Box frame at spring hangers only?

THIS is what Im trying to resolve....
2.5" lift 33" tires V8 Engine and compound gears... I just want to throw bit of steel in the RIGHT place to complement the design of the 72-75 Jeep... not re engineer it..

 

Ya'll know this is how Jeep made the heavy duty frame right from the factory right.........

Not an owner modification. Not saying whether it's the best or not, just how Jeep did it.

 

I've been using my Jeep mostly off-road, on some serious trails. With 15 year of use. I have not developed any cracking of the frame. This is my fix.

 

Personally I think that anything you can do to add a little strength in and around the spring hangers makes perfectly good since.......as mentioned above some of the later frames came with a brace at the hanger from the factory.
Once into the eighties most frames were fully boxed adding several times the level of strength but also adding problems to owners who live in area's that have snow and salt to contend with............lets face it in a perfect world we would all have 2"x 3.5" square tube mandrel bent frame's and never again have a discussion about cracks or bent rails on our Jeeps.

Proper placement & design of braces & gussets along with proper welding & heating techniques could fill up several chapters in a good Book.

In all reality no one knows your driving skills better than you nor how aggressive you work your Jeep off road , notwithstanding the techniques that you may employ in gusset placement , welding and fabrication skills which may work for you but not others for obvious reasons.

 

I used to fight frame cracks continuously. I tried the plate welded in at the hangers and my frame cracked at each corner of the plate. If the plate were considerable longer it might distribute the stress out over a larger area. It also matters what kind of trail you do. Twisty or not. When I did my last build I discovered that all my hat channels were trashed and the tub was really kinda floppy. The lack of support from the tub probably contributed to the excessive frame flex and cracking. I decided to make the frame and the tub as rigid as possible but have a supple suspension that flexes well. On the twisty stuff I feel no flexing in the floorboards anymore. I know everyone talks about the frame flex the old Jeeps have, but a lot of that flex is because these frames are 40+ years old. If your tub is firmly mounted to the frame and your frame has lots of flex your tub has to flex too. As was stated in a previous post if you flex steel enough it will crack eventually.

 

I'd like to respond to a couple of points re Posimoto's post -

First I think it's just not correct to say that CJ frames are rubbery because they are old. To the contrary, even brand new CJs had quite a lot of frame flex brand new from the factory. The '70s era CJs with mechanical clutch linkage were notorious for losing any clutch action when flexed out, because of the flex of the body wrt the frame. There has always been a lot of movement in the design. Look at the door openings of many of the CJs and you'll see a buckle that's specifically due to the body flex that transfers from the frame. The flatty were like this too - read on the CJ-3B page about how the frames pop and groan when flexed out.

It's just the way they are - stiff springs and rubbery frame. They have to have stiff springs to control body roll, with the springs as far inward as they are. In '76, when Jeep tried to make the CJ ride better with wider and softer springs, that led to significant other changes like the repositioning of the rear springs outward, and front sway bars.

One advantage of coils on modern Jeeps is that they can be positioned further outboard without impacting turning radius. With leaf springs, you have to compromise ride with body roll - so getting that flexy suspension with a stiff frame is going to require a sacrifice of highway safety and handling. And you'll also have to address the highway performance and safety issues.

Second, steel does not fatigue unless it's stressed past its fatigue limit. This is strictly a property of iron alloys - not other metals. If the steel is never stressed beyond the fatigue limit, its service life is indefinite. Other metals like copper and aluminum accumulate fatigue regardless of the amplitude of the stress - this is why airplane fuselages must be retired after a fixed number of pressurization cycles. Even with the best design, aluminum accumulates fatigue. Steel does not. So, just because your frame is old, it does not mean it's going to crack.



(from the Oregon State engineering curriculum).

Yes, this could fill a book.

 

Tim, all good points, but are we not talking about the same thing when you say "steel does not fatigue unless it's stressed past its fatigue limit" I would venture to say that any old Willys CJ frame that has a crack or fracture has met that limit more than once!

Steel does have a working life especially in the category of zero-to-max-to-zero loading whereby a part or frame goes from zero load to max and back as some of these frames do while off road. It should also be acknowledged that fatigue cycles are Cumulative!
Lab tests although important have for the most part taught engineers that the endurance values found in optimized conditions do not really fully apply to real-world components simply because of all the variables and the lack of ability to repeat real-world conditions..............
Lets face it , the early Jeeps & frames were made as a throw away component.............not one engineer thought in 1942 that 71 years later these would still be on the road! Obviously the later double welded C channel frames are a major improvement over the single C open style early frames.
This is really a no one size fit's all discussion..........everyone should make informed decisions regarding there use factor and how they should or should not strengthen there frames.

 

Smileys noted - reply given in the spirit of the post it replies to.

What are you saying? Stress cycles above the fatigue limit are cumulative? That's what the chart shows. Stress cycles below the fatigue limit are not cumulative. That's also what the chart shows. You can find plenty of references about this. I guess my main point about this is that you shouldn't have to go nuts with hundreds of pounds of extra steel and try to make the frame 100% stiff, because some movement will not degrade the frame steel. As long as the (reasonable) stresses are evenly distributed along the frame, the frame will not degrade.

Clearly there are portions of the Jeep frame that accumulate stress. Reinforcements should strive to distribute that stress so that the resulting stresses do not exceed the fatigue limit. We have heard how some repairs cause cracks elsewhere. Clearly, some section of the frame that was not stressed (or was stressed somewhat less so that it had not yet failed), was then stressed beyond the fatigue limit.

Determining the distribution of stresses is a daunting task. Modern automotive engineers likely use powerful computers and model their frames by finite element analysis. Clearly, the engineers in the 40s and 50s did not have these methods at their disposal, so you would expect their results to be inferior to modern equivalents. Lacking computer simulation, we have to use mechanical intuition, trial and error, and maybe some simple experiments, as Howard suggested.

We do have one advantage the engineers of the 40s and 50s did not have - we can share our cumulative experience with these modifications. So let's keep this going, and see some more examples of frame repairs.

Sorry if this sounds like a lecture -

 

I have been researching this for a few days and still am nowhere closer to figuring out how I will address my frame...

LOVE IT!!!

 

"Modern automotive engineers likely use powerful computers.....engineers in the 40s and 50s did not have these methods at their disposal, so you would expect their results to be inferior..."

Wow. That is quite an indictment of empirical engineering. The Golden Gate Bridge, the Zippo lighter, the P51 Mustang, the Colt 1911, the Empire State Building, The Ford GT40, the DC3, and yes the Jeep --- all "inferior?"

People are fond of pointing out we went to the moon before computers of the power of today's pocket calculators; I also note that we haven't made it back there since!

;-)

 

Fair comments, Howard.

On the other hand my sense is that computer-driven design tends towards complexity, just because it can. And in going "faster, cheaper, lighter," sometimes cuts too close to the edge. The Shuttle was an example of both problems, and the consequences.

One reason cars today are heavy to begin with is because, again, we pack them with ridiculous amounts of egregious complexity, just because computers enable that. The Model T Ford was pretty darn light, and got fair milage too. Many parts of it's design were pure genius, and it's "cheap" price was legendary.

"Faster?" The Empire State Building was completed in less than two years - try that today.

How do we define "better?" I wish we could apply all our modern computer abilities to achieving elegant simplicity, and enduring functionality. That would be real progress.

 

I believe that both of you have valid points. I have to agree however that computers only improve things, it is the engineers who make things complex not the computers. There is nothing to say they would not have tried to over engineer things without computers also. Don't over look all the laws regarding emissions, saftey, ect either though - the majority of stuff in new cars, that us old school folks don't like, is there because of those laws.

For the record guys we did not go to the moon on an inferior computer. The one in Houston was like a calculator because that is all it needed to be - the cluster of mainframes that was used to develop the technology was far ahead of its time and because of the simplicity of design (I am sure PeteL would approve) the code running was getting a lot of processing power dedicated to it. We could have gone back to the moon anytime we wanted - instead we choose to use our new found technology to go all the way to Mars. The distance difference there is 248 million miles to Mars and about 240,000 miles to the Moon. Big difference, getting to the moon with modern technology is almost trivial.

But, all that said PeteL, I also wish they would just apply the technology to do simple things. In my opinion, one of the best vehicles created in the last 50 years was the Suzuki Samurai and it was based on the CJ3A.

I would definitely start a car company the produced very simple vehicles if not for all the legislation. I would just make a 2 or 3 models and never change them unless something forced the change.

Regarding the OP - I have a DJ frame on my CJ5, boxed all the way is definitely not always better. I have a lot of cutting and welding to do because of that frame being boxed

"Second, steel does not fatigue unless it's stressed past its fatigue limit"

As the son of someone who worked in a steel mill for over 30 years, and was the chief metal engineer and researcher, I just want to say this, I don't care what the book says steel will fatigue even when it has not be pushed to the fatigue point. Resonance alone will fatigue steel. The Uniform Material Law definitely states steel will suffer from fatigue even when not pushed beyond it stress limit

In fact I would argue that more often than not, when these Jeeps are heavily modified it is the result of long term fatigue that is causing them to crack and not the stress. If you use a frame on a modified Jeep that has hardly seen offload use prior you will find that you have far less problems - that to me points to nothing but fatigue.

 

Well, if I were you, I would have a serious issue with this. For me, it's a case of "Who do you believe? Me or your lyin' eyes?" It's not hard to find a lot of references that confirm what I posted in the chart. I have no reason to doubt that the textbook claims are true - it would be easy to prove them otherwise, and there's no motivation (ie power or money or ideology or religion) to falsify this claim.

This fatigue limit thing seems to be to be a rather mundane and well-accepted issue in metallurgy and mechanical engineering. I'm an engineer (my degree is in engineering, though my job title is physicist), but not a mechanical engineer or material scientist, so maybe there is more to it than I appreciate. But somehow I doubt that the textbook claims are a lie.

In my experience, when you find a contradiction like this, there are important extenuating circumstances that are being left out of the argument. These circumstances reconcile the opposing claims. What this might be in this case, I do not know - if I were in your place, and I felt as strongly about this contradiction as you do, I would try to resolve it with some more detail than you have.

A few seconds of search came up with this paragraph from Wikipedia about the Uniform Material Law - http://en.wikipedia.org/wiki/Fatigue_(material)#Fatigue_life
One method to predict fatigue life of materials is the Uniform Material Law (UML). UML was developed for fatigue life prediction of aluminum and titanium alloys by the end of 20th century and extended to high-strength steels and cast iron. For some materials, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit, endurance limit, or fatigue strength.

This points to another Wikipedia article on Fatigue limit, which doesn't help much because it basically supports what I wrote. http://en.wikipedia.org/wiki/Fatigue_limit
Fatigue limit, endurance limit, and fatigue strength are all expressions used to describe a property of materials: the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure. Ferrous alloys and titanium alloys have a distinct limit, an amplitude below which there appears to be no number of cycles that will cause failure.

So I'm wondering how to resolve this. Likely the fatigue limit depends on the alloy under discussion... though it seems that all ferrous and titanium alloys have some form of fatigue limit.

Another interesting issue which these articles point out is that the notched fatigue limit is much lower than the unnotched fatigue limit - ie, the geometry affects how much stress can be applied with no expectation of failure. To me, this is something of a distinction without a difference, because the notch simply focuses the stresses locally, without actually changing material properties. We very often see blobby welds on our Jeep frames and melt-back notches where two pieces are joined. The melt-backs are particularly bad because they form a stress riser at the notch. So quality of welding matters. An article on the CJ-3B page claims that you can strengthen these frames by just rewelding the poor factory welds on the original steel... and I think that's likely true.

 

been toying around with some frame reinforcement ideas...
in talking with much smarter fabricators than me and a few measurements at lunch came up with this......



just to box right above the spring hanger and extend a little bit outwards..
kind of a tweak on the original HD renegade frame design.. the squared off box created stress points that would cause cracks..
the fish plate design of the above plate should disperse the load over a broader area..
Still need to work on something possibly for the front and rear shackle hangers..

 

"the fish plate design of the above plate should disperse the load over a broader area.."

Makes sense. Repair plates are recommended to be diamond shape for the same reason. I might suggest you go one further and cut them with curves instead of the acute interior angles - reduce those stress-riser notches Timger referenced.

Or in real life... fix whatever breaks when it breaks... *if* it ever does. Might be less work - if any at all..

 

I don't feel strongly about it at all. I just know that steel will break even if it never goes beyond the stress point. My father built skyscrapers for many years and calculated stress points of steel his entire life. But try to resolve it in more detail? You are assuming a lot there I think. After about 20 years of studies done not only designing and building the structures, not to mention working directly with NASA they pretty much exhausted every test that could be reasonably done.

Anyway, I see it as all off topic, feel free to agree to disagree on the stress point. I don't think it will matter much to the OP what we think about it other than he might be curious about where the metal will break.


The fisheye design looks good but I am curious do you think it will break if it is boxed, or whatever method you use, or is that you are just trying to get it best as you can? I am just curious

 

just looking to add a little extra oomph where it is commonly needed..

I have a NOS frame I’m working with.. so it hasn’t been stressed or dressed ever..
Just sitting in a warehouse…
The front section already being boxed is not such a critical area in my mind.. but the rear has no support..

For how the jeep will be driven and wheeled I don’t think it will be a problem, but I am leaning more and more toward Hot Dip Galv… in that case I would like to get all my welding/fab/mock up done before hand… if it cracks 15 years from now.. I’ll deal with that then..

I agree with the radius corners.... just got crazy with the ruler and protractor

 

For what it is worth my DJ frame came from the factory boxed all the way to just after that transmission cross-member. After that it is basically open and just like a CJ. They did that on purpose to allow flex. This frame is just about as strong a frame I have ever seen on a CJ saving the tube ones. I suspect it would be fine with just minor changes, and it might even be fine without.

I am actually with PeteL on this, I would just leave it and fix what breaks if it did but I understand wanting to try to prevent it breaking in the first place