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tube chassis - HREW vs DOM

Discussion in 'Center Of Gravity' started by KRAZIE87K5, Mar 24, 2004.

  1. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    I know the differences between the two have been covered in the past, but I'm really curious about using 1.75 .156 or .188 wall for a tube chassis. The real question is if DOM is really worth all the additional cost when building a tube chassis that is bound to use quite a bit tubular footage. If HREW is just ~10% variance in wall thickness, I still would have minimum of .120 wall with the .156, and .160ish with the .188.

    Is it worth twice the price to get the DOM? Am I missing something here?

    -Dan
     
  2. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Of course there is the old standby of "DOM is more rigid due to the DOM process, and it's usually found constructed of stronger alloys". So it's not just a matter of wall thickness tolerances. But in general HREW is more than strong enough with proper design (avoid mounting "in bending", i.e. triangulate with nodal joints). This difference comes into play primarily when the "frame" is subjected to impact loads from below. Similarly sized DOM (especially of superior alloy like 1028 my tie rod is made out of) will be more able to withstand deformation than HREW. But, skids often make that irrelevant as well.

    So, as usual, the answer is, “It depends…”

    Me, I prefer 2 x 3 x 0.188 square for the base frame and tube building off that. Not quite as “sexy” but more functional for the typical loads as well easier to work with and faster to build. If I do decide to ditch the truggy, this is how I’ll build the replacement.
     
  3. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    So, lets say that I was considering a buggy made from 1.75"x.156 wall. Links will be from 2"x.250 wall. I'll be building a 4 seater chassis. Its shortly thereafter that I start to lose track of where I need to begin design. I know many people use CAD to design their chassis - but believe it or not, I don't have access to that REALLY expensive software. Is there something else I can use, that won't cost me $$$$?

    I think that with a little design time and LOTS of reading, I could design an interesting chassis. Now I just need the resources to do so... /forums/images/graemlins/thinking.gif /forums/images/graemlins/pimp1.gif

    -Dan
     
  4. willyswanter

    willyswanter 1/2 ton status GMOTM Winner

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    Just bust out the triangles, ruler, and pencil and paper and get to work!
     
  5. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    I generally set around doodling on a note pad. Sort of brain storming, putting down anything that comes to mind. Take some time looking at pics of similar things that I like. Repeat.

    Once I get something like I think I want it, I take it to the next level. For simple parts, I draw them to scale in Visio (2D stuff). Before I used Visio, I used drafting tools (used to work as a "design engineer" in a tooling shop years ago). For the truggy I cut the body off and took some side pics, converted to gray scale, washed it out so it sort of looked like a water mark, printed copies, and started sketching. Each page had side, front, back, and top views. That kept the scale and proportions close.

    For more complex spatial relationships and construction (like a cage or buggy), I build a scale model out of bailing wire (mechanics wire, whatever). Each wire is cut to size and bent exactly like I envision the real process coming together. This helps identify the construction patterns and gives you something to play with. You’ll see exactly what order to do things, where seems will be, etc. For the truggy, it also helped me verify that it could handle point loads from any direction and remain rigid without complex FEA tests. I guess it worked ok, this was my first cage and it’s taken some serious smacks with no distortion (unlike another local guy who us surprisingly still alive). Once I moved to building the actual cage, I knew exactly how things were going to look and how it was going together.

    This is basically how I will approach a buggy when or if I build one.
     
  6. az-k5

    az-k5 1/2 ton status

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    If you want to get a life size model (I did with my cage) Get some cheap conduit from home depot and a bender ($20) and go to town with tubin. I also play with pencil/paper a lot and bend up bailing wire models. Pirate has a bending article by bill avista that can help a lot for your first time.

    Also as mentioned above a properly triangulated HREW tube structure will be plenty strong fot most uses. I would take russ' advice about the square tube main frame.

    Just my .02
     
  7. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Yes, I highly recommend the “Bending 101” article, but it's not by BillaVista. That was written by Tin Bender (aka Rob Park).

    Bending 101

    I had never used a bender before I built my cage, and this article gave me all I needed to know (when combined with some common sense and a little Trig). Well worth a read even if you have done some bending before…
     
  8. az-k5

    az-k5 1/2 ton status

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    Oh... oops. I had just glanced at the article after reading up on the king pin rebuild noticed they were on the same page and made an assumption. Well we know what happens when we assume. Credit to Rob Park.
    Thanks Russ. /forums/images/graemlins/grin.gif
     
  9. marv_springer

    marv_springer 1/2 ton status

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    [ QUOTE ]
    unlike another local guy who us surprisingly still alive

    [/ QUOTE ]
    /forums/images/graemlins/whistling.gif /forums/images/graemlins/doah.gif
     
  10. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    /forums/images/graemlins/whistling.gif
    /forums/images/graemlins/histerical.gif

    No, not you Marv, any cage that took that whack would be sporting a bruise. I was actually talking about "elf" and his cruiser. Funny thing, he was with us when I flopped with Jessica (her first time over on the side, she loved it and wants to ride (or drive) every obstacle now) in the truggy and I mentioned the lack of lateral stability but he kinda blew it off. Something like 2 weeks later he flopped hard (or maybe went all the way over) and that thing folded over like a card board box without the ends locked...
     
  11. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    I understand the need for lateral strength, but what's the difference between lateral support and triangulation (if any)?

    I like the idea of building something from piano wire first too... maybe I need to head out to the hardware store for a few feet.

    Visio... interesting. That I have already. For 2D drawing I would suppose that it would be fine. Once I get past that though, will most CAD programs calculate strength in a design? Are there applications available (at a hobbyist price point) that can assist in this? Or should I ask a third party co. to test my design? Just questions from a guy just getting my feet wet in the design world.

    Thanks for humoring me guys.

    -Dan
     
  12. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Triangles are the most stable simple geometric structure. Without triangulation (or a shear plane which is really just a special case), squares have no lateral support. They fold over with any significant force applied from any direction. Just take a box and open the ends (which would otherwise form a shear plane) and see how well it maintains it’s shape. So basically, all the roll cages (and “sport bars”) you see that look like a box with no diagonals, will fold relatively easily in any direction that does not have a diagonal.

    The strongest case (fore a given material) is when the diagonal meets at the perpendicular (or otherwise) joints forming a “node”. When a proper “node” is formed, any force not having a significant component perpendicular to a span and located between the nodes will be directed into compressive or tensile force vectors along and parallel to the other tubes. And, even when force component is applied perpendicular to a span, the bar can not give very far because bending the bar attempts to pull in the ends, effectively shortening the bar. This in turn pull on the nodes which transfer the forces as described above because triangles can not deform without deformation of one or more of the sides, which don’t want to deform because forces are transferred parallel to those spans. This is the strongest way to form a stable structure, but it must have proper triangulations to withstand impact forces and direct them along the spans rather than deforming.

    And, this is why coarse, angular cages (if designed properly) are much stronger than cool monkey bar cages with “aesthetic bends” in spans to make it look “cool”. Those bends cause the transferred forces to have some leverage on the span and (assuming the bend is not restrained, i.e. a node) it can collapse. Try this, take a welding rod that is straight and stand it on its end on a hard surface standing vertically. Now push straight down on it from above. It will probably go through your hand long before bending. Now make a small 15* bend or so and repeat. Big difference eh? That’s what happens then you have arbitrary bends in the middle of a structural span, it becomes much weaker when nodes transfer forces, or when it takes a hit in the right direction.

    Gussets are a degenerative case (no nodes) that also lend additional strength to the “nodes” they brace, reducing the chance of failure at or near a node.

    And you can forget FEA without an engineering degree and a mortgage on the house. Good quality packages shoot quickly over $10k, and those are generally add ons to AutoCad and such which in turn cost more than many new cars. Never even played with one, but they are the shiznit when it comes to figuring out EXACTLY how a structure will respond to various simple point loads and as well as more complex scenarios. Paying someone to do it will not save you much given the investment required (the degree and time to model as well as equipment overhead) unless you have a very close friend who owes you a lot. Me, I have to use a model, my finger pushing on it, and my imagination…

    This is my best stab at an explanation based on what I think I understand of what I’ve seen and “learned” over the years, but I’m certainly no engineer. There are others here who could probably give a better and more accurate description in fewer words.
     
  13. Triaged

    Triaged 1/2 ton status

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    [ QUOTE ]
    This is my best stab at an explanation based on what I think I understand of what I’ve seen and “learned” over the years, but I’m certainly no engineer. There are others here who could probably give a better and more accurate description in fewer words.

    [/ QUOTE ]

    I don't know if anyone could explain it any better than you just did.

    Forget FEA...even if you have easy acess to it (like I do /forums/images/graemlins/woot.gif).

    FWIW I think piano wire is just too strong/hard to use for a scale modle. Russ' sugestion of welding wire or bailing wire is a very good and cheep way to go...have fun brasing!
     
  14. BlazerGuy

    BlazerGuy 3/4 ton status

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    Man, my brain hurts now.... /forums/images/graemlins/grin.gif
     
  15. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    That all makes perfect sense. I'll drop the idea of having it tested for strength... I think I have a fairly good head for figuring these things out once I start working on them.

    You really think piano wire is too strong? I just figured that most bailing wire comes in a reel, meaning that its ALL been bent. Every little piece would have to be straightened out before building it into the model. Hrmm...

    I think I got the basics down.

    #1. Use straight tubes where ever possible
    #2. Build in triangulation where ever possible
    #3. Use sq. tube for lower frame rails
    #4. .156 wall tube is good for most chassis applications, for the best of strength and weight.
    #5. Model first, build second /forums/images/graemlins/wink.gif

    Looks like I'm off to begin design on the new chassis. /forums/images/graemlins/woot.gif

    -Dan
     
  16. az-k5

    az-k5 1/2 ton status

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    To add to Russ. As far as where to put a gusset. There are two main week points on a cage. The bends and longer sections of tubing. An easy way to keep a bend strong is to put a smaller support tube that is cut @ 45° from the edge of a bend to the nex perpendicular piece to it (parallel works to but keeps the stress in one plane 2D) Also try to avoid welding on a bend. It weakens it further. If you have to gusset both sides of the bend. The actual use and over all design plays in real heavy. Russ and I use similar *halo* bar designs (I find a halo design is more rigid and has a multi point strength). He has no cab or top and runs spreader bars. I have my half cab to distribute the load so I have no extra spreader bars. This is what can happen if you don't plan for every angle.
    This is a cage with little multi point strength. The double hoop design is good sideways but poor in an endo and horrible in a head over heals roll. A little gusseting in the middle and that would still be a near strait tube section.
    and this is over kill but good example of triangulation. clicky to another big cage pic.
     
  17. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    Do I need to have a member login @ POR to see those pics? It tells me forbidden.

    I would like to see those designs. Thanks for posting.

    -Dan
     
  18. big pappa b

    big pappa b 3/4 ton status

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    That's not it Dan. I'm a member there and I can't see it either /forums/images/graemlins/dunno.gif
     
  19. BIGJ

    BIGJ 1/2 ton status Author

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    I open it in a new window and it shows up fine for me. I'm on a mac though.

    BIGJ
     
  20. az-k5

    az-k5 1/2 ton status

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    I can't get them to post up as a pic (why I don't know) but they open fine for me. I am not a pirate member either.
    Here I'll try again.

    Both pics are in a pirate post about cage construction.

    Here is a link to that page
     

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