Sunday, March 16, 2008

How a Bicycle Helmet Works

Hi guys. I don't have too much time today on a detailed post. I raced today and I have a hilly one tomorrow as well. I imagine I'll be putting a good 70 miles in racing and close to 100 miles in riding on both these days alone.

I think I need every bit of outdoor hills now, even if the weather is in the sub 30's. I believe I did mention that I'm racing the Tour of Battenkill in an earlier post. FYI, it has become the biggest one day road race in America! And its hardly a month away!

Anyway, just in case you still didn't get it, here's a slightly old but nice article describing what a helmet is made of, how it works and a little bit of interesting history to go along with it. Enjoy!


Helmets work by bringing the head (and brain) to a relatively gradual stop upon impact. When an unprotected rider strikes his head against a hard surface, inertia causes the brain to slam forward against the skull, which can cause bruising and internal bleeding. Helmets soften the shock by gradually crushing to absorb impact energy. The outer shell will crack but still remain intact. The helmet's body, composed of fused polystyrene beads, is compressed as it absorbs energy to cushion the blow.

What Helmets Do : This picture will make you appreciate better how helmets save lives

A GIRO Helmet Construction

Cyclists realized early on that headgear was a good idea. In the 1880s, the direct correlation of bicycle-related head injuries with the increasing miles of hard-surfaced asphalt and macadam roads gave rise to riders wearing pith helmets. Pith is a crushable material that is able to absorb some shock, but it virtually disintegrates upon impact. Still, a disposable helmet was better than none at all.

Pith gave way to a padded leather covering--a design that lasted into the 1970s. Worn primarily by racers who referred to them as "hairnets," these helmets were quite attractive, with their soft Italian leather exterior attached to a foam liner. Unfortunately, they offered little or no impact protection, but they did save a rider's ears from being ground off as he slid over the pavement.

'Hairnet' Helmets

The soft shell helmet was the next major development. Looking like a salad bowl with few vents, this headgear used a shell made of polystyrene, a material similar to that used for picnic coolers. A Lycra cover and inner foam pads attached by Velcro were added later.

Bicycle helmet standards for impact resistance and strap retention were introduced by the Consumer Product Safety Commission (CPSC) in the mid-'70s. These and other performance parameters were drawn from various sources such as the Snell Foundation, the American National Standards Institute and the American Society for Testing and Materials. At the time, only light motorcycle helmets met the standards, but cyclists refused to wear them because the helmets were too hot and heavy.

Bell's Biker was the first bicycle helmet to meet the CPSC standards. It was constructed of an ABS plastic shell taped over polystyrene foam. Newer shell materials, such as PVC in lower-end helmets and polycarbonate in more expensive models, have since reduced weight without compromising performance. A denser form of polystyrene has cut weight as well as bulk. Helmets must have the integrity to withstand primary (initial fall) and secondary (getting hit by other bikes) impacts.

Stylewise, round shaped bicycle helmets are as out-of-date as Charo and the hoochie-coochie. Everyone is going with the low-profile angular look. Only a geek would ride around with something that looks like an overgrown walnut on his head. Vents are part of the style too, but their true purpose is to make the helmet cooler to wear. In case you hadn't noticed, there's a vent war going on. How many vents can you punch into a helmet? Limar's F111 ($155) introduced at this year's Interbike Show holds the record at 37 vents, up from last year's F107 model, which has 23.

A Limar F-111

Mountain bike helmet vents, such as in the Bell X-Ray, are larger and less aerodynamic than their road bike counterparts'. They provide for more passive sweat evaporation at low speeds under a hot sun. CoolMax liners, almost universal in helmets these days, also help to wick away moisture.

Helmets don't work if they slide back on your head or are ejected upon impact, so a lot of attention is being paid to a secure fit--especially at the back of the head. Bell has developed a Geared Positioning System to secure the back-of-head cradle inside the helmet. The Roc Loc--twin levers that lock onto a toothed strap--is Giro's answer to secure retention. And Limar uses a 10-position click-adjustment system to provide a snug fit across the board--this helmet comes in only two sizes.

Sportscope helmets depart from standard one-piece construction and use six segmented panels joined by a strong, flexible nylon mesh. The separate panels expand and contract for a customized fit. The design eliminates the need for fit pads and reduces the distance between the helmet and the skull, so the helmet is more stable.

A Sportscope Helmet

Bike helmets are a proven lifesaver. According to the Insurance Institute for Highway Safety, 98 percent of bicyclists killed in 1999 weren't wearing helmets. In the event of a crash, wearing a bicycle helmet reduces the risk of serious head injury by as much as 85 percent. Helmets not only look cool, they are cool. Those aero vents on road bike helmets reportedly keep your head cooler than riding without one. And you know that other old saw--cooler heads will prevail.

Source : Popular Mechanics


tOM Trottier, VP Citizens for Safe Cycling said...

There wouldn't be any crushing inside, would there? Because the inside conforms to the head shape, while the outside is what gets hit by curbs, flat surfaces, etc., which would distort the surface. On the inside, the load gets distributed over the entire hemisphere of the head.

Richard Keatinge said...

After testing on the standard rigs, what we get, I'm told, is crushing on the inside. What I've seen after accidents is fracturing of the rather fragile material, and superficial grazing. I presume that this should avoid scalp grazes, which can be quite nasty, but will equally dump energy inside the head, rather than using the scalp as a shock absorber for the brain.

The Conehead helmet is only available for motorcyclists, but may actually absorb energy as intended. The Phillips helmet - again for motorbikers only - is designed to absorb rotational energy. There seem to be no plans to make either of these advantages available for us. I doubt I'd bother with a helmet even if they were - the risk just isn't high enough - but if there were helmets that worked I'd certainly consider them.

Craig Miller said...

I would have to agree with Richard.

It is a pity that people still ride the dangerous "safety bike".

In discussing the types of injuries received from recumbent bike crashes with the BicycleMan (in NY, USA), it was mostly broken ankles.

ksteinhoff said...

I'm late to the party, but that doesn't mean I don't get a piece of what's left of the punch.

Like some of you, I'm skeptical of a lot of the "my helmet saved my life" stories.

On the other hand, here's an account of where NOT wearing a helmet changed my riding partner's life forever.

Will a magic foam hat protect you from an encounter with an 18-wheeler? Of course not. On the other hand, in this case it would have been the difference between bruises and road rash versus five skull fractures.

Your picture of What Helmets Do almost exactly illustrates my friend's crash, except that she landed on her right temple, then flipped over and had a second blow the the back of her head.

Anonymous said...

Hmmm, rotation- certainly a smooth/rounded shape can rotate best.

What I think you are all missing is the psychological aspects of helmet wearing. Not only does it remind the cyclist that they are on a bicycle, it also is a visible sign to a driver that there is a cyclist in view.

The visibility of the cyclist compared to the background environment may even be more important. Perhaps studies should focus on the color of the helmet.

Anonymous said...

Is it not the point that helmets DO reduce head injuries, hut making them compulsory reduces the number of cyclists (particularly women who hate 'helmet hair.')

Health costs for illness from people not exercising (cycling) and extra deaths exceed the reduction in deaths from helmet wearing. Those statistics are available.

I choose to wear a helmet - I have to wear something to keep the sun off, it might as well protect my head as well.