I am in search of a chart for strength comparison of seamed tube, dom and chromoly. I need to know how .065 x 1.5 chromoly compares to .095 x 1.75 seamed tube. Any help would be great.
What do you mean by "strength"? It mostly depends on the application...but in bending, you can compare the stress from applying 1000 ft lbs to both sizes of tube the larger tube has about half the stress as the smaller tube. You can look up the yield and ultimate strength for the materials you plan to compare. Consider weight and fabrication difficulty when you make your decision.
I am building a light weight ch***is for a road car. The car will make less than 300 hp, and will weigh less than 2000 lbs. I have done this before, with the 1.75 seamed welded tube, and I am thinking that the chromoly should be about the same overall strength and considerably lighter. I have found psi ratings for 1.5x083 wall, but nothing for .065. The .083 is around 20% stronger than the 1.75x.120 wall , but I have the .065 allready.
In my mind it will depend on a number of things, like the actual layout of the ch***is, the number of diagonal bracing, crossmembers, etc. Just for fun, here is ONE example.
I'm sure there are smarter people than me reading this, and I'm looking forward to what they have to say. Until that time however, I got two cents to spend. What you get to compare is material composition and it's metallurgical properties. The whole "strength" thing, that is something you need to define, tensile, impact, compressive, yield, elastic limit... What I'm alluding to is carbon content, alloy content, welding process, physical changes to metallurgical properties (grain structure), any heat treatments...and somewhere there is ridged strength, weight, size, and load or strain being applied, not to forget the design and any bracing. Using a seamed tube, if the seam is located on top, bottom or a side, does it matter? With DOM being the golden child for such things, is it really necessary? What's the application, and would it better than say, the seemed tube with more structure bracing? And the chromoly...that light weight high strength alloy wonder... A material that will resist bend and flex until it doesn't. I heard where they would braze a chromoly race car ch***is because it was easier to find torches than a tig machine. But as far as strength goes...that's a tough one to answer I think.
I am trying to get an idea how chromoly tube compares to seamed welded steel tubing in overall strength. The best I can figure, 1.5 x.065 wall chromoly should be comparable to 1.75x.120 seamed tubing. I have seen charts for psi strength test, but the chart I have only goes down to .083 in chromoly.
This is from ChatGPT, but I am curious what the resident fabricators will say. (Everything below is from ChatGPT (AI)) You’re on the right track conceptually, but the comparison needs to separate material strength from section (geometry) strength, because tubing size often matters more than alloy. Below is a clear way to think about 1.50 × .065 4130 chromoly vs 1.75 × .120 ERW/seamed tubing. 1. Material strength (typical values) Normalized 4130 chromoly Yield strength: ~63–75 ksi Ultimate tensile: ~90–100 ksi Typical ERW / seamed mild steel tubing (A513, A500, etc.) Yield strength: ~40–50 ksi Ultimate tensile: ~58–62 ksi So chromoly is ~1.6–1.8× stronger per unit area than mild steel. 2. Geometry matters more than people think For tubes in frames/roll cages, bending strength and stiffness depend on: Section modulus (Z) → bending strength Moment of inertia (I) → stiffness (deflection resistance) Let’s compare the two tubes. 3. Cross-section properties (approximate) 1.50 × .065 4130 OD = 1.50 ID = 1.370 Area ≈ 0.293 in² Section modulus Z ≈ 0.111 in³ Moment of inertia I ≈ 0.083 in⁴ 1.75 × .120 ERW OD = 1.75 ID = 1.51 Area ≈ 0.614 in² Section modulus Z ≈ 0.247 in³ Moment of inertia I ≈ 0.216 in⁴ 4. Bending strength comparison Bending strength ≈ Yield strength × Section modulus Chromoly tube 70 ksi × 0.111 ≈ 7.8 kip-in ERW tube 45 ksi × 0.247 ≈ 11.1 kip-in The 1.75 × .120 ERW is ~40% stronger in bending, despite weaker steel. 5. Stiffness comparison (deflection) Stiffness ∝ Moment of inertia 0.216 / 0.083 ≈ 2.6× stiffer The ERW tube deflects far less under load. 6. Weight comparison (per foot) 1.50 × .065 4130 ≈ 1.2 lb/ft 1.75 × .120 ERW ≈ 2.5 lb/ft ERW is ~2× heavier Your ***umption—how close is it? “1.5 × .065 chromoly comparable to 1.75 × .120 seamed tubing” Strength-wise: ❌ Not equivalent Weight-wise: ❌ Not equivalent Use-case-wise: ⚠️ Depends A closer strength match would be: 1.75 × .083 chromoly ≈ 1.75 × .120 ERW 1.50 × .095 chromoly ≈ 1.75 × .120 ERW (lighter but closer) Why charts don’t go down to .065 At thin walls: Buckling Denting Weld heat-affected zone effects …become dominant failure modes, not pure tensile strength. That’s why sanctioning bodies (SCCA, NHRA, SCORE, FIA) usually forbid .065 wall chromoly in primary structures unless diameter is increased.
One thing to think about is how you want your structure to behave in an extreme event. Do you want it to deform, absorbing energy before fracturing, or withstand a higher energy level, with the potential for catastrophic failure if the incident exceeds the maximum stress that the material can withstand. Further, what safety factor should you use? I have a background in a subset of mechanical engineering, in applied mechanics. The best way to understand the difference is in traditional strength of materials design, we design our structures ***uming the material is perfect, no flaws of any kind, then we apply a safety factor. By contrast using the tools of applied mechanics, we know that we will have flaws. So we design the structure to withstand the stresses that we expect, but ***ume that we have a flawed structure. How large of a flaw and in what critical cir***stances can we tolerate. And we still apply a safety factor, but because we are looking at the problem from the other side as it were, we can design components closer to the point of failure, somewhat safely. Without these new tools, we would not have put men on the moon. NASCAR has the viewpoint that they want the structure to absorb a lot of energy, lowering the peak accelerations experienced by the driver, through large deformation before failure. They use dom tubing to effectively do this. weight is not a factor at the design level, because everyone has the same weight. NHRA has a different viewpoint. The top cl***es require 4130 with specific heat treatment, and only tig welding is allowed, with specific characteristics that have to be applied in the welding process. They want the part of the ch***is around the driver to survive, and use the failure of the rest of the structure as the energy absorption part. Long winded and probably not very helpful, however I hope you will give this some thought in determining your material selection and the design of your structure.
Thank you all for the input. I have decided that .065 is not tough enough for my application. After much thought I have changed plans. I will be using .083 wall chromoly for a lot of the ch***is and use the .065 where I see fit.
There is a book available for building homebuilt airplanes which will explain a lot about not only the materials and that common names do not always mean all forms of the material "chromoly" are created equal. It will give lots of info on the welding processes used and how brackets and gussets and braces should be placed. The book is "Construction of Tubular Steel Fuselages" Aircraft Technical Book Company PO Box 270 Tabernash CO 80478 970 887 2207 www.ACtechbooks.com OR Amazin Amazon has it for $27.95 or Used for $16 As has been mentioned above, the placement of bracing is a major factor in what will work and what may not.
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