LocostUSA.com

Marcus,

don't get me wrong, i too think this was good work for anyone, let alone for a student, but a complete understanding of the chassis in the real world is the only way to interpret the gained data.

when i look at some of the old chassis design by Chapman, i just give up in disgust, the way he did things was in some cases dangerous and i do not agree with his philosophy of making one part do 3 jobs, i could go into detail but that is not the discussion here.

when you look at the car 9 chassis, it is obvious that it is far superior to anything Chapman or Costin produced with seat of the pants designing, i put this down to computer modeling and a lot of effort on your part, how long it took you to finalize the design shows how much there was to it, but in that time, you gained a lot of experience.

having designed two dragster chassis, i was faced with how to make them actually deform on demand and in the right place, this was many years ago now and if i had any computer modeling i would have been considered a genius, but it was all educated guess work, based on experience and testing, and using a video camera to see the thing bend in real time, Chapman didn't even have the video luxury, just Jimmy Clark giving feed back to a then very young Beaky Simms to fix the car.

these men were considered and still are heros in my eyes and if i had the intuition that they had then, i would be a clever man, alas i can only ride on their backs.

the way you used the modeling in car 9 seems to me that you are a fair combination of that student and myself, having atributes of both, a very good combination.

i am too old to even consider using comuter modeling for anything these days but i know it is the future of design, i guess i'm a luddite in some respects.

i do believe however that technology is often used for the sake of technology when a good eye can see what is needed.

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this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Hi John, I agree with all you're saying right above. For me I have to try and look at problems from different directions or I get blind sided. So I agree with your points but also have to look at things other ways if I can figure out how to do it.

Doing drawing with SketchUp takes some work to learn, and I am not very good at it. Simple parts are not too hard for me, but I can't really do bodywork with real curves. Doing the FEA is pretty simple though. If you can post here and have some mechanical knowledge it goes pretty quickly. If I could sit down with you I think you would be up and running in probably less than a couple of hours. It basically comes down to typing in the measurements for where the nodes are and then drawing in the connections and saying what the size of the tube is. You can move the nodes around so you do not even have to all that accurate to start. Once you have it going you can just click on the tubes and change their size.

I saw someone working on some type of rail dragster a few months ago and stopped and talked to the guy for awhile. It was interesting how much flexibility he was putting in the chassis. His connections to the bottom chassis rail where done by welding to little metal sleeves on the bottom frame rail. Not rigid at all… I think he had stops so the sleeves couldn't slide along the rail but they were allowed to rotate freely.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

dragster chassis have a lot of flex to transfer weight to the rear on launch, the difficult part is to allow the flex but not allow the chassis to rotate where the engine is attached, otherwise it will plant one tire and not the other, some people use tensioned cables in the chassis to achieve this instead of a diagonal tube.

the amount of flex has to be in a controlled manner as though shock absorbers were employed.

the whole thing only bends in one plain, but has to be rigid enough to support a violent decent from a wheel stand whithout the pan hitting the ground.

chrome moly has a lot of rigidity and will flex if long enough but will permanently deform without warning if pushed beyond a given point especially at radial welds hence the sleeves.

it is usually good practice to eliminate radial welds on the lower tubes where ever possible.

even in my locost i was mindfull of this in the case of roll bar mounts to the lower frame rails where i placed an elongated plate between the rail and the roll bar tube, welding the plate along the axis of the tube but not across the tube and then weld the roll bar to the plate, the plate has two purposes, spreading the load of the roll bar if it ever happens and eliminating the axial weld which can act just like a saw cut in the tube.

this also applies to main frame tubes in any structure where a node is formed by multiple tubes around the main tube, when stressed, the tube will deform or indeed break immediately next to the node so it is best practice to never completely capture a main frame tube through 360 degrees of weld at a node.

this is one reason for making diagonal bracing tubes smaller than main tubes, the weld is more axial on the main tube.

for instance, if i have a i" main tube and a vertical tube, meeting at right angles, and at each side of the vertical tube are smaller round diagonals, i would not weld the vertical tube across the main tube but only down the edges, then install the diagonals and weld them in, this leaves a weld pattern like an elips on one face of the main tube and the integrity of the main tube is improved, i do not feel that any weld strength is lost by this method but tube integrity is improved.

consider a standard bracket for mounting a coil over to a top tube of a locost, if the bracket is weded vertically to the side of the tube, it will reduce the strength of the top tube but if the bracket is welded along the top and bottom edges, it will act like a stiffener to the top tube.

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this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Using a knife is simple. Doing brain surgery with it is hard. Modern FEA is simple to use and that is its biggest fault. Making the mesh took ages back in the 70's and 80's and we were limited in the size of the model so everything had to be carefully thought out. Access to the programs was limited to the engineers or researchers whose job it was to do that work. I work in an engineering department of 60 now and everyone has at least limited access to Ansys through Inventor. Thankfully only the structural guys have full Ansys. I cringe when one of the new college kids tells me he has checked something out on FEA when he means that he stuck a load on his Inventor model and got some results. When I ask what he used for boundary conditions, I get a strange look.

I'm not trying to rain on anybody's parade but it is very easy to get good looking but incorrect results from FEA. In the old days models were often verified with strain gauge results on test pieces or on the actual hardware but that seems to have gone away as it costs too much.

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My car viewtopic.php?f=37&t=16434
1930s Style Sports Car viewtopic.php?f=36&t=16888

Well I'll stand by my comments. I'm not offering to teach anyone how to do useful work with any FEA problem, I'm offering specifically to teach John Hennesey who has designed and built several space frame cars and who is quite knowledgable on this subject. I don't think he needs to be in fear of his simple computer skills at limiting him from doing this. The harder part is knowing what to do with the FEA not employing it.

Additionally we are talking not about general FEA work here, but analyzing a space frame and even then in regards to the paper mentioned above and the program "Grape" that I use just doing beam and truss work which doesn't use a mesh at all. In the threads on this forum about using FEA for this we have models of the Locost chassis and it's results appear to correlate well with actual measured chassis. The paper above also models and measures a chassis with good agreement.

All in all I think we are on pretty firm ground. I do worry more when I see modeling being used on complex shapes that are milled or cast. Then again you can get in plenty of trouble without using FEA also, so there always a general issue of being "real world aware". The FEA is a tool to apply a lot of math to a problem and it is a good thing that this is available. I am not sure that just raising the bar for entry makes for a better final result, because that doesn't really prevent people from trying anyway…

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

I didn't know that. So, it treats everything as a beam that is essentially rigid from end to end? At our level, that is non-aerospace and not F1 or Indy, that is undoubtedly just fine.

FEA is definitely in my future. I've been waiting for the companion FEA product for my 3D design package to be updated to handle thin plates and shells, i.e., the tubes (round or RHS) plus the thin sheet we're all using before getting serious about it. Having the right underlying FE models is part of that not getting "pretty answers that happen to be wrong."

If John doesn't take you up on that offer, I might in future.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

Tubes are well understood shapes with simple equations that descibe them. Tension, compression, twisting and bending forces are applied to the joints of the tubes. The tubes are not rigid though, the equations for the forces mentioned give you how much the tube deforms and how it trasmits loads to the next joints in the structure.

So the finite elements are the tubes themselves and the shapes they form. Grape works at this level as did the program the student in the above paper used. More complicated shapes require much more complicated approaches but don't offer much benefit for us on our frames. The exception here is modeling the affects of gusseting, paneling and brackets on the design.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Thanks, Marcus. I know you're super busy, so I'll poke around the GRAPE documentation and try to figure out what you so graciously just passed on to me. My current chassis design is done, so it's more of a need for a conceptual understanding on my part rather than an immediate need for FEA analysis.

There's are still many things left to design on my Locost. I am hoping to apply FEA to some of those remaining tasks, but I'm dependent on the software gurus updating the FEA package for my design suite.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

For comparing the effects of tubing sizes on stiffness, it isn't that hard to get it pretty close. Everything is a beam element and as long as you apply your constraints correctly you get something good enough for comparative purposes. Even if the final value is wrong, you can see that increasing that tube from 3/4" to 1" increased the stiffness by 5% - you're going in the right direction.

I don't disagree with the point you are trying to make (I really prefer hand calcs over FEA whenever possible) but what we are doing is pretty much the crudest FEA you can possibly do. I could probably create a system of equations for a space frame chassis and solve them by hand if I hated myself enough.




Grape and LISA do a pretty good job of eliminating the mesh options from the user but by selecting your number of elements (not to be confused with the thing that goes between two nodes to represent a tube) you are essentially defining the mesh. In Grape choose File > Options > Element and select size from the drop down menu. LISA has a few other options under the Elements menu. Either way not touching it seems to work fine.

I haven't really played with the option much in either of these programs but in ANSYS the element size really didn't have much effect on the results for spaceframe chassises. I think the highest we typically went was 3. I'm guessing the tension/compression loading you mentioned really minimized its benefit.

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-Andrew
Build Log
Youtube

The nice thing about the discussions here is that you get the benefit of other people's mistakes and the wisdom of the very experienced. I don't normally post because I haven't got anything to offer. Usually I don't even read the FEA posts because it reminds me of work and I'm tired of that after all these years. I decided to say something this time because I wanted to say "hey, be careful of this." I've done it myself and I've seen the young engineers at work do it. It is now very easy to make a nice model and put incorrect boundary conditions on it. My point had nothing to do with whether you had simple beam elements or solids or plates. If you can define your problem with equations and solve it then you don't need my advice. I was speaking to anyone that doesn't know the difference between a fixed end and simply supported. That was what I meant about the general availability of FEA these days. People without the technical backround or relevant experience can make models that produce professional looking results. Most people believe what the computer tells them is fact without understanding that the way the problem is set up affects the answer. Part of my job is mentoring the new kids. I'm not sure if it is punishment or a compliment but I try to show them what to look out for.

Many years ago, a coworker modified the results file of an ANSYS run, ran it through the printer ( this was back when printouts came on the wide green and white striped paper with the holes on the sides) and showed his results around. The results violated the laws of physics and included several comments supposidly made by the computer. It was amazing how many people thought that if it came out of the computer, it was true. He did it as a joke but it ended up as a sad commentary instead.

Enough of this, I think I'll go out and sit in my Locost and make engine noises.

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My car viewtopic.php?f=37&t=16434
1930s Style Sports Car viewtopic.php?f=36&t=16888

at times the situation as described can be the result of self doubt, when confronted with something official looking that is alledged to be from the computer "god" but appears to them to be incorrect, so they are reluctant to give an opinion as they doubt that there correct opinion is trully right.

i only ever made one mistake and that was thinking i may have made a mistake.

actually that's not true, i've been married twice, is that one or two mistakes?

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Thanks Run78k and sorry if I seemed too defensive. I share your concerns and will admit up front I am not a mechanical engineer, I do software and some electronics. I don't believe anything a computer tells me and was the last of the kids going thru school using a slide rule. So I am used to making mental notes for orders of magnitude and also reasonableness.

I can do simple equations for mechanical things but am afraid that doing the whole frame would involve more than self hate, it would require regular beatings to improve my moral.



I think you have a great deal to offer and think you made helpful points above. I think even pictures of you making engine sounds in a chassis would be an inspiration for others. These are daunting projects for most of us and we all stand to benefit from more people completing their projects. Simple pictures and progress notes are an encouragement to others and help keep our site alive.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

I feel I should mention another FEA software, Mecway http://mecway.com which should look very familiar to some people on this forum. It can do static and dynamic analysis of space frames and is fully units aware which makes pounds and inches easy to work with.

It doesn't however solve the problem of getting wrong results that look nice. You've still got to recognize what the curvature of a simply supported beam should look like in case you accidentally used a fixed support, etc. But it's very graphical and easy to see what's going on.

Disclosure: I work for Mecway

Hello everybody.

First, I have to learn to post on this site so here goes. Attached, I hope, is an FEA file of a box structure I'm thinking about developing into a car chassis. Its an aluminum plate box, 20 inches wide, 13 inches high, and about 96 inches long. There's on opening in the top for an engine bay. This thing is crude but I want to see if I can submit it. It was done in Risa 3d and I have to cut and paste into Word, then make a pdf because Risa's printing is poor. I'm getting over 4000 ft lb per degree torsional deflection with less than 100 pounds of plate.

Next, I've posted yet another suspension set up. I haven't had a chance to run this on Vsusp or anything.

I'll come back and report in if this comes through.

Bob


Well, that didn't work. PDF extension not allowed.

Yes, there have been several times I've wished we could upload pdfs too.

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mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.