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Still beating my box beam concepts. More important, trying to get some printouts for all to see. Viewing distorted plate shapes is very graphic as to where the stresses go. Anybody make a model out of rubber, mark it with little squares, and bend it? Kinda like that.

With most FEA there is a difference between plates and shells. As you would guess, plates are thick and shells are thin. In reality, plates take out-of-plane bending as well as in-plane bending and shear. Shells don't resist out-of-plane bending or in-plane bending, only in-plane shear. They both behave differently in FEA. (I like to use plates - but I used to design concrete walls). Shells have the advantage that they can still resist loads even though they have failed in shear. They go into that diagonal tension mode (where you see the sheet ripple). You don't want your car to do this normally but it's reserve strength there if you crash or something. We do this for earthquake loads in bridge girders.

Yes, plates and shells are difficult to model successfully, at least the first time. I used to work at big engineering outfits and with the advent of computers, we always had engineers checking engineers.

For everybody's info, I use Risa 3d for my FEA. The 2012 version goes for US1500. I picked up a reduced fee copy where I used to work. The company also used SAP, spelled over five grand. But you should see it shake those high rise models for the big one.

This is a great forum. I spent hours with my plate model and learned the same thing y'all already know - open engine bays are chassis torsion stiffness killers. I'm trying to design an engine bay brace that will also carry motor mounts.

Great book on basic torsional design: "Design of Welded Structures" by Omer Blodgett.

Bob

Bob

A few pages back we talked about about this paper. He has some interesting comparisons of different contraction methods and their stiffness in the appendix. It also has the below picture and I thought there was a more complete picture but I don't see it now. It might be buried in the paper somewhere or there may be a footnote pointing to something that describes this project.

http://www.scribd.com/doc/192898005/Vehicle-Structures-Development-of-the-Sports-Car-Chassis-and-Stiffness-Analysis-of-the-Westfield-Sports-Car

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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.

How much did they charge you for that paper, Marcus?

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

I vaguely remember an FEA program called SAP4 from the 70's, is it related to that? You don't mean the business program SAP that sponsors McLaren and is currently ruining the company I work for.



Definitely a good book. I went to a seminar hosted by Mr. Blodgett back when I was a little engineer. He made you think about designing a weld not just plopping a weld symbol on the drawing. He gave us examples of how a smaller weld can actually be stronger than a larger weld. Surprising how I can remember that after 40 years but I forgot my lunch today....

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

Hi Lonnie, didn't need to pay for it. I read it on the web page, in order to download it you need to register. It does seem a little harder to read because of an advertisement than it used to be. It's interesting paper though and has a lot of background on sports cars in general. The appendix comparing a space frame to a single tube etc. is interesting too.

Searching for the author might find an original copy at a university or something.



I know what you mean. I've been carrying a spare lunch on my waistline for a couple of decades and trying to get rid of it...

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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.

Horizenjob, I appreciate the link and I’m spending a bunch of time absorbing it. Although, those kilonewtons are a killin me! His quick torsional analysis of the stressed skin chassis on page 95 was fascinating. He’s getting a torsional rigidity of 47,000 Nm per degree, or 35,000 ft. lb. per degree. That’s pretty stiff. He’s considering 0.9 mm thick steel. I've been experimenting with aluminum plate which is one third as rigid as steel so if you replicated his stressed skin steel chassis in aluminum, you would get about a third of the rigidity or 12,000 ft. lb. per degree – still pretty good. Theoretically you can get this from a 12 inch x12 inch x 1/8” aluminum box (at 60 lb. bare weight).
I’m leery of going with stressed skin vs. plate, though. You start getting into buckling, shear lag and other aerospace stuff that may be fine for airplanes but the loads aloft don’t come from going sideways into a curb.

I’ve been experimenting with using aluminum plate for a box chassis mostly because I used to dive for several of the firms I worked for and we built diving support equipment to assist us in our dives. It was easy to fab something up from 1/8” plate and one structure we made had a boom 8 feet long extending out between some piling. We made it out of aluminum plate, bolted together, and it had to support both bending and twisting. It was remarkably strong. Why not use this principle in a car chassis?

Run87k, the SAP program was strictly structural. I’ve about worn out my Blodgett book. I suspect you’re like me – watching the modern stuff we built, now being torn down because it's outa date.

Lonnie, I finally finished reading your build – NICE WORK!

Bob


Tall tales:
Did I ever tell you the about the time I designed some excavation supports for a construction job? They had kicker frames welded up from box tubing sections and each frame looked remarkably like a dune buggy chassis. I ordered some extras in case they had to excavate further, and took home an extra. Just in case. Percs of working for a contractor.

I’m going to try again to submit some drawings here. I’ve developed the aluminum box beam chassis concept to a Locost configuration. I hope the drawings come through – they’re four pdf files in a zip file.

It is just a very rough structural concept right now. Preliminary calcs get 13,000 ft. lb. per degree and a total bare weight of 180 lb. Needs some fine tuning.

I spent too many hours in front of VSusp so I just designed the suspension to cover all practical roll center heights and instant center ranges.


Bob

“Give me my laptop on my deathbed, and Lord, let me just finish my suspension design first, OK?”

Attached is a copy of aluminum section properties from the Aluminum Association Design Manual. It doesn't cover the smaller sections (called bar shapes in steel design); you can get these from metals online or others.

I've got a copy of ASTM B209 which covers allowable aluminum stresses if you are interested.

Bob

I’m still working on that aluminum box beam chassis concept. Structure’s good but aluminum costs are likely over US$2000.

Well, if I can design an aircraft carrier wharf for Coronado, I should be able to come up with a concrete Locost. Concrete is pretty cheap around here and I can just pour it from the leftover ready-mix that gets sent back to the yard at the end of the day. They’ll probably give me the concrete.

For guidelines, I tried to pattern this after the McSorley. Had to shrink it up a bit to fit those Super Transport Firestones, though.

Bare Weight is lookin’ at around 2300 pounds. Still lighter than an Escalade. Easy to mount components using expansion anchors.

Bob

2300 lb. ? Well, hell, just take out some of the rebar!

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

too late Bob, i just flew my first concrete airplane, NOT

perhaps if you include vermiculite in the mix, a lighter weight could be achieved

or indeed tubes instead of rebar?

now all you have to do is make some concrete "a" arms and we're good to go, i fire up the mixer.

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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.

That'll keep the CoG very low.

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Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

Here's the latest Aluminum Box Chassis concept. The FEA file now has over 14,000 plate elements and takes over 45 seconds to load! I've got the bare weight down to 144 lbs, down from 180 lbs. Torsional rigidity is 8200 ft. lb. per degree (without roll cage), good enough for me. I don't want to go thinner on the plate, now at 0.10", to avoid buckling issues. Shear stress is less than 10,000 psi at the worst so I could go to welded construction and a weaker (cheaper) grade of aluminum. Like steel, the rigidity of aluminum is the same for all strength grades. High strength aluminum lacks ductility and doesn't absorb a lot of "crash".

The plate elements are really sensitive. With a coarser mesh, I was getting impossible local deflections. Plate analysis is a PAIN. Manual calculations are now proving the latest run to "appear" to be accurate. I would still do a full scale test, though.

The chassis now consists of the central "tub" with suspension mounting "boxes" on either end. I may revise these boxes to fabricated tube assemblies but I like the plate concept. A nice thing about plate construction is that there is always a surface nearby to mount something on to. If you need to, you can just add a stiffener to the plate for more strength. The bad thing about plate construction is that there is always a plate in the way, just like tube construction.

I can't decide whether to continue this design or go back to my concrete chassis concept. I've now revised the concrete chassis to a more appropriate lifted 4x4 concept, and extended the chassis all around for general parking lot bashing. I'm thinkin' maybe orange construction cones pointed fore and aft for bumpers. I suppose I should go diesel with this thing. My dream is to slowly cruse through the local mall parking lot and let them crash into me all they want. I'll still drive home.

Bob

It's really hard to see the structure with all the mesh data shown. Can you upload a JPG of the structure as thin plate, so we can see what it looks like? Perhaps even do a cut-away down the centerline as it appears to be symmetric.

The following may seem like an unrelated structure, but I think this simple blind rivet and adhesive technique could be applied to a Locost chassis too. This is a solid tail bike that will take a lot of abuse.

Bike ==> http://www.gizmag.com/voiroo-zero-sheet ... pictures#3

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

Hi Lonnie

The configuration is very similar to the one shown in my “Das Boot” post. I’ve changed the driveline tunnel for more butt-room. I haven’t figured out seating/pedals/steering wheel positions yet. I really don’t want to go sit on a cold garage floor with a pot lid steering wheel in my hands and three bricks at my feet for pedals. Anybody publish these dimensions? Couldn’t find on the internet.

I like that aluminum fab you posted. I’m considering countersunk solid aluminum rivets to assemble the panels. Remember those riveted sheet metal dustpans that we all had to make in metal shop? And YouTube makes riveting look so easy! Countersinking them should keep it from looking like it has zits. We used countersunk screws in our marine equipment fabrications so they wouldn’t snag our lines and that worked well.

Welding of the basic structure would be good if I could weld aluminum. I used to live up in Seattle and they had some exquisite aluminum boat builders up there. Well, this thing looks like a boat so I’ll have it built up in Seattle. Put a jet drive in place of the differential, drive the thing off the coast to San Diego and put the wheels on down here. Wifey’s hot for that; we used to do a lot of white water rafting.

I’ve started in on the final drawings although I still haven’t figured out the suspension. Sound familiar? As with a real build, there comes a time to throw all the engineers out and just start building.

How is your’s coming? I’ve been enjoying everyone’s builds.

Bob