For true gearheads, a car is never just a car—it’s a four-wheeled heap of possibilities. Whether you’ve got an old Triumph Spitfire or the latest Mitsubishi Evo, the options for customizing your vehicle are just about endless. Some of our favorite enhancements include splitters, wings and undertrays. However, when it comes to the materials you’ll use to build these parts, your options are much more limited.
Different applications require different materials, and choosing the right one for your fabrication job is crucial—it can even be a matter of life and death. That means you need to look at certain material properties: strength, weight, cost, durability, elasticity, resistance to corrosion, heat tolerance and ease of fabrication. Understanding these eight material properties will help you bring that homemade component to life.
ABS Plastic
- Strength: 4/5
- Weight: 2/5
- Cost: 3/5
ABS plastic is a common thermoplastic with an extremely long list of uses. You can find ABS in automotive trim, bumper bars, whitewater canoes and even Lego bricks.
The advantage of ABS is that it combines the strength and rigidity of plastic with the toughness of rubber. Available in sheets, pipes, rods or rectangular tubes, ABS plastic can be purchased in just about any form. Plus, those base stocks can be easily machined, turned, drilled, milled, sawed, die-cut, sheared or routed.
The most important mechanical properties of ABS are impact resistance and toughness. Pieces can withstand a lot of deflection upon impact before ultimately breaking, and professional race teams have used ABS to make front splitters and dive planes for years. ABS is also used on many interior trim pieces, as softer plastics would fatigue over time.
ABS’s Achilles heel, however, is heat. With a melting point of 221 degrees Fahrenheit and a maximum service temperature of 192 degrees, it simply can’t be used in high-heat applications. In addition, once ABS has been molded, it’s incredibly tough to change its basic shape without weakening the material. That means a flat piece of ABS isn’t going to work for those curvaceous dive planes.
Carbon Fiber
- Strength: 4/5
- Weight: 2/5
- Cost: 3/5
Carbon fiber is probably the most desired material in motorsports today. It’s extremely lightweight and can be stronger than steel, making it appropriate for body panels, wings, diffusers and interior panels.
Exactly what is this wonder material? Carbon fiber consists of extremely thin strands of carbon atoms. The carbon atoms are bonded together in a microscopic crystal arrangement that is aligned parallel to the fiber.
The crystal arrangement is what makes the fiber so strong. Several thousand carbon fibers are twisted together to form a yarn, which is usually woven into a fabric. Carbon fiber has many different weave patterns and can be combined with plastic resin to create an impressive strength-to-weight ratio. Thanks to its high tensile strength, low weight and low thermal expansion, carbon fiber has become very popular in motorsports despite the relatively high costs.
However, carbon fiber has its own shortcomings. It isn’t the easiest material to fabricate on your own, as most creations need to be vacuum molded—and that vacuum-molding process is what gives the material much of its strength. DIY vacuum-bagging systems can be made, however, and quality results are incredibly simple with the right tools.
Carbon fiber is very strong when stretched or bent, but it’s weak when compressed or exposed to high shock. In other words, those fancy carbon fiber splitters work well—until they experience an impact and shatter.
Plywood
- Strength: 3/5
- Weight: 3/5
- Cost: 1/5
We jokingly call plywood the original carbon fiber, as both materials feature a matrix of carbon strands. Some may label plywood as a subpar material, but we couldn’t disagree more. After all, World War II fighter planes used plywood in many of their structural assemblies.
The trick to plywood is understanding its properties and knowing its limitations. Plywood is a type of manufactured timber made from thin sheets of wood veneer that are bonded together. The sheets get their strength because of the way they’re arranged: the grain patterns run at right angles to each other. Compared to plain wood, plywood offers more resistance to cracking, shrinking and warping. On top of that, it’s relatively cheap.
Plywood can be used to make many interior panels, including rear seat deletes that are usually covered with carpet. Some may recall the episode of “Top Gear” where they made a splitter out of wood. Unfortunately, that splitter caught fire. Thanks to an autoignition temperature of around 400 degrees Fahrenheit, plywood isn’t the best solution for applications where contact with exhaust pipes may occur.
Sadly, plywood’s time in motorsports may have expired. There are so many automotive applications where ABS, fiberglass or carbon fiber is better suited. Fortunately, if you need a material quickly and inexpensively, plywood will always be waiting for you at the local hardware store.
Fiberglass
- Strength: 2/5
- Weight: 4/5
- Cost: 1/5
Fiberglass is a common material often used to make body panels and various other components. Similar to carbon fiber, fiberglass is a composite that features a fiberglass mesh reinforced by a polymer. Fiberglass isn’t as strong or as rigid, but it can handle more elongation before it fails.
Fiberglass still has two big advantages: It’s inexpensive and simple to work with. Other than being messy, fiberglass is easy to form—simply shape it to your heart’s desire or apply it to a mold. Once a mold is made—usually out of fiberglass—replicating a part multiple times becomes a breeze.
Polycarbonate (Lexan)
- Strength: 5/5
- Weight: 2/5
- Cost: 4/5
Lexan is a trademarked name for polycarbonate, a particular group of thermoplastics commonly used to replace glass on vehicles. This polymer is easy to work with, as you can mold and thermoform it quite easily. It’s also a very durable material that’s highly resistant to impacts. Polycarbonate is similar to acrylic, but it’s stronger and usable in a wider temperature range.
The maximum stable temperature for most polycarbonate is around 176 degrees Fahrenheit, similar to ABS plastic. In addition, polycarbonate expands and contracts due to heat at a higher-than-average rate. To compensate, bolt holes should be enlarged in order to prevent cracking.
Unlike most thermoplastics, including ABS and Plexiglas, polycarbonate can undergo large plastic deformations without cracking or breaking. As a result, polycarbonate can be processed and formed at room temperature using sheet metal techniques—bends, for example, can be formed on a brake. However, tools must be kept at high temperatures, generally above 176 degrees, to produce strain- and stress-free parts.
Polycarbonate is excellent in electrical applications where a nonconductive, flame-retardant and heat-resistant material is necessary. When sheet metal isn’t an option, consider polycarbonate. It’s also a go-to material for creating transparent components. Common uses include windshields, replacement window and headlight lens glass, and mounting surfaces in the cockpit. Be aware that it scratches easily, though.
Acrylic Glass (Plexiglas)
- Strength: 2/5
- Weight: 2/5
- Cost: 2/5
Plexiglas, Lucite and Perspex are trademarked names for acrylic glass. These synthetic plastics are an economical alternative to polycarbonate and are useful when extreme strength is not required. Acrylic glass’s benefits include moderate strength properties, ease of fabrication, and low cost. However, the material can be brittle when loaded, especially during an impact, and is just as prone to scratching as polycarbonate.
Acrylic glass will also dissolve in many organic solvents and has very poor resistance to other chemicals. Even Windex or soap and water can easily produce undesirable stress crazing.
In addition, when acrylic glass is cut, the edges become stress-fractured and should be annealed. The annealing process, which involves controlled heating and cooling, makes the glass resistant to solvents and stress-induced cracking.
Acrylic glass should also be thermoformed, as it can’t be worked at room temperatures like polycarbonate can. If you’re deciding between acrylic glass and polycarbonate, realize that acrylic glass has only recently started to be replaced with polycarbonate—and that’s typically only in extreme applications, such as the canopy for the F-22 Raptor fighter aircraft.
For most vehicle applications, acrylic glass is still immensely popular due to its affordability and general ease of fabrication. Safety items such as the windshield, however, should be constructed out of polycarbonate.
Aluminum
- Strength: 4/5
- Weight: 2/5
- Cost: 4/5
In the early 1900s, manufacturers referred to aluminum as “winged metal” thanks to its popularity in aircraft. Since then, research and development of aluminum alloys has allowed us to fly in supersonic jets and venture into space. While we all have lofty goals for our cars, we’ll settle for using aluminum to shed precious pounds from our chassis.
As a rule of thumb, 1 pound of aluminum can replace 2 pounds of steel. With returns like that, aluminum will definitely find its way into your DIY projects. Aluminum can be found just about everywhere, from brackets and catch tanks to chassis and bodies.
Nearly all forms of aluminum are alloys, containing trace amounts of other elements to improve the material properties. These mixtures are called series, and they’re known by numbers ranging from 1000 to 8000. The alloy’s designation is determined by the major alloying element, and the most common alloys used in motorsports are 2024, 3003, 6061 and 7075.
- 2024 is commonly used in the motorsports industry. While it isn’t weldable or exceptionally formable, it has tensile strength levels and stiffness well beyond those of most other aluminum alloys. Tilton produces their flywheels from 2024, as it’s much stronger than 7075 at high temperatures. 2024 requires a bend radius of at least four times its thickness.
- 3003 is the most widely used low-strength alloy and is popular with automakers. It can be easily formed and welded, making it practical for nearly any application except structures—its weakness means it should never be used for critical parts. Common uses include forming brackets, tanks and boxes.
- 6061 is a weldable alloy with a decent degree of formability. It can be a savior for trackside repairs, as it’s one of the few alloys that can be worked with a wooden block and hammer in sheet form. 6061 is excellent for making brackets and other formable parts. In bar form, however, 6061 doesn’t machine very well.
- 7075 is the strongest and stiffest commonly available aluminum alloy. It is also incredibly machinable, making it well suited for bushings, spacers and even suspension and steering parts. Aftermarket transfer case and differential housings are routinely machined from 7075 aluminum. However, 7075 is not very formable and can usually only be spot welded. Still, whenever we see 7075 aluminum, our minds flood with visions of billet goodies.
Most aluminum alloys are heat treated to improve their tensile strength. While there are many different tempers available, the most common are T4 and T6. A T4 temper is naturally aged at room temperature, which makes the material stronger but generally soft and formable. A T6 temper is artificially aged via extended baking period. T4 is better for tight-radius bends and forming, but T6 is ultimately stronger.
Steel
- Strength: 5/5
- Weight: 4/5
- Cost: 2/5
It would take an entire book—or even books—to fully explain the complexities of steel and its many alloys. Not all steels are created equal.
Carbon steels are common in motorsports and can be used in various applications. The trick to working with steel, as is the case with most materials, is using the correct grade. Since the properties of steel are so intricate, our best advice is to do your own research. You should know and understand the steel’s tensile strength, toughness, fatigue resistance, heat treatments and inherent ability to be welded and machined.
Also, homemade steel parts destined for structural use should be heat treated. This process is imperative for getting the ultimate strength out of steel. A common mistake is creating a part using SAE 4130 chrome-moly steel and not heat treating the finished product. Without that last step, you’ll end up with a part only as strong as SAE 1020 steel—and with brittle weld areas.
How do you heat treat steel? Your best option is to send your part to a professional. Be explicit about how the part will be used, then let the professional take it from there.
Titanium
- Strength: 5/5
- Weight: 1.5/5
- Cost: 5/5
Titanium is a fantastic material: It’s light, incredibly strong, resistant to heat, will not corrode at normal temperatures, and looks incredible. So, why isn’t titanium everywhere? It’s expensive and difficult to fabricate.
Titanium contaminates itself with oxygen and nitrogen rather quickly at temperatures around 1300 degrees Fahrenheit. Once it’s contaminated, it becomes very brittle and breaks nearly as easily as glass.
Welding that beautiful metal requires a TIG welder, titanium filler rod and an enclosed box—much like a sandblasting cabinet—which must be filled with pure argon to properly shield the weld.
If there’s an upside, titanium can be treated like low-alloy steel during forging, grinding and machining, providing some hope for the do-it-yourselfer. Even so, chances are that another material will be more suitable for your projects.
[Please Note: As using the wrong material can have serious consequences, please consult an expert and do your own research before hitting the fab shop.]
