Thoughts on frame crossmember

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superbee70
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Thoughts on frame crossmember

Post by superbee70 »

Started tear-down and looks like something made a nest in the cross-member just ahead of the rear axle. Options are to replace or patch in place. Prefer replacing, has anyone done this? I have the trans cross-members out, at least the flat pieces, not the frame brackets. I think I can remove the radiator cross-member and spread the frame pieces enough to slide this cross-member in. if not would have to remove the two cross-members located closest to the rear of the frame. really prefer not to patch since the riveted section of the member is really ratty.
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Elwood
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Re: Thoughts on frame crossmember

Post by Elwood »

I'm not sure that you'd need to remove one or more of the other crossmembers. Remove the rivets holding the box section crossmember together and to the frame, grind off the welds at each end of the crossmember, and rotate the crossmember out. You might have to push the frame rails apart just a bit, but they're flexible enough to do so.
“When a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, IT IS THEIR RIGHT, IT IS THEIR DUTY, TO THROW OFF SUCH GOVERNMENT...” -Declaration of Independence, 1776
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Re: Thoughts on frame crossmember

Post by superbee70 »

Thanks for the reply. Have some time today, will see if the old one comes out with the others in place.
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Re: Thoughts on frame crossmember

Post by superbee70 »

Worked like a charm. Got all the rivets out, moved on to my BFG and was able to bank out the crossmember diagonally.
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Re: Thoughts on frame crossmember

Post by Elwood »

Glad it worked for you.

Have you decided how you're going to install the replacement cross member? Rivets or bolts?
“When a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, IT IS THEIR RIGHT, IT IS THEIR DUTY, TO THROW OFF SUCH GOVERNMENT...” -Declaration of Independence, 1776
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m-37Bruce
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Re: Thoughts on frame crossmember

Post by m-37Bruce »

I used nut/bolt for my cross member swap, it was the final one with the Pintal Hitch, I have no intention of towing anything what-so ever. I removed it to straighten it, it had pulled way to much in the past, almost 1.5 inches out of square.
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Re: Thoughts on frame crossmember

Post by Elwood »

Bruce, a proper nut-bolt-washer connection is at least as strong, if not stronger, than a steel rivet. I have an Ingersoll-Rand 6A pneumatic rivet gun and half-round rivet headers, plus a small forge for heating the rivets, but I'm using Grade 8 bolts and nuts on my frame. I think Chrysler specified rivets on the original frames because they're cheaper and faster to assemble in a production situation, not because they're better.
“When a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, IT IS THEIR RIGHT, IT IS THEIR DUTY, TO THROW OFF SUCH GOVERNMENT...” -Declaration of Independence, 1776
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m-37Bruce
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Re: Thoughts on frame crossmember

Post by m-37Bruce »

Cha-Ching, good news, grade 8 all around on re-install.
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superbee70
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Re: Thoughts on frame crossmember

Post by superbee70 »

Bolts here. May tack weld the nut to the bolt when done tightening. Also will wells the top and sides to the frame.
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Re: Thoughts on frame crossmember

Post by NAM VET »

I am certainly no metal structural engineer, but back when I had my Superformance Cobra, several owners decided to cut out and then re-bolt a main cross frame member so they could remove and install their transmissions without having to pull the engine. The company who makes the cars was adamant that the cross member had to be welded (or in the case of our trucks, riveted) because bolts would allow flex and movement of the union. Makes sense to me. So it is probably best if bolts, no matter how stout, be used on a frame member, and that union be also somehow welded.

But then, how would I know.....

all the best, guys.

NV
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Re: Thoughts on frame crossmember

Post by Elwood »

NAM VET wrote:I am certainly no metal structural engineer, but back when I had my Superformance Cobra, several owners decided to cut out and then re-bolt a main cross frame member so they could remove and install their transmissions without having to pull the engine. The company who makes the cars was adamant that the cross member had to be welded (or in the case of our trucks, riveted) because bolts would allow flex and movement of the union. Makes sense to me. So it is probably best if bolts, no matter how stout, be used on a frame member, and that union be also somehow welded.

But then, how would I know.....

all the best, guys.

NV
Riveted frame connections are not especially rigid, and the rivet joints used on these frames are not of the highest precision. Using the same small welded joint at the top of the crossmember, a properly installed bolted joint is probably stronger than the factory riveted joint. As to the desirability of an inflexible frame in an off-road 4x4 vehicle, hmm...
“When a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, IT IS THEIR RIGHT, IT IS THEIR DUTY, TO THROW OFF SUCH GOVERNMENT...” -Declaration of Independence, 1776
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Re: Thoughts on frame crossmember

Post by NAM VET »

yeah, adding some weld is likely added insurance against frame movement. Also a good idea to ensure the bolt holes are a precise fit, perhaps even threaded, for the bolts.
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Re: Thoughts on frame crossmember

Post by rickf »

You don't want to weld in crossmembers that were riveted or bolted. They were do so for the main reason so the frame could flex. If it could not flex it would break. A long frame has to have the ability to fex and bend along its length a small amount or it will get stress cracks in it. Ever watch a more modern Ford or Chevy pick up out four wheeling? Talk about twist!!! But they will do it all day, every day. I know a guy that welded in the cross members on his 78 Ford pick up and within 6 months he had a broken frame. And even before that the truck rode hard as a rock and handled terrible.
Last edited by rickf on Thu Sep 07, 2017 1:34 pm, edited 1 time in total.
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Re: Thoughts on frame crossmember

Post by just me »

A riveted joint is much stronger than a bolted joint. And it is flexible. Both are desired in a truck frame. A welded joint is too rigid and the weld itself will cause stresses that can (and usually do) crack the frame. The rivet, when properly installed, fills the holes and a secures the parts completely. Most automotive rivets are cold formed with "squeezers". Since we generally don't have access to tooling like that, I install them with hot rivets and a buck. When the joint T cools the rivet contracts and really holds the parts together!
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Re: Thoughts on frame crossmember

Post by superbee70 »

My thoughts...

In a perfect world you would design a very rigid frame. You don't want frame flex, you want suspension flex. frame flex is bad and leads to thinks such as doors and tailgates not being able to be opened, broken body mounts, fuel and brake lines compromised.

Is frame flex desired by some off road guys? yes but only because they don't have a good enough suspension. Do the M37 frames flex? Probably, but that doesn't mean it is desired. I'm sure Dodge engineers had no way of calculating, much less designing into the frame a certain amount of flex.

Whether riveted, bolted or welded, a flexy frame will still cause problems. Cracked welds, cracked frames at points of high SIF's (stress intensification factor), wallowed out bolt or rivet holes. The crossmember I'm replacing has 6 rivets per side, plus a bead of weld. Any flexing at this joint will have to result in some deformation of the bolts or rivets, otherwise flexing would not be possible. This is assuming flexing is due to movement at te connections, not due to torsion/bending of the frame members.

I will bolt and weld my connections to make the frame as stiff as I can. Same reason you will see people box their frames, add torque boxes (go Hemi), etc.
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