Folded Honeycomb Construction - FSAE/GRM Constructions

Alright, this thread is an out-growth of some discussion in the "what would a GRM prototype look like" thread:
https://grassrootsmotorsports.com/forum/grm/what-would-a-grm-daytona-prototypelemans-race-car-/150789/page1/

I'm starting this thread because the original one has moved on... but I think there's still benefit in discussing this method of construction... both for chassis as well as body-replacement panels, as well as canards/wings/spoilers.  We're going to get technical in here, there will be discussion of FSAE design papers, shear and tensile moduli, bonding agents, etc.  Some math will probably be involved.

To begin with, I'm going to pull in some of the relevant comments via quotes from that first thread, specifically starting with alfadrivers' comments on page 3:

alfadriver said

In reply to GTXVette :

While not in the same ballpark as the Challenge, the original FSAE goal was to make a $5k race car.  So combining older FSAE work with Challenge restrictions nets you a pretty interesting car.  But not that much unlike some of the 70's era inexpensive sports racers.  I can't recall the brand name of the car Per found quite a few years ago- but it's not a super expensive material make.

I also don't see the repeatability as a challenge, ether.   What it means is that the first chassis will be expensive, as you have to build some jigs around cutting and welding them.  But from there out, they won't be too hard.  OR- the design changes to a simpler to assemble process- instead of putting a lot of tubes together, you get sheets, bend and connect- which should reduce the amount of connection work.  Heck, one could even find some of the less expensive structural sheets out there- like honeycomb structure sandwiched by aluminum.  The point there is that there are other way to make a chassis than just steel tube welding.  In terms of other parts- brakes, fuel systems, electrical, etc- while not the top of the top stuff- circle track parts tend to be quite reasonable in cost, even brand new.

In terms of the car- mid engine for sure.  And I would not throw aero work out- there are some plenty good CFD tools out there that are cheap enough to make a good body and aero devices around.

So in summary- take a midlandia, alter the shape to fit under a easy to make but aero capable body, and choose a powertrain.  Done.  You'll end up with what is very close to a spec racer of whatever era (Renault, Ford, Honda...).  Which is a very good car.

sleepyhead said:

In reply to alfadriver :

The challenge with using sheets to build up a chassis with is that cars tend to have a number of “point loads” introduced into them.  This is based in part from following palatov’s original dp1 development back in the day (which I followed independently from joining here), where he looked at going with carbon & nomex (?) honeycomb panels.  But anytime you went to mount something, it required cutting the sheet out oversized and dropping in a plug that would help distribute that load out into the panel without creating a localized over-stress.  Which is something you’re going to have to work through since all the GCR’s that I know of specify steel-tube roll-over/cage structures.

now, perhaps aluminum sheet with aluminum honeycomb can mitigate this... I mean, Porsche did it with their prototype cars back in the ‘60’s, right?

interestingly enough, palatov’s cars grew out of reading FSAE forums, and people proposing that an Awd FSAE was... impractical ?  Impossible?  Something like that.

page3:

alfadriver said: 

In reply to sleepyhead :

My limited knowledge of structural engineering would make me think that sheet aluminum honeycomb would be very similar to carbon honeycomb.  Which would say that the loads would have to be designed around.  

IIRC, Carroll Smith book used sheet design for it's race car book.  The ONLY reason to go down this path is if it's faster and cheaper to make in bulk.  Otherwise, cutting and welding does a good job.  

The on really interesting development I've seen are some FSAE teams using folding sheet design for the base chassis- where the big issue is how to cut the folds so that they fold correctly- there was an interesting article in Racecar Engineering about it.  Seemed like something that could be translated to mass production if perfected.  Making bulk sheets that can be later cut and folded would be a whole lot faster than laying out and curing every single chassis (it seems to me).

page4: 

sleepyhead said:

Aluminum has two advantages over carbon... it is much less brittle than carbon, and it has a higher shear modulus.  I don't have the numbers handy, so I could be wrong... but I believe that Aluminum has higher shear and compression strength than carbon.  Carbon is primarily a 'miracle worker' in tension only.  And point-loads generally transfer loads into a panel in multiple load types at the same time... iirc.  caveats:  structures was not a... {ahem} strength of mine in school, and it's been a while since I had these things/numbers at my fingertips.  so, I could definitely be wrong.

re: 'folding sheet design for chassis'
I went bombing around trying to find that article, with no luck.  if you come across it again, I'd be interested in reading it.  I agree that a sheet-based chassis could be a haven for 'production'... especially since it could leverage cnc cutting.  but, as Exocet and Palatov have 'proven' to a degree... cutting tubes and welding into a fixture is still a valid approach.

GTXVette said:

We Have a Member here that has built A Chassis,And a Gourgous Car, Uses Production  Suspinsion, C4

    Ya'll Read Up On the Cheetah Being Built, It's a Super Car And he has Designed it for several engine tranny options. I Doubt It Could be (Forgive me)   Dummied down for this but maybe ?

  Having Grown up Next to Lockheed and A&P school I am Familiar with Honycomb Materials ...Monocoque Construstion... And Box Folding 101.

          Lets stay with Easy Repairable Ol' Fashion DOM.

      READ THE CHEETAH BUILD.

page 4 cont

alfadriver said:

In reply to sleepyhead :

I'll have to go to our library to look it up- but I could not see reference to it on the Racecar Engineering site.

alfadriver said (later):

So it's the Sept 2018 edition of Racecar enginneering.  I scanned it, but not the title page, as it had no text.

How does one post a .pdf file? I'll post it up when I get home.

The article is very relevant to this thinking thread- as it's a debate in the FSAE world about a space frame vs. the monocoque.  And the big separator in the technology tends to be the resources available to the teams.  And this idea of folding sheets of honeycomb brings the monocoque to more teams.

If it works, it also brings the idea to more of a backyard builder, too- where you mostly need a CNC mill to cut the sheets prior to folding.

I'm going to skip a couple of messages where we discuss Racecar Engineering and accessing the article in question "Skeleton Key", when alfadriver links to some other technical "FHC" papers

alfradriver said (edited a tiny bit):

[...] a quick google search for the term "cut and fold aluminum honeycomb" results in some interesting items- some which are public research papers.

http://ojs.unsw.adfa.edu.au/index.php/juer/article/viewFile/1023/651

http://www.universalmetaltek.com/pdf/Honeycomb-Panel-Fabrication.pdf

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.821.4355&rep=rep1&type=pdf

But the article does debate the pros and cons of the various methods which is interesting.

stroker (original thread's OP) said:

This is edging into a discussion of how to morph FSAE autocross design into an Endurance Prototype.  I like it!  It echoes of Bruce McLaren's "whoosh-bonk" design philosophy.  We assume street car suspension (e.g. Miata, C4) with a FSAE inspired frame.  If we project transverse mid-engine our options are almost myriad.  If we assume Longitudinal engine then the really big question comes down to affordable transaxle, assuming we're using a Challenge-style budget limit.

alfadriver said:

In reply to stroker :

It should.  Again, FSAE does have a budget goal that is pretty inexpensive as part of the challenge.  And many teams compete with similar constraints that many challenge cars are built.  So it makes sense to see how FSAE does it, and if that can translate into a 2 seat sports racer or not.

We all know that getting a great transverse powertrain that fits into the challenge budget is totally possible, making a nice mid engined package.  Longitudinal is more tough, as the number of transaxle choices are far more limited- but given the Corvair middie in 2003/4 and the few 914's with big engines- we know it's at least possible- just harder.

I don't really see the powertrain as the main constraint if this is feasible or not.  It's the chassis.  And even then, I'm pretty sure that a Midlandia chassis is possible- since it's just like making a Locost, which has competed a few times.  

But it's interesting to figure out how far one can find the state of the art. 

GTXVette said:

Monocoque Construction has been around a Minute I think the 1962 Lotus 25 was first then F1 ,  Indy cars , Even 'Little' John Buttera Built a Top Fuel Dragster  late 60's. we could do a tub pretty easy, at Least a basic design With add on Susp. mount points. that could weld/ Rivet to the tub

Unless Honycomb cost has dropped Drasticly How can we consider it.

And again repairability.

I Have a cobra chassis here Engine setback Is Such that the Drive shaft is only about 13" long On a 90" wheelbase Car,So A Roadster Car could be easy enough,  I would class that with a Mid engine car built from Factory parts.No Fear. 

There is a small Road race Track At Gainsville I hear!!   A 3-4 Hour event could be enough, Open to all Challengers. 

alfadriver said:

https://www.ebay.com/i/192520006956?chn=ps

May need a few of them bonded together, but they are not hugely expensive.

Not sure how much a basic tub would cost in materials. 

fanfoy said:

 GTXVette has got it.

Repairability makes the aluminum honeycomb chassis an absolute no-go for a grassroots effort unless it would be incredibly cheap to make. 

Also, a honeycomb chassis is always harder to get approved by the insurance for safety. 

And a tubular chassis, while simple, is very labor intensive to fabricate.

[... ...]

At this point, the original thread started discussing SCCA SRF's, and I started reading slowly through the linked files alfadriver had provided through a google search.  I'm still working through those.  I feel like this construction concept deserves more discussion, and I don't feel like the original thread is the right place do to it, so I've started this thread here... however, I'd like to open the discussion up beyond just considering chassis, since I feel the materials/techniques discussed so far could lend themselves to additional motorsports uses.

I was going to ask a person here at work about it- as it seems to me that making a structural sheet of this + cutting and folding it would be a faster and cheaper overall process than trying to make a full sized car out of a CF layout.  

Even for normal cars. 

The "how to build a motorcycle powered racecar" book has a good section on honeycomb construction.

Whe I was in fsae, we were doing honeycomb sandwiched in carbon fiber. Much easier than an all honeycomb mono construction as you could do complex bends, all honeycomb only allows simple bends.

Challenge is many sanctioning orgs won't allow non-tube construction for safety reasons.

Haven't done a cost analysis of honeycomb construction, but remember you need really really good adhesives and they don't run cheap. For our fsae we used hysol. 

If there were a formula car challenge that would be awesome! Easy button, get old autox only f500 and add Aero, springs shocks, and bigger motor. Hard button...  you can mate a Ford 302 to an early 90s Audi transaxle... should be good to 350HP, f5000 wannabe!

Seriously, grm, you should make a purpose built racecar supplemental challenge class happen, even as a once off. Didn't someone bring a formula vee in the early years? I get that it's absolutely unfair to those building a sedan, but tow cool to see in its own right.

Oxford Brookes University in the UK builds folded aluminum honeycomb chassis for FSAE.

Based on everything I've read it is the simplest, fastest, least equipment-intensive way to build one of these cars. 

Design, and particularly passing the side-impact rules, is harder. You need to put the loads into the corners and spread them out.

They end up heavier than the best steel tube frames (60# is a rough figure) but lighter than most. A really good tube frame has so little unnecessary material that the lightest FSAE car to finish endurance (California-Berkeley 2013, 275#) was built that way. 

If a few people wanted to pony up for an extrusion die for corner joints the weight could be reduced further and some really elegant engineering could take place. As it is they're reinforcing them with off-the-shelf open-section extrusions and using spreader plates to mount the roll hoops. 

When I was doing FSAE, we did two different types of somewhat-GRM carbon fiber monocoques.  Here's why:

What we had:
-Large CNC and manual mills
-Lots of free carbon fiber fabric
-Dedicated but unskilled labor

What we didn't have:
-An autoclave

This led us to make cut and fold carbon tubs for a few years.  We would take a big flat sheet of core material, route our "flat pattern" and drill holes, put in phenolic spacers everywhere we planned to bolt through it, then lay up the whole thing on a flat table with carbon and vacuum bag it.  Looked like this:  

Then, you could route out the inside CF layers and core of the bend areas, heat gun the outside, and bend it like this:  

We would lay fiberglass on the inside once the bends were complete, and form the chassis into a big tube around (iirc) some plywood bucks.  Then we'd ratchet strap the whole deal into place, adhere the "open" corner together, and leave it to let it cure.  On the shot from the cockpit of the '09 car you can see the sharp edge where the chassis was folded together- the others are bends so they have a real radius to them:  

After a few years of that, we started doing "rail" type frames.  Basically, instead of vacuum bagging a flat sheet and folding it, we let the CNC machine do all the precision work and routed out the entire chassis shape from foam as the first step, then add our phenolic inserts, then hand lay the CF over the chassis.  This allowed us to make a big perimeter frame and hang everything between the rails, and the cars actually had no floors or bottom at all other than the seat and pedal areas:  

Overall, for someone working from home, it would depend on resources.  If you can get access to a giant CNC machine, the rail type frame would be easier- it puts most of the difficult work of locating everything in the hands of the machinist, and can then be relatively simple to lay up.  If you have no CNC capability, the cut and fold method is probably easier but may require a couple of tries to get right.

 I  JOKINGLY re ferred to the sheet design as 'Box Folding 101' and with some math and forthought, thththat's all Folk's.

    Pretty simple, Think of your finished tub look, fuel tank mounts, Where suspinsion and engine mounts will be, all that needs to be seen in your head or on Paper somewhere Then Un Fold it,

         You now have an Unfolded Box that you transfer to your SINGLE sheet, then dependent on the folded angle needed you groove the sheet with the correct angle going through the honycomb but not OUT side skin , if you know how Aircraft attach supports to a bulk head that is the method I would use for the rest.

     It's simple ,It's Light, it will Colapse on you if you mess up and don't just start over, I Suggust an A&P be on our Friends List.

       Really I Like it and if had the Extra Income to build a 3rd Car I wouldn't hesitate as I do have an Extra C-4 sitting Here, A Simple Cradle that Emulates a G M Or your Choice FWD attached to the Back LS/6speed w/c4 front. Don't really know others without saying something else stupid,

In reply to chaparral :

That's one of the interesting things to me, that FSAE actually has a side-impact strength requirement, and have a number to put against it.  I need to go dig into their rules, and see what the test is like (i.e. how "point load" it is), because it wasn't clear to me from reading two of the FSAE papers alfadriver linked above.

re:extrusion die
another thing that's interesting to me, based on those two papers (and a little bit the "Racecar Engineering Skeleton Key" article... is that they're not discussing putting adhesive into the joint before folding, and simply bonding a reinforcement to the inner skins.

In reply to ¯\_(ツ)_/¯ :

just to be clear, y'all were making up dual carbon skin panels with a phenolic honeycomb core (i.e. not aluminum core)?

another benefit of your teams' original approach could be that shipping a "base chassis" kit, could be flat-pack like a certain Scandinavian furniture

however, as alluded to already... finding panels easily available seems non-trivial

In reply to sleepyhead :

The side impact structure equivalency is exactly what it sounds like- it needs to be mathematically proven to be equivalent to the 3 spec tubes.  I believe we did our official test in bending, claiming that the section between the front and rear hoops could be approximated as a beam supported by pivots at both ends with a load applied in the center.

We also did a "dynamic" side impact test on a cut and fold chassis by dropping a 55 gallon drum with a bunch of weight in it from ~10ft onto the chassis.  The drum bounced 

One thing to keep in mind is the folded & riveted sheet aluminum framed race cars from the '70s tend to slowly tear their own frames apart from regular use and need to be remade after a while...CF frames on the other hand have held up well so far.

Also you could do your own foam core CF frame without a CNC machine, you'd just have to make your foam core very carefully by hand, perhaps by cutting and gluing pieces of the stuff together.

sleepyhead said:

In reply to ¯\_(ツ)_/¯ :

just to be clear, y'all were making up dual carbon skin panels with a phenolic honeycomb core (i.e. not aluminum core)?

another benefit of your teams' original approach could be that shipping a "base chassis" kit, could be flat-pack like a certain Scandinavian furniture

however, as alluded to already... finding panels easily available seems non-trivial

Actually a Diab Divinycell foam core- smaller voids than honeycomb.  Then adding phenolic inserts where the core would be clamped.  We did use aluminum honeycomb in a bolt-on crash structure at the nose of the car.

I would like to be able to discuss this intelligently but I don't have any experience whatsoever with the materials being discussed.  I know what aluminum honeycomb is but have no experience on how it's formed into a chassis with a "tub" or "tubular" shape.   The most common metaphor I can think of would be corrugated cardboard.  For the sake of my limited experience let's discuss building a tub, tube or channel of corrugated cardboard instead of honeycomb aluminum.

Let's assume a sheet of corrugated cardboard which you want to bend at 90 degrees in two parallel lines to make a channel.  You can't just bend it in a brake because it will crimp the corner and destroy the integrity.  You'd have to cut a 90 degree notch on the inside of the material at the bend point or it will crimp, which I'm assuming is the same problem as honeycomb.  Optimally you'd have to remove a line of both the interior sheet AND the corrugation sandwiched inside and THEN make the bend.  I knocked this out in Paint:

which then becomes this

You'd glue the bulkheads to offer some sort of rigidity to the new shape of the channel.  However, you'd still have a weak point at the bend because the interior corrugation and the inside sheet isn't bonded at the joint.  You could do that with glue (or weld) but it's still not as strong as it could be.  The sheet integrity of the panel is good but the strength a the fold would be junk.   So it sounds like you're bonding carbon fiber to the inside and the outside of the honeycomb aluminum bend like this:

To continue our cardboard analogy, you'd effectively be gluing a sheet of cardboard to the inside and the outside of the corrugated structure.  That would serve the same purpose as the carbon fiber over the honeycomb aluminum previously described, correct?  Am I on roughly the same page as you guys at this point?

In reply to stroker :

You can make the folds less weak by giving them a decent radius- so instead of cutting a notch out, imagine cutting a bunch of grooves so that you get a more gradual bend.  A lot of the strength comes from the continuous outer skin.  

In reply to stroker :

You are on the right path with your examples, with one small difference. The addition of material after the bend on the car Nonack referenced in the pictures above are only done to the inside of the bend. These could be laid up carbon/glass of bonded riveted aluminum/steel sections to reinforce the inside corners. If these doubler were extended locally you can use them for suspension pickups as well.

Some of the chassis I have worked on used billet/fabricated pocket inserts for the suspension pickup points. Basically, the bolt receiver is a machined part that get bonded/bolted/riveted into a hole in the honeycomb chassis tub walls. That allows the pickups to be replaced/repaired is there is damage that does not tear the pickup completely out of the tub.

In reply to ¯\_(ツ)_/¯ :

Okay, that's consistent with the cardboard analogy I used, but I understand that the circumstances will differ depending on the material.  My goal is to understand how "hardpoints" like suspension bearings, roll bar mounts, etc. are attached to an aluminum honeycomb tub.  If we continue with the corrugated cardboard analogy, how would you attach a roll bar base or a suspension point?  Glue a sheet of wood to the cardboard or cut a notch in the cardboard then insert and glue in a wood plug?

In reply to GameboyRMH :

If they are properly glued they do not come apart in service. Many early English aluminum monocoques were only riveted (Chevron) and loosened themselves immediately. Hammer & dolly between every session according to someone who crewed one when new. McRae F5000 cars were glued, and I dis-assembled one that had been crashed repeatedly and then left in a moist basement for several decades. After drilling all the rivets it was still well stuck together. When I built a replacement for a Chevron I used Hysol to remove the maintainance issue for the vintage racer. Photo documentation available on request.

In reply to stroker :

The wood plug, most likely- but imagine it being inserted into the corrugated section before the outside sheets of cardboard are attached.  So you have whatever you're attaching, then the outer skin, then the insert, then the inner skin, then some sort of washer or backing plate.  Other designs are possible but that's the way we did it, and it was relatively easy to make phenolic inserts in the right thickness, and phenolic bonded to the outer layers nicely.

¯\_(ツ)_/¯ said:

In reply to stroker :

You can make the folds less weak by giving them a decent radius- so instead of cutting a notch out, imagine cutting a bunch of grooves so that you get a more gradual bend.  A lot of the strength comes from the continuous outer skin.  

Thank you for posting that picture, as I had a question about corners that this illustrates well.  

One of your earlier pictures shows one of the big issues with this technology, where you have a huge gap in the corner thanks to the bulk being cut out.  

Then (thankfully) you mentioned that you changed to a foam core design.  

And with this picture, you give me a good segway for my question- when you did the foam core, did you change the cut out pattern to keep the corner filled?  

Plus- with this picture, it's very possible to have a cutter that penetrates the exact right amount so that the resulting corner would be 100% filled (at least in theory),  so then one could develop an adhesive for the foam so that it retains some level of internal stiffness.  Which would them make the adhesive "tape" that is put on the inside more effective, I would think.

The other thing that is nice about this design, rounding a corner keeps high stress points down.

On a more global comment- the reason this looks so inviting for the GRM'ers is that many of us have builder clubs locally, where a large CNC machine is very available.  So it's more possible to make a chassis using this method than a full CF layup system.  Also- look at the required skill level for this- in theory, this is about running a CNC and then properly applying an adhesive to fill the corners.  To me, that skill looks more accessible than cutting and welding a steel frame.

And if the resulting tub can be strong and stiff enough, then, theoretically, this allows more manufacturers of chassis.  Then it's up to some company to demonstrate that to organizing bodies, and one instantly brings back the "back yard" engineer that most racing series have been missing since CF tubs have become so common.

In reply to alfadriver :

I believe the corners were routed in a slightly more clever way than pictured to reduce the void space- but some extra space was needed since the bends would never be perfect.  I think there was some sort of filler material applied as well, but I don't recall exactly what it was.

A big part of the problem with DIYing this is that it's still expensive- carbon fiber isn't cheap, and neither is a good core material.  If you have a cheap/free source of carbon (like our team did) it opens up a lot of options.

stafford1500 said:

In reply to stroker :

Some of the chassis I have worked on used billet/fabricated pocket inserts for the suspension pickup points. Basically, the bolt receiver is a machined part that get bonded/bolted/riveted into a hole in the honeycomb chassis tub walls. That allows the pickups to be replaced/repaired is there is damage that does not tear the pickup completely out of the tub.

I think I understand what you're describing but pictures would be very useful.

alfadriver said:

And if the resulting tub can be strong and stiff enough, then, theoretically, this allows more manufacturers of chassis.  Then it's up to some company to demonstrate that to organizing bodies, and one instantly brings back the "back yard" engineer that most racing series have been missing since CF tubs have become so common.

This makes me wonder if we can come to a consensus on a common design template that might be used (ala Ron Chapman's Locost) as "open source hardware".  We could develop variations to accommodate differing suspension/drivetrains and post or share them.  

In reply to ¯\_(ツ)_/¯ :

the two papers I've read, and then RCE article both have pictures/descriptions that imply they were using "straight" 1/8" (or cm equivalent) bits for their routers... and no discussion of angled bits, which surprised me.

I follow Frank Howarth on youtube, who a) does awesome video stuff with wood working, and b) utilizes an large CNC to facilitate both

that CNC is around $5k for a 4'x8' "bench top" setup... which, yeah is expensive... but a lot cheaper than it used to be, and is "in reach" of someone who wants to setup a side-hustle; and... who knows, with smart panel layout, you might be able to do it out on a cheaper 4'x4' setup?

one thing that I'm curious about, is if any of the FSAE teams studied their Carbon skins corroding their AL honeycomb cores?  That'd probably be as big an issue as the lifetime Fatigue stress reduction?