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  1. #21
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    Quote Originally Posted by Lizardracing View Post
    a 100 years ago I got curious about this topic and tried something that Rocketbonehead mentioned.
    I chained down 3 corners of the car at the springs pickup points on my dirt modified and hung weight on the unsupported corner and wrote down the flex number after each increment of 50lbs. What I discovered was the first 25 or so percent of the total weight hung, flexed the car the most and after that, each 50lb increment has less and less effect up to where I ran out of suitable weight. It seemed once the chassis was loaded up, it didn't move around as much. I didn't ever go further to perfect the two car garage hilly billy test, I still have the car but I've re-clipped and re-stubbed since then.
    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.

  2. #22
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    Quote Originally Posted by Ghopper View Post
    I am impressed that Billet cut that whole car up! Must have been one destine for the shelf.

    We are setting up for some tests this season. We have measured on-track deflection and are quantifying the stiffness changes for several braces. Next do blind studies with a driver of using those braces in different pre-measured configurations. Data system with load cells on some key locations.

    It may not yield any noticeable changes. In that case, additional stiffness is better for repeatability. In the opposite case, complementary suspension changes could be necessary and will need understanding.

    Why is this an issue at all? Because some drivers say car to car variation matters. That points to a manufacturing tolerance that needs specification and validation test for each new car. Such that new cars are within a window that is smaller than a driver can discern.

    What is a dirt car vehicle stiffness window? Around 2000 lb*ft/deg to 4000 lb*ft/deg. Now you can populate your models or do some napkin calculations. Maybe this can be your teams specification to your chassis builder when you order your next car. "I would like 2500 lb*ft/deg with +/- 100 lb*ft/deg over 40 midwest 1/4mile nights and max 0.125' plastic deformation". Or chassis builder could specify for you "this design is one of our favorite in the 2500 lb*ft/deg variety, with notes of strong restarts and smooth finishes". (corny)
    Are your tests for a dirt chassis builder? Just curious if chassis stiffness is "engineered" into their chassis's!
    An army of sheep led by a lion(Trump), is mightier than an army of lions, led by a sheep(Biden).

  3. #23
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    yes, and not flexing is better,

  4. #24
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    hey billet , what was the actual wall thickness for each of your examples ?

  5. #25
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    Quote Originally Posted by fastford View Post
    hey billet , what was the actual wall thickness for each of your examples ?
    If you mean on the cutting the bars out, all tubing was .065 moly but the main frame rails so all that was cut was either 1.5 or 1.25 OD tube that was .065 wall.

    If you mean something else, I didn't follow

  6. #26
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    so all three examples on tensil strength were .065 wall ?

  7. #27
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    Quote Originally Posted by fastford View Post
    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.
    Last edited by MasterSbilt_Racer; 01-24-2022 at 09:46 AM.
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  8. #28
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    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.
    Last edited by billetbirdcage; 01-24-2022 at 01:30 PM.

  9. #29
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    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

  10. #30
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    Quote Originally Posted by fastford View Post
    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.
    Last edited by MasterSbilt_Racer; 01-25-2022 at 10:19 AM.
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  11. #31
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    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

  12. #32
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    Quote Originally Posted by fastford View Post
    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#

  13. #33
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    Sorry, I forgot those 2 where actually 2.0 inch round frame rails not 1.75 but wall was still what I listed

  14. #34
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    Quote Originally Posted by fastford View Post
    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
    You have to have triangulation. But you always have unsupported spans. And no matter how much triangulation you have, the triangles with the larger cross section are stiffer, assuming we have materials with basically the same modulus of elasticity.
    Last edited by MasterSbilt_Racer; 01-25-2022 at 06:05 PM.
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  15. #35
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    Quote Originally Posted by billetbirdcage View Post
    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#
    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 ...

  16. #36
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    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.
    Last edited by MasterSbilt_Racer; 01-26-2022 at 07:09 AM.
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  17. #37
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    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 .....

  18. #38
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    Quote Originally Posted by fastford View Post
    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.
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  19. #39
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    agreed ..........

  20. #40
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    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.

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