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PostPosted: July 7, 2007, 9:52 am 
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Right, we can't forget that adding all the othr parts will make a difference in rigidity. At this point, I'm not too concerned about that, not to mention that it's MUCH quicker to do a beam analysis in COSMOSWorks, than a solid analysis with all the other parts.


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PostPosted: July 8, 2007, 8:40 am 
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Thanks Thawa!

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PostPosted: July 12, 2007, 10:09 am 
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THAWA and d_dejohn81, what program(s) are youse guys using for your FEA work? What you're doing is pretty amazing, I'm thinking I may have to take the plunge.

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PostPosted: July 12, 2007, 11:47 pm 
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I'm using COSMOSworks. I'm going to try Grape again soon, as I'm sure it'd be much faster than COSMOSworks once I can figure it out.


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PostPosted: July 13, 2007, 8:29 am 
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I have Grape, unregistered. I downloaded the geo files and they work. For some reason, although I download to the sample file under the Grape directory, the sample 12 does not appear. I called WhittleBeast Sunday but he is on vacation this week.

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PostPosted: July 13, 2007, 1:13 pm 
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Is there any advantage to gusseting all the joints and can these FEA programs model that? Or maybe just some of the joints if it would help them distribute the load?
John...


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PostPosted: July 13, 2007, 9:42 pm 
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Gussets would definately increase the rigidity of the chassis, how much? Depends on where they were placed, and how big they were, thickness and such, but as you can see from the quick analysis I did for chetcpo just putting two small tubes on the corners of the B-H tubes added as much rigidity as boxing the top of it.


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PostPosted: July 14, 2007, 12:18 pm 
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I have to admit to being skeptical when comparing computer models to real world fabrication, but this is very interesting. I have no doubt its a good way to make improvments.

A few things I really wonder about:

- a model with the panels bonded to the chassis. I'm sure they are lighter and stronger than a diagonal tube.

-Including a bonded sheet aluminum driveshaft tunnel with no diagonal tubes

- using an aluminum panel much like one used in the lotus 23 to strengthen the dash area

- using aluminum sheet to completely box in the top area ahead of the dash (footbox, scuttle area)


-double bonded panels-- sheets of aluminum bonded to both sides of 1" or 3/4" tubes.--maybe filling the void with foam-- could be applied to the seat back area, cockpit sides, dash hoop, footbox top, and maybe the floor.


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PostPosted: July 14, 2007, 1:38 pm 
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I was a powertrain guy on my FSAE team not a frame and suspension guy, so I haven't spent alot of time learning this stuff, but my understanding is that.

1. In an ideally designed/triangulated space frame all the tubes are in almost pure tension or compression, therefore gussets would add little stiffness. How close are we to that ideal?I have no idea, if you look at the pictures that show the stress in the tubes, then tubes that are the same color along their entire length wouldn't benefit much from gussets, but tubes that have more stress at their ends are probably taking some bending loads and would benefit from gusstes.

2. The effects of solid panels are very hard to model in FEA, because they deform by some weird shear buckling, where they bow out of plane, as soon as they aren't flat anymore they have almost no stiffness. For example a flat piece of paper on edge should be able to hold the same weight as one rolled into a tube, but you'll never get it to work in practice although it probably would in FEA.

3. A good compromise between FEA and the "real world" for spaceframes is to make a scale balsa model, you will easily be able to see which area of the chassis is contributing most to the deflection and easily test adding/removing tubes/panels/gussets. Have people done this for a locost?


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PostPosted: July 14, 2007, 4:28 pm 
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Herb Adams has a neat section in his book "Chassis Engineering" that shows a basic frame and roll bar, roll cage, shear panels, etc., and demonstrating the subsequent increase in chassis rigidity. The difference in a shear panel of steel over a diagonal is impressive. I'll see if I can scan the pictures this evening.

G

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PostPosted: April 7, 2008, 9:36 am 
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I wanted to know the efficiency of the models i made in grape, but it didnt give model weight. The grape .b3e files are very standarized, so i made an excel spred sheet to calc the weight of a model. It should ask you for the .b3e file of your model 3 times and then the .geo gometry file you used, and then it will display the weight in the results page. It currently only works for files up to 500 nodes, and (nodes+elements) of 1000, but can be expaned by dragging all formulas down. If anyone trys it let me know if it works.


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File comment: grape model weight calculator
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PostPosted: April 7, 2008, 5:39 pm 
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Glen wrote:
3. A good compromise between FEA and the "real world" for spaceframes is to make a scale balsa model, you will easily be able to see which area of the chassis is contributing most to the deflection and easily test adding/removing tubes/panels/gussets. Have people done this for a locost?


They talk about it, and show the results with different things in Herb Adams' Chassis Engineering book. Maybe one of us will scan the pages up :lol:

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PostPosted: April 11, 2008, 3:32 pm 
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d_dejohn81 wrote:

I'll summarize the results. Note that we want to see tubes "turn green" in the pics; this indicates they are loading up. Blue means they aren't carrying much load and red means they are carrying disproportionately high loads.


Very interesting, why is the firewall section so do deflected in pictures 6 - 8? The color indicated loads don't look that high.

A request for additional analysis, I have considered using 1.5 x 1 or 2 x 1 rectangular tubing in the cockpit sides (very tight fit side to side already, thus don't want to give up cockpit space for bigger square tubing). The top tube must be aligned so the tall leg is vertical, but the bottom tube could be vertical or horizontal without significantly impacting driver & passenger room. Which is structurally better? I tend to think vertical but would like some confirmation.


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PostPosted: April 11, 2008, 4:06 pm 
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Grintch wrote:
Very interesting, why is the firewall section so do deflected in pictures 6 - 8? The color indicated loads don't look that high.


Ah, I see now you said you moved those tubes to try and make fitting an engine easier with the engine bay bracing.


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PostPosted: April 11, 2008, 5:01 pm 
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Hi Grintch. I do see rectangular tube used for the top rail in some older formula fords. It is oriented with the long side horizontal. The reason is that it is the top surface of the cockpit that is not diagonally braced. So you to try to help that as much as you can by putting diagonals in the corners or making the edges wider to help compensate. I have also seen round tube welded to the side in the middle and a few inches towards the center at the ends to try and triangulate.

The newer formula fords address this by making the cockpit skinnier and taller. They use the forward braces from roll bar to dash hoop to about double the height of the cockpit. The opening in the cockpit becomes proportionally smaller, and matters a little less.

Design would be easier, were it not for the problems drivers present..

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