A helmet is the most important item of safety gear in any type of motorcycle riding or auto racing. In the event of an impact, helmets are designed to minimize the G-forces that act on the head, and they're rigorously tested to ensure they do their job. To get an insight into the materials technology behind this crucial safety device, we caught up with Gary Peters, Director of Engineering at Simpson Performance Products, who has more than 20 years of experience in the design and development of helmets.
The protective outer shell of a helmet is made from fiberglass or carbon fiber. The key differences between the two are price and weight.
The shells of carbon-fiber helmets are created using prepreg material sheets that are laid up into the correct form and baked in an autoclave. Carbon helmets are more accessible than they were on their introduction 20 years ago, but still carry a price premium over their fiberglass counterparts because top racers value their lightness. Let’s take Simpson’s famous Bandit design as an example. Here, the Carbon Bandit is 1/3-lb lighter than the regular Bandit but is around $500 more expensive than the fiberglass model. “The carbon shell is lighter in weight and tends to be a little stiffer as well,” says Peters. “That changes a few things in the design, primarily the density of the expanded polystyrene (EPS) foam beneath the shell, which tends to be a little more forgiving.”
The lighter weight of a carbon shell is often valued by drivers in open-wheelers or shifter karts, where the higher lateral G-forces take a greater toll on the neck muscles. However, there’s no drop-off in safety when it comes to the fiberglass-shelled designs. Like carbon helmets, they’re tested to the demanding Snell SA2020 standard, whose requirements have now been brought closer in line with FIA 8859 international standards.
During SA2020 tests, which can be replicated in Simpson’s own labs, an instrumented head form wearing the helmet under test is subjected to impacts from both flat and hemispherical anvils. An accelerometer inside the head form measures how much of the impact is carried through to the head. “Designing a shell is a balancing act,” Peters explains. “We use our experience to create a ‘ply book’ that describes how the material layers are put into the shell, how they’re intertwined and oriented. We combine that with the shape of the shell and the density of the EPS foam, and tune all three elements to get good, balanced results between the hemispherical impacts and the flat impacts.”
Simpson helmets are assembled in Texas and are certified by the Snell Foundation, which sets the prevalent safety standard in the US. Helmets from Simpson’s Italian sibling company, Stilo, are sold throughout the world so they’re also certified to FIA standards.
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Expanded polystyrene (EPS) foam has been used to absorb impacts in race helmets since the 1950s. Today, it’s used in a variety of densities for different applications, although – unlike the differences in shell materials – the weight changes between densities are negligible.
“It’s still being used today because it’s not really affected by temperature change,” says Peters. “As such, it is probably the best impact-attenuating material that we have available to us. We test helmets at ambient temperature, we have a hot test, a cold-cycle test and a wet test. There’s a broad range because we create real-world scenarios. I’ve been thinking about alternate materials for EPS foam since 2003 and I’ve tested many different candidates. EPS foam works very well but that doesn’t mean we stop looking.”
The EPS inside a race helmet is usually in two parts: a ring around the sides and back of the head, and a separate, lower-density ‘cap’ that sits on top. This two-piece setup was developed in response to the tightening of industry test standards around 15 years ago.
“People were able to change their layups or their shells and EPS, to help with the hemi-anvil impacts” he continues. “But this caused the flat impacts to be more difficult to pass. Putting this cap in a certain area to have coverage on top helps to lower the Gs from the flat-anvil impacts. Between that and the greater curvature of the helmet on the top, you’re able to have a lower density of EPS on top and still have good impact results with your hemispherical impacts. Again, it’s about ensuring a balance of good results from the two different impactors.”
Next to the EPS is a layer of urethane foam that forms the comfort padding for the head and, when deployed in different thicknesses, it determines the final fit and helmet size (note that more than one size may share the same exterior shell).
There will typically be a urethane pad on top of the head, usually removable, that can be thicker or thinner to adjust how the helmet sits on the driver’s head. “You want the top of the eye port to be right at the top of your eyebrows,” advises Peters. “That top pad can be important from a fit standpoint. If you’ve put a helmet on and you’re getting pressure in the temples, you can sometimes put a thicker pad up top and it will move the helmet up just enough to ease the pressure.”
Removable or not, all of the urethane pads and areas within the helmet that come into contact with the skin are covered in a fire-resistant, Nomex or CarbonX-type material. The same goes for the helmet skirts used by drag racers. These carry an SFI fire-resistance rating of SFI 5 or, for the heavier, multilayered skirts typically used in disciplines like NHRA Funny Car, SFI 10. In other cases, flexible chin flaps are sometimes used to seal the helmet under the chin, preventing air or debris from blowing up into the face. Again, these are made from a fire-retardant fabric.
The chin straps affix the helmet securely to the head, anchored to the inside of the helmet on each side and connected at the bottom by double-D-rings. For strength, chin straps are typically made of Kevlar webbing, which is naturally yellow in color but dyed black on Simpson helmets. Any comfort padding toward the end of the chin strap, to prevent rubbing, is again made of urethane foam with a fire-resistant Nomex or similar material sewn to the outside.
Helmet visors are generally made from 3mm-thick (0.118-inch) polycarbonate. On the outside of the visor is a hard coating while on the inside is usually an anti-fog coating. There may be posts for tear-offs, too. Perhaps surprisingly, helmet visors are not made from laminated sheets. Peters says that Simpson has extensively tested laminates and found no benefit over the regular thermal-formed or injection-molded polycarbonate visors.
“A good visor is very important,” he stresses, “especially if you are in a car without a windshield like a sprint car or a dirt late model, and constantly have rocks and chunks of dirt being thrown at you. We’re always looking to improve and test new materials against what we currently have.”
