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| 2 x 3 vs tube chassis | Login/Join |
DRR Trophy |
Really?? That amazes me considering NHRA seems to think Mild Steel is inferior. Or is that just due to wall thicknesses and weight issues? Even though NASCAR's crash every weekend, other than the occasional rollover; a NASCAR crash isn't nearly as violent as a 1/4 mile wreck, even on 150 mph vehicles.[/QUOTE] It is, essentially MS is carbon based and is soft and malleable, on the other hand CM is alloy based and is stiff and brittle. This can get pretty deep, but the theory I understand, is that in NASCAR they want the chassis to flex/bend and absorb some of the impact without physically breaking. With CM the impacts will ultimately break the chassis before it would bend and therefore not absorb much. The threat of parent material failure isn’t as high in drag racing as it is in NASCAR. Then you have shape “memory”, so with MS, it’s kind of like Biden, it has no significant memory and when it reaches its yield it will end up where ever the energy takes it. CM has great shape memory and would prefer to break opposed to bend….kinda like Trump (sorry had to enlighten this a little – lol) . Keep in mind that this is talking dynamic load failures and not fabrication bending of CM with a slow controlled process. So ultimately Big power is going to like CM as it will not flex to the point of changing its memory (imagine a fuel car made of MS- the chassis would have a different shape/geometry every pass.) Jerry Kathe | |||
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| DRR S/Pro |
I am not trying to spin anything Jerry. Both static and dynamic loads from normal racing apply to my statement that you are incorrect with you statement as noted. Once you start including a crash/high impact loads, then it becomes much more complicated. You never stated a crash situation. | |||
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| DRR S/Pro |
You don't need math equations here, you just need to understand punching shear along with a basic understanding of what's going on at a connection of two or more parts in a chassis. This is especially critical if you are connecting a 1.5" round tube onto the 3" side of a 3x2! 4130 round tube has it's crack issues too from fatigue right next to the welds! It would also suffer from the same crack issues you describe if you welded a 1" tube to a 3' tube! Don't get me wrong here, I am not saying all race cars should have 3x2 frame rails! Most DIY'rs and some chassis builders don't understand the mechanics of connections in a chassis, just because they can weld parts together doesn't necessarily make for a good long lasting connection. | |||
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| DRR S/Pro |
Yes there is a difference between strength and stiffness. Here's a good example, take a wheelie bar and fabricate it from 4130 and another using the same sizes and wall thickness but use titanium. Both have basically the same strength/the ability to support the same loads but the titanium bar will flex twice as much under the same load! | |||
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| DRR Trophy |
You are correct, I did not. I was not steering the subject matter here, I was trying to help with some of the Ops questions. Also you are absolutely correct, I am wrong on the SAE comment. However what I said wasn’t; the ASE board who reached that conclusion , it was a physicist (or team of). I’m not about to argue this as I could care less, you jumped in here with objection and I’m only attempting to express what I have learned and used during my career in aerospace. It’s been nearly 20 years since I have authored any FAA approved repair techniques, but the life limit of liability has no statute of limitations….maybe those repair techniques that I authored should be revoked and the FAAs authority handed over to the CAA or the JAA due to incorrect physic analysis? (just for clarity - that was a wise guy comment – lol) Its all good Top, I’m not here to steal any of your thunder, just trying to help and kill some time. Jerry – out. Jerry Kathe | |||
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| DRR S/Pro |
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DRR Pro |
Interesting comments from some learned people. One of the reasons I like this forum. I would venture that a square tub chassis "may" be easier for the home builder to construct at home. Anyways, I have a mild steel square chassis car. It's been a working race car for twenty years (working race car, I love that term). It had been certified to 7.50 when I got it and another inspector here okayed 7.50. I noticed some flexing later on and prior to adding tubes purchased the SFI book. I was shocked when I read it. There was no fooking way that chassis should have gone 7.50. No X-member between the frame rails, no straight bar under the driver's seat, and most important no ladder bar support going to the seat back bar. I added some other bars to stiffen things up. Forgive I don't recall the SFI bar numbers right now. I added sixty pounds of mild steel tubing to get it right. With Dave Covey's advice. It must have needed it as the car never slowed down. There are some 4130 pieces on the car. Chassis crossmembers' that is. They cracked and Elite Racecars sorted that out for me and we're good to go. I'm not a metallurgist by any means but, I feel I'm at the limit with this chassis. 155 is fast enough for what it is. nomad Bruce Guertin Easily distracted by bright shiny objects. Wife says I'm a new adventure every day. Call Automotive Performance Engines for all your complete engine building, dyno service needs 863-967-8781 | |||
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| DRR Sportsman |
Some mentioned characteristics about mild steel and chromoly in earlier posts i won't rehash, but from a metallurgy perspective the real issue with 4130 tubing is the weld's heat affected zones. When you weld chromolly your alloy elements migrate away from the weld zone leaving a heat affected zone that ends up being brittle. If you want to fix that it requires re-heat treatment at specific temps and times. I think tempering may be enough to recover nearly all of the parent materials strength, but without it you are left with a brittle zone in the weld affected areas which is why chromoly cars crack often times near welds. Tidbits: SAE didn't credit chromoly over mild steel in roll cage construction on SAE sponsored/governed college level competitions back in 97 (mini-baja). We had 2 choices Mild steel 1" diameter or 1.5" diameter aluminum tubing. I had the idea we'd use chromoly for our chassis and since NHRA allowed thinner pipes when going with chromoly, thinking we could do the same, i'd be able to use thinner tubing on our buggy. Not the case, bending modulus was the key factor and in Mild steel and chromoly its the same or real close to it. Their main concern was crash protection and preventing roll cage collapse/failure. How or why SFI and NHRA allow it, i understand after working with molly tubing years later and seeing how tough and rugged it is (over bend on a chromoly tube vs MS to get the same bend is like 3-4X). Being our senior mechanical engineering project was a buggy i still stand by my original idea to use chromoly for the chassis, heat treat it after welding, then have a lighter yet steel (4130) based frame. The answer from SAE at the time was NO. Aluminum was an option but tubing requirements were much bigger in diameter and thicker. The aluminum option yielded a lighter frame by 1/2X so that's what we went for. Knowing what i know about aluminum welding and the metallurgical impacts in the HAZ for aluminum, i'm not real sure how SAE thought that was a good option. Wonder what they are doing these days. Tig welding is required on molly cages in race cars as per SFI/NHRA, that welding process generates more heat than does a mig weld. Joining two materials together mig will do the job just like tig will, the filler rods are usually the same or close to it depends on what builders select ER70S or ER80S, so thats not the reasoning. My thoughts are the fits must be 10X better for a good quality tig weld vs. a mig, better fit usually yields a better weld. Square vs. round profile well, we'd need to break out the engineering book and figure that one out both are good, but i've never seen a 6.0 legal square tube car. That was many years ago but i think its something like the centroidal area (I) or something of that sort and bending modulus (material property). Forgive me its been 25 years since i was in mechanical engineering school..... To be honest, given that 2x3 has a larger cross section and likely the same thickness as the roll cage bar, if it were chromolly and we could address the concerns someone else has pointed out bout load distribution at attachment points, i'm thinking it may be just as good, but like i said we'd need to break out the engineering books to prove it. Finding 4130 square tubing i'll leave that one up to the internet experts, i've never seen it in person in any chassis builders shop or at any of the places i bought tubing, not saying it's not out there, just saying i've never seen it. | |||
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| DRR Sportsman |
Triple Great comments especially on welding. If memory serves the term would be MOMENT OF INERTIA with units of in^4 for calculating stiffness along an axis. (But it's been a few decades so forgive me if I'm off on that) BG | |||
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| DRR Sportsman |
These comments are why I'm a member on this forum | |||
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| DRR Trophy |
Positive contributions for sure. A couple of things I would further elaborate on (I would generally only touch on the gist of the matter without getting too technical. I wish I had only been out of school 25 years – lol, my memory only visits when it wants too). I sometimes struggle with how deep to get “into the woods” with these types of topics, as I don’t want to lose anyone with technical jargon that is confusing, but also don’t want to provide inputs that could lead to misinterpretation. After all, if any one person could paint this topic in a clear vision with a few technical insights on a chat board, it wouldn’t take the amount of class room hours to teach or learn it. Welding; HAZ, General description this, is the zone where the parent material elements are modified, some of the elements are lost during the property change and some have a serious change in the grain boundaries. The job of the filler material is to not only help “fuse ” the materials together but to also replenish those elements in the weld bead from face through the root into the toe of the weld bead. The weld zone encompasses the HAZ on both sides of the bead and the weld itself, general rule of thumb is the weld bead face should be 2x the material thickness in width, fully penetrated (except for lap joints) and evenly fused (weld centered on actual joint). The HAZ is expected to be twice the width of the weld face on each side, so for example if the bead face is 1/8” the weld zone should be expected to be 5/8” total. Annealing the weldment after completion will help restore the grain boundaries in the HAZ but can also lower the strength properties of the remaining base material (I think the jury is still out on this in some circumstances). I would caution on the MIG vs TIG process with CM however, the byline there is that the alloys in CM have a much higher liquid to solidus point (weld going from solid to liquid and back again) so using a MIG process you will not properly fuse the materials together and align the grain boundaries without voids, with TIG you can focus the heat and assure you get that homogenization required, obviously with MIG, your heat is provided through the filler and you have no heat control….more weld doesn’t fix that – it just piles more filler on top. With that said, the heat control is in the hands of the welder (person) and not the materials or process. Please never MIG a CM weldment that could involve safety. (I would only use TIG process and ER70S-2 filler on CM – JFWIW) Fit IS extremely important with TIG as it plays a big role in the amount of time spent on that joint fusing/filling it, ultimately affecting the HAZ due to heat dwell time. Ill make one last pitch on the round vs square tube subject – look into the physics of “moment of inertia” and load axis, this is a very complex subject and has had a great deal of research involved – also don’t imply variations of loads, stick with the things that a race car chassis will endure. Once understood you will agree round is far better than square in that arena….and if you don’t….well that’s OK too. Jerry Kathe | |||
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| DRR Sportsman |
Really good stuff. Thank you I enjoyed reading and learned something. BG | |||
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| DRR Sportsman |
Thanks to everyone who responded here. Learned a bunch of information here and while I always made sure to have my car tig welded with the CM tubing, for safety reasons, it was the right decision. | |||
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