Sway bars....Solid VS Hollow

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  1. Kern Dog

    Kern Dog FBBO Gold Member FBBO Gold Member

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    I saw this video recently and find it surprising.




    Being a Carpenter, I have some basic engineering knowledge in terms of construction materials. I find it very strange that a hollow bar can provide greater strength than a solid bar. If this is actually true, why hasn't anyone made hollow torsion bars?

    I like the idea of weight savings. I'd consider one of these if I could hear from someone that could explain in basic terms why a hollow member is stronger than a solid one. Is it in the metallurgy?
     
  2. 1 Wild R/T

    1 Wild R/T FBBO Gold Member FBBO Gold Member

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    Well you missed the fact that the solid bar is 1 1/4" & the hollow bar is 1 3/8"....

    But it does weigh less & provides a stronger bar....

    Personally I run the Hellwig bars, they are also hollow, virtually the same design as the Hotchkis & less than half the price....
     
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    • Kern Dog

      Kern Dog FBBO Gold Member FBBO Gold Member

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      I did notice the difference in diameter but even if the sizes were the same, could a hollow bar still be stiffer? I've seen this topic discussed elsewhere and some think so. I just wish someone had a rationale explanation as to why.
       
    • 1 Wild R/T

      1 Wild R/T FBBO Gold Member FBBO Gold Member

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      Stronger? No... Virtually the same? Yes... The torsional strength of the inner area is minimal... Take a 5" long 1/2" steel rod, Bend each end 1' in at a 90 degree angle.. Mount three points solidly like in the sway bar test.. Now how much force would it take to deflect the free point 1"? I don't know but I'm willing to bet I could do it with one hand...... Now try doing the same thing with 5' length of 1" pipe with a 1/8" wall.... Bet it takes allot more effort...
       
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      • Kern Dog

        Kern Dog FBBO Gold Member FBBO Gold Member

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        The video above would be easier to believe if I knew it wasn't paid for by Hotchkis. I don't begrudge them or mean to bad mouth their stuff but just like when your Mom says that you are the best looking boy in the world, you have to question the whole deal.
         
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        • Kern Dog

          Kern Dog FBBO Gold Member FBBO Gold Member

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          I know that GM started using hollow bars in the 80s with the Camaro and Firebirds , maybe other models too. I'd like to know the wall thickness of these Hellwig and Hotchkis bars.
           
        • 66 Sat

          66 Sat Well-Known Member

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          [QUOTE. If this is actually true, why hasn't anyone made hollow torsion bars?

          [/QUOTE]
          Do a search online and you'll learn a lot. It's not the metallurgy, more the physics of stiffness with the hollow tube coming out on top (too complicated for me to explain, I sort of understood when I read it though and I'm sure you will too). It wouldn't work on torsion bars though as that's a different force, twisting and not bending. Another area the solid tube would win is in compression. I think it's only in bending stiffness the hollow tube comes out on top (and weight of course). Interesting stuff though, thanks for posting, I learned something new.
           
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          • HT413

            HT413 Semi Pro Bowler

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            Kern, it’s simple as carpentry. A 2x12 joist is stiffer and stronger than 2) 2x6 next to each other despite the amount of wood being roughly the same.

            The strength of the beam increases as cube of the height. So a joist that is 2x taller (such as a 2x12) would be 2x2x2 = 8 times stiffer than a single 2x6.

            The other thing is I’d guess the solid one might be lower quality steel. Quality 4130 is way stiffer than most lower grade bar stock steel.
             
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            • Stanton

              Stanton Well-Known Member

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              Often times things are not made out of particular materials because they're stronger but because they're available. I build rotisseries and I use 3/16" wall material - not for its strength but because a smaller tubing fits inside better. We used to race go karts and the European karts were always lighter and allowed better tuning than American karts. Why? Because in the US chrome moly was produced in 1/8" increments Whereas in Europe its made in 2mm increments. So the karts could be built with a diameter that worked best as opposed to what was on the market. Also size and shape comes into play. Take bridge trusses for example. They build a truss out of multiple pieces of steel and the truss is 20' tall. If they just used a solid bar of the same weight, regardless of its dimension it wouldn't have anywhere near the strength of the truss. So while in itself a solid piece may be more rigid than a tube, you have to look at the intended purpose.
               
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              • Dennis H

                Dennis H FBBO Gold Member FBBO Gold Member

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                As a novice it sounds like the weight savings and improved stiffness exist, but are both so marginal as to barely be detectable when driving. That was a run-on sentence for the Kern.
                 
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                • LMSC33

                  LMSC33 Active Member

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                  From my experience in circle track racing, spring rates on hollow bars are significantly higher than solid bars.
                   
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                  • Kern Dog

                    Kern Dog FBBO Gold Member FBBO Gold Member

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                    THanks for the responses but what I am not seeing anyone clearly explaining is WHY....At least in a manner that helps me understand.
                     
                  • HT413

                    HT413 Semi Pro Bowler

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                    a solid bar would be stiffer than a tube of same material and dimension. The reason the hollow bar you posted was stiffer was the increased diameter and probably better steel.

                    But it’s true that a hollow tube can be really close in stiffness to its solid bar companion; but it can’t be as stiff and certainly not stiffer. Go back to the 2x12. As the 2x gets smaller in height, it’s stiffness decreases by a cube as I mentioned. Well as its thickness approaches zero, its stiffness also approaches zero.

                    Now if we imagine the solid bar as a series of tubes nested within each other, you can see that each smaller tube is successively less stuff, until the most inner tube’s stiffness approaches zero. It’s in this way that the inner material of solid bar stock is of less structural importance than the outer skin.

                    So well designed tube can be super stiff - so long as the wall is stiff enough to hold its form without kinking under load.
                     
                    Last edited: Dec 8, 2019
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                    • Kern Dog

                      Kern Dog FBBO Gold Member FBBO Gold Member

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                      I have read that the closer you get to the center of the bar, the less effective the metal is. Is that maybe also due to leverage? In other words, can that principle compare to a large versus small steering wheel? A long handle breaker bar versus a stubby handle ratchet?

                      The 66-69 models could use the 1 3/8" bar but the '70-'72 models had a tighter fit through the K member and it seems the biggest that anyone makes as a "Bolt in" is an 1 1/4" bar. The lever arms are shorter in the 70-72 so even without doing the math, I'm guessing that the later one while 1/8" smaller, is probably stiffer.
                       
                    • HT413

                      HT413 Semi Pro Bowler

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                      the concepts are related, though leverage is a 1:1 with diameter / length while bending stiffness is diameter cubed. Incidentally, the torsional stiffness is diameter to the 4th power which really translates to a big improvement as you step up even the 1/8”
                       
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                      • DeltaV

                        DeltaV You are about to participate in a great adventure FBBO Gold Member

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                        • biomedtechguy

                          biomedtechguy Accelerati Rapidus Maximus FBBO Gold Member

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                          Well Dennis, obviously Kern Dog is "slacking" on his demand for precise language...:poke:
                          Otherwise the title would have been
                          "ANTI-sway bars"...:poke:
                          Unless you are inducing sway...lol:lol:

                          I think that the diameter is a bigger factor than "common sense" would suggest, and the best explanations are in Stanton and HT413's posts.
                          I'd be concerned about longevity of a hollow torsion bar vs anti-sway bar. Different physics at work.
                           
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                          • YY1

                            YY1 Well-Known Member

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                            Agree (and thanks) on HT413's "center of bar/tube is less significant" explanation but believe the word "bar" in the last paragraph should be "tube".
                             
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                            • HT413

                              HT413 Semi Pro Bowler

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                              fixed. Was on my second bourbon by then.
                               
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                              • coloradodave

                                coloradodave FBBO Gold Member FBBO Gold Member

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                                Math teacher?
                                 
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