Crossmember Bars POLL

[haygood at myway.com (Brian Haygood)]

If people are still interested, I can do a 3D model and FEA of the bar and see how much stress it can take before it breaks.  Have to know the dimensions and material.  I wouldn't bother to include effects of weld HAZ, etc., but it would speak to a lot of issues Nate is raising, like whether the plates are in danger of bending, what happens with more bend in the bar, etc.



As for the fatigue arguements, I'm gonna side with Nate so far.  The brittle fractures Allyn describes (haven't seen them myself) fit the fatigue scenario well enough.  As Nate said, fatigue and brittle often go together.  I'd have to get all microscopic on one to really have any useful input beyond that.  



The weightlifting bar analogy doesn't fit.  There simply aren't enough cycles there for fatigue to be at play.  The weightlifter's bars, like so many coil springs, are changing shape because their yeild strength is being reached.  If it were fatigue, they would experience cracking and brittle failure, not just a little bend.  The HoR springs my PO put on the rocco did the same.  Through use they plastically deformed little by little because they couldn't support the highest loads they were hit with.  Such springs sag because they are reaching yield strength occaisonally, not fatigue.



The real question is, how many cycles should this bar be able to survive before it fatigues to death?  From there you look an S/N table for that material and it tells you what stress (S) you have to stay under to reach a number of cycles (N).  If we know the material, any engineering student, or other interested student, would be able to find the diagrams in their school library.  



So I've missed most of this topic, probably because the topic was never switched to "Crossmember Bar Engineering Analysis" or some such, but anyway, some of the arguements about the bar causing more stress on the crossmember seem off the mark.  Are we talking about lateral stresses generated from the wheels bending the new crossbar, thus killing the original crossmember?  If so, forget it.  The front A'arm mounts would be the source of such a force, and they are suspended on those darn horns, well away from the crossmember.  I don't think those forces are enough to do anything at all to the crossmember.  Just a crude guess, but doesn't strike me as something that would happen.  Most people using the crossbar will also have a K-brace between those horns that will keep the whole front end moving together anyway.  Consequently, I don't think there is much worry about the plates bending out of the plane of the plates (i.e. the bumper supports getting closer to each other).  



As for me and my rocco, I'll put a thin piece of sheetmetal across the crossmember to build up the crossmember, if I ever do anything.  Shooting from the hip, though, Eric's bar should work just fine.



BH

 --- On Sat 06/18, Nate Lowe < nlowe79@gmail.com > wrote:

From: Nate Lowe [mailto: nlowe79@gmail.com]

To: sad_rocc@yahoo.com

     Cc: scirocco-l@scirocco.org

Date: Sat, 18 Jun 2005 10:40:08 -0400

Subject: Re: Crossmember Bars POLL



On 6/17/05, Dan Smith <sad_rocc@yahoo.com> wrote:<br>> It's nice to see people discussing and analyzing the bar rather than just saying it's >useless but come on, we're not talking about a space age subframe for the next >Challenger or anything. Does it work? Maybe, I'd even go as far as to say probably. It's >worked for Tobias definitely.<br><br>I don't dispute this. If fact, I'd agree that the bar looks like a<br>winner from what has been observed so far.<br><br>>Those endplates are big hunks of expertly welded steel. Sure, someday<br>they may fail. But >if you're abusing your car enough to make them<br>break, I'm sure you'll have replaced plenty >of other parts before the<br>bar. It's quite possible that there could be areas for design<br>>improvement but don't fix what isn't broken.<br><br>Failing doesn't mean only breaking. I was referring to that curve on<br>the end plates, that would be the weakest link and the first place the<br>bar would bend (if it ever did). If 
that plate were to bend it would<br>cause the bar to put the crossmember into constant tension, not the<br>best situation for a cracked piece of metal. If this is a possibility,<br>then I don't consider strengthening that area useless improvement. It<br>would be easy to do some calculations to find out what kind of load<br>over what period of time would make that plate bend, no rigorous<br>testing involved.<br><br>> Like I said, if you want to argue the theoretical merits of 2 degrees more bend here or a >slight sizing change there, cool! <br><br>You might be surprised how much of an improvement 2 degrees might<br>make. Again, only a few calculations would be needed.<br><br>> Dan - overworked, stressed and tired of the insane amount of nitpicking lately (and not >just this topic)<br><br>I'm sorry it came off as nitpicking, but I don't think people trying<br>to give help to improve a product that will benefit many of us is a<br>bad thing. Hell, some of this is trying to help 
you figure out if it<br>even needs improvement, which is still uncertain.<br>Nate<br><br>_______________________________________________<br>Scirocco-l mailing list<br>Scirocco-l@scirocco.org<br>http://neubayern.net/mailman/listinfo/scirocco-l<br>

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