There are few activities you're likely to do in your life where a helmet is more important than when you're on a motorcycle. Think about it - compared to when you're in a car, a motorcycle accelerates faster, generally has a higher top speed, is inherently less stable (if you hit a patch of ice in a car, you might do a pirouette if you're lucky - on a bike, you're skating across the tarmac), is less visible in traffic, and (with extremely few exceptions) has no crash protection worth mentioning. Finally, there is rarely such a thing as a 'small' motorbike accident. In other words, any protection you get in case of an accident is based on whatever you wear - boots, leathers, and a helmet.

A few words about motorcycle accidents

Obviously, the best possible outcome of having a helmet is not needing it. However, for all the reasons described above, motorcyclists are particularly vulnerable, and do have a lot of accidents. It doesn't matter if you are the most observant, passive motorcyclist in the world: There might always be some idiot out there who fails to spot you, with nasty consequences. In the UK, failing to be seen happens so often that it has its own name; a 'SMIDSY' - short for 'Sorry, mate, I didn't see you'.

Anyway, when you come off your bike, there are typically two main types of accidents; those involving other vehicles, and those not involving other vehicles. If other vehicles are involved, the accident generally involves crashing into something, flying for a bit, landing, sliding for a bit, and then stopping. If no other vehicles are involved, it's generally coming off the bike, sliding for a bit, and then stopping, hopefully without hitting any trees or other relatively solid objects in the process.

Either way, your helmet has to protect you from an impact (which, statistically speaking, will be either against a part of a car, or against the asphalt), and abrasion damage (as you slide along the road for a bit). Finally, your helmet should be smooth enough to not snag on anything - if it has ventilation inlets etc, these should break off as you hit the tarmac, because if they don't, and your helmet ends up snagging on anything, it'll snap your neck like a dry twig - with a broken neck, it doesn't really matter if your helmet protected your head...

Helmet construction

Basically, a helmet has a hard outer shell which stops sharp pointy bits from sticking into your brain, and which protects you from abrasion.

It also has a softer inner layer, which is often made of polystyrene (yup, the same stuff as the coffee cups, just compressed in a different way) designed to bring your head to a stop as slowly as possible - in effect, it's a crumple zone between your head and the inside of your helmet. These inner layers come in different stiffnesses. "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." (http://kamps.org/g/?zyof)

Types of MC helmets

There are a few different types of motorcycle helmets. A full face helmet is the type which you probably associate with motorcycle racing - it has a visor, and covers all of your head. An open-face helmet protects the sides, back, and top of your head, but leaves your face behind a plastic visor. A flip-up helmet is a combination of the two, where the chin-bar part of the helmet can flip up (great for motorcycle instructors and police officers, as it is easier to communicate when there isn't anything in the way of your face). When flipped down, the helmet offers chin protection, but due to the hinging mechanism, it isn't quite as safe as a full-face helmet.

There are other types of helmets as well, such as a skull cap etc (often seen on Harley Davidson riders), but seeing as these are practically useless, they won't be covered any further here.

Choosing a helmet

There is an old adage in motorcycling that goes 'if you've got a £5 head, get a £5 helmet'. There's something in that, but seeing as helmets cost up to £700, thinking that expensive automatically means better protection isn't necessarily correct.

A lot of focus is placed on helmets, but the truth is that even a cheaper helmet (as long as it passes certain safety criteria) generally gives enough protection in an accident: As it turns out, most accidents where helmet-wearing motorcyclists sustain enough head trauma that it kills them, even the most expensive, best helmet in the world wouldn't have helped, as the damage to the rest of their body (back, torso, and neck, especially) would have killed them anyway.

You would be forgiven for thinking 'hey, the MotoGP guys use helmet X - if it's good enough for them, it's good enough for me'. Which might seem reasonable, until you think about what these guys go through: Motorcycle racing is done in a beautifully controlled environment: Even if you're going at 180 mph, it's possible to survive a crash. Why? Well if you come off your motorbike in a race, it's generally in a turn, so you may go flying for a bit, but you then land on asphalt, and slide on asphalt, grass, sand, and end up in a stack of haybales. With all the body armour these guys wear, it can mean broken bones and being seriously winded, but generally, it's not really worth worrying all that much about. On the road, however, the situation is quite different: There are cars, sharp kerbs, buildings, trees, sharp pointy things (fences etc), traffic light poles, etc.

What I'm getting at, is that it's important to choose a helmet that is suitable for the kind of environment you are likely to crash in. You're unlikely to survive a crash doing 150 mph in an uncontrolled environment (due to trees and other traffic, mostly), and it's generally illegal to go that fast, most places, anyway. For those reasons, it's relatively pointless to buy a 150mph-certified-kevlar-racing-helmet.

More realistically, you'll be going about 50mph, being cut off by a taxi, crash into the side of it, fly over its bonnet, bounce off the roof, and land ungracefully on the edge of a kerb, before sliding into a phonebox, which smashes into pieces, showering you in broken glass.

So, no racing helmets, unless you are, er, racing.

Also, open-faced helmets are a Bad Idea. Sure, you'll look cooler on your little Vespa, but there are regular reports of people who note that their chin bar and visors have started melting from the heat and friction of even a relatively undramatic crash. Melting fibreglass is probably preferable to leaving half your face along the road, so choose a flip-style or (even better) a full-face helmet.

The construction of the helmet is less important - they come in all sorts, from plastic and fibreglass to kevlar and carbon fibre. If you do a lot of long-distance riding, a lighter helmet might be a good idea (so a more expensive carbon fibre helmet might be a good idea), but apart from that, it doesn't really matter all that much: if it adheres to the appropriate safety standards (see below), you're good to go.

One last thing worth considering is ventilation and noise - more expensive helmets tend to be quieter and better ventilated than cheaper models, so for comfort and long-distance touring, it's worth looking to the more expensive and better-known brands.

Helmet sizes

Basically, a helmet needs to be as tight as it can be without giving you a headache. To really get a feel for it, select a helmet that seems to be snug, then wear a smaller size for about 10 minutes. If you can bear it for 10 minutes or so, you've probably found the right helmet size for you: It'll loosen a little bit as you wear it in, anyway.

Safety markings / certificates

In the US, expensive helmets are tested and marked by the SNELL foundation - but the tests used here are debated, and many physicists and physicians argue that the tests are too academic, and not realistic in the real world - which means that manufacturers creating helmets to pass the SNELL tests, are, in fact, creating helmets that are dangerous in real motorcycle accidents (see http://kamps.org/g/?zyof).

In the UK, a new government-appointed test body have developed the SHARP test (see also http://kamps.org/g/?ikrh), which gives motorcycle helmets a rating from 1 to 5 stars. These tests are designed to closely reflect 'real life' motorcycle accidents, and so should, at least in theory, be the most useful rating system we have for MC helmets to date. It's worth noting that expensive helmets (such as the £500 Arai RX-7) don't necessarily fare all that well, whilst some cheap helmets (Such as the £60 Lazer LZ6) get a full score, and would be safer in the real world.

Different countries have different mandatory safety markings for motorcycles: In Britain, all road legal motorcycle helmets must conform to ECE 22-05 or the older British Standard 6658, and will have markings identifying such conformity. Other countries have other markings, so check what is required for you in your country.

Expiry date

One last note - when you buy a motorcycle, make sure that you replace it if it is dropped (even from a small distance), or after 5 years - helmets do deteriorate over time, and it's about your life, after all...

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Re the write-up below: Barring a few points that don't sit well with me, everything the man says is true. Choice of helmet should probably also be influenced with the roads you ride on. I ride a small-engined (125cc) scooter that does 75mph with a hind-wind, down-hill, so I am not at risk of a high-speed impact. I do, however, ride in inner-city London, arguably one of the most congested cities in the world, during rush-hour (this route every day, plus whatever I decide to do in addition..), which means a high risk of a low-speed (under 40mph) impact against a car, van, or other motorbike.

The first (and only, knock on wood) time I came off my bike, I was wearing a cheap Lazer helmet, and it saved my head and my face, without a doubt. Your mileage may vary, but my personal choice is pretty easy: I'll never be caught out without a full-face helmet.

One final point: Do a search on YouTube on MotoGP accidents (or just motorcycle accident). You'd be surprised the kind of stuff people shrug off and walk away from. Take especially close note of how and how hard the bikers hit the tarmac with their heads. Pretty amazing stuff. Also note what type of helmets they are using.

There are few issues so contentious as helmet use in motorcycles.

At risk of sounding like a bulletin board response, I will present a counter-argument to "There are other types of helmets as well, such as a skull cap etc (often seen on Harley Davidson riders), but seeing as these are practically useless, they won't be covered any further here." above, as this is about helmet selection, and there are VALID reasons to select such a helmet.

Disclaimer: I ride with a helmet. It's a half-helmet of a type described above, the minimal helmet one is allowed to get away with. It's light carbon fiber with some padding and is a hair away from being a novelty helmet.

Here's a test for you - go to a motorcycle store and pick up the most expensive Mighty Morphin Power Rangers full face SNELL/ANSI/DOT helmet you can find. The kind where all you can see is out of a tiny slit cause your head is encased in a bowling ball. Have a look at what the tag says that's attached to it. I'll tell you what it says in a minute.

The bare-bones, almost illegal half-helmet was the kind of helmet I was wearing when I was knocked from my motorcycle at 55mph and hit the ground eyebrow first, sliding about 55 feet. Yes, I sustained some facial abrasion, but you can't see it today, there are no scars. And I'm glad I was wearing that.

Physics fact of the day: a body falling towards the ground hits the ground accelerating towards the ground at 10 m/s2. If you're going FORWARD at 55mph, your head doesn't hit the ground ANY HARDER than if you're standing still. Yes, you can knock yourself out and die falling off a motorcycle head first at standstill, but it's rare.

Sharq's commentary about leaving your face on the road: well, if you're stupid enough to KEEP it sliding there rather than lift your head, sure. (Of course, if your head is weighted down and you can't...) But something far more sinister can happen.

Chin impact with a full face helmet often causes the head to roll forward, causing a hyperextension of the cervical spine with subsequent quadriplegia and/or death. In other words, the helmet don't flex, so it pulls your head off your spinal column, and breaks your spinal cord. Thanks, I'd rather risk a cut to the chin.

What else can a helmet do? Hmmm. Let's see, an impact to the forehead can snap back the helmet, and the hard ridge at the back (in the MMPR helmet) is in the perfect spot to come forward like a paper cutter and break your cervical spine, meaning you're Christopher Reeve from a relatively small impact.

It's now time to tell you what the tag says on that $500 ARAI POWER RANGERS HELMET with the three inches of padding on the inside and the lightning bolts decalled onto the side. It says, basically, that no helmet can protect your head from forseeable injuries over 15 miles an hour. In fact, It's legalese on there so that when you hit the deck and the massive weight of that helmet breaks your neck, your family cannot sue. Sure, your head's intact, but you're a quadriplegic or dead. Or when the helmet shatters when you're sliding down the road and hit something at 35 miles per hour.

Now, let's look on the other side of the coin, what does a helmet do for you?

Helmets cause rider fatigue You've got that giant bowling ball on your head. It's heavy, it's bulky, it costs you a lot of energy to turn your head. Not at first - at first you don't notice much. But cumulatively, at the end of a long ride, you DO notice the extra weight, and it contributes to rider fatigue, which is a bigger cause of rider death than anything else.

Helmets blunt your senses You need your wits about you when riding on a motorcycle, in every way - which is why I do not, cannot, and will not wear anything that covers my ears preventing me from hearing, and limits the peripheral vision of my eyes. The standard says 118 degrees of vision from the central point of my head, blah blah blah. Yeah, well I'm not Cyclops, thanks. I have two eyes, on either side of my head, and HAVE RIDDEN into the path of a speeding ambulance, sirens blaring, because I neither heard nor saw it with my "sensible" helmet, which was tossed in the trash the moment I got home.

Helmets heat your head Here's a good one - a black bowling ball around your head while riding through the summer. Three inches of insulating foam. Sounds like a recipe for heatstroke. And it is.

The sad fact of the matter is, rider deaths FROM HEAD TRAUMA are way down thanks to mandatory helmet use, but the rates of actual death have not gone down appreciably. In other words, impact that would have killed you from head trauma now kills you from a broken neck. Or, in most cases, you're travelling fast enough to hit something (sliding forward) with sufficient force that a helmet would protect your head, the rest of your body, so unprotected, takes a similar impact and you bleed out internally of a ruptured spleen.

If you're sliding down the road and something makes your body stop suddenly, they've found there's no real benefit to the helmet. Either your neck breaks, something inside ruptures, and... well, the helmet you're wearing has AT MOST been tested to an impact of thirteen miles an hour. Chances are any real impact will cause rupture, hence the legalese written above.

Most accidents involve the rider having the time to brake the motorcycle hard. He might go down, he probably will go down, but he's probably going at a very low speed when he hits the ground. If he's blindsided, comes around a curve and into the path of a semi, what have you, he's SMOKE, regardless of what he's wearing. Similar impacts in a car don't leave the drivers looking too good either.

So to summarize, I don't really see the benefit in wearing something that's not tested beyond 13 mph, that's proven to cause neck injury and death, will prevent me from seeing and hearing properly, will weigh me down, tire me out and heat my head, and bring its own hazards, like trapping a stinging insect that might fly into it.

I also don't see the benefit in overstating the benefits of a helmet either. The attitude "expensive helmet good, Harley rider half helmet stupid" one stated above, apart from insulting and condescending, is illusory. You see a lot of squids race off at 140mph wearing flip flops, a T-shirt and shorts, because their MIGHTY HELMET OF ANSI STANDARD PROTECTION +4 gives them a CONE OF PROTECTION around their body, a halo of security that will save them in any kind of crash. I wonder if they would be so foolhardy if they knew just how much that helmet ends up really helping when the rubber meets the road.

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NOTE: Downvotes and some rude comments require me to clarify something: Gravity pulls you towards the earth at a rate of ten meters per second, per second. The downward pull and therefore the impact of you against the ground from where you start out from IS THE SAME regardless of whether you are moving forward or not. I am not talking, in the above paragraph, about how hard you'll slam into the brick wall in front of you or how long you'll slide along the road. Being on a motorcycle does not alter the laws of physics no matter how ad hominem you want to get about it.