In for some more tech: I understand arm length, diameter of the bar, and length of the bar change rate.
I see some use bigger bars but gun drill them vs a smaller solid bar thats turned down in the middle. Why? It softens rate I assume but maintains twisting yield strength from a bigger OD?
Is there a difference in rate or strength from a 1in bar at the splines but .75 at middle vs 1.25in at the splines but .75 in the middle.
Should a front bar always be thicker than a rear bar? OEMs usually use thicker front than rear but I’ve seen some U4/buggy’s do the opposite.
How much does material change rate? Oem cast bars vs bars from Currie or TK1. I noticed TK1 mentions heat treating changes rate too. How does that work?
I guess that’s all my questions for now
Disclaimer: I am not a material scientist. I am just throwing out the circular bar equations from my Mechanics of Materials textbook, which to be fair I've used at work before and no one yelled at me
I don't design sway bars, but a shaft is a shaft. All of this applies to axle shafts also (Spidertrax even mentions this in some of their promo videos in a sentence explaining why they gun drill their shafts).
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I would ASSume that gun drilled vs turned down in the middle mainly comes down to manufacturing capability. Gun drilling is usually superior because it removes the least efficient section---the center. Stress (ie torque generated) is proportionate to the distance from the center. This means the center (neutral axis) does nothing and has zero stress, where as the outside sees max stress and does the most work. If you remove the center of a bar you remove a lot of material that isn't doing much. I don't see a scenario where turning down the middle is better, which is why I suspect it's a manufacturing thing--it's easier to turn down a bar than it is to gun drill it, so you can easily dial in your spring rate even if it's not the most mass efficient.
To throw up some numbers to prove that, here are two bars putting out the same torque:
The left bar is solid with an OD of 0.88in, and an area of 0.6in^2 (effectively the mass for all purposes). The bar on the right has an OD of 1" but a 0.8" diameter gun drill, so only 0.12" larger OD but a massive hole in the middle and an area of 0.28in^2 so weighs less than HALF the bar on the left---but both of them have the same spring rate, the angle of twist is ~12deg @ 1000ft-lbf. You can think of the left bar as "necked down" and the right bar as being gun drilled. Same stiffness, but the gun drill weighs half as much because that whole center section of the bar that doesn't do much is removed.
The one thing that should be noted though, is the max shear stress is higher with the gun drilled bar. Why? Because it's outer diameter is larger, and both bars are deflecting the same amount, and as stated above stress is proportionate to diameter so a larger diameter bar at the same deflection will experience more stress. So the gun drilled bar weighs less, has the same stiffness, but it's max angle capability is lower.
Here's another picture to compare another setup:
The left bar is unchanged, 0.88" OD. The right bar is NOT gun drilled and has a 1.0" OD. Two solid bars side by side. Only 0.12" between them in diameter. Going down the equations, you can see Moment of Inertia is way different, nearly double just increasing the diameter by ~14%! Max shear stress has decreased also, even though the outer diameter has increased and we know stress increases with diameter---however as you go down, angle of twist has DECREASED with the larger diameter. Same torque, the larger bar deflects less (intuitively makes sense) and less deflection usually means less stress. Now if we set the deflections to be the same, both bars twist 12deg---then the right bar has a higher max stress because the deflection is the same but the larger outer diameter is moving more due to it's distance from the bars centerline (shown below):
But note in this photo, angle of twist is the same at ~12deg, but to get to that 12deg we had to increase the torque on the right bar. The right bar is stiffer. Stiffer bar = more torque to twist = more stress to twist further.
I could show more screenshots to show differences how you change sizes, but all the equations are there if you want to punch the numbers. The main takeaways are this: the center of the material has low stress so isn't doing much. The outer diameter has the most stress. A larger outer diameter at the same deflection will have MORE stress. A larger outer diameter has a huge effect, as you can see Moment of Inertia is a 4th power equation so a small change makes a big difference----this doesn't just apply to sway bars either! Links, brackets, if you make something thicker it makes a drastic difference in performance even though the geometry might not change much (albeit heavily dependent on loadings and setup, so don't apply that idea to everything).
How much does material change rate? Well in the above equations there are two material values: shear modulus and max stress. Max stress is how much does the material need to be able to handle and cycle and be fine. Usually this is lower than yield by a factor of safety. Max stress allowables are determined by the material, such as chromoly or DOM being able to handle more stress before yielding than A36 steel. Shear modulus though is harder to change. Chromoly and A36 steel both have the same shear modulus, and the same modulus of elasticity. Similarly 6061 aluminum and 7075 aluminum have the same modulus, even though 7075 can handle way higher stresses without issue. This is a key discrimination: there is STRENGTH and there is STIFFNESS. In the above example, you could pick any steel and the angle of twist would be the same. But if you used A36 steel you would yield it quickly, you couldn't deflect it very far and because of that wouldn't withstand a lot of torque. Or you could use chromoly and it could twist further and handle more torque. But regardless if it was A36 or chromoly, they would behave the same until you hit the yield stress of one of them since their stiffness is the same.
Stiffness is how much force you get for an amount of deflection, and doesn't strictly have to do with stress (as you can see in the above equations, Angle of Twist has nothing to do with stress). Yield is how much raw force can you ultimately handle.
Tk1 mentions their heat treat changes rate. I'm not a material scientist, but I'm slightly skeptical of that claim. The heat treat might change what the maximum stress (and thus the max angle of twist) is, but modulus doesn't usually change for materials in my experience. With that said, I may be wrong, TK1 is pretty guarded with what he does since I've asked about what material he uses before so I could calculate what an allowable angle of twist was and he wouldn't give me any information to do my own analysis.
thanks
