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Learning how to build Lotus Seven replicas...together!
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PostPosted: June 30, 2016, 2:53 pm 
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Thank you, Marcus, I was able to find the files. If I have a chance to enter your coordinates, I would like to sub in some plates for tubes to see what happens. I typically design to AISI Specs which limit the span of sheet steel panels to about h/t=200 so any 0.063" steel panel longer than 12" will "theoretically" buckle at less than full strength. But it is still there for the rigidity and still has strength after buckling. As you know, it is difficult to attach effectively. The plate in the model I showed would need 1/8" solid aluminum rivets spaced at 1" centers just to develop the shear rigidity.


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PostPosted: June 30, 2016, 10:23 pm 
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I took the liberty of downloading Car9 into my Risa analysis program. I haven't entered tube sizes yet. Is there supposed to be a strut from the sides of the cowl to the mid engine bay?


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PostPosted: June 30, 2016, 11:12 pm 
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It looks like you might have some extra tubes. Also I'll put an updated model here in the next days. I just have to go thru it make sure I don't have any temporary 12" tubes added somewhere.

Here's a screenshot of my Car9 in SketchUp. SeattleTom and Briggs have their own models with slight changes for their cars. My car has some changes I made while trying to fit the engine and transmission pretty far back. You can see the rectangular tube that would cross the bottom of the dash hoop has been cut in the middle and and moved back to form a V shape. This cut area now has vertical tubes rising to the middle of the dash hoop.

The upper and lower frame tubes are long straight sections with a bend at the cockpit or a hoop shape in the rear. So some of the bends in your picture look like they have errors in my model. The width of the car at the nodes 1 foot back from the front looks wrong. I'll put a close up of that area in the next post

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The frame rails are straight here and there isn't really a second bulkhead 1 foot behind the front bulkhead. The rear legs of the control arms go into those 1x2 that are sort of flying a couple of inches in from the rails. The front legs of the suspension are perpendicular to the front bulkhead. So the full cornering load goes in there and the braking loads go thru the rear legs.

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SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


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PostPosted: July 1, 2016, 2:02 pm 
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Here’s the latest Car9 with all the tubes added. I realize that there have been changes to this design but I’m checking how long it takes to enter coordinates and get a model ready on this program. It was a little tedious but I got Car9 ready to analyze in a very lazy day. I’m going to try to run a model of another car for someone on this forum but it’s got some major curves in it.
Anybody else got a chassis I can try? I do plates (if I have to).


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PostPosted: July 5, 2016, 2:03 pm 
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I added a hypothetical roll cage to the generic Locost to see what that would do to the torsional rigidity. I guessed at the cage dimensions. I made it 36” high which I think is too much. Anybody have any suggestions for roll cage dimensions? Also, it needs more bracing. I like the external roll cage eh3Civic is using.


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PostPosted: July 6, 2016, 1:12 pm 
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The engine bay is a torsional weak spot for these cars so I added a “spider” torsional support above the engine bay to see what would happen. I sized it the same as the roll cage for simplicity. As expected, it made a big difference.


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PostPosted: July 6, 2016, 1:16 pm 
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Correction: The weight with spider is 250 lb.


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PostPosted: July 6, 2016, 3:24 pm 
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Consider another related experiment. In your model you have a pair of fore/aft tubes at the top of the engine bay. These tubes also include a diagonal beneath them. Try moving the node at the front of those tubes to the outside rail. These fore and aft tubes will then form a triangle on each side of the engine bay. I think that will give you a big stiffness bump without adding any metal. It's basically what I did on Car9, but I went a bit further and those top tubes became a diamond shape.

On Car9 I started with what I describe above but then moved to the diamond shape which is the same triangulation but with less acute angles.

These engine bay triangles then tie into the W brace shape under the scuttle. That all has to work together.

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Car9 Roadster information - models, drawings, resources etc.


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PostPosted: July 6, 2016, 10:10 pm 
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Moving the framing as you noted is a golden point. I haven’t made other changes you recommended so I can see one result at a time.


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PostPosted: July 7, 2016, 1:05 am 
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Isn't that fun when you can get a good result without actually adding things, just adjusting! :)

I think with the way those tubes were moved, you can remove those inner diagonals now. I think those tubes ( the inner diagonals ) will have that job done by the outer diagonals next to them. You might get a small drop in stiffness, but much less than the about %50 you just gained. Your going to need room around that engine anyway.

The twisting forces from the coilovers and the cornering loads and can go into one bulkhead in front instead of two. I use a single bulkhead to mount the front legs of the wishbones, the coilovers and the steering rack. The rear legs of the wishbones just put the braking loads into the frame and they don't need a bulkhead because they just push back along the rails. Just an idea to simplify it a bit. You'll need to get radiator hoses and steering shafts etc. thru there and it was harder than it seemed it would be, you can ask SeattleTom about that...

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SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


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PostPosted: July 7, 2016, 3:01 pm 
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Here’s the latest generic Locost with a generic tail added. It seems the rear ends of these cars are different for everyone so this is just something to keep the design moving along. I’m satisfied with the car design so far. It looks adaptable to different ideas. The diagonal bracing needs improvement but I suppose final brace locations are dictated by car pieces that have to be mounted or cleared. I haven’t played with different tube sizes yet.

I’m including a sketch of the basic dimensions I’m using. I just made these up from a composite of the other designs I have found. I hope this is generic enough to adapt to a specific design if anyone has anything in mind.

Marcus, omitting the diagonals as you mentioned dropped a little rigidity (100 ft lb) and saved a little weight. My concern is welding all those tubes together at a single joint. If I get into the details, I would design in some offset at these connections as is done in buildings.


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PostPosted: July 10, 2016, 1:23 pm 
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I was still getting some severe out of plane bending so I changed to a cross brace as shown. Cross bracing is good but you have to be able to transfer bending with it.


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PostPosted: July 12, 2016, 7:02 pm 
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I finally got these files open and made up a preliminary analysis model. I don’t have tube sizes yet and the curves could use some improvement. I’m getting this down to a system and this this was done in less than a day. But no analysis yet.


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PostPosted: July 13, 2016, 9:03 am 
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Here is the chassis that he is referring too. Some tubes are missing on the one above but we can add those in. The main curved tubes and floor i think are going to be the hardest parts to get right for simulation. The one above looks a bit off but again its a tough chassis. Also there are varying tubes sizes that will change the simulation as well.


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PostPosted: July 13, 2016, 10:22 am 
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If you can get me some tube sizes, I can add the additional tubes and stress it out. Do you have any equations for the curves? I just used coordinates - these should be checked but it won't likely affect structural that much. Still planning on the Big Block? What plans are you working from?


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