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Strapping Vs. Boxing A Frame - Pros And Cons?

Discussion in 'Builds and Fabricators Forum' started by aallison, Jul 16, 2016.

  1. Jul 16, 2016
    aallison

    aallison 74 cj6, 76 cj5. Has anyone seen my screwdriver?

    Green Cove...
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    I have always wondered about strapping a frame. I don't understand how this really helps much as compared to boxing the frame. Or even adding plates to an already boxed frame.

    I just don't see how adding a 12g strip of material and stitich welding it gives much strength to the frame. And it seems like the strap would hold dirt and moisture between the strap and the top of the frame and cause it to rust.

    Or do you run the weld beads the length of the strap so it is sealed? Even then the amount of strength added seems marginal.

    It just seems like boxing the frame adds much more support with about the same amount of work as strapping. Help my understand this my jeep buddies.
     
  2. Jul 16, 2016
    a42pb

    a42pb Member

    atascadero ca
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    Has anyone here boxed a willys frame and missed any flexibility off road? Or is it worth the trade off
     
  3. Jul 16, 2016
    wheelie

    wheelie beeg dummy Sponsor 2019 Sponsor

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    I can't speak for strapping but, I tend to agree with your thoughts on the benefits of it, Allison.

    While my '71 cj5 is no Willys or flat fender, the frames are similar. I had mine boxed when I built the JEEP many moons ago. I miss nothing in terms of flexibility. Especially the pedals being sucked down toward the floor and and the clutch linkage binding up. Yes, my old frame had several cracks in it which exaggerated the flex and the issues I mention. The boxed frame lets the suspension do all the work, as it should in y opinion. Unless you're driving a pick up truck. If I ever build another JEEP to see moderate or worse trail use, the frame will be boxed, without a doubt. Just my opinion. I do have aftermarket springs so, that's part of the equation. Can't say how it would work with factory springs.

    I'd love to hear real world experiences with strapping.
     
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  4. Jul 16, 2016
    ITLKSEZ

    ITLKSEZ Volvophilic

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    Define strapping. I'm not sure what's meant by that?
     
  5. Jul 16, 2016
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    My thoughts about this - I'm an engineer, but not a mechanical or structural engineer - and I have a history with the Baja racers that influences my thinking.

    The C-channel consists of a horizontal top and bottom rail, and the vertical web that connects the rails. In theory, a C-channel carries load like an I-beam, with the top rail in compression and the bottom rail in tension, and the web only serves to hold the rails in place with respect to each other.

    Strapping adds material to the top and bottom surfaces of the rails. Typically you choose material about the same thickness and width as the rail, and stitch weld it along all or much of the rail. Typically you'd weld for an inch or two, leave a gap, weld some more. It's not required that you weld the entire length of the rail.

    From a load-carrying POV, the strap along the bottom rail makes the most sense. Adding material here most effectively resists the bending and possible buckling of the beam (channel). Some CJ-6s have strapping along the bottom rail of the frame, extending the full frame length between the rear front and front rear spring hanger.

    Unlike boxing, I think the strapping does not affect the lateral deflection, ie the resistance to twist, as much as boxing does. So if you want to make the frame stiffer, boxing is likely the way to go.

    However, in my opinion, frame breakage is a bigger problem for these Jeeps than frame stiffness. In my opinion strapping will succeed well at keeping the frame from breaking. The way this works is not by making the frame stiffer, but instead by mitigating the stress on the rails. Steel is a unique material - it can be repeatedly stressed and not fatigue, as long as the strain limit is not exceeded. To reduce the likelyhood of the frame rails cracking, the thickness of the rails is increased. This both makes the rails bend up and down less - less strain - but it also distributes the stress across a thicker section, additionally reducing the magnitude of the local strain.

    Back when I worked at the Jeep dealership (Brian Chuchua Jeep in Placentia CA), Brian had a fleet of Jeepster race cars. These cars were all fully strapped to keep the frames from breaking under the repeated stress of Baja racing. The frames of these Jeeps had a lot of extra steel added besides the strapping, but they started out as a regular C101 frame. I recall a '72 Renegade that he prepared for journalists to run the 500 race in. This was intended as a production car that could actually finish the race, with minimal changes, as a promotional effort by Jeep. The main modification to the frame was strapping. No one discussed this with me at the time - I was not involved in the work other than watching it progress - but it was clear that the steel was being added so the frame would not break during the race. So I kinda think that it's a good option if you want to make the frame stronger while preserving some of the flexible nature of the factory frame.

    So this is getting pretty long. I'll add some more if I think of any additional issues.

    hth!
     
    Last edited: Jul 16, 2016
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  6. Jul 16, 2016
    Daryl

    Daryl Sponsor Sponsor

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    My 57 FC150 is strapped top and bottom.
     
  7. Jul 16, 2016
    aallison

    aallison 74 cj6, 76 cj5. Has anyone seen my screwdriver?

    Green Cove...
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    Ok, that makes sense. I also understand the bottom is more effective than the top when strapped. Kinda like the substructure of a bridge supporting the span. I also see how tension stress would effect the strap on the bottom of the frame and compression force would effect the top strap. I would also assume closer welds in the top of the rail would be more beneficial that the bottom of the frame rail. But it also seems like the strapping should go from end to end. I would assume where the strap ended, stress would tend to build up and cause frame cracking. Would a fish plate end help reduce the tend to crack?

    Now I wonder what the strength difference is between strapping and boxing? I guess it would be similar to doubling the top and bottom thickness of the frame rail strength wise.

    Now that Tim got me to think about it (thanks Tim) I see both have their good points. Boxing would be stronger all around but stiffer, strapping would also add quite a bit of strength and still allow flexing.

    Strapping - welding a strip the width of the frame along the top or bottom of the frame using a stitch weld. This frame is already boxed but the C-clamps should explain what it is.
    [​IMG]

    Boxing - cutting steel and filling the C channel of the frame to make it a box.
    [​IMG]
     
  8. Jul 16, 2016
    wheelie

    wheelie beeg dummy Sponsor 2019 Sponsor

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    Excellent first hand information, Tim. Thanks for the lesson. Anxious to learn more as this thread progresses.

    I have seen factory strapping on the inside of the bottom of the JEEP frame that ran basically just on the flat section between the spring hangers (give or take a few inches). I suppose to strengthen the weakest area of the frame as the trans cross member is the only cross member in that area.
     
  9. Jul 16, 2016
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    I would guess the top-bottom thing is because steel strains less under compression than under tension. So the bottom rail is thereby the weaker of the two, since it's more likely to approach the strain limit. The stress on the top and bottom rails should be the same (as long as you don't stray too far from the idealized beam geometry).

    I don't see why the weld length should differ top to bottom, though it's probably theoretically true that more shorter welds are better. There may be practical welding reasons for the stitch interval. They say that the beginning and end of a weld is always the weakest part, so maybe having more stops and starts is weaker. Maybe it's a case of diminishing returns - once you start a bead, it's easier to keep going for a while than to make lots of little welds. Also, you're not supposed to weld across the rails unless you have to, but welding along the rails is benign, because the stress is along rather than across the weld. So I suspect it does not matter, as long as you weld enough to keep the strap and rail from straining independently.

    Any change in rigidity is going to make a stress riser, so there has to be a focus where the strap ends. I think the usual approach is to continue the strap till you reach a location where there is not a lot of strain. Not sure how you determine this point if it's not obvious. Certainly we have an idea of where these frames tend to crack, so you definitely would not end the strap (or your boxing) there - you'd extend beyond it.

    My feeling is that strapping is definitely the stronger method if load carrying is your objective. The web of a beam does not see much vertical stress compared to the rails, so adding another web does not seem like the most effective way of increasing strength ... as long as you are only concerned with vertical deflection. If boxes were that much stronger, skyscrapers would be built of square tube rather than I-beams. Another web obviously improves vertical deflection some, but I think the main effect is that lateral deflection is reduced substantially - now you have two rails into the horizontal axis that strain in compression and tension, just like the vertical load strains the top and bottom rails.

    Yes, each has good points, but I think your analysis of relative strength is wrong - see above.
     
  10. Jul 16, 2016
    timgr

    timgr Jeepin' Nerd Sponsor 2019 Sponsor

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    I wonder if that's because of the number of holes in the bottom rail in that area.
     
  11. Jul 16, 2016
    jpflat2a

    jpflat2a what's that noise?

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    You will notice a lot less frame flex with strapping.
    I think my 2A is strapped top and bottom from the factory.
    I only boxed the front frame horns when I did the Saginaw conversion years and years ago.
     
  12. Jul 17, 2016
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member Sponsor

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    FWIW the CJ6 frame I had had strapping between the spring mounts with the strapping extending underneath the mounts that were riveted through the strap to the frame. one thing to consider if operating in a wet environment is that moisture will get between the strapping & frame & cause rot.

    H.
     
  13. Jul 17, 2016
    AlexCold

    AlexCold Member

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    So running it through some online calculators of material properties (figured it's at least close and I'm lazy) the properties for flexibility and rigidity against bending are very close, with rectangular tube being better than c-channel by only a small percentage.

    Personally both can work but I've yet to see anyone make a c-channel survive rigorous use off-road. It comes more from the fact that while the C is difficult to bend as a whole, the individual flanges are not. So in areas of high inputs, say your rear spring hanger, you're very likely to see that area bent inwards. This is not an issue with box tubing (too often).

    Building stock class desert race trucks (Hummer H2 & H3) the stock rectangular tube frames never cracked over years of races and many victories. And I don't think a single manufacturer uses C channel frames anymore. There's something to say with that.

    Plus, in my opinion they (boxed frames) are easier to work with and on.
     
  14. Jul 17, 2016
    tarry99

    tarry99 Member Sponsor

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    If your suspension works , you don't want the frame to flex...........Most racers want a tunable suspension..........you cannot tune a flexy frame. Although a top fuel dragster is the exception to the rule whereby the tire becomes the suspension working with a wing to apply down force.

    Strapping the frame would add some rigidity and I believe Jeep did this as a cheap alternative to boxing the frame to vehicles that were either stretched or was to be equipped with additional equipment and weight.
     
  15. Jul 17, 2016
    a42pb

    a42pb Member

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    Now if you don't have the money for frame and suspension mods don't worry. That early Fhead 5 won't make much power up in the mountains so ditch all excess steel strapping and boxing from your frame along with the roll bar and back seat. It won't be to expensive to put some hiem joints in that clutch linkage just in case you neet to feather that baby. [​IMG]
     
  16. Jul 17, 2016
    rejeep

    rejeep Well-Known Member Sponsor 2019 Sponsor

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  17. Jul 17, 2016
    PeteL

    PeteL Member Sponsor 2019 Sponsor

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    Take a look at an M38A1 frame, to see how Willys engineers felt about reinforcing. For instance, boxing the front horns, etc..
     
  18. Jul 17, 2016
    a42pb

    a42pb Member

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    Agreed boxing best for a off-road "build" Throttle Down Kustoms frame would be even better than having firewood delivered.
     
  19. Jul 17, 2016
    cj6/442

    cj6/442 Sponsor Sponsor

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    I cracked my 5 frame constantly ......so when I built the factory frame which was crack free for the 6 ,,,I boxed the frame and stitch welded ....so far so good . The bottom is factory strapped . And I have alot of suspension movement.
     
  20. Jul 17, 2016
    oldtime

    oldtime oldtime

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    I'm a welder by trade so I need know at least a little about structural engineering.

    From a molecular point of view we are dealing with 2 main forces.
    Steel granular compression and grain tension.

    First let's envision a common leaf like one that's removed from a leaf spring.
    The leaf is relatively thin and also relatively wide.
    I will disregard commenting on the leaf width for now.
    The top of the spring (called the belly) is under compression (push) and the bottom (called the back) of the leaf is under tension (pull).
    The middle of the leaf is neutral.
    The middle is not being compressed nor is it being tensioned.

    If the back and belly are somehow separated then the force upon the back and the belly are increased.
    In the case of a "C" channel we have separated the back and the belly.
    In the case of a "C" channel the vertical forces are top flange compression and bottom flange tension.
    The "C" channel Jeep frame varies in height being notably taller near the middle of the jeep.
    That implies the mid section of frame should be able to better withstand bending from vertical loading .
    And bending is caused from over stressing the grain structure.

    Different shapes or forms of steel provide differing strength characteristics.
    Of all forms the "I" beam provides the greatest resistance to vertical loading.
    Even here there exist many different types of "I" beams.
    Some have wide flanges while others are narrow flanged.

    With the jeep frame we are mainly concerned with vertical loading.
    But a jeep frame also encounters a certain amount of side loading.
    So an "I" beam is probably not the ideal form to use.
    Even the body tub is being used in part to detur some of the force received from side loads.
    That's because the tub is used to reduce the channels susceptibility to sideways bending.
    The "C" channel has a lesser resistance to side loading than does rectangular box tube.

    I believe Jeep used "C" channel because it makes perfect sense.
    Simply stated the "C" channel provides the greatest strength to weight ratio for the forces that are most likely to be encountered.
    A boxed tube greatly increases the resistance to side stress loading with minimal vertical benefit.

    Unlike the spring leaf I mentioned earlier; the web (vertical portion) of a channel or the web of an "I" beam receives some of the compression and tension stress.

    Not sure where I'm going with this post.... Ha Ha Ha !
    Let's just consider it as being relative.
     
    Last edited: Jul 17, 2016

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