Truths and Myths about Materials and Aerodynamics

[EarlQHan]

Originally Posted by CA2

Carbon fiber composites will act differently than metals in a crash. They fail differently. They respond to damage differently.

Also, composite structures are difficult to accurately model. Is any sort of model verification being done (ie. testing w/ strain gages, etc.) on these parts? To have any real confidence and to eventually establish design allowables, you should have some sort of iterative process of analysis, test, tweak model, repeat.

Correct, the way I see it in today's world, you design on the computer the best you can, prototype it, then test it. Make changes where they're needed during the computer modeling phase and after the testing phase. However, budget constraints and delivery dates are always an issue.

I am not a composites expert, SubieNate is so he might be able to give better insight. I am not sure what goes into designing a carbon structural piece, but for aero pieces, it's easy to calculate the kinds of loads it will generate based on cd and target velocity. You're right composites are incredibly difficult to analyze on the computer, but that's why there's (very expensive) software to help engineers determine layup schedules and FEA the part. And composite experts are fairly expensive to contract for consulting.

I don't know what parts you are referencing, but in the aftermarket industry, I highly doubt there are companies verifying their parts with real world test data. The time and budget constraints are outside the scope of practicality.

[SubieNate]

I wouldn't quite call myself a composites expert, but thanks for the confidence.

In failure, your biggest issue with composite parts is liberated material becoming shrapnel. Unlike metals, which give before break and then bend or snap, composites have a very sharp transition between "okay" and "kaboom" and fail very spectacularly. Best case, the liberated material is extremely small (e.g., dust) and has little energy to cause serious injury. Worst case a large section delaminates and you have a giant flying sheet of razor sharp material. Bad news.

F1 cars use a combo of carbon fiber and zylon. Unlike carbon, zylon has a very high strain capability (Carbon tends to be very strong and stiff, but somewhat brittle). By surrounding the driver in a cell made of a zylon/carbon composite, in an accident the zylon prevents the carbon parts it's bonded to from being liberated. You end up with a bunch of broken carbon still held together with the zylon portion of the laminate. This is similar to one of the advantages of the carbo-tanium system Pagani uses.

The biggest issue I see with most aftermarket composite body parts is hoods. Many of the designs don't look like they have proper breaking points designed into them, and are bolted in via the stock mounting bolts. Carbon is not so great at holding fasteners in that fashion (big stress riser) so my worry would be in a head on collision, the fastener locations shearing off and the hood coming back towards the cockpit in one piece (or several large pieces) as opposed to the hood buckling up in the middle as the stock one is designed to do.

Cheers
Nathan

[CSG Mike]

Originally Posted by SubieNate

I wouldn't quite call myself a composites expert, but thanks for the confidence.

In failure, your biggest issue with composite parts is liberated material becoming shrapnel. Unlike metals, which give before break and then bend or snap, composites have a very sharp transition between "okay" and "kaboom" and fail very spectacularly. Best case, the liberated material is extremely small (e.g., dust) and has little energy to cause serious injury. Worst case a large section delaminates and you have a giant flying sheet of razor sharp material. Bad news.

F1 cars use a combo of carbon fiber and zylon. Unlike carbon, zylon has a very high strain capability (Carbon tends to be very strong and stiff, but somewhat brittle). By surrounding the driver in a cell made of a zylon/carbon composite, in an accident the zylon prevents the carbon parts it's bonded to from being liberated. You end up with a bunch of broken carbon still held together with the zylon portion of the laminate. This is similar to one of the advantages of the carbo-tanium system Pagani uses.

The biggest issue I see with most aftermarket composite body parts is hoods. Many of the designs don't look like they have proper breaking points designed into them, and are bolted in via the stock mounting bolts. Carbon is not so great at holding fasteners in that fashion (big stress riser) so my worry would be in a head on collision, the fastener locations shearing off and the hood coming back towards the cockpit in one piece (or several large pieces) as opposed to the hood buckling up in the middle as the stock one is designed to do.

Cheers
Nathan

I'd like to add to this re: hoods

Modern cars all have "hooks" behind the rear mounting points so that in the case of a frontal collision, the hood catches on the hooks, and fold up in the center. That would be a very localized force on CF hoods...

Fortunately, the FRS/BRZ's windshield is pretty raked, and hopefully deflect the windshield up...

[EarlQHan]

Originally Posted by SubieNate

I wouldn't quite call myself a composites expert, but thanks for the confidence.

In failure, your biggest issue with composite parts is liberated material becoming shrapnel. Unlike metals, which give before break and then bend or snap, composites have a very sharp transition between "okay" and "kaboom" and fail very spectacularly. Best case, the liberated material is extremely small (e.g., dust) and has little energy to cause serious injury. Worst case a large section delaminates and you have a giant flying sheet of razor sharp material. Bad news.

F1 cars use a combo of carbon fiber and zylon. Unlike carbon, zylon has a very high strain capability (Carbon tends to be very strong and stiff, but somewhat brittle). By surrounding the driver in a cell made of a zylon/carbon composite, in an accident the zylon prevents the carbon parts it's bonded to from being liberated. You end up with a bunch of broken carbon still held together with the zylon portion of the laminate. This is similar to one of the advantages of the carbo-tanium system Pagani uses.

The biggest issue I see with most aftermarket composite body parts is hoods. Many of the designs don't look like they have proper breaking points designed into them, and are bolted in via the stock mounting bolts. Carbon is not so great at holding fasteners in that fashion (big stress riser) so my worry would be in a head on collision, the fastener locations shearing off and the hood coming back towards the cockpit in one piece (or several large pieces) as opposed to the hood buckling up in the middle as the stock one is designed to do.

Cheers
Nathan

The hoods usually have metal inserts for the fasteners, at least the ones I've seen. They're typically weighted between OEM steel and aluminum hoods (about 90% FRP, 10% CF for finishing, lots and lots of resin). On more than one occasion, I've see people who forget to latch the hood, drive off, then have the hood flip up. Cracks the windshield, but I've never seen the hood take serious damage. I'm not sure how they'd act in a front end type of collision though.

[SubieNate]

[ame="https://www.youtube.com/watch?v=IyC9xMbhs2U"]Pagani Huayra Crash Test - YouTube[/ame]

[ame="https://www.youtube.com/watch?v=6eXU-tPt6Gg"]Koenigsegg Agera Crash Test - YouTube[/ame]

With cars at this level, the passenger compartment is made so strong that as long as the front and rear have good crumple zones you're safer than in almost anything else. The Pagani one brags about the carbotanium, but there are also many different specific carbon fibers you can choose to get more/less flex capability without shattering. Fiber orientation matters a lot as well.

Nathan

[EarlQHan]

That actually gave me a great idea Nate. The truths and myths of safety.

"Speed has never killed anyone, suddenly becoming stationary… that’s what gets you." – Jeremy Clarkson

First a PSA, I want to say please drive responsibly and safely on public roads. We must share the roads with other people, so if you kill yourself, then it doesn't matter (in the metaphorical sense), but your direct actions can injure or kill others so please be careful.

Modern cars are safe, there's no question, but automobile accidents caused over 30,000 deaths last year in the US alone. This is largely due to the (relatively) safe driving laws in our country (Switzerland will jail DUI offenders [.01 BAC] for LIFE on the account of attempted manslaughter). If any of you watch Top Gear (so everyone), you probably remember hearing the fact that over 100,000 people in India die a year due to automobile accidents.

Nothing known in the universe, so far, is immune to the laws of physics. As I said earlier, in time, science will reveal the truth. To give you a few quotes by men who started some of the most legendary car companies in the world:

"Aerodynamics are for people who can't build engines." - Enzo Ferrari

"I build them to go, not to stop." - Ettore Bugatti upon criticism of poor braking performance in his race cars

Now we know they're both wrong. Aerodynamics are braking are much more important to fast lap times than the engine. To appreciate the history of safety, you must know the history of motorsport. This is why (OEMs, if you're reading this) motorsport is important, it drives innovation and technology for cars. Back in college, we had to learn a lot about the history and man was it eye opening. This is a 1903 Premier



Notice original Indy 500 cars were two seaters. This is because they had manual fuel pumps, so a co-driver would constantly be pumping the fuel. They didn't have front brakes, and the rear brakes they did have were leather straps used as friction surfaces. It wasn't until about the 40's when hydraulic brakes became normal, and even then they used drums as seen in this 1948 Alfa Romeo



They didn't have seat belts, because they believed it was better to be ejected out of the vehicle, rather than risk being crushed by the car or trapped under it and be burned alive. And largely, they were correct. This is a 1954 Mercedes W196 in streamlined bodywork



The body was made of magnesium skin, and the following year, a 300SLR would crash at LeMans, causing the magnesium skin to ignite, killing 80 people instantly, 3 more would die from injuries, and left 120 more injured; you can see the footage here (WARNING: IT IS GRAPHIC AND CONTAINS FATALITIES):

[ame="http://youtu.be/FXtb5eDUuQw?t=1m7s"]http://youtu.be/FXtb5eDUuQw?t=1m7s[/ame]

Roll hoops didn't even become common until the 60's as seen in this 1963 Lotus



Then, through more and more deaths, eventually came the mandate of harnesses, fuel cells, crash structures, etc. In parallel with open wheel racing, through LeMans came the headlights, windshield wipers, etc. Motor racing in the early days was a blood sport and people came to watch the crashes and ensuing injuries and deaths as much as the racing itself. The drivers in the past were cut from a different cloth, a breed apart; some may glamorize it as courage or bravery, I'll just say they were bold and willing to put their lives on the line to win. Now why is this history important? Because new safety features were invented and specialty groups were formed to better understand the nature of car crashes.

According to Newton's laws, massive bodies have inertia, and when in motion, they have momentum. When you crash, there are actually three crashes that happen. First, the car comes to a stop. Then, your body comes to a stop. Lastly, your internal organs come to a stop. That last one is why HANS devices are used, to prevent a basilar skull fracture, which is the medical term for when the base of your skull detaches from the spine. The two most famous drivers that died from this kind of injury are: Ayrton Senna and Dale Earnhardt Sr. The threshold for which this kind of injury happens to drivers is about a 120G deceleration rate. Fatal brain injuries will happen at about 250G's. However, some of the more violent crashes will actually be the reason why drivers survive. The worst one I can remember of the modern era is Robert Kubica's crash in 2007 at the Montreal GP.

[ame="http://youtu.be/AtrzvwayniM"]http://youtu.be/AtrzvwayniM[/ame]

Initially he hits the parked car, so some energy is removed, then he hits the barrier and the barrier absorbed some energy and the deceleration there did not kill him, then when parts broke off and got sent flying into the air, that took energy with it, then as he flipped, the act of flipping removed, energy, the roll hoop removed some energy as it hit the ground, then it hit the guard rail and skidded until finally all the kinetic energy was removed. If you look closely, his feet were exposed and Robert was very lucky not to lose his legs. Luckily, there weren't any permanent injuries from that incident.

Using the average Asian weight male of 61 kg (least amount of force required for BSF) and F=ma, the force needed to cause instantaneous death via BSF is 61*(120*9.81) 71.8 kN or about 32000 lbs. Now let's say the BRZ/FR-S/GT86 is about 1250 kg (2756 lbs for those of in the non-learned parts of the world). and your velocity is 100 km/h (roughly 60 mph) which means you are traveling at about 28 m/s. Kinetic energy, is equal to 1/2*mass*velocity^2 (KE = 1/2*m*v^2). That means the total energy is 490 kJ. Work, a form of energy, is the sum of forces along a path (work = force*distance or W = F*d). Because energy equals work, and in this case (car crash), KE = W, the human body would normally need about at least 7 m to come to a stop to stay under the BSF threshold. Now that is not practical, but when you are in an impact, everything between the object and you will absorb some energy reducing that necessary distance. Then there are safety features like crumple zones, safety belts, and air bags.

Now, because F=ma, many people automatically assume the more mass a car has, the safer it is. NOT TRUE, BSF is not the only way a person can die in a car accident. A more massive car will slow down your rate of deceleration for sure, but at the same speed, you also carry more momentum and have more energy. This is a crash test of a 95-04 Chevy Blazer/ 98-01 GMC Jimmy, and a 96-01 Oldsmobile Bravada

[ame="http://youtu.be/W-StFZKnCHo"]http://youtu.be/W-StFZKnCHo[/ame]

Notice how body literally collapses into the dummy. Even if the driver was not killed upon impact, there would be several external and internal injuries caused by blunt force trauma. This is a crash test of a 2013 Scion FR-S

[ame="http://youtu.be/16CIDz4pAxo"]http://youtu.be/16CIDz4pAxo[/ame]

Notice how everything in front of the firewall crumples or breaks away, but the cabin remains intact and still looks largely unaffected besides the airbag deployment. Everything worked as it should. As a generality, larger is safer than smaller (more distance for deceleration), but lighter, is safer than heavier (less kinetic energy).

Crumple zones are what they sound like, they are meant to crumple because it slows down the rate of deceleration for the occupants inside the car. Since it takes time and distance for this structure to collapse, this reduces G's upon impact. Now, these crumple zones can be tuned, because you know the general direction of loading, so after they crumple, they send the rest of the energy to a prescribed direction (the chassis, not the cabin).

The unibody chassis is made of high-strength steel so it does not bend and protrude to potentially injure the occupants. To my speculation, rather than BSF related death, this is probably the #1 cause of death to occupants in the cabin under normal use (not motorsports related). The seat belt prevents you from blunt trauma and the air bags deform, slowing down the rate of deceleration.

Now, people have been asking about replacement body panels. Bumpers contribute very little to safety so I don't think they are inherently dangerous. Hoods however, are a bit different. If it crumples and shatters into a bunch of pieces, there's not much to worry about. I suspect, the OEM hood transfers energy to the chassis, but does not absorb much because there is little mass (relative to the car). An aftermarket hood however, might end up shearing at the bolts and could become a projectile. Is this likely? I do not know, but it is a possibility. However, I have never heard of a case like this, so I think it's safe to assume it is unlikely.

Another issue I saw debate on was the crash beam or bumper beam. It is my understanding that federal laws in the US mandate that a front or rear end collision cost a certain percentage of the original car cost to fix. That means the damage amount must remain under a certain dollar amount. That is why in the US, and to my knowledge, only the US, there are foam bumpers sandwiched between the bumper beam and the bumper. The rest of the world does not have the same law so they tend to come without the foam bumper, and sometimes even have lighter crash beams. One case I know for a fact is that the USDM Subaru STI has a foam bumper and a crash beam. The JDM STI has no foam bumper and a lighter crash beam. If the crash beam does make first contact, then it will transfer energy to the crash structure, but AFAIK, that is not its primary purpose. Looking at the BRZ/FR-S/GT86, it looks low enough that the beam may not make contact at all with some SUV's, trucks, and semis (tractor or lorry depending on where you're from). Let's say the crash beam weighs about 40 lbs or 18 kg and with a 55% front weight for a weight of 687 kg, it is a very small percentage of the total front end weight. Plus, you are heavily relying on the strength of the bolts that hold the beam in place do not shear. I could do the math for absorption, but I don't know the exact shape of the beam, which is important, because a collision loads the beam in bending. I am almost positive you can remove the bumper beams (front and rear) without safety concern. Do I recommend this? No, not because of potential safety issues, but because in the case of accident, this will increase the damage caused and increase the cost to fix it.

Once again, materials, by themselves, have nothing to do with safety. It is in the design. The BRZ received a perfect 5 star crash rating and the chassis is made of steel. The Audi A8 also received a perfect 5 star crash rating and the chassis is made of aluminum. But don't take my word for it, the US government deems it so.

Now, lowering the car with low offset wheels and extreme alignments (aka "hellaflush"). Does a lower car make it unsafe? Yes and no, there is a possibility an extremely low and extremely short car could have the hood entirely go under the bumper of another, taller vehicle leaving nothing but the A pillars and front windshield between you and impact, but this is highly unlikely. Class 6 and higher trucks typically have a bumper height of 30" or 76 cm so if your hood is lower than that, you probably don't have a production car and you are on your own. Having a car low enough to scrape, I also don't believe it's dangerous. It damages the car and the street for sure, but having to slow down to cross uneven patches in the road, whether it be speed bumps (sleeping policeman), railroad tracks, etc., isn't "dangerous" if you are aware of your surroundings and your surrounding drivers are aware of you. In fact, I think lifting a truck is more dangerous than lower a sports car. The center of gravity gets raise, making the truck more prone to tipping over. The bumper gets raised, making it more likely to decapitate someone that hits you from the rear.

Now, extreme camber. Does X amount of camber make the car unsafe to drive? No, in my opinion it does not. Why? Physics. Friction force = the friction coefficient * normal force (Ff=mu*Fn). That means they generate a similar amount of friction force as car with a normal alignment if the tires are the same. Is this completely accurate? Don't wider tires give you more grip? Yes, but that's because tires don't have a completely linear relationship, and heat changes the coefficient. However, for all intents and purposes, a reduced contact patch does not automatically make it unsafe. By the same argument, making the car lighter is making it "unsafe" since you are reducing the normal load and therefore getting less grip. As a blanket statement, "hellaflush" builders (the ones who are building it only to be "hellaflush") are generally nice, show car people. Sure they cruise on the street and will get on it every once in a while, and take a corner fast (like we all do sometimes), but they're not mindless idiots who try to race everyone, everywhere. This is 2013, the days of the original Fast and Furious are over (at least I think. Never got into drag racing, much less street racing). In fact, they typically have to drive at slower speeds than normal cars for fear of rubbing or breaking stuff.

Toe is generally a non-issue because the "hellaflush" people stay in the ball park of 0 toe, it's usually the racers that change toe settings to dial in handling characteristics.

Low-offset wheels. It wears out your wheel bearings faster, that's a known fact, but how fast and how soon depends on the wheel bearing. AFAIK, the BRZ/FR-S/GT86 uses a "Type III" interlocked bearing system so it can't fall apart under normal circumstances.

Stretched tires. This is up for debate. It largely depends on the situation. I've seen a 195/55/15 stretched on a 8" wheel, with no ill effects. I've seen a 275/35/19 on a 19x10 come off its bead. It will depend on wheel design, typical G's the tires see, the tire pressure, and how much the tire is stretched. However, as a rule of thumb, you can subtract 10mm from whatever is lowest recommended tire size and be safe.

So, once again, I want to stress the importance of the human element. Engineers design cars to be safer, and drivers take it for granted and drive with less awareness. Just because a car is not built for performance does not make it unsafe. If you know your car well and drive within your level of talent, it is not any less safe, just slower. I really couldn't care less what you do with your own car. I might have differing opinions, but it's not my place to tell you what to do with your property. However, if you drive like a jack ass, you can be sure I will cuss you out. So to all who read this, please be safe and make good driving decisions.

"Driving fast on the track does not scare me. What scares me is when I drive on the highway I get passed by some idiot who thinks he is Fangio." - Juan Manuel Fangio

[Calum]

This reminds me of about half of the drivers here. They just don't understand that the only way to get in an accident is to have a different vector and something else. If you're going the same speed and in the same direction as the rest of the drivers you're MUCH less likely to get in an accident. SO, when you're merging at 40 km/h slower then the traffic, you're going to have a much worse time then if you were going the same speed.

Now, how do you know the exact speed the traffic will be going? You don't! But, you can create a much bigger change per unit of time in the speed of your car with your brakes then with the throttle. So, why not set your speed a few km/h faster then you think traffic might be going, breath on the brakes to adjust to the correct speed and merge into the traffic with the same vector as the rest of the cars.

But NOOO, people here will come to a compete stop while trying to merge onto a highway.

[Dezoris]

I can't believe I read this whole thread. What is the goal here? It's so lengthy I don't know what conclusion is to be drawn.

[EarlQHan]

Originally Posted by Dezoris

I can't believe I read this whole thread. What is the goal here? It's so lengthy I don't know what conclusion is to be drawn.

it was originally started as a way to inform people of the difference between solid engineering and marketing hype. it strayed a bit, but i think we're getting back on track. bottom line is to shed light on internet hype and urban legends.

[SubieNate]

Originally Posted by Calum

This reminds me of about half of the drivers here. They just don't understand that the only way to get in an accident is to have a different vector and something else. If you're going the same speed and in the same direction as the rest of the drivers you're MUCH less likely to get in an accident. SO, when you're merging at 40 km/h slower then the traffic, you're going to have a much worse time then if you were going the same speed.

Now, how do you know the exact speed the traffic will be going? You don't! But, you can create a much bigger change per unit of time in the speed of your car with your brakes then with the throttle. So, why not set your speed a few km/h faster then you think traffic might be going, breath on the brakes to adjust to the correct speed and merge into the traffic with the same vector as the rest of the cars.

But NOOO, people here will come to a compete stop while trying to merge onto a highway.

Dude. Come out to California. People either A. floor it and merge with no regard for the drivers already on the freeway, or B. try to merge at 40 mph and thus make everyone else behind them feel like a sitting duck.

Nathan

[EarlQHan]

Saw this pop up on Sauber's Twitter this morning so I thought I'd share

[ame="http://www.youtube.com/watch?v=P3ulqzRR9UA&feature=share&list=PLBA90185C6 3B95B3B"]http://www.youtube.com/watch?v=P3ulqzRR9UA&feature=share&list=PLBA90185C6 3B95B3B[/ame]

"Did you know that (depending on the aero/wing setup) the tyres of a Formula One are account for as much as 30% to 50% of total drag?

If you like this, we might come up with more similar tech bites.

CFD (Computational Fluid Dynamics) is a great tool which is not only used for aerodynamic research, but also for simulations of all kinds of fluids in the race cars. Different from the wind tunnel, CFD also allows to calculate and simulate temperatures. The two complement each other perfectly."

Paul was planning on doing a similar video for the FR-S but it takes about a week straight of processing so it might be a while before the work station frees up...

[Dezoris]

Originally Posted by EarlQHan

it was originally started as a way to inform people of the difference between solid engineering and marketing hype. it strayed a bit, but i think we're getting back on track. bottom line is to shed light on internet hype and urban legends.

I have to speak my peace on this issue.

To be successful in sales you rarely need the best products or innovative engineering. Mass market consumers don't care about all the technical details, they just want value and simplicity. The esoteric engineering crowd are the 5% who want all of this. Even then only about 1% put their money where their mouth is. On this site there are only a hand full of guys who will go balls out designing and testing a better mouse trap and most of them have products to sell.

As all of this pertains to the FR-S/BRZ these are mass produced cars with standard issue stamped/cast steel everything. Not one aluminum suspension component. Material wise it's about as exotic as a Chevy Aveo.

If we started with a chassis from someone like Lotus, purpose built, I might be a bit more excited for advanced engineering for this car.

But really after following progress on most of the fastest guys on here, they don't need anything more than standard issue materials.

Over the past 5 years I have spent over 20k on karting, cars and projects.
The pay off?

A $60 Ikea display case filled with 40$ worth of Chinese made trophies.
Most anyone here with experience will tell you, does not matter what you spend on motorsports, sometimes less is more.

[CSG Mike]

Originally Posted by Dezoris

I have to speak my peace on this issue.

To be successful in sales you rarely need the best products or innovative engineering. Mass market consumers don't care about all the technical details, they just want value and simplicity. The esoteric engineering crowd are the 5% who want all of this. Even then only about 1% put their money where their mouth is. On this site there are only a hand full of guys who will go balls out designing and testing a better mouse trap and most of them have products to sell.

As all of this pertains to the FR-S/BRZ these are mass produced cars with standard issue stamped/cast steel everything. Not one aluminum suspension component. Material wise it's about as exotic as a Chevy Aveo.

If we started with a chassis from someone like Lotus, purpose built, I might be a bit more excited for advanced engineering for this car.

But really after following progress on most of the fastest guys on here, they don't need anything more than standard issue materials.

Over the past 5 years I have spent over 20k on karting, cars and projects.
The pay off?

A $60 Ikea display case filled with 40$ worth of Chinese made trophies.
Most anyone here with experience will tell you, does not matter what you spend on motorsports, sometimes less is more.

The chassis is a bit special. Totally serious. Makes IS-F guys cry.

[OrbitalEllipses]

Originally Posted by CSG Mike

The chassis is a bit special. Totally serious. Makes IS-F guys cry.

1000lbs of weight will do that.