Jesse Ransom
Jesse Ransom UltimaDork
4/26/22 6:25 p.m.

My searches aren't turning up much. I've found one article noting that Lamborghini got the structure of the Essenza SCV12 itself homologated by the FIA as a roll structure, obviating the need for a separate cage. I have it in my head that this isn't the only example of this type of thing...

But what I'm really hoping for info on is roll structures in the sense of structure added to an existing vehicle for the same purpose as a traditional tubular steel roll cage.

In the case of the Lamborghini, they worked with the FIA to do a bunch of tests on this specific car. I wonder how far they are from being able to issue a set of rules for construction, in the same way that you can, as far as I know, build a cage to meet FIA rules by applying their specifications to your application.

It's certainly made more difficult by the variations in shape and material properties which composites introduce. It's much harder to inspect and see you've done something in the same way you can look at a two foot span of 2" dia 0.125" wall steel tubing and know how it will behave.

For me, the interest here is in moving roll structure improvements onward from "cage or no cage" so that we can integrate the structural improvements in ways that don't require a helmet in order to avoid getting your skull dented in a minor accident. Round is a great shape for tubing, but composites could allow us to make something just as strong or stronger, and no heavier, which could fit relatively tightly against the roof and pillars, allowing the basic shape of the interior to be no more dangerous than normal.

As a stopgap, I wonder about making filler panels or fairings so that while the tubing is still there, it's part of a panel that presents as flat or slightly concave toward the driver, much like the existing roof... (That's right! Terrible Paint Skillz Time!). Maybe a compressible standoff between that panel and the tube so there isn't still a totally unforgiving spot? OTOH, it's not like the uncaged structure of a car is a great place to bang your head... We're really looking for not significantly more dangerous than stock for an un-helmeted driver, while retaining the cage benefits of being able to hold the roof up and not have concerns about running harnesses and HANS devices and so forth. I mean, having the roof stay up is also good even if you weren't planning to be upside down and thus didn't wear your helmet.

...but I digress. Anybody know anything about composite roll structures? I'm thinking something which in the same view as above might look something like this:

Maybe the simplest question around feasibility is "can we substitute superior materials and more efficient use of available volume to make up for a lack of cross sectional depth?" We're absolutely talking about greater cost, but A) for some cars just being able to do both is worthwhile even at significant cost, and B) in some cases we're talking about the ability to obviate an entire car now that one car is safe to drive around VIR and to Taco Bell.

Keith Tanner
Keith Tanner MegaDork
4/26/22 7:55 p.m.

I don't think you'll ever eliminate the needs for helmets. Heads are important and don't like impacts, and there's just too much chaos possible to say you will never hit your head. 

Outside of that, I think the problem for a full composite structure is verification.  You nailed it when you said that tubes are a very well-known quantity, and there are just too many variables in efficient composite design to evaluate a one-off cage. Designing hood roll structures into the tub at the manufacturer level, there's a lot of potential there.

Your fairing idea is a good one, and Mazda added something similar to the factory hardtop (and soft top?) in the 2003-05 Miatas. Basically a flat plastic panel that covered some less friendly topography. Of course, it also took up space that was at a bit of a premium. 

Jesse Ransom
Jesse Ransom UltimaDork
4/26/22 8:06 p.m.

In reply to Keith Tanner :

The hope isn't to obviate the need for a helmet when doing "helmet stuff." It's just to avoid that choice between "it's a street car that I can't use a harness/HANS device with at track days" or "it's a caged track car that's no longer safe to drive to coffee."

I've also wondered about alternative steel tube shapes, but that's more weight, less (structural) performance, and still invovles shapes that don't make it easy to sanity check for safety.

I suppose rather than hoping to get okayed for competition, I wonder whether it's possible to get to a point where you can be confident you've considerably improved the car's rollover safety so that you can use harnesses in arenas that don't require a cage (and thus don't require a cage meeting certain specifications).

The ideal is to be able to have a streetable car that can also be taken to the UTCC or other time trial stuff, but maybe just being able to do non-competition track stuff is the best we can hope for for now.

GameboyRMH MegaDork
4/27/22 9:15 a.m.

Due to the tubes being a well-known quantity vs. custom rollover structures being hard to test issue, I think the closest we'll see to composite rollover structures as addon cages would be carbon fiber tubes pressed and bonded into conventional tubular chromoly joints.

Jesse Ransom
Jesse Ransom UltimaDork
4/27/22 11:20 a.m.

In reply to GameboyRMH :

You may be right; I wonder what standards exist for production tubes (or rods)? The joints could at least be inspected in the same way as conventional cages.

That path doesn't really help with the main advantages; it might get you a lighter cage, but it doesn't really help with using the available space better. Unless we can show that, say, production composite rods of known pedigree and smaller diameter could be paired/tripled and packaged more like my beam/blade against the roof. Even then, you're not really able to follow the roof curvature.

The frustrating thing is that from where we sit it seems obviously a huge challenge, effectively prohibitive. And yet, it's one of those items where I'm certain composite and manufacturing tech will move on to the point that this is just how it's done X number of years in the future, assuming the interest in track cars remains. We just don't have the body of knowledge needed about how these sorts of structures work, where the big wins are in safety margins, etc... Well, I certainly don't have it; I don't know what the folks at Lamborghini (or Audi) know, or what the composites experts advising them know. We may be closer than it seems from here in the peanut gallery, but it's not an arena anybody's going to want to wade into lightly, for obvious reasons.

frenchyd MegaDork
4/27/22 2:20 p.m.

In reply to Jesse Ransom :

The tech guys once bought aluminum tubing and afterwards several drivers had bad results that got aluminum banned.  This time for good.  
     If they want new seatbelts every few years for a car that spends zero time in the sun.  ( yet street cars with 20 + year old seatbelts are fully legal ).  I doubt you'll ever get alternative roll structures ever approved. 

ProDarwin MegaDork
4/27/22 2:23 p.m.

Those shapes in the first post can be formed with Aluminum and Steel structures as well.  I would think that would be the intermediate step... non-standard shape/fabrication process, but easy analysis. 

Composite I agree is further down the road due to analysis and standard fabrication processes.

WonkoTheSane UltraDork
4/28/22 10:59 a.m.

The other problem is that a round tube is strong the same in any direction of impact... Going to a tapered blade design gives fate all sorts of opportunities to put a crack/load on the smallest leading edge because of the way THIS particular accident happened a piece of sheetmetal sliced the epoxy. 

I doubt you'll ever see one approved for rally or the highest ends of NASCAR or the like.. One of the problems with composites is that like glass, once you break the surface tension there's not nearly as much strength.  You may be able to take a single hit (the car rolls onto the wall), but not subsequent ones (the car continues over the wall and lands on it's roof).

Jesse Ransom
Jesse Ransom UltimaDork
4/28/22 12:00 p.m.

In reply to WonkoTheSane :

Remember that we're not just trying to sub in a composite tube; we're designing a composite structure for this purpose. There are mitigating options for all these issues.

Besides, the issue I'm trying to address here isn't top-tier motorsport, or in fact any dedicated racecar, where there isn't really a downside to a normal cage. It's trying to put a roll structure in which doesn't require a helmet to run out for a loaf of bread.

frenchyd MegaDork
4/28/22 9:40 p.m.
ProDarwin said:

Those shapes in the first post can be formed with Aluminum and Steel structures as well.  I would think that would be the intermediate step... non-standard shape/fabrication process, but easy analysis. 

Composite I agree is further down the road due to analysis and standard fabrication processes.

Actually some of those numbers are known and well documented. Formula 1 Indy car, Sports racing, cars are all composite now-days.   As is modern aircraft, which has left the riveted aluminum structures and moved on to composite.  
      Many if not most racing groups that do accept composite cars actually require proof of suitability.  Something the one off guy is not likely to be able to do.    
As long as MASCAR still plays with pushrods don't expect them to suddenly embrace modern tech. Same with NHRA. 

Driven5 UberDork
4/29/22 10:46 a.m.
frenchyd said:

Actually some of those numbers are known and well documented. Formula 1 Indy car, Sports racing, cars are all composite now-days.   As is modern aircraft, which has left the riveted aluminum structures and moved on to composite.  

To the best of my knowledge, roll structures (including formula car halos) are all still metal, even if something more exotic like titanium sleeved in an aero composite shell where budgets and rules allow. Modern aircraft also still use more structural metal under that 'all-composite' skin structure than the marketing teams want to admit, and that's with absolutely massive engineering resources.

This is because the properties of composites are not superior to metal in all situations. Composite strength in tension comes mostly from the high strength fibers, while in compression it comes mostly from the much weaker binder. Modern airplane designers would actually love to be able to combine metal upper wing skins (compression) with composite lower wing skins (tension). Unfortunately, CTE is a B on a structure that size. Composite safety cells use a body of knowledge that exists primarily outside public domain, or at least the parts of it that most people have access to... Especially when trying to integrate it as a reinforcement into a metal structure that would primarily be loaded in compression in any type of wreck. 

I've thought about doing similar to the original idea before too, as it would make driving a classic car on the road today a much more palatable option. Look at any classic car with those flimsy doors and toothpick pillars holding the roof over head like as an umbrella rather than a structural member. Now look at all the metal in the trunk and engine bay. Even though incorporated as a 'rollover structure', I think a large part of the ultimate goal would also be to make the middle of the car stronger than the ends, to change which is the actual 'crumple zone'.

In my opinion, metal reinforcement with an energy absorbing layer seems like the obvious way to go here.  Remember that if it's never going to be trying to conform to any rule set, reinforcement does not necessarily even have to mean tubular, even though that may often be the most practical solution. 


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