Motorcycle
Helmet Performance: Blowing the Lid Off Part 1.
Searching for the truth behind motorcycle helmet
design, helmet standards and actual head protection by Dexter Ford. Photography:
Jim Brown (except I cut them all out
…Ed.)
How good is
your helmet? Will it actually protect your brain in your next crash? These seem
like easy questions, ones you probably think you can answer by reciting the
lofty standards your helmet meets and the lofty price you might have paid for
it. But the real answers, as you are about to see, are anything but easy. There's
a fundamental debate raging in the motorcycle helmet industry. In a
fiberglass-reinforced, expanded-polystyrene nutshell, it's a debate about how
strong and how stiff a helmet should be to provide the best possible
protection.
Why the
debate? Because if a helmet is too stiff it can be less able to prevent brain
injury in the kinds of crashes you're most likely to have. And if it's too
soft, it might not protect you in a violent, high-energy crash. What's just
right? Well, that's why it's called a debate. If you knew what your head was
going to hit and how hard, you could choose the perfect helmet for that crash.
But crashes are accidents. So you have to guess.
To understand
how a helmet protects—or doesn't protect—your brain, it helps to appreciate
just how fragile that organ actually is. The consistency of the human brain is
like warm Jello. It's so gooey that when pathologists
remove a brain from a cadaver, they have to use a kind of cheesecloth hammock
to hold it together as it comes out of the skull.
Your brain
basically floats inside your skull, within a bath of cervical-spinal fluid and
a protective cocoon called the dura. But when your
skull stops suddenly—as it does when it hits something hard—the brain keeps
going, as Sir Isaac Newton predicted. Then it has its own collision with the
inside of the skull. If that collision is too severe, the brain can sustain any
number of injuries, from shearing of the brain tissue to bleeding in the brain,
or between the brain and the dura, or between the dura and the skull. And after your brain is injured, even
more damage can occur. When the brain is bashed or injured internally, bleeding
and inflammation make it swell. When your brain swells inside the skull, there's
no place for that extra volume to go. So it presses harder against the inside
of the skull and tries to squeeze through any opening, bulging out of your eye
sockets and oozing down the base of the skull. As it squeezes, more damage is
done to some very vital regions. None of this is good.
Helmet
designers have devised a number of different liner designs to meet the
different standards. The Vemar VSR uses stiffer EPS
than most, but has channels molded in to soften the assembly (to ECE specs) and
enhance cooling. To prevent all that ugly stuff from happening, we wear
helmets. Modern, full-face helmets, if we have enough brains to protect, that
is.
A motorcycle
helmet has two major parts: the outer shell and the energy-absorbing inner
liner. The inner lining is made of expanded polystyrene or EPS, the same stuff
used in beer coolers, foam coffee cups, and packing material. Outer shells come
in two basic flavors: a resin/fiber composite, such as fiberglass, carbon fiber
and Kevlar, or a molded thermoplastic such as ABS or polycarbonate, the same
basic stuff used in face shields and F-16 canopies.
The shell is
there for a number of reasons. First, it's supposed to protect against pointy
things trying to penetrate the EPS—though that almost never happens in a real
accident. Second, the shell protects against abrasion, which is a good thing
when you're sliding into the chicane at Daytona. Third, it gives Troy Lee a
nice, smooth surface to paint dragons on. Riders—and helmet marketers—pay a lot
of attention to the outer shell and its material. But the part of the helmet
that absorbs most of the energy in a crash is actually the inner liner. When the helmet hits the road or a curb, the outer shell stops
instantly. Inside, your head keeps going until it collides with the
liner. When this happens, the liner's job is to bring the head to a gentle
stop—if you want your brain to keep working like it does now, that is.
The great
thing about EPS is that as it crushes, it absorbs lots of energy at a
predictable rate. It doesn't store energy and rebound like a spring, which
would be a bad thing because your head would bounce back up, shaking your brain
not just once, but twice. EPS actually absorbs the kinetic energy of your
moving head, creating a very small amount of heat as the foam collapses.
The helmet's
shell also absorbs energy as it flexes in the case of a polycarbonate helmet,
or flexes, crushes and delaminates in the case of a fiberglass composite
helmet. To minimize the G-forces on your
soft, gushy brain as it stops, you want to slow your head down over as great a
distance as possible. So the perfect helmet would be huge, with 6 inches or more
of soft, fluffy EPS cradling your precious head like a mint on a pillow.
Problem is,
nobody wants a 2-foot-wide helmet, though it might come in handy if you were
auditioning for a Jack in the Box commercial. So helmet designers have pared
down the thickness of the foam, using denser, stiffer EPS to make up the
difference. This increases the G-loading on your brain in a crash, of course.
And the fine points of how many Gs a helmet transmits to the head, for how
long, and in what kind of a crash, are the variables that make the
helmet-standard debate so gosh darn fun.
The helmets
are mounted on a 5-kilo (11 pound) magnesium headform
and then dropped from a controlled height onto a variety of test anvils to
simulate crash impacts on various surfaces and shapes. In the real world, your
helmet actually hits flat pavement more than 85 percent of the time.
Standardized Standards To
make buying a helmet in the U.S as confusing as possible, there are at least
four standards a street motorcycle helmet can meet. The price of entry is the
DOT standard, called FMVSS 218, that every street helmet sold here is legally
required to pass. There is the European standard, called ECE 22-05, accepted by
more than 50 countries. There's the BSI 6658 Type A standard from
The helmets
are dropped, using gravity to accelerate the helmet to a given speed before it
smashes onto a test anvil bolted to the floor. By varying the drop height and
the weight of the magnesium headform inside the
helmet, the energy level of the test can be easily varied and precisely
repeated. As the helmet/headform falls it is guided
by either a steel track or a pair of steel cables. That guiding system adds
friction to slow the fall slightly, so the test technician corrects for this by
raising the initial drop height accordingly.
The headform has an accelerometer inside that precisely records
the force the headform receives, showing how many Gs
the headform took as it stopped and for how long. If
you test a bunch of helmets under the same conditions, you can get a good idea
of how well each one absorbs a particular hit. And it's important to understand
that as in lap times, golf scores and marriages, a lower number is always
better when we're talking about your head receiving extreme G forces.
All the
Snell/DOT helmets we examined use a dual-density foam liner. The upper cap of
foam on this Scorpion liner is softer to compensate for the extra stiffness of
the spherical upper shell area. Some manufacturers, including Arai and HJC, use
a one-piece liner with two different densities molded together.
On The Highway to Snell On
the stiff, tough-guy side of this debate is the voluntary Snell M2000/M2005
standard, which dictates each helmet be able to
withstand some tough, very high-energy impacts. The Snell Memorial Foundation is a private,
not-for-profit organization dedicated to "research, education, testing and
development of helmet safety standards."
If you think
moving quickly over the surface of the planet is fun and you enjoy using your
brain, you should be grateful to the Snell Memorial Foundation. The SMF has
helped create standards that have raised the bar in head protection in nearly
every pursuit in which humans hit their heads: bicycles, horse riding, harness
racing, karting, mopeds, skateboards, rollerblades,
recreational skiing, ski racing, ATV riding, snowboarding, car
racing and, of course, motorcycling.
But as helmet
technology has improved and accident research has accumulated, many head-injury
experts feel the Snell M2000 and M2005 standards are, to quote Dr. Harry Hurt
of Hurt Report fame, "a little bit excessive."
The killer—the hardest Snell test for a motorcycle helmet to meet—is a
two-strike test onto a hemispherical chunk of stainless steel about the size of
an orange. The first hit is at an energy of 150 joules, which translates to
dropping a 5-kilo weight about 10 feet—an extremely high-energy impact. The
next hit, on the same spot, is set at 110 joules, or about an 8-foot drop. To
pass, the helmet is not allowed to transmit more than 300 Gs to the headform in either hit.
Tough tests
such as this have driven helmet development over the years. But do they have
any practical application on the street, where a hit as hard as the hardest
single Snell impact may only happen in 1 percent of actual accidents? And where
an impact as severe as the two-drop hemi test happens just short of never? Dr.
Jim Newman, an actual rocket scientist and highly respected head-impact
expert—he was once a Snell Foundation director—puts it this way: "If you
want to create a realistic helmet standard, you don't go bashing helmets onto
hemispherical steel balls. And you certainly don't do it twice.
"Over
the last 30 years," continues Newman, "we've come to the realization
that people falling off motorcycles hardly ever, ever hit their head in the
same place twice. So we have helmets that are designed to withstand two hits at
the same site. But in doing so, we have severely,
severely compromised their ability to take one hit and absorb energy properly. "The consequence is, when you have one
hit at one site in an accident situation, two things happen: One, you don't
fully utilize the energy-absorbing material that's available. And two, you
generate higher G loading on the head than you need to. "What's happened
to Snell over the years is that in order to make what's perceived as a better
helmet, they kept raising the impact energy. What they should have been doing,
in my view, is lowering the allowable G force.
"In my opinion, Snell should keep a 10-foot drop [in its testing]. But
tell the manufacturers, 'OK, 300 Gs is not going to cut it anymore. Next year
you're going to have to get down to 250. And the next year, 200. And the year
after that, 185.'"
The Brand Leading The Brand "The Snell sticker," continued Newman,
"has become a marketing gimmick. By spending 60 cents [paid to the Snell
foundation], a manufacturer puts that sticker in his helmet and he can increase
the price by $30 or $40. Or even $60 or $100.
"Because
there's this allure, this charisma, this image associated with a Snell sticker
that says, 'Hey, this is a better helmet, and therefore must be worth a whole
lot more money.' And in spite of the very best intentions of everybody at
Snell, they did not have the field data [on actual accidents] that we have now
[when they devised the standard]. And although that data has been around a long
time, they have chosen, at this point, not to take it into consideration."
The Z1R ZRP-1
uses a soft, one-piece liner to soak up joule after joule of nasty impact
energy.
A World Of
How does the
Snell Foundation respond to the criticism of head-injury scientists from all
over the world that the Snell standards create helmets too stiff for optimum
protection in the great majority of accidents?
"The
whole business of testing helmets is based on the assumption that there is a
threshold of injury," says Ed Becker, executive director of the Snell Foundation.
"And that impact shocks below that threshold are going to be non-injurious. "We're going with 300 Gs because we started with 400
Gs back in the early days. And based on [George Snively's,
the founder of the SMF] testing, and information he'd gotten from the British
Standards Institute, 400 Gs seemed reasonable back then. He revised it downward
over the years, largely because helmet standards were for healthy young men
that were driving race cars. But after motorcycling had taken up those same helmets,
he figured that not everybody involved in motorcycling was going to be a young
man. So he concluded from work that he had done that the threshold of injury
was above 400 Gs. But certainly below 600 Gs.
"The
basis for the 300 G [limit in the Snell M2000 standard] is that the foundation
is conservative. [The directors] have not seen an indication that a [head
injury] threshold is below 300 Gs. If and when they do, they'll certainly take
it into account."
So nobody is
being hurt by the added stiffness of a Snell helmet, we asked. "That's
certainly our hope here," answered Becker. "At this point I've got no
reason to think anything else."
European Style The Snell
Foundation may have no reason to think anything else. But every scientist we
spoke to, as well as the government standards agencies of the
The European
Union recently released an extensive helmet study called COST 327, which
involved close study of 253 recent motorcycle accidents in
If your brain
is injured, swelling and inflammation often occur. Because there's no extra
room inside your skull, your brain tries to squeeze down through the hole in
the base of the skull. This creates pressure that injures the vital brain stem
even further, often destroying the parts that control breathing and other basic
body functions. If you're hit very violently on the jaw, as in a head-on
impact, the force can be transmitted to the base of the skull, which can
fracture and sever your spine. It's a common cause of death in helmeted
motorcycle riders—and a very good reason to wear a full-face helmet and insist
on thick EPS padding—not resilient foam—in the helmet's chin bar. When your
brain collides with the inside of your skull, bony protrusions around your
eyes, sinuses and other areas can cause severe damage to the brain. And if your
head is twisted rapidly, the brain can lag behind, causing tearing and serious
internal brain injury as it drags against the skull. A helmet is the best way
to avoid such unpleasantries.
How
Hurt is Hurt? Doctors and
head-injury researchers use a simplified rating of injuries, called the
Abbreviated Injury Scale, or AIS, to describe how severely a patient is hurt
when they come into a trauma facility. AIS 1 means you've been barely injured.
AIS 6 means you're dead, or sure to be dead very soon. Here's the entire AIS
scale:
AIS 1 = Minor AIS
3 = Serious AIS 5 =
Critical
AIS 2 = Moderate AIS 4 = Severe AIS 6 = Unsurvivable
A patient's AIS score is determined separately for each different section of
the body. So you could have an AIS 4 injury to your leg, an AIS 3 to your chest
and an AIS 5 injury to your head. And you'd be one hurtin'
puppy. Newman is quoted in the COST study on the impact levels likely to cause
certain levels of injury. Back in the '80s he stated that, as a rough guideline,
a peak linear impact—the kind we're measuring here—of 200 to 250 Gs generally
corresponds to a head injury of AIS 4, or severe; that a 250 G to 300 G impact
corresponds to AIS 5, or critical; and that anything over 300 Gs corresponds to
AIS 6. That is, unsurvivable.
Newman isn't the only scientist who thinks getting hit with much
more than 200 Gs is a bad idea. In fact, researchers have pretty much agreed on
that for 50 years.
The Wayne State Tolerance Curve is the result of a pretty gruesome
series of experiments back in the '50s and '60s in which dogs' brains were
blasted with bursts of compressed air, monkeys were bashed on the skull, and
the heads of dead people were dropped to see just how hard they could be hit
before big-time injury set in. This study's results were backed up by the JARI
Human Head Impact Tolerance Curve, published in '80 by a Japanese group who did
further unspeakable things to monkeys, among other medically necessary
atrocities.
The two tolerance curves agree on how many Gs you can apply to a
human head for how long before a concussion or other more serious brain injury
occurs. And the Wayne State Tolerance Curve was instrumental in creating the
DOT helmet standard, with its relatively low G-force allowance.
According to both these curves, exposing a human head to a force over 200 Gs
for more than 2 milliseconds is what medical experts refer to as
"bad." Heads are different, of course. Young, strong people can take
more Gs than old, weak people. Some prizefighters can take huge hits again and
again and not seem to suffer any ill effects other than a tendency to sell
hamburger cookers on late-night TV. And the impacts a particular head has
undergone in the past may make that head more susceptible to injury.
Is an
impact over the theoretical 200 G/2 millisecond threshold going to kill you? Probably not. Is it going to hurt you? Depends on you, and
how much over that threshold your particular hit happens to be. But head
injuries short of death are no joke. Five million Americans suffer from
disabilities from what's called Traumatic Brain Injury—getting hit too hard on
the head. That's disabilities, meaning they ain't the
same as they used to be.
There's
another important factor that comes into play when discussing how hard a hit
you should allow your brain to take: the other injuries you'll probably get in
a serious crash, and how the effects of your injuries add up. The likelihood of
dying from a head injury goes up dramatically if you have other major injuries
as well. It also goes up with age. Which means that a nice,
easy AIS 3 head injury, which might be perfectly survivable on its own, can be
the injury that kills you if you already have other major injuries. Which, as it happens, you are very likely to have in a serious motorcycle
crash.
The COST
study was limited to people who had hit their helmets on the pavement in their
accidents. Of these, 67 percent sustained some kind of head injury. Even
more㭅 percent—sustained leg injuries, and 57
percent had thorax injuries. You can even calculate your odds using the Injury
Severity Score, or ISS. Take the AIS scores for the worst three injuries you
have. Square each of those scores—that is, multiply them by themselves. Add the
three results and compare them with the ISS Scale of Doom below. A score of 75 means you're dead. Sorry. Very
few people with an ISS of 70 see tomorrow either.
If you're
between 15 and 44 years old, an ISS score of 40 means you have a 50-50 chance
of making it. If you're between 45 and 64 years old, ISS 29 is the 50-50 mark.
And above 65 years old, the 50-50 level is an ISS of 20. For a 45- to 64-year
old guy such as myself, an ISS over 29 means I'll probably die.
If I get two
"serious," AIS 3 injuries—the aforementioned AIS 3 head hit and AIS 3
chest thump—and a "severe" AIS 4 leg injury, my ISS score is ...
let's see, 3 times 3 is 9. Twice that is 18. 4 times 4 is 16. 18 and 16 is 34. Ooops. Gotta go.
Drop my AIS 3 head injury to an AIS 2 and my ISS score is 29. Now I've got a
50-50 shot.
Obviously, this
means it's very important to keep the level of head injury as low as possible.
Because even if the head injury itself is survivable on its own, sustaining a
more severe injury—even between relatively low injury levels—may not just mean
a longer hospital stay, it may be the ticket that transfers you from your warm,
cushy bed in the trauma unit to that cold, sliding slab downstairs.
To be continued …