| All About Carbon Fiber
It's lightweight. It's strong. It's expensive. It's carbon fiber, the miracle material that shows up in everything from Formula 1 car tubs to golf club shafts. Chances are good, if you don't already have some carbon-fiber pieces on your car, that you would probably want some, since carbon fiber has the built-in cred and high gotta-have-it factor that come with a racing heritage. But do you know where that cred comes from? Know why the racers use it? And what can get lost in the translation from the track to the street? Well, Super Street's "All About..." department is here, once again, to give you the story behind the product, all in the selfless quest to make sure you know what the hell you're talking about.
A Little HistoryThe first use of carbon fiber goes waaaaay back to Thomas Edison and his invention of the light bulb. While testing materials to use as a bulb filament, he baked a piece of cotton thread and produced the first carbonized fiber strand. The carbon fiber we know today isn't derived from cotton, though. It's a man-made material that was first produced during the '60s, a boom period in synthetic-fiber development that spawned fabrics like Spandex. (Jonny is forever thankful for that invention.-JN) While Spandex went disco, carbon fiber moved into aerospace. Because of its tremendous strength-to-weight ratio (more on that later), carbon fiber was used in spacecraft, and later adapted for airframe structures on military and commercial aircraft.
The McLaren Formula 1 team is credited with bringing carbon fiber to auto racing. The 1981 McLaren MP4 was the first race car with a carbon-fiber monocoque (body tub), and in 1982 McLaren experimented with carbon-fiber brake discs. The material was adopted by other race teams, and it is still an integral part of today's F1 car. (See "The F1 Connection" on the next page.)
Even though it's widely used in racing, carbon fiber hasn't really made the jump to mass production. Why? Cost. Back when carbon fiber was being used exclusively for the space race, only NASA could afford the stuff at about $150 per pound. By the time McLaren started playing with it, its cost had decreased to about $65 per pound. Currently it runs about $5 per pound. Quite a drop, but consider this: Steel used in car bodies costs approximately 40 cents per pound. Even if the cost of carbon fiber lowered again, it would be comparable to the per-pound cost of aluminum, not steel. So it's going to be a while before we see any mass-produced carbon-fiber vehicles.
What Is This Stuff?The carbon fiber used for automotive parts is a fabric mat made up of woven strands of carbon. Those strands are produced through a chemical process that occurs when a compound called polyacrylonitrile is heated several times to temperatures as high as 2,300 Fahrenheit. The heating and subsequent cooling result in carbon ribbons that bunch together to form fibers. Those fibers are woven together to produce mats in a variety of thicknesses and patterns.
The industry refers to the thicknesses of the individual fibers with a "k" designation, which denotes how many thousands of carbon filaments are contained in the fiber strand. A 3k fiber has 3,000 carbon filaments in each fiber, a 6k fiber has 6,000 filaments, and so on.
To turn the carbon-fiber mats into an automotive part, they must be combined with a bonding agent-typically epoxy resin-and then cured via heat and pressure to form the desired piece.
Now, the world is full of fabric mats-fiberglass, Aramid, Kevlar-that can be molded into various shapes. What makes carbon fiber special is its strength-to-weight ratio. Though estimates of this ratio vary, depending on the thickness of the fibers themselves, carbon fiber can have as much as 10 times the strength of steel with only one-fifth the weight. Its strength-to-weight ratio beats aluminum, too. Even a modest 3k carbon fiber is four times stronger and more than twice as stiff as the alloy.
Strength and weight have a lot to do with the type of carbon-fiber mat that's chosen for a particular part. As fiber thickness increases so does its strength, but there's also a corresponding rise in weight. So if you're building a hood, for example, you have to strike the right balance between keeping the part light and making it strong enough to withstand repeated openings and closings.
Fiber thickness also plays a role in how the carbon-fiber piece looks. A small piece -like a fuse box cover-will look better with a tight, small weave than a big chunky one. Speaking of weaves, there are a number of different patterns that carbon fibers are woven into. The most common is the standard or plain weave, which is a simple under/over pattern where the strands are at 90 angles to each other; and the twill weave, in which the strands form little "v" shapes. This weave is more delicate than the standard weave, and it conforms better to complex shapes (like a dashboard or battery box).
Some carbon-fiber mats have no weave. Called "unidirectional," these sheets are made from carbon strands going in one direction and are used in race car construction. By computer modeling and stress analysis, car builders can determine how to orient several layers of the unidirectional mat to build maximum rigidity into the part.
The F1 ConnectionThe current state-of-the-art in automotive carbon-fiber is in Formula 1, where its use was pioneered 20 years ago. Allianz, sponsor of the BMW Williams F1 team, provided this cool illustration of how and where carbon fiber is found on a current F1 car, and it provided us with some inside info on F1 carbon-fiber technology.
According to Allianz, some 60 percent of the car is made from carbon fiber, including the monocoque (body tub), underbody, nose assembly, wings, front wishbones, driver's seat, transmission housing, clutch disc, brake rotors, and linings. All this is done with an aim to remove weight from those parts, so that the saved weight can be "distributed to optimum advantage" on other parts of the car.
F1 manufacturers start with pre-preg sheets of unidirectional carbon-fiber mat. More than 300 square feet of carbon-fiber sheeting go into forming the monocoque alone, and the layers of mat are aligned and oriented in a certain way (determined by computer modeling) for maximum rigidity. Instead of vacuum bagging, the F1 constructors use an autoclave to heat and pressurize the mold at the same time. The high-temp resin systems cured in the autoclave are more stable than those cured in a heat box, and the 100psi of pressure generated in the autoclave makes the part incredibly dense.
Making The MoldA carbon-fiber part is only as good as its mold. So a lot of time and effort are put into constructing the mold that'll hold and shape the mat during its curing process.
There are a number of ways to build a mold, ranging from splashing a fiberglass copy of an original-equipment part to cutting the shape using coordinates from a CAD/CAM system. Fiberglass (or some other sort of epoxy material) is commonly used to make molds, as it's relatively inexpensive. But fiberglass doesn't hold tolerances as well as other materials, and it may not withstand the heat and pressure used to cure some carbon-fiber mats. Molds that are more stable and precise are made from aluminum.
Wet Lay-Up Or Pre-Preg?Those phrases may sound like they're from some of the sites Rikdaddy frequents, but actually they describe the two means of joining the carbon-fiber mat with its bonding resin.
In the wet lay-up process, a dry carbon-fiber mat is placed in the mold, and then resin is poured and brushed into the carbon cloth. Pre-preg refers to a particular kind of carbon-fiber mat that has resin pre-impregnated in the cloth. Heat releases the resin from the cloth, so pre-preg mats must be kept cold until ready for use.
Each has its advantages and disadvantages. Dry carbon-fiber mats are less expensive than pre-preg and don't require special handling before use. But the brush/pour method of adding resin is inexact and can be tough to control, so there's a risk of having too much or too little resin in the cloth. Plus, the act of brushing the resin onto the cloth can cause the cloth to shift in the mold, which could result in an uneven weave pattern or a snag in the cloth.
With pre-preg, the right amount of resin is already in the mat, so there's zero risk of over- or under-coating the cloth. And since there's no brushing, the mat won't move once it's in the mold, which will keep the weave even. But pre-preg is significantly more expensive than the dry cloth, which only adds to the cost of an already spendy part. So to keep these pieces within reach of most enthusiasts, many aftermarket companies use the wet lay-up method, especially for large parts that require a lot of mat.
Hybrids And ReinforcementsNext to its cost, one of the downsides to carbon-fiber is its unforgiving nature when it takes an impact. While urethane bends, carbon fiber is prone to shearing and damage from abrasion. And carbon-fiber can't be patched like fiberglass. So hybrid materials were developed that weave carbon-fiber with high-strength materials like Kevlar. As an added benefit, Kevlar strands can be dyed, introducing an element of color to the otherwise black carbon fiber.
And then there are the instances where carbon-fiber sheets need a little support. When forming a large, flat surface (a hood, for example), the broad expanse of carbon fiber mat doesn't have the same strength as would a smaller piece conforming to a more complex shape (like a valve cover or battery box). Over time, the large carbon-fiber sheet could wobble or "beer can," just like a sheet of steel or aluminum would, which could damage the piece.
There are a couple of ways to keep that from happening. One is to reinforce the piece with extra sheets of carbon fiber-effective, but expensive. A more cost-efficient solution is to add a reinforcement layer to the carbon fiber while it's in the mold, like a sheet of Nomex honeycomb material or fiberglass. Or, some sort of external frame can be added to reinforce the piece. Kaminari, for example, reinforces its carbon-fiber hoods with a fiberglass inner frame, carbon-Kevlar pieces at the hood mount points, and a zinc-plated latch plate. It's not the lightest hood on the market, admits Kaminari, but the maker feels it is one of the most durable for a daily-driven car. The new line of Wings West hoods also has inner liners for reinforcement.
The Right FinishNotice we said the "right" finish, not the "best" finish. How the surface of your carbon-fiber piece should look depends on a number of factors. The most important is a question of form versus function. Are you building a dragster and using carbon fiber to shave every ounce of weight off the car? Then you don't want the same kind of glossy, mile-deep finish that the show-car guy has. Why? Because the sheen adds weight. There are two ways to make carbon-fiber shiny: loading resin in the mold that cures to a glassy finish, or painting layers of clear-coat on the piece once it's made. Either way you're adding material.
Actually, there is a third way to add shine to carbon fiber, and it doesn't cost you an ounce of extra weight. It's in the mold: The smoother its surface (that faces the mat), the smoother the piece will be. For the ultimate in smooth, some manufacturers make their mold surfaces from glass. What comes out is a carbon-fiber sheet that looks like an opaque, black pane of glass. Of course, making a mold from glass has its own challenges, but the finish is remarkable.
In The BagWe were curious to see how carbon-fiber parts were made, so we visited Troy Sumitomo at 5Axis Models to see how he does it. 5Axis is not your typical aftermarket parts maker; instead, the design and manufacturing company builds custom parts (and entire one-off cars) for clients that range from private parties to auto manufacturers. The widebody Acura RSX that hit the show circuit last year carried several carbon-fiber accessories crafted by 5Axis. One of those was the rear diffuser, and Troy used that part as an example of how he crafts from carbon.
The Bottom LineSo when you're shopping for carbon fiber, what should you look for? That depends on how you want to use it. For those of you buying pieces for show, its technical details aren't all that important. Chances are good the guys at your local parts house will think a wet lay-up is a muffed b-ball shot, and wonder why you're asking about 3k or 6k when Y2K was two years ago. Doesn't really matter anyway. Eyeball the piece for a straight, even weave with no irregularities or snags. Run your hand over the finish; you shouldn't be able to feel the mat's texture, nor should you feel high or low spots.
If you're buying a hood, check for reinforcements at impact and load points. Also make sure the hood actually fits your car. Some of the cheaper parts on the market may be made from tooling that's not as precise as it should be, and the result can be hoods that'll fly off as you go down the road.
And keep in mind this advice from Wings West's Ernie Bunnell: "If the carbon-fiber piece looks good initially it will stay good. But unlike fiberglass, which you can sand, fill, and paint, you can't make carbon-fiber look any better than it does originally."
If, on the other hand, you're buying carbon fiber for competitive reasons, none of that show stuff applies. Instead, you want parts that are purpose-built for racing, which means they were constructed with strength-to-weight in mind, not cosmetics. For example, look for pieces that started as pre-preg sheets, for better resin control. And a part cured in an autoclave will be denser, which not only helps the carbon fiber's integrity but also means it will require less paint to cover it (again, saving weight). All this adds to the price of the part, of course, but everyone knows that going fast costs money, right?