag of support, where the line from point A>B>C is non-straight on 2 planes, not just one, since the mount is behind and below the factory mounts. It’ll put unnecessary twist on the crossmember.

    If it were me, I’d hollow out the top of some 2 x 3 x 3/16” tubing to allow the mount to drop inside. I’d grab 2 of those mounts in the link and mount them up as high as possible in the inside of the frame directly in line with/square to the trans mount. Wider tubing like 2 x 4 would work, but if using the mount in the link, it would need to be tapered to fit the 3” wide sleeve in the mount.

    [​IMG]

    (Note: If you go this route, make sure there’s enough space to pull the pin and drop the sleeve straight down and slide it backward/forward. If you don’t have clearance to do this, once it’s assembled, it will be stuck between your frame rails. If that’s the case, you’ll need to box that section of frame and mount it to the boxing in order to drop the unit straight down, but you *should* have enough clearance inside a wagon frame. It would be better to avoid boxing a section, unless you’re doing the whole frame.)

    The rest is just a matter of cutting/bending/welding pieces of the tubing in whatever manner you see fit to connect the dots.

    Don’t forget about clearance for the front driveshaft, and accommodations for the t-case mount. Also with the top of the tubing open, make sure you drill drain holes or leave the ends of the lowest section of tubing open.
     
    Rich M., Twin2 and Fireball like this.
  7. Jul 9, 2021
    Twin2

    Twin2 not him 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Virginia Beach, VA
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    forgot to take art class did you :D
    but your a master of CAD (y)
     
  8. Jul 9, 2021
    melvinm

    melvinm Member

    Arvada Co. 80003
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    I have A length of 2" X 2" with 3/16 " wall if you need it

    If I don't answer the phone leave A message .

    303-420-8310

    Mel.
     
  9. Jul 9, 2021
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
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    I have a bachelors degree in art. :p
     
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  10. Jul 9, 2021
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
    Joined:
    Jul 18, 2013
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    As stated in the OP, not looking at the end connections - just the drop section construction....

    there was a time when other people's nice work was inspiring...now its just depressing and a reminder that I cannot even come close building something much more simple...

    Guess I don't understand this...fore-aft the new mount is in the exact same place as the old mount, just lower....its not behind. Instead of sitting 2" above the cross-member with space for the isolator, it sits 1/2 above the cross-member so there is no space to fit the isolator. Plus with the current positioning and wide cross-member, the T-case side mount is perfectly aligned.

    I guess that is what I was trying to avoid doing by dropping the middle section....I cut-out out the old cross-member to allow the bushing to slip through - but thought that was pretty marginal and weakened the crossmember too much. Maybe i should go back to that....

    I am worried about mounting stuff on the inside of the driver frame rail as I have clutch linkage and stuff mounting in there - so while I at first thought it was a cleaner install inside the frame rails, not sure it will work....plus, with the crossmember under the frame rails (as was OEM), there is room to fit the exhaust above the cross-member on the driver side (the passenger side is tough with the D18 etc so probably single exhaust on the driver side). Finally, one last challenge with inside the frame rail mount - as it would be mounting exactly where the old mounts are (but on the inside), the old mounts are also part of the body mount and have 4 rivets through the frame in the area...Even if I cut the lower cross-member mounts off, The body mount and rivets are still in the way.

    I am pushing over 3 weeks....still staring...

    Thanks for trying to help...the issue is me...my brain just doesn't seem to work this way...and even when it comes up with a decent idea, my skills can't complete the job...
     
  11. Jul 9, 2021
    cayenne

    cayenne Member

    central Texas
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    276
    Here is what I did, and I don't have fabrication skills or tools. I did it with an angle grinder, poor tack welds, alot of trial and error, and a buddy who let me take it to him for welding.
    I would have much preferred to put the ends inside the frame rails like mentioned above because I think the ends are hang-up points, but that would have required me to box the frame, or put a deeper bend in the kit's supplied tube, neither of which I wanted to tackle. I just lucked out the kit's pre-bent tube put everything where I needed it.

    Your idea of square tubing would allow for a more secure skid plate mount, because what I have run into is that mounting a skid plate will be hard since I can't attach it to both the frame and the crossmember (since the crossmember is isolated). Also, the added complexity of intersecting round tubing was hard even with templates, etc you can print out online.

    Cayenne's Calamity Build.
     
  12. Jul 9, 2021
    Rich M.

    Rich M. Shoe salesman 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Maryland
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  13. Jul 9, 2021
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
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    You started talking about using tubing. Any tubing will have more rigidity than the factory crossmember, and if you’d mount it solidly to the existing factory frame tabs, you run the risk of fatigue.

    I think your best bet is to just reenforce the factory crossmember.
     
  14. Jul 9, 2021
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
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    I understand the issue with more rigid crossmember and using poly bushing mounts....didn't understand the non-straight on 2 different planes piece...didn't see how going to the universal tubing style would cause that.

    yea - its sad to say but true given what I can do...I wish I could make nice stuff...:(
     
  15. Jul 9, 2021
    colojeepguy

    colojeepguy Colorado Springs

    At the foot of...
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    Once you figure it out, make two, I'll take the second one:D
     
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  16. Jul 9, 2021
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
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    Disregard that; I was thinking of others’ where the trans swap pushed the mount back further.
    I think it’s a confidence issue more than a skills issue. Your firewall work looks great from here! You can do it!!
     
    fhoehle likes this.
  17. Jul 9, 2021
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
    Joined:
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    chatting with someone on OWF with the same swap, and they fought the same issue...basically the extra length of the sm465+adapter is all taken up by pushing the engine forward 3 inches...the adapter mount literally sits on the cross-member right were the old mount was....They put larger body lift on to make the clearance, but also went to hanging pedals (and probably hydraulic clutch)...I don't want to go that route, and need the through the floor pedals to work normally. It would have been easier had the mount been moved back 1.5 inches as a simple piece of angle iron welded on the back of the cross-member would have been easy....

    This is how I am modified the original cross-member - just don't really like it and was looking for a better solution:
    [​IMG]

    I welded in a vertical flat bar to help provide vertical strength - I need all the room I can get inside the notch, so had to weld the re-enforcement on the outside of the notch and then drilled holes through and plug welded it. I tried to overlap the bar a couple inches on each side to distribute the stress away from the opening. I tacked in the horizontal 'floor' inside the notch for the mount to sit on, but I didn't extend the floor laterally like I did the vertical support to distribute the stress away. I need access to the underside for the parking brake mount nuts...so I stopped and am debating whether I need to come up with a better way to extend that horizontal support and maybe add a couple access holes....Maybe it needs a lot more support - especially as it already been bent the eff up from a PO.
     
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