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Thread: Box frame at spring hangers only?

  1. #21
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    Default Re: Box frame at spring hangers only?

    Quote Originally Posted by tarry99 View Post
    But if you opt to have larger tires , more Horsepower , lower gears and lift then perhaps that old frame also has to evolve some to also meet those new challenges.

    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..
    2012 WK2 SRT8 - 6.4L of fun
    1988 YJ Sahara - 4.2L,AX-15,4:1,D30 w/aussie, 8.8, 4.11, 6"Lift 35"BFG/KMT2
    1974 Land Rover Series 3 - fully restored
    1949 CJ3a - 6 Volt and stock, staying that way!

  2. #22
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    Sep 2002
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    Minnesota
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    Default Re: Box frame at spring hangers only?



    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.
    Mark
    70 CJ5 225 V6
    M416

    "We hold these truths to be self-evident, but apparently not to everyone!"

    "I ain't got time to bleed"

  3. #23
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    Default

    Quote Originally Posted by Zoomer View Post


    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.
    1964 M38a1 w/4.3 ltr TBI Vortec V-6, SM465, D30 w/ARB Locker, D44 w/Detroit locker, FF axles, 4.27, 33x12.5x15s KM2's
    2005 Colorado w/220 hp 5 cylinder. (The power of a 6, with the economy of a 4)
    2014 SS Sedan w/ 415hp 6.2 ltr LS3 V-8.

  4. #24
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    Default Re: Box frame at spring hangers only?

    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.

  5. #25
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    Default Re: Box frame at spring hangers only?

    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.
    Last edited by Posimoto; 03-20-2013 at 11:44 PM.
    '67 CJ5 w/'73 frnt clip, 94 1/2" wheelbase, D44 FF, Warn Drive Flanges, 30 Spline, OX locker, D30, ARB Locker, 4 Wheel Discs, Ford T-18, 5.38 gears, 13.50 x 35 Krawlers, SOA w/ YJ springs, Powerwelder, OBA, FI V6 Dauntless, Warn M8000 winch, PS, HEI, Chevy High Steer Knuckles, Fully Boxed M38A1 Frame, Hydro Clutch, Tera Low 18 T-case

  6. #26
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    Default Re: Box frame at spring hangers only?

    Quote Originally Posted by Posimoto View Post
    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.
    Last edited by timgr; 03-21-2013 at 09:32 AM. Reason: added graph
    Tim Reese
    '75 CJ-6, Copper Polly 304/T15
    '77 J10 type 45 (131" WB) 258/T15/3.54
    '82 J20 type 27 360/T18/NP208/3.73 7600 GVWR
    '95 VW Golf Sport 2D 2L

  7. #27
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    Default Re: Box frame at spring hangers only?

    This is getting interesting

    C channel is inherently twisty, i.e it flexes.

    Try this as an experiment- take a piece of project board cut into a 6" strip. Bend it into a C channel with 2" top & bottoms, tape a piece to each end to make a box- this keeps the top & bottom locked into position with each other just as the crossmembers do. Tape a piece of paper to the top edge so that it hangs down just short of the bottom edge & make a mark on both aligned with each other.

    Now, twist the ends in opposite directions- this is what happens when the frame flexes- note that the top & bottom edges remain nice & straight and that the marks on the paper & bottom edge move apart from each other. This is what wants to happen to your frame rail when one corner wants to go up or down.

    Now, tape a ruler somewheres along the bottom edge & try twisting again. The ends of the ruler represent the spring hanger points & the ruler acts like the springs that fight against the frame rail twisting. Grab the ruler in the middle & push up & down on it- notice the bends that develop at the ends of the ruler, this is the spring stressing the bottom of the rail as you hit bumps. Now try twisting- in both cases the bends that develop at the ends of the ruler are where the frame cracks appear.

    Next, tape about 2" long pieces of board across the top & bottom edges overlapping the ends of the ruler- this represents re-inforcement plates being welded to the frame rail. Try pushing on the ruler again- no more bends but now give it another twist- now we've really got some funny bends developing at the re-enforcement plates & that's pure hell on the metal.

    Take off the "reinforcement" plates off and install some "D" plates instead, try the pushing & twisting again & see what the difference is.

    Now take out the D plates & add a strip of project board all the way along- i.e. "box it"



    My $.02- Either go with the d plates or box the whole thing.

    H.
    Tonka
    1955 CJ5
    F134
    S/N 378
    tantel.ca/Tonka.html

  8. #28
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    Default Re: Box frame at spring hangers only?

    Quote Originally Posted by timgr View Post
    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.

  9. #29
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    Default Re: Box frame at spring hangers only?

    Quote Originally Posted by tarry99 View Post
    ... It should also be acknowledged that fatigue cycles are Cumulative! ...
    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 -
    Last edited by timgr; 03-21-2013 at 04:48 PM.
    Tim Reese
    '75 CJ-6, Copper Polly 304/T15
    '77 J10 type 45 (131" WB) 258/T15/3.54
    '82 J20 type 27 360/T18/NP208/3.73 7600 GVWR
    '95 VW Golf Sport 2D 2L

  10. #30
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    617

    Default Re: Box frame at spring hangers only?

    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!!!
    2012 WK2 SRT8 - 6.4L of fun
    1988 YJ Sahara - 4.2L,AX-15,4:1,D30 w/aussie, 8.8, 4.11, 6"Lift 35"BFG/KMT2
    1974 Land Rover Series 3 - fully restored
    1949 CJ3a - 6 Volt and stock, staying that way!

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