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Stiffen Rear Frame

Old 03-15-2019, 09:39 AM
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01 4 door XJ.

I'm looking for a way to stiffen up the rear cargo area/rear hatch area of my XJ.

I was out wheeling last night and stopped on a really flexed spot (I was checking suspension component clearance/etc. after making changes). I opened the rear hatch to get something and then couldn't get the rear hatch to close correctly (it latched fine after I got back on flatter ground).

Now yes, the simple answer is "don't open the rear hatch when the body is flexed out" but I'm taking this "issue" as an indicator that the rear of the body is twisting and that's something I'd like to prevent (less twist = longer lasting body).

I've already done 3/16" welded frame stiffeners from front to back. I'm thinking of welding a piece of 3/16" plate on the back side (fuel tank side) of the rear crossmember (where the bumper bolts onto). If anyone else has some suggestions/experience in stiffening the cargo area, I'd appreciate the insight. I'm trying to avoid a cage as it's also a daily driver and I don't want to loose the versatility.
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Old 03-15-2019, 11:08 AM
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This may seem silly but a hitch might do the trick. I have hooligan offroad stiffners front to back and hitch mounted to the unibody rails using new nut strips and all the bolt holes keeps mine squared up pretty well. or a beefy fuel tank skid should also give you more torsional rigidity.
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Old 03-18-2019, 10:54 PM
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I also 3/16 plated the area behind the rear bumper. that adds some serious strength. welded the edges and also made a few rosettes. I'm hanging a 35" BFG spare from the rear stock bumper with no sagging or flimsy feel about it. Also welded in frame stiffeners.
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Old 03-18-2019, 11:10 PM
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Originally Posted by neverenuff View Post
I also 3/16 plated the area behind the rear bumper. that adds some serious strength. welded the edges and also made a few rosettes. I'm hanging a 35" BFG spare from the rear stock bumper with no sagging or flimsy feel about it. Also welded in frame stiffeners.
That's what I wanted to hear!
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Old 03-19-2019, 06:38 AM
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They are designed to flex like that, most SUVs are that way. You can add stiffeners if you want to, but it isn't a problem the way it is.
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Old 03-19-2019, 08:43 AM
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Originally Posted by 00t444e View Post
They are designed to flex like that, most SUVs are that way. You can add stiffeners if you want to, but it isn't a problem the way it is.
Factory vehicles flex because it's not cost effective to make them not flex.

Any flex in a vehicle's frame/body is undesirable as it stresses the weld/bolt joints, lessens the performance of the suspension, and over time increases noise (interior panels). Take a piece of metal and exert a load on it that causes it to flex back and forth. Eventually it will crack. Now exert the same load and loading cycles on a gusseted piece (that can't flex) and it will last forever.

Flex in suspension = good
Flex in body/frame = bad

Last edited by CLSegraves1; 03-19-2019 at 08:49 AM.
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Old 03-19-2019, 10:28 AM
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Originally Posted by CLSegraves1 View Post
Factory vehicles flex because it's not cost effective to make them not flex.

Any flex in a vehicle's frame/body is undesirable as it stresses the weld/bolt joints, lessens the performance of the suspension, and over time increases noise (interior panels). Take a piece of metal and exert a load on it that causes it to flex back and forth. Eventually it will crack. Now exert the same load and loading cycles on a gusseted piece (that can't flex) and it will last forever.

Flex in suspension = good
Flex in body/frame = bad

I would rather have a chassis that flexes and absorbs the forces, even if it starts to crack the welds....than one so stiff it doesn't flex, it just suddenly fails.

Flex is fine, and it lets you know when it needs work and when you are getting close to the breaking point. Too stiff, and all it does is catastrophically fail....with no warning.

How do you think tall trees get so tall...they can bend & flex when the wind blows so they don't break.


.

Last edited by TRCM; 03-19-2019 at 10:30 AM.
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Old 03-19-2019, 11:06 AM
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Originally Posted by TRCM View Post
I would rather have a chassis that flexes and absorbs the forces, even if it starts to crack the welds....than one so stiff it doesn't flex, it just suddenly fails.

Flex is fine, and it lets you know when it needs work and when you are getting close to the breaking point. Too stiff, and all it does is catastrophically fail....with no warning.

How do you think tall trees get so tall...they can bend & flex when the wind blows so they don't break.


.
Right.... go tell that to any professional racing team (asphalt/dirt/off road) or professional car builder and see what their reaction is....
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Old 03-19-2019, 11:17 AM
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Originally Posted by CLSegraves1 View Post


Right.... go tell that to any professional racing team (asphalt/dirt/off road) or professional car builder and see what their reaction is....

1) I am not a professional racer or car builder, and neither are 99.9% of the people on here

2) The cost to build a professional level chassis that has no flex is beyond many here, both money and talent wise

3) Have you not ever seen dragsters launch doing a wheelie, and let off, and come back down hard...the chassis flexes up & down to absorb the energy and not break stuff

4) When you make something so stiff it can't flex, it becomes brittle...and brittle breaks instead of bending/flexing

5) Ever watch an airplane and the relationship to the wings to the main body....tons of flex......

But I digress...and I am 100% sure my post started with...."I would rather have....", which means it is my choice, not necessarily a scientific law......but simply what I prefer and why........
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Old 03-19-2019, 01:47 PM
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Originally Posted by TRCM View Post
1) I am not a professional racer or car builder, and neither are 99.9% of the people on here

2) The cost to build a professional level chassis that has no flex is beyond many here, both money and talent wise

3) Have you not ever seen dragsters launch doing a wheelie, and let off, and come back down hard...the chassis flexes up & down to absorb the energy and not break stuff

4) When you make something so stiff it can't flex, it becomes brittle...and brittle breaks instead of bending/flexing

5) Ever watch an airplane and the relationship to the wings to the main body....tons of flex......

But I digress...and I am 100% sure my post started with...."I would rather have....", which means it is my choice, not necessarily a scientific law......but simply what I prefer and why........
Technically, your initial post was that vehicles "are designed to flex like that, most SUVs are that way. You can add stiffeners if you want to, but it isn't a problem the way it is." I argue that they don't design them to flex.

Per several GM engineers, it's why GM put millions into analysis and design (and was the driving force for why they moved the transmission to the rear axle rather than using a traditional transmission mounted behind the block) to minimize the body flex of my C5 Corvette. VEHICLE MANUFACTURERS DON'T WANT THE BODY TO FLEX. Hell, people put frame stiffeners on an XJ body to PREVENT body flex induced cracking. If flex were good, then the original XJ flexible body should last forever without cracking.

My opinion is based on having been through several chassis building classes (both drag race and oval track), 2/3 of a mechanical engineering degree (I interned and realized it wasn't for me), input from OEM engineers from GM and Ford and advise from a professional race car builder (not directly related to this jeep build but prior performance oriented builds). CONSISTENTLY, when it comes to wheeled vehicles, the goal is always to build the chassis as stiff as possible. You don't need flexibility in the body to prevent damaging the body, that's why you have a suspension (shocks/springs/etc.). Stiff does not mean brittle, brittle means brittle. Brittle is a function of the molecular structure of the material.

Yes, aircraft wings are designed to flex (the fuselage isn't), but aircraft are an entirely different animal.

But yes, you did state that "(you) would rather have" and I accept that this is your opinion (never my intent to make an argument, just a discussion). At the end, you build your vehicle with a flexible body. I'll build mine with a stiff body.

Last edited by CLSegraves1; 03-19-2019 at 02:30 PM.
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Old 03-19-2019, 03:00 PM
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Originally Posted by CLSegraves1 View Post
Technically, your initial post was that vehicles "are designed to flex like that, most SUVs are that way. You can add stiffeners if you want to, but it isn't a problem the way it is." I argue that they don't design them to flex.

Per several GM engineers, it's why GM put millions into analysis and design (and was the driving force for why they moved the transmission to the rear axle rather than using a traditional transmission mounted behind the block) to minimize the body flex of my C5 Corvette. VEHICLE MANUFACTURERS DON'T WANT THE BODY TO FLEX. Hell, people put frame stiffeners on an XJ body to PREVENT body flex induced cracking. If flex were good, then the original XJ flexible body should last forever without cracking.

My opinion is based on having been through several chassis building schools (both drag race and oval track), 2/3 of a mechanical engineering degree (I interned and realized it wasn't for me), input from OEM engineers from GM and Ford and advise from a professional race car builder (not directly related to this jeep build but prior performance oriented builds). CONSISTENTLY, when it comes to wheeled vehicles, the goal is always to build the chassis as stiff as possible. You don't need flexibility in the body to prevent damaging the body, that's why you have a suspension (shocks/springs/etc.). Stiff does not mean brittle, brittle means brittle. Brittle is a function of the molecular structure of the material.

Yes, aircraft wings are designed to flex (the fuselage isn't), but aircraft are an entirely different animal.

But yes, you did state that "(you) would rather have" and I accept that this is your opinion (never my intent to make an argument, just a discussion). At the end, you build your vehicle with a flexible body. I'll build mine with a stiff body.

I never said that...00t444E did........

Funny you mention corvettes....they have a fiberglass body, which is WHY the engineers didn't want the chassis to flex...fiberglass doesn't survive well with flex in most configurations. XJs aren't made of fiberglass.

People put frame stiffeners on XJs to strengthen the frame so you can:
1) add different suspension components because the frame is little more than sheetmetal
2) stiffen up the handling
3) make the suspension work better

A stiffer body is just an added bonus.

If you make a piece of metal so thick that when 1 side gets heated up, it can't flex to spread that energy across the thickness of the metal, it will catastrophically fail.....there is such a thing as too stiff. Even roll cages and such have flex to them.

All I am saying, is in stock form, the XJ chassis will work just fine if you don't have the money or means to improve it. Also never said flex was good, as in a desired quality you build in....but did say it was fine, as in it is a quality that results from the way it was built, and for 99% of us out here, it works.

The chassis (frame) should be stiff...the body, well, it is what it is. Again, even frames/chassis/rollcages flex some, so I'd rather have a body that will flex with it and not fail. Build it too stiff, and when the frame/chassis/rollcage flexes and the body can't....what do you get ?? pop.

The larger something gets, the more it must flex....no way around it. Look at the floating airports I work on daily.,.,they are so huge, and way stiffer than any vehicle chassis...but yet they must be designed to flex to avoid catastrophic failure when moving thru the water.

A corvette or cherokee, can be stiff, but the longer something (think 300" long dragsters), the more flex must be designed into it to avoid failure.


Also, don't forget, if you make the body/chassis too stiff, then in any accident, they will not absorb energy well, they will transmit it instead...which often means more pain for the driver.


Anyway, as you suggested, you build it stiff, I'll use it the way it is built, but I do have frame stiffeners to go on mine so I have enough metal to attach the 4 link front & rear suspensions when they get added.

Just various thoughts from different points of view & needs.



.

Last edited by TRCM; 03-19-2019 at 03:35 PM.
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Old 03-19-2019, 03:49 PM
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Maybe I should have worded that different. Every vehicle is designed to have some flex in the frame and body. Like TRCM said flex is necessary to prevent stress failures, even big rigs frames flex. The same principle applies to bridges, roads and buildings.
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Old 03-19-2019, 04:14 PM
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Originally Posted by TRCM View Post
I never said that...00t444E did.........
My bad

Originally Posted by TRCM View Post
I never said that...00t444E did........

Funny you mention corvettes....they have a fiberglass body, which is WHY the engineers didn't want the chassis to flex...fiberglass doesn't survive well with flex in most configurations. XJs aren't made of fiberglass.
Look at any performance vehicle (Viper, Z4M, M3, F-bodies, etc.), stiffness is the name of the game. Even Ford/Chevy/Dodge/Toyota are in constant pissing matches about who has the stiffer chassis/frame. Automotive engineers do not WANT the chassis to flex, they just have to deal with it on some level.

Originally Posted by TRCM View Post
I never said that...00t444E did........

Funny you mention corvettes....they have a fiberglass body, which is WHY the engineers didn't want the chassis to flex...fiberglass doesn't survive well with flex in most configurations. XJs aren't made of fiberglass.

People put frame stiffeners on XJs to strengthen the frame so you can:
1) add different suspension components because the frame is little more than sheetmetal
2) stiffen up the handling
3) make the suspension work better

A stiffer body is just an added bonus.

All I am saying, is in stock form, the XJ chassis will work just fine if you don't have the money or means to improve it. Also never said flex was good, as in a desired quality you build in....but did say it was fine, as in it is a quality that results from the way it was built, and for 99% of us out here, it works.

The chassis (frame) should be stiff...the body, well, it is what it is. Again, even frames/chassis/rollcages flex some, so I'd rather have a body that will flex with it and not fail. Build it too stiff, and when the frame/chassis/rollcage flexes and the body can't....what do you get ?? pop.
The XJ body IS the frame, it's a uni-body construction. They aren't two independent items. In order to stiffen the chassis (frame) in an XJ, you are stiffening the body.

Originally Posted by TRCM View Post
If you make a piece of metal so thick that when 1 side gets heated up, it can't flex to spread that energy across the thickness of the metal, it will catastrophically fail.....there is such a thing as too stiff. Even roll cages and such have flex to them.
Yes, if you have a piece of material and you induce an non-uniform heat into the material such that one part grows/moves and another doesn't, something is going to fail. But that's not the situation we are discussing when we talk about whether or not a chassis (or specifically a uni-body vehicle's body structure/frame) should be rigid of flex.

Originally Posted by TRCM View Post
The larger something gets, the more it must flex....no way around it. Look at the floating airports I work on daily.,.,they are so huge, and way stiffer than any vehicle chassis...but yet they must be designed to flex to avoid catastrophic failure when moving thru the water.

A corvette or cherokee, can be stiff, but the longer something (think 300" long dragsters), the more flex must be designed into it to avoid failure.
TECHNICALLY, it doesn't HAVE to flex. You can keep adding materials to the system to prevent flexing, but it gets to be a point of diminishing returns (weight of the material itself starts to result in flexing) and/or it gets so heavy that it isn't practical (a 5000 lb "tube chassis" dragster isn't going very fast). Also, returning the the thermal expansion issue, any material will have some coefficient of expansion (inches/degree F). So something that's big enough (like a bridge) has to have some flexibility (or specifically ability to handle growth/shrinkage) otherwise yes, something will crack/fail.


Originally Posted by 00t444e View Post
Maybe I should have worded that different. Every vehicle is designed to have some flex in the frame and body. Like TRCM said flex is necessary to prevent stress failures, even big rigs frames flex. The same principle applies to bridges, roads and buildings.
It's not that they design in flex, they design to compensate for flex. Engineers recognize that the system is going to flex, so some compensating system has to be in place to deal with the flex. It's why you don't rigidly mount a radiator to the frame/body, it will crack. In a perfect world, the frame would be 100% ridged and not flex. Then the body would never twist, the interior panels would all stay tight, etc.

I also recognize that there are instances where flexing is important to deal with applied stresses or impacts (ex: an airplane wing), but again, in the case of a vehicle frame/chassis, that's not the place to have flex. The purpose of the frame/chassis is to remain rigid so that all other components remain in a consistent (relative to each other) location. The frame/chassis needs to be built of a material that can handle stresses without fatiguing and developing stress fractures (ie: why steel is easier to deal with in vehicle frames than aluminum).
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Old 03-19-2019, 05:43 PM
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Originally Posted by CLSegraves1 View Post
The XJ body IS the frame, it's a uni-body construction. They aren't two independent items. In order to stiffen the chassis (frame) in an XJ, you are stiffening the body.

Pretty sure I know that....but strengthening the 'frame' on an XJ as with frame stiffeners, won't do much for the body in comparison.

Take a cardboard box 2' x 2' x 2'...and put a 1/2" thick steel plate in the bottom.....you just strengthened the 'frame' a lot....but the top and sides still move around almost as much as before.

An XJ isn't much different when you consider the sheetmetal it's made of. That's basically what I am saying.

You can make a chromoly frame for an XJ and insert it inside the existing frame rails (or under them if you will), and the body will still flex. To significantly change the amount of body flex, you'd have to have uprights from the frame. so it has torsional stability in all 3 planes.

and as for steel vs aluminum....yeah, 1 sure flexes better repeatedly without breaking......................
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Old 03-19-2019, 07:25 PM
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Originally Posted by CLSegraves1 View Post
Even Ford/Chevy/Dodge/Toyota are in constant pissing matches about who has the stiffer chassis/frame.
All that is just marketing hype, just because one trucks frame flexes more than another doesn't mean it is weaker. You would be surprised at how much the frame flexes on a loaded semi truck, but they aren't weak by any means, and most of them still use regular c channel frames, not fully boxed frames like you hear about all the time in the pickup truck advertisements.
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