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PostPosted: Tue Feb 21, 2012 2:15 am 
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This isn't for a locost, it's for a Mid-engine Lola t-70 bodied car, chassis questions nonetheless though. I'm looking for info on doing a tube chassis that also has integrated honeycomb. The idea is from Daytona Prototypes, and as far as I can tell, they get some very impressive rigidity numbers. It strikes me as a good compromise of easy-to-produce + cost + weight. I have searched around and not found much info about the specifics of how these chassis are done? I don't know if I would just design a regular tube chassis, then somehow bond the honeycomb into all the open spaces? Or if it's better to get away with the bare minimum of tubes needed to connect to each other, and you're filling in these larger open spaces between the tubes with the honeycomb? Basically I don't know where to begin, there doesn't seem to be great info online, even on tooling/ working with honeycomb. I used to know a couple guys who built honeycomb chassis, they said it's surprisingly easy to work with, self-jigging, etc. But I've lost touch over the years. Also, I know the assumption is to use Alum honeycomb, but there even seems to be a lot of options these days on using other aramids, or even plastic honeycombs, and they really aren't too expensive for the types of quantities you need to do a car. Any info, experience, resources you've got, please share.

Thanks!

edit: Searched more, and partially answered my question with this thread: viewtopic.php?f=39&t=6136&hilit=honeycomb

Still wondering on the types of honeycomb to use & best low-buck ways of bonding/ attachment to steel chassis.


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PostPosted: Tue Feb 21, 2012 4:08 am 
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When we build a steel tube space frame we do it on the shared experience of our members and decades of other's experience. It isn't clear what your goal is if you just want to build a body or a chassis with tube and honeycomb. I wouldn't worry so much about the body, but if your trying to do something structural and you're going your own path - then you are in fact the designer.

When you're the designer you need to do your own testing and verification etc. You can;t use other peoples numbers for composites. You have to see what you get with the resins, materials and procedures *you* develop. Researching this will take more time then building a steel space frame. Doing the testing you should do will also take more time then building a steel space frame. I think it's an interesting subject - but that is sort of a warning sign for me anyways... :)

Is the rigidity useful to you if your car doesn't see the thousands of pounds of aero loads the real Daytona Prototypes do? What if you land up taking a couple thousand extra hours of effort and the car is 20 pounds lighter? Or possibly 20 pounds heavier? Space frames for our cars run well under 200 lbs., in general. I think the FEA model for the Locost we have came in at 118 lbs.

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PostPosted: Tue Feb 21, 2012 7:15 am 
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+1 to Horizenjob's post.

But hey, all the best of luck to you, different can be exciting as well as beneficial but I would seriously add into the equation what you are going to do after you crash it in terms of repairing it .... that's the key point for me to avoid it.


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PostPosted: Tue Feb 21, 2012 1:59 pm 
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if you can bond the honeycomb together, then you can build a tub and mount subframes at front and rear for the working bits

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PostPosted: Tue Feb 21, 2012 5:39 pm 
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I'm just talking theory. We can relate it to a locost chassis though... Say we took the diagonal tubes out of the chassis sides (leaving just rectangles, not triangles), and instead bonded-in honeycomb to fill in the open spaces, and didn't bother to skin the sides either, would you expect to see a gain? I know I didn't find any FEA numbers in searching old posts, but I'm sure some of you engineers could at least explain this as good idea/ bad idea a jumping-off point. Personally, I like this from a safety standpoint, because you've got a frame holding everything together, so a bonded joint could fail, and the chassis would just get floppier, but would not pose a danger.


I'll get more specific though, and post a couple pics- I have the car body, and a tube chassis (the real cars were thin aluminum sheet metal, riveted together, and would need constant chassis checking/ servicing). The chassis was thrown-in, and I know nothing of it's design quality. It seems like a horrible design to me, and it's quite heavy, but it is straight. By the time it were fully triangulated, it would be about 100 lbs heavier, than an alternate tube chassis I've designed, (but haven't built). I was just thinking I might be able to fill in these fairly large areas with honeycomb, and end up with a chassis as stiff, or stiffer as if I were to go through and triangulate with steel tubes. This is an obvious answer if I could just draw it up in FEA software, which I have no experience in, and would need buy & learn some piece of software to get accurate numbers I suppose (which I'm looking into, this stuff really does get expensive). Again, I'm just looking for what your thoughts are... hopefully the pics help.


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PostPosted: Tue Feb 21, 2012 6:58 pm 
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I'm sure you get some other replys from some of our expert build community, in the mean time here's my advice.

Take a look here http://www.hexcel.com/resources/technology-manuals , specifically the sandwich design and fabrication ones.

By just bonding a block of h/c (honeycomb) in an open bay you get no structural benefit. Look at your example chassis above and one of square bays. If you put a load on your chassis that bay would try to "longinize" or deform from a square into something else (parallelogram, diamond, etc.). If you riveted a skin (aluminum, composite, etc.) all the way around the periphery of one side of the bay, assuming you sized the skin thickness and fasteners correctly, you get some structural benefit. Now when you apply a little more load to your bay the square is going to longinize, and the skin will buckle. Add another skin to the opposite side and you get more structural benefit, but the skins are still going to buckle. Now imagine you had something inbetween those two skins that was fastened (bonded) to both skin faces that would keep the skins from buckling.............welcome to the world of h/c sandwich panels.

Now the part were an engineer earns their money. Size the tubing, skins, honeycomb, fasteners, adhesive, surface prep, etc. to optimize weight vs. strength.........and also don't make it cost a fortune.

Don't let me or anyone elese deter you from this construction method. I'd love to someone get this work for our chassis, without costing a fortune. For me, I think a sub 100lb chassis is what you have to shoot for........since most people can build 125-150lb steel tube chassis all day long.

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PostPosted: Tue Feb 21, 2012 8:28 pm 
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You've probably already stumbled across the following, but here are a couple bits of info:

The book "How to Build Motorcycle-engined Racing Cars" (http://www.amazon.com/Build-Motorcycle-engined-Racing-Cars-Speedpro/dp/1845841239/) devotes chapter 8 to the subject of honeycomb chassis construction. While there's not enough depth to build a tub from, it's at least a starting point.

Also Dennis Palatov has built at least one car this way. Check out http://dpcars.net/ and http://www.palatov.com/index.html; I haven't looked at his sites for a while, but it should probably there somewhere.

I think the idea is pretty fascinating, but unfortunately the group I plan to race with would never let a car built that way go wheel to wheel without a whole lot more engineering data than I could provide.

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PostPosted: Wed Feb 22, 2012 1:09 am 
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Thanks for the replies, this is all really really helpful stuff!

I just found/ read this old student paper from the FSAE world: http://www.firearmz.com/user/SAE_983055.pdf

Very relevant to what we're talking about here, and would be very useful for you guys considering hc panels to do this on your locost too.

It discusses how they tested different ways to attach the panels (they happen to be using carbon fiber, as opposed to alum skins), and determined which was the best way to attach to a steel tube frame, then they failure tested. Very useful info!


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PostPosted: Wed Feb 22, 2012 2:11 am 
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We did it on our 2003-2005 FSAE cars thanks to that paper. After being intimately involved in the composite part of the 2005 car, I will never, ever, ever, ever do it again. If you had full access to proper composite equipment it would be a different story but we had what a typical builder would have available plus a huge vacuum pump. Making flat panels on the bench in this environment was actually pretty successful since you could easily vacuum bag it. The panels were typically really flat, very consistent, and very light. The chassis ended up not much stiffer but significantly heavier due to its complexity making it impossible to vacuum bag.

Everything was done with room temperature wet lay-ups to mimic the papers panel #5. We used two pieces of polycarbonate on both sides to give it a nice shiny finish albeit this did reduce the ability to remove resin to an extent - peel ply would have made it lighter but less attractive (hind sight is 20/20). It is also darn near impossible to vacuum bag an entire chassis and have the bag seal so getting all of the extra resin out was impossible. The only way to apply pressure to work out excess resin was with large sand bags and weights. Without removing the excess resin whatever small advantage composites offered was lost.

Going with the mechanical fasteners is definitely the best way but for the small weight reduction, the headaches don't seem worth it for the amateur builder. I've been happily adding diagonals since 2005 and have no plans on changing. I think if you rebuild the entire chassis using properly sized tubing and diagonals you will be much happier.

As an aside there are enough health issues with carbon fiber that I became aware of after the fact (thankfully I was wearing a respirator while doing all of the trimming following the wet lay-ups) that I go out of my way to avoid the stuff.


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PostPosted: Wed Feb 22, 2012 2:33 pm 
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I also have experience with this on our FSAE car. It was very successful for us but it is a huge pain in the ass and you really can't do it without helpers. I didn't read that SAE paper but hopefully it mentions that you need to pre-stress the tubes before riveting for best results. Good luck. I'm not going to bother with the work involved.


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PostPosted: Wed Feb 22, 2012 3:47 pm 
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Making your own panels is certainly well out of the grasp of most home-builders, but there are pre-made structural panels available which can be worked with a jigsaw and a router (and especially a respirator), and can use the cut-and-fold method. Not very practical for round tubes due to the profile, but cutting a panel to fit the entire floor/side/what have you and slotting it to fit around 3 sides of the square tubes, then applying a backing/reinforcement layer ~4" wide over the open side/exposed tube to bond it fully to the tubes sounds like a pretty reasonable plan of attack to me, an OCD-level of precision aside. That said, I haven't actually tried this, but I do plan to use the method in one of my projects, so critique/thoughts welcome. :)

His existing chassis looks too awkward (not to mention positively beastly, good god that looks heavy for zero triangulation) to just add panels to, but something made exclusively from 1x1 and/or other standard sizes would be workable.


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PostPosted: Wed Feb 22, 2012 4:05 pm 
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Going back to Andrew's contribution, ....

One of the sections I used to direct in Defence R&D Canada conducted aircraft structures research in collaboration with the Canadian Institutue for Aerospace Research. The composite materials workshop was a hazmat area. I should be more careful than I am when I play with carbon fibre laminates for my iceboat .... although it's the CF machining dust that gets ya, not the resins or the bulk carbon fibre (he said hopefully)

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PostPosted: Wed Feb 22, 2012 4:09 pm 
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Warren Nethercote wrote:
Going back to Andrew's contribution, ....

One of the sections I used to direct in Defence R&D Canada conducted aircraft structures research in collaboration with the Canadian Institutue for Aerospace Research. The composite materials workshop was a hazmat area. I should be more careful than I am when I play with carbon fibre laminates for my iceboat .... although it's the CF machining dust that gets ya, not the resins or the bulk carbon fibre (he said hopefully)


Not that resins are terribly safe without ventilation either, but he's right. CF fabric is fine, cured CF is fine, but the small particles produced when cured CF is machined or sanded behave very similarly to asbestos in our lungs, as it turns out, besides being incredibly itchy and giving you the Worst Splinters Ever. A respirator is a must, long sleeves and gloves are just a good idea. :cheers:


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PostPosted: Wed Feb 22, 2012 10:10 pm 
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I know the guys who wrote that paper, I've even seen those test panels. As the paper shows, the sandwich panel should be both lighter and stiffer than the triangulating tube and aluminum skin they replace. As mentioned, the Pashley book has a nice section on monocoque construction from sandwich panels, which should open your mind up to some interesting possibilities. I personally have ideas about using aluminum core/skin panels in a locost type chassis, it's a few projects down the line from even being started, but I was thinking of welding a steel flange around the perimeter of the opening and bonding one face of the panel to that and then running a bead of adhesive in the corner where the other face of the panel meets the edge of the fame tubes (panel thickness is less than tube width).


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PostPosted: Wed Feb 22, 2012 10:27 pm 
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OrangeCrusader wrote:
Warren Nethercote wrote:
"... the small particles produced when cured CF is machined or sanded behave very similarly to asbestos in our lungs, as it turns out, besides being incredibly itchy and giving you the Worst Splinters Ever. A respirator is a must, long sleeves and gloves are just a good idea." :cheers:


And I learned long ago, hanging around in Indycar paddocks, that damaged CF bits were sharper than razor blades. Or aluminum sheet metal, for that matter.


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