Pretty much what I see in backyard hillbilly tests I've done too.
Side note: Rocketbonehead is now racing/driving a modified instead of crewing. I talk to him via text about twice a week.
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yes, and not flexing is better,
hey billet , what was the actual wall thickness for each of your examples ?
so all three examples on tensil strength were .065 wall ?
Tensile strength is a material property. Size doesn't really matter except for seam weld where the weld would be a higher percentage of the cross section. Then it's going to depend if a tube was tested or just a section cut from the wall.
All of my materials research and certification experience is from the aerospace industry. I'm not sure how they tensile test structural tubing. The applicable ASTM spec doesn't make allowances for different strength based on diameter. But, the strength ratings are just minimum, where composition of steel and other things have a range to fall inside.
For reference: HREW (IE muffler tubing)
The seam in the tubing while I'm sure it has an effect, let me say I have never seen a seem split from a wreck. The only time I ever saw a seam split down the length of the tube was on a header so I'm sure heat is what caused that. So why is HREW or CREW so much weaker then say DOM? it's generally not the same material, Dom is 1018 or 1020 and HREW can be several from 1008 to 1010.
Here is a general list for HREW strengths (again it varies due to what material it actually is)
40,000 to 45,000 psi 30,000 yield - 15%
You can see why you don't want to make a racecar out of this stuff unless your using super thick wall stuff and even then it's not worth it.
maybe I should have said structural strength , and that may not be correct because I am no metallurgist , but my understanding is .o65 moly has as much structural strength as .125 mild steel plus memory , which makes for a lighter chassis that will flex and come back , I may be thinking of .095 MS , cant remember what the article I read was
It can flex and come back. But as I have argued for years, chassis should be designed for a certain rigidity. If that is the case, all steel tubing should be the same diameter unless it is a driver protection part of the car where the extra yield strength is of value.
All steel has roughly the same "spring rate" known as modulus of elasticity. And the stresses should be far enough below the yield point, when you have achieved proper stiffness, that a moly car and DOM car should behave the same when built with same size tubing. If you go thinner tubing with moly, the car can live, but it's now less rigid.
i think rigid in a chassis as for as its normal function is controlled by triangulation more than any thing , now backing one into a wall is a different scenario , like i said , my methods are a bit crude , but I took a piece of .065 moly tubing years ago and clamped it in a pipe vice , hooked a chain hoist to it and pulled it and let off until it didnt go all the way back , then took a piece of dom mild steel and done the same , the dom would not stand half of what the moly would and come back , but that is not what convinced me to use all moly , the weight was , our dom chassis weighed 150 lbs more than its exact counter part did in moly , we were running an all steel ford engine and that weight advantage was wonderful , we could get to min weight and have great percentages , jmo also I am pretty sure the dom was .125 but could have been .095 , not sure now , been a long time ago
A LM stye typical in the 2010 era: (I built these 2 as exact copies of each other to see DOM vs Moly difference on the stop watch vs each other)
Dom car 1.75 main rails .095 - 1.5 was .083 - 1.25 was .065 = no bumpers, powder coat, ect was 324#
Same car but with moly = 1.75 main rails .083 - 1.5 was .065 - 1.25 was .065 = 274#
Sorry, I forgot those 2 where actually 2.0 inch round frame rails not 1.75 but wall was still what I listed
dont know about your chassis , but in 2006 we had a thoroughbred dom LM chassis and switched to a moly TNT that had basically the same amount of bars and weight was just shy of 150 lbs less , I still have my notes where we weighed the two and looked again tonight to be sure , but either way , I will never use any form of mild steel again , even on our modified ...
I think Ghopper is the only other guy here that says you design for stiffness. If you do, weight will be the same for both cars, aside from making the cage of the stronger material. But even there, if you downsize tube wall, torsional stiffness will be impacted.
You build a DOM car of typical tubing and the moly of the typical thinner tubing and compare the torsional stiffness, they will be different cars. The moly car will be less rigid. At that point, they are 2 different designs and one is less optimized. You can't even compare them. The moly car is lighter because it is a lighter design, not an equal design.
If you use all moly for better crash worthiness, that's fine. It will likely have better fatigue life as well, provided it was welded correctly. But, there is nothing to support going lighter and having the same car. It's a misunderstanding of the end goal.
I respect both you guys opinion's , but I know what my scale says , I know of no new LM chassis that is not built out of moly or docol , either way , I appreciate both of your contributions and have learned a lot from you both , one more thing , both my chassis were scaled rolling , but all components were swapped from one to the other , so IDK .....
I'm not arguing it's lighter. Of course it is. But it was just randomly lightened with the thought the material was stronger. You could use thinner DOM too, and may not see any difference until it got hit. It's yet another thing that hasn't really ever been approached correctly in our sport and smart people are changing that.
agreed ..........
at 150# lighter, that DOM frame had to be made with some really thick wall stuff. Which is why I listed the wall thickness on mine which is more typical of what most guys use on Dom vs moly. It's usually same or one wall thinner size on thickness for the moly.
Right or wrong, that is sort of the industry standard as I have seen it.