Plastic Fantastic, the enigmatic composite chassis...or how to build a car like a boat or plane

I've read some past forum threads on DIY composite chassis and aluminum monocoque chassis here that unfortunately fizzled out. I would like to at least rekindle the conversation...

I've been interested in kit cars for at least as long as I've been interested in exotic cars, which is to say practically since my recall memory began. I distinctly remember watching Group B races on late night TV when my babysitter was watching me...yet I can't remember her face or name, or anything else I may have done that day...but there, basking in the blue-glow of that old tube TV, I remember being enthralled by these machines.

At first kit cars were this kid's dream as how to acquire an exotic car for hot rod/junkyard prices (ah, how simple ideas like this can infect a mind...), then dawned a realization that a kit car could be a platform for something equal to, not just a low-rent knock-off of, an exotic car. After owning a few exotic cars, my goal quickly became to build something BETTER than, superior to, those poster cars.

As time passed, experience grew, and my own tastes emerged, I realized I wouldn't be satisfied with owning a knock-off, no matter how much better it performed or how much more affordable to own or how much more reliable I could make it. I started suffering the hubris of making my own car!

And so after I built a few component/kit cars of my own, that bitter taste of disappointment from (kit car) engineering flaws, repurposing OEM equipment without understanding the impact of geometry during dynamic vs. static demands, financial constraints and just sheer laziness on the part of the manufacturer, led me to fantasize once again about building my own car. I'd do it right, if only!

There is a coherent point to all of this rambling, I promise. 

Everyone oohs and ahhs over the supercar/hypercar carbon fiber tubs from the like of the Ferrari F50, Porsche Carerra GT, McLaren F1, the alphabet soup McLaren cars, Koenigsegg, Pagani, (did I leave any out?) etc. But did you know there were much smaller manufacturers that used composite tubs/monocoques too?

The Murtaya seems to be known by at least a few members here, as I found a thread regarding them. Fiberglass monocoque shell, Subaru AWD powertrain. Looks kinda like the BMW M coupe (clown shoe).

Then there is the Davrian/Darrian, the GTM Libra, various Marcos and a Caterham. All of these were made in England...makes sense, between their national infatuation with kit cars and aerospace/racing pedigree.

Over here, this side of the pond, we have fewer examples, but one notable manufacturer: Consulier/Mosler. I know we have at least one member who has a Consulier GTP.

I'm not suggesting that it's as easy as slapping some resin and cloth together in a shed...but what if it's not much more complicated than repairing/rebuilding/designing a boat? There are several forums (forii?) that cater to building composite boats, from kayaks to mega-yachts, and many channels on YouTube of guys in their back yards rebuilding fiberglass boats, again from kayaks to yachts.

I was a voracious consumer of enthusiast/niche print magazines when those were still a thing. In fact, until the advent of the internet, it was often the only means one had of seeing what was going on in an industry/hobby elsewhere. Besides GRM, I discovered the British kit car magazines, racing magazines, kitplane magazines and boating magazines. An idea had gelled in my head in the early 2000's...what if a guy could build a car like a boat or kitplane?

Besides not knowing anything about boats, planes or composites, the idea of an "affordable" exotic chassis kept sticking in my craw. Discovering the Davrian/Darrian and later the Consulier GTP/Intruder/Raptor then the Mosler MT900, further whetted my appetite. Not much info was out there, but then the power of the internet started to allow enthusiasts and experts to cross paths, sharing info, ideas and build techniques.

Finally, 25 years after my first inklings of a DIY monocoque composite chassis, I'm prepared to attempt what I once thought was an impossibility...building my own.

I'm not a composites expert. I'm not even an advanced composites amateur. I have some experience professionally with composites from my stint in NASCAR, but getting paid to do something, even under the tutelage of experienced experts, doesn't mean that I have all the answers. I know best practices and have been keeping up with the latest developments, but the means I have available to me are sub-optimal, compared to pre-pregs and autoclaves.

My goal is to develop my own composite monocoque using readily available, affordable materials, using techniques that a dedicated DIY'er could accomplish with a modicum of equipment. Skills build with experience, so I'll be learning as I go...a lot of this is theory to me, again, my experience was with NASCAR composite work in the early 2000's and they have advanced much since then, whereas I have not.

This thread won't get much traction at first, as I'll be trialling different materials, resins, layups, cores and compaction techniques. Much of this will be a refresh for me, as I'm rusty as hell and digging out reference materials to refresh my rather trivial knowledge.

But, if you'd enjoy watching and interacting while I experiment with composites towards an end goal, I encourage you to follow along, give feedback, ask questions and offer ideas, information and experience so that we can build a larger body of DIY knowledge.

My first question to the group...are there any folks with composite experience amongst us? Boats, kitplanes, wind turbines, racecars, etc? Feedback can help not only myself, but other people who may dare to dream, imagine and attempt.

My second question to the group, if I choose a path that is only partially DIY versus full DIY, would that be less interesting to read and interact with? Pre-made composite panels offer serious structural advantages regarding their bond strength and lack of manufacturing deficiencies. After making several sample panels, I may find that I trust off-the-shelf components to my own home-brew construction...and since it'll be my ass in the thing, with some high-performance plans (albeit totally street-legal) for this chassis, I'm tending to favor whatever puts my abilities/resources at ease.

My third question to the group, is what level of "exotic" materials usage would be most interesting to you? Would "plain" fiberglass be too prosaic? Would nomex honeycomb and carbon/kevlar be too outside of your comfort zone to experiment with? Would Polycore or foam core be interesting, or outside of your novelty range?

My overarching goal is to create a composite monocoque chassis, running steel subframes for suspension and drivetrain components. My goto example and reference source material is the Consulier GTP. Obviously adequate materials considering the performance envelope and competition history!

As stated previously, my first forays for the benefit of myself and the forum will be to procure various sample fabrics, core materials and resins, for the purpose of creating test panels. I will experiment with basic hand tools like brushes and rollers for compaction, then applying peel ply over handlaid materials to reduce excessive resin content, moving on to vacuum bagging handlaid materials, before attempting resin infusion.

My first vehicle may be a mostly utilitarian, bare-bones design to prove the concept, as well as my knowledge/comfort using the materials. Most likely lots of flat panels and an easy/affordable to acquire drivetrain. From there I hope to progress to an actual styling buck, so that I can then build out a reasonably attractive vehicle that will incorporate all that I've learned, using an appropriately sporty/reliable drivetrain. This build will take place around my schedule and budgetary constraints. It may happen relatively quickly, or it may drag on for years. Depends on time/money/resources/skills/comfort level. Sometimes things happen at their own pace, and sometimes they can be forced & cajoled into existence. Life happens and this is part of my growth as a car guy, as well as skills that I may be able to use towards future business/product goals.

Yikes. So many questions. I used to build human-powered fiberglass boats, but the loads on automotive chassis are exponentially more. So I don't have a lot to add. The Berkeley used a fiberglass chassis with metal reinforcements, but it had tiny motors.I would think that whatever you're doing, you're likely to want some sort of core - probably honeycomb, although in some cases, foam is all you need. For some key components such as suspension arms, I'd want to go vaccum-bagged prepreg carbon fiber, which, while a lot cheaper  than it was in the past is still damn expensive

Good luck! 

In reply to Kreb :

The same can be said of the Lotus Elite- which was a fiberglass chassis with a small motor powering it.

OP- you may want to look into how Chaparal CanAm cars were made- although they benefitted a lot by having the engine in the back- simplifies the heat flow and structure. 

Kreb said:

Yikes. So many questions. I used to build human-powered fiberglass boats, but the loads on automotive chassis are exponentially more. So I don't have a lot to add. The Berkeley used a fiberglass chassis with metal reinforcements, but it had tiny motors.I would think that whatever you're doing, you're likely to want some sort of core - probably honeycomb, although in some cases, foam is all you need. For some key components such as suspension arms, I'd want to go vaccum-bagged prepreg carbon fiber, which, while a lot cheaper  than it was in the past is still damn expensive

Good luck! 

Cores for structural panels are an absolute must, doubling the thickness quadruples the stiffness, core thickness adding very little weight for the stiffness accrued.

If I were to make my own suspension arms, rather than adapting OEM components, they will be steel fabrications or billet aluminum. I do not trust DIY carbon composites to those kinds of loads or forces. That is WAY outside my comfort zone.

alfadriver said:

In reply to Kreb :

The same can be said of the Lotus Elite- which was a fiberglass chassis with a small motor powering it.

OP- you may want to look into how Chaparal CanAm cars were made- although they benefitted a lot by having the engine in the back- simplifies the heat flow and structure. 

The Lotus Elite was one of the few examples I found of how NOT to build a fiberglass monocoque. I'm surprised Colin Chapman didn't insist on using core material like balsa, rather than the add-on steel members that were added to later models.

Mid-engine is a must with me, it's the only layout that really trips my trigger. To justify the expense both in time and money, this has to be something that pushes me more than simply the challenge of trying something I've never done before.

Interestingly, the Consulier GTP used a rear-mount radiator, and I've been unable to find any info on them overheating, so the cooling system seems to have been well designed. I'm just unsure as to how I could incorporate it on my build, since that is typically a low-pressure zone and the utilization of pressure differences has the most impact on cooling.

Composites repair tech when I worked in aviation.

Since I've been restoring cars for a living, I've built a fiberglass 1934 Ford and restored a 1967 Marcos GT.

Boss and I both have vowed to never do another fiberglass car.

Nothing is the same left vs right, body keeps moving and changing shape even though the glass should have cured a year ago. Old fiberglass deteriorating and falling apart, wood core full of dryrot.

Never again.

Tube chassis and aluminum skin. Colin Chapman created absolute perfection in the Lotus 7. 

That said, the aircraft level stuff with a foam or honeycomb core and vacuum bagged / autoclaved curing is some strong, light, tough stuff but it costs. $$$

ShawnG said:

Composites repair tech when I worked in aviation.

Since I've been restoring cars for a living, I've built a fiberglass 1934 Ford and restored a 1967 Marcos GT.

Boss and I both have vowed to never do another fiberglass car.

Nothing is the same left vs right, body keeps moving and changing shape even though the glass should have cured a year ago. Old fiberglass deteriorating and falling apart, wood core full of dryrot.

Never again.

Tube chassis and aluminum skin. Colin Chapman created absolute perfection in the Lotus 7. 

That said, the aircraft level stuff with a foam or honeycomb core and vacuum bagged / autoclaved curing is some strong, light, tough stuff but it costs. $$$

Great feedback!

Having experience with several fiberglass bodied cars, some factory, some aftermarket, I agree with you in regards to CSM or even lightweight weave. Throw a core like Soric or foam in the mix and the story changes. Also consistency of resin brand, type and mixing ratios makes a big difference too.

I'm trying to avoid any exotic manufacturing methods, though I feel that vacuum bagging and even vacuum infusion are established and supported enough that a high-end DIY'er can manage them at home.

I'm leaning toward purchasing core "kits" where a distributor CNC cuts the foam panels into sections and sizes that you provide via CAD. It's not quite as easy as LEGO, but they are consistent and symmetrical, which reduces concerns somewhat.

As for body symmetry, it's frustratingly simple to maintain, and the bane of every shed-built fiberglass body bought by the end user. Starting with station bucks and progressing to a gantry stylus, which can be as simple as measuring tapes on sticks that are used to mark and denote elevation changes left/right. Something else I hope to share for those who might be curious about building their own car.

I have my custom designed and built 3-wheel EV car.   I have test novel core materials and have considering using them in the car body.

What I really want to do for production is come up with a new way of doing filament winding that can do concave shapes and do a whole car body this way.   

I'd have a look at the British hillclimb folks. They have a tendency to do some crazy stuff with some real engineering behind it. I think Staniforth talks about DIY composite monocoques in at least one of his books as well.

nimblemotorsports said:

I have my custom designed and built 3-wheel EV car.   I have test novel core materials and have considering using them in the car body.

What I really want to do for production is come up with a new way of doing filament winding that can do concave shapes and do a whole car body this way.   

Please share! Pics and details are welcome. I think that this topic (composite chassis) gets blown out of proportion as being too difficult for home build status, but then I see some guy sail a boat around the world that he built in his backyard, or some guy flying a plane he built in his garage...drowning or falling out of the sky is infinitely worse than your car not being stiff enough!

Concave shapes would be near impossible with filament winding due to the tension of the tow being pulled against the form. It would require some kind of mix of a segmented core with different densities and vaccuum bagging to allow the bag to compress the tow against the deformable core...hmmm, food for thought.

Keith Tanner said:

I'd have a look at the British hillclimb folks. They have a tendency to do some crazy stuff with some real engineering behind it. I think Staniforth talks about DIY composite monocoques in at least one of his books as well.

Thanks for that. I've been drooling over this European mid engine all wheel drive rig...flat panel carbon fiber chassis layed up on a home made mold...a professional shop did the vacuum bagging and autoclave work, but the chassis was designed by the owner. I don't want anything that crazy, but it gives hope.

I have all of the "DIY" bookshelf must-haves, along with some DOD composite FM/PM's...Navy and Air Force have some really competent composite workers and training. There is actually a lot of information out there available to the hobbyist/professional, but it's scattered and people tend to get locked into silo-ing of information...marine stuff/boats, aviation stuff/planes, racecar stuff/cars...when really it's interchangeable depending on being fit for purpose. Sussing out what qualifies as fit for purpose can be a bit of a punt when you're first getting into this stuff!

What about using off the shelf carbon fiber tubes and then using epoxy for joints? Maybe also some type of reinforcement.

I mean, if the tubes were say just half price id be strongly looking at this for a challenge car.

https://dragonplate.com/carbon-fiber-truss

In reply to Robbie :

Trek, Univega and I'm sure others used this in bicycle frame construction with good success. 

Carbon fiber tubes bonded to aluminum lugs.

I still have my Univega Carbolite frame hanging in the rafters.

Robbie said:

What about using off the shelf carbon fiber tubes and then using epoxy for joints? Maybe also some type of reinforcement.

I mean, if the tubes were say just half price id be strongly looking at this for a challenge car.

https://dragonplate.com/carbon-fiber-truss

There was some kind of "future" hypercar built using tubes like those and 3D printed titanium lugs. Looked like one of the cars out of Minority Report. Blade by Divergent Technologies was the name.

I guess the idea has some merit, after all spaceframes using steel and aluminum tubes have been done for ages. But I remember reading something about load-sharing and how if one of those tubes was damaged, it was a catastrophic domino affect. I think that's why almost all composite chassis cars use big tubs and spread the loads across as much material as they can, so that damage in one area isn't necessarily game over for the whole structure.

ShawnG said:

In reply to Robbie :

Trek, Univega and I'm sure others used this in bicycle frame construction with good success. 

Carbon fiber tubes bonded to aluminum lugs.

I still have my Univega Carbolite frame hanging in the rafters.

I think bikes get away with spaceframes because of the triangulation built into the structure and point loads being shared by big nodes and energy dissipated line-of-sight through the big tubes. Some people love them, some people hate them...bicycle folks remind me of audiophiles "vinyl's just so real and warm man, especially through rare and expensive tube amps" 

I remember watching some Moto GP races and watching Ducati tripping all over themselves with their carbon fiber frame...the riders hated it and it had to keep being modified to handle the stresses it was seeing. Meanwhile other teams like Suzuki could do no wrong with their carbon fiber chassis. 

You might have all the right ingredients and the best oven, but if you don't follow the recipe correctly, you won't end up with a cake worth eating. And the eating, so I've been told, is where the proof lies.

In reply to Gingerbeardman :

I might be one of those bike guys. I have aluminum, carbon and chromoly steel frame bikes. My favourite one to ride is the 25 year old, fully ridgid, hand-built chromoly steel mountain bike. Aluminum seems to have a harsh feel to it, steel is nice and supple and carbon is somewhere in between. 

ANY tube failure on a bike frame tends to be catastrophic. 

We've been dealing with aluminum and steel nearly forever so we've pretty much perfected it now. I think we're still learning things with composites, not necessarily in a bad way.

I read an interview with Keith Bontrager (MTB frame builder). He laughed and said that if we had started with titanium, people would be treating chromoly steel like it was some sort of wonder metal.

ShawnG said:

In reply to Gingerbeardman :

I might be one of those bike guys. I have aluminum, carbon and chromoly steel frame bikes. My favourite one to ride is the 25 year old, fully ridgid, hand-built chromoly steel mountain bike. Aluminum seems to have a harsh feel to it, steel is nice and supple and carbon is somewhere in between. 

ANY tube failure on a bike frame tends to be catastrophic. 

We've been dealing with aluminum and steel nearly forever so we've pretty much perfected it now. I think we're still learning things with composites, not necessarily in a bad way.

I read an interview with Keith Bontrager (MTB frame builder). He laughed and said that if we had started with titanium, people would be treating chromoly steel like it was some sort of wonder metal.

This is interesting to me. I grew up with BMX bikes and later dirt bikes, but I don't consider myself a biker or a bicyclist. I even have friends who do the whole road bike and mountain bike lifestyles (different folks), but I just can't seem to enjoy more than an hour of playing at the skate park, or dooting around on my friends back yard course.

I can see (mind's eye) what you're talking about with the different materials, but I don't know that my ass is sensitive enough to notice. I would likely chalk it up to suspension or tires.

Do you think it's about frequency vs. mass, joint design or something more esoteric? It seems like it should be quantifiable, not subjective, but I've read so many opinions on it that it just seems to come down to which camp you call your own.

Regarding frequency, I know that that is how most auto companies do their suspension tuning, figuring out the masses and picking springs and dampers that let the suspension (cycle?) operate at the frequency. I've seen figures like 2.0-2.5Hz for most passenger cars.

I'm sure someone who knows more than me could explain it.

I probably couldn't even tell if I rode them all back-to-back but I can certainly tell if I've been riding my aluminum hardtail for a couple weeks and then switch back to the steel bike. Bumps and small stuff on the trails feel more harsh on the aluminum bike.

ShawnG said:

I'm sure someone who knows more than me could explain it.

I probably couldn't even tell if I rode them all back-to-back but I can certainly tell if I've been riding my aluminum hardtail for a couple weeks and then switch back to the steel bike. Bumps and small stuff on the trails feel more harsh on the aluminum bike.

 

I wonder if it's the stiffness to diameter ratio that plays into it. Aluminum is 1/3rd as stiff as steel and 1/3rd the weight, so you either have to up the section size or increase the total material, which plays into your previous comment about chromemoly. If steel were invented today, it'd be considered amazing, instead of staid.

By increasing the section size (diameter/cross section) it changes the distance from the neutral axis that the force is reacting on. My SWAG is that this can amplify certain types and magnitudes of impacts.

Aluminum is much more subject to fatigue than steel, so it has to be made rigid, whereas you can build some compliance into cro-moly frames since they'll take a lot more flexing without cracking. 

In reply to Gingerbeardman :Curves generally provide a stronger structure than flat panels,which is why that Tesla truck is so poorly designed...The Aptera was a car that was built as a composite foam sandwhich like a boat hull, although it looks like an airplane.     

Concave filament winding is only impossible until someone makes it possible.  

Making scale models is always a good start.  I made a 1/10 scale of my 34ft boat.   I plan to make a 1/3 scale of my 64ft boat..if I can ever get back to boats.

FSAE teams are allowed to build either a welded steel tube frame or a composite sandwich-panel structure. By the time adequate energy absorption and intrusion protection are provided, the composite tubs end up at the same weight as the tube frames. End-to-end they are stiffer in torsion and bending, but local stiffness at suspension mounting points is not improved.

You could certainly build a car as a composite "safety cell" around the driver and steel tube subframes at the front and rear ends to handle all of the point loads from the suspension and powertrain. You'll want to use Zoltek unidirectional fabric to build the laminates and aluminum honeycomb and pine for most of the cores, with magnesium or aluminum inserts for bolts.