Strapping Vs. Boxing A Frame - Pros And Cons?

The quality of Jeep frame engineering is self evident.
These old Jeep frames most often crack at the frame horns where stress from the suspension pushes vertically onto the shackle bracket.
That portion of the frame ideally should have been "double butted".
To my knowledge double butting of jeep frame horns has never been administered.
If metal is added then weight is added.
The strength to weight ratio is thereby effected.
So metal must be added only where it is required otherwise the addition of metal is somewhat self defeating.

 

You guys are smart. I feel enlightened reading this.

So, I'm getting that vertical load on the frame is most common and what was most expected by the engineers who designed the frame. Not side load or (and more to my point) a twisting sort of load. Seems a bit odd to me as an off road vehicle may see a twisting type of load often. I believe that boxing of a C channel counteracts the twist more than strapping would but, maybe I'm thinking wrong again.

In the manufacturing of these things, I would suspect that weight vs benefit has something to do with the choice of C channel. Not sure how common boxed frames were back then either. Maybe cost prohibitive at the time.

As to the wet environments mentioned earlier, I can vouch for this first hand. Many of our trucks at work are doubled C channel frames. The salt from winter snow removal operations has many of them in very poor condition because there is no way to get in between those sandwiched together frame rails to clean them. The trucks still run fine and operate fine but, the frames are rotted. Many heavy truck manufacturers are getting away from selling trucks with double frame rails to municipalities, suggesting instead that a single rail frame will last longer.

 

Dave , without overthinking how the Jeep chassis came to be C-channel or otherwise one only has to look back at how and why it was first created.........back around 1940 with the World in turmoil the US War Department put out an urgent call for folks in auto manufacturing to come up with a vehicle that was cheap , light and all terrain and gave a 130 odd companies 49 days to submit there first test vehicle...........only 1 made the deadline and that was Bantam. Tucked into those specs somewhere I have heard that the useful life of this little vehicle was only supposed to be 90 days................

Although we are all both critical and appreciative of it's overall performance no one at the time really set out to build anything more than just a throw a way vehicle...........

 

Oldtime - Thank you for taking the time to give such a through explanation. Can you give a little information on the use of strapping and how that benefits the strength of the frame?

Tarry, that is an excellent point. I also know the supply of steel was short. By using a C frame instead of a box, I would assume they would have enough steel after 5 or 6 to make another whole frame, just be not having a boxed frame.

 

The 90 day thing may be something of a distinction without a difference. All car manufacturers build cars to a level that they think meets the expectations of their customers. It's just that the military spells it out. I would expect that 90 days is under combat conditions, with no access to service. A 90 day cycle time in normal service would be a huge drain on war resources. The Willys and Ford factories made many thousands of these Jeeps, but my impression is that the services needed that many because they were being blown up, abused or wrecked and abandoned at a high rate at the fronts. Redesign for civilian use impressed me as mostly down-specification to a less sturdy and lower cost version. Look at the M38A1 compared to the CJ-5 frames, specifically.

 

I have built a fair number of frames for Jeeps now, usually out of 2x4x3/16 or 1/4 box, or 2x5x1/4" box. I can put any of them up on one corner and another corner and not have any appreciable frame flex. None have had any cracking issues, and they've all been wheeled, some beat every weekend, depending on the owner. I'd prefer the springs to do all the work, and wear out springs rather than fix cracks in frame rails, or repair bodies due to the frame flex tearing them apart. My creations usually come out a bit heavier than I want, but if it's offroad durability, I'd go boxed. All heavy truck frames are C channel, but they can work with a 10, 12 or bigger rail section. I once installed a triaxle body on what I felt was the best compromise for the salty/snowy northeast....it was a Ford LTL9000 former milk tanker. It had a single 5/8 inch thick 12 inch tall C channel frame, all really heavy crossmembers, cast iron spring hangers, on Hendrickson rubber block rear suspension, with locking differentials and super singles all around. No rust to get between double sections. What a truck! Wow, reading my post I really got off track. Sorry.

 

Let me back up a little....
The center of either a channel of an "I" beam is called the web.
In general, the taller the web the greater the structures resistance to withstand vertical loads.
The extensions outward from a channel or an "I" beams web section are called the flanges.
The thicker the flange the greater its resistance to over stress.
The wider the flange the greater its resistance to side loading.

Yes absolutely most of the frames structural loading will be vertical.
A wide flanged double webbed "I" beam (AKA center cell) would provide the best vertical load service except that it is totally impractical.
That's because the drivetrain must occupy the same space as the structure.
Imajine the structural frame of a semi trailer or a train car.
It runs directly down the centerline of the structure.
So for vehicles with a drivetrain the double frame rail design becomes the obvious next best design.

Side loading of the jeep structure is fairly minimal but does occur to a limited degree.
Channel provides sufficient strength whenever minimal side loading occurs.
Extreme offroading provides much increase side loading over normal.

Twisting occurs as a combination of vertical and side load forces.
This is where boxed tube frame becomes far superior.
The boxed tube will resist twisting loads (vertical x horizontal) almost as well as round tubing.
Round tube excels with twisting loads but lacks a sufficient amount of vertical integrity.

With top and bottom strapping of a channel, an "I" beam or even a tube we are only adding to its ability to withstand additional vertical loading.
A wider strap provides better resistance to side loading and twisting than can a narrow strap.

So because of drivetrain configuration we are basically forced to utilizing a double railed framework.
For a Jeep expected to encounter a lot of rough terrain be must expect an increased amount of diagonal loading (twisting).
Here the boxed tubing far excels over most all except the tubular rail buggy frames.
The only draw back to Jeeps using rectangular tubing is that the tubing restricts overall frame flexing.
I feel that frame flex can be a good thing so long as it is not overflexed.
If the frame does not flex well then the suspension travel must be greatly increased in order to compensate.

Under extreme offroading conditions the twisting forces become tremendous and rectangular tube frames become the only sensible solution.
A properly built tube frame will not become damaged by the frames overall twisting forces.
An open "C" channel frame could survive severe twisting too but only by increasing the gauge or the wall thickness.

Aquiring the highest strength to weight ratio for the forces encountered is the real challenge.

 

Henry Ford knew something about that. And titanium steel.

 

Wow.............this is getting interesting...................... A class in Metallurgy, Structural analysis , weight distribution , Torsion , beam , Stress & Strain Calculations
Structural steel, load dynamics , compression and rebound and perhaps we should even get into the proper welding techniques ........................Nah , maybe another day!

I'm not a big fan of frames flexing do to the inherent work hardening of the joint that will eventually lead to cracks and failures.

I think the bottom line is this: Flex from an antiquated C-Channel frame such as a Jeep can be acceptable depending on how you use the vehicle.

Parade and Grocery trips not a Problem.........

Off road depending on the level of use , if you don't mind welding on the chassis around winching or pulling points , spring hangers or power steering mounts and fixing the body from time to time........it can also be acceptable.

If your tired of fixing these areas then perhaps boxing the chassis is the next step , while adding steering braces along with added support where you tow or pull from.

Or build a 2x4 rectangular tube chassis along with adding a decent suspension and most if not all of the issues go away.....................

Round Tubing is always an option but requires a different level of skill...............This is the last Drag Racing Chassis I built.......1.625x.083 chomo tubing , main frame rails and roll cage........all Tig welded with 309 stainless rod.

 

Like.

(Just clicking the "like" tab didn't seem sufficient on that one.)

 

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This is interesting to think about. It seems like the web doesn't add a lot to the load carrying by itself, but it increases rigidity with more distance between the flanges. The web height changes the amount of deflection (rigidity) for a given strain in the flanges - but only because a taller web increases the differential stress (compression vs. tension) in the flanges for a given load. A flange will withstand a specific stress before the strain becomes excessive - you can make the flange thicker or wider, and both will distribute the stress over a larger cross-sectional area of the flange, increasing the stress threshold.

So this pretty much supports what I was saying earlier about strapping vs. boxing. If we are worried about breakage, it's the flanges that break because that's where all the strain is. Adding more material to the flanges reduces the amount of strain in the flange cross section simply by virtue of it being more material. Breakage comes from work hardening in response to repeated strain. Unlike aluminum or copper, small strains (less than the strain limit) are not cumulative in steel. So steel will not work harden and crack if the strain limit is not exceeded. It's not about rigidity - you can make an I-beam that is very rigid but weak (just make the web very tall and the flanges thin and narrow). Instead it's about the material properties of the steel. Local concentrations of strain are bad. Strapping reduces local strain in the flanges by the increase in cross section. Twice the flange cross section, half the strain.

 

A few years back I bought a rebuilt offset flanged dana 44 Detroit locker nice axle from a 70 cj5. The jeep it came off of had 1 ton axles, exoskeleton,armor, etc. I said to the guy those Buick v6 engines are sweet in a cj aren't they? Yes it was before the jeep got too heavy need a v8 now. Intended use,budget, do you have a tow rig, etc

 

where is the 6 frame different...?
curious at this point..

very interested in the convo.. wish I had more to contribute..

the only part of this process I ever considered was just a gussett above the rear spring hanger..
I figured like many of you have said... intended use..

 

Funny this is where I arrived after googling frame flex and cj6. My 6 is seemingly super flexy. I can stand on one frame horn and when bouncing slightly it moves about an inch. I did this because it seems the frame might be bent with one horn being slightly higher. Thought maybe I could bend it back. Fat chance!!

But to answer your question the 67 cj6 in my garage has strapping the entire belly section six inches in front of the spring mount to the back directly below the rear shock mount.

So should I worry about the flex? I'm not building a crawler. Will have a v6 and auto trans.

Thanks.