A cycling helmet is a ventilated polystyrene shell that's intended to provide some protection to the wearer's head in the event of a fall. Because helmets need to be light in weight and relatively inexpensive to buy, the protection they provide is limited.
Helmet standards are typically designed so that conforming helmets protect the wearer from the consequences of a fall on to a flat surface at low speed, with no other vehicles involved. In practice this means that a helmet will prevent a visit to hospital for a typical mountain bike crash, but is inadequate to prevent death in the high-speed motor vehicle collisions that typically kill road cyclists. In jurisdictions that have made cycling helmets mandatory the cyclist death rate has not fallen significantly, or has mirrored the fall in pedestrian deaths attributed to simultaneously introduced measures such as random breath testing of drivers and stricter enforcement of speed limits.
None of this is to suggest that you shouldn't wear a helmet. The protection helmets provide is valuable, especially for mountain bikers - it's in the nature of off-road riding that falls are part of the game. But a helmet won't make you invulnerable and is no substitute for general riding skills and, on the road, for specific traffic skills.
What to look for
Given that all helmets conform to some sort of standard, you're guaranteed a certain level of protection, whatever you buy. When it comes to choosing a helmet, you should therefore consider areas such as weight, fit, and ventilation, and decide whether you need specialist features such as the chin-guards of the full-face helmets intended for mountain bike downhill racing.
If you're into types of riding where hard, fast crashes are more likely, such as downhill mountain biking then you might want to look for a helmet with a thicker shell and more coverage. Unfortunately, however, there are no tests or standards to guide your choice.
As you go up the price range, helmets tend to be lighter, better ventilated and have more easily adjustable retention systems. Reducing weight and increasing ventilation while maintaining a level of shock absorption requires more expensive materials and manufacturing techniques.
That said, budget helmets are as good as they've ever been thanks to top end technologies quickly filtering down.
A helmet should be snug without being tight or pinching. When the retention system is properly adjusted, the helmet should stay in place if you shake your head quite hard. A helmet that doesn't fit well won't provide as much protection as one that does because your head will still be able to move around inside it in the event of a crash.
The shape of the inside of the helmet differs between manufacturers, so it's important to try them on. Most people find that their head shape - be it rounder or more oval - corresponds to one or two helmet brands.
The pads and cloth lining the helmet make it comfortable against your skin and absorb sweat. This means they will get smelly over time, so they should be removable so you can wash them. Many people find the sweat-absorbing capacity of typical pads to be inadequate and supplement them with a bandana anyway.
This is usually made from expanded polystyrene and is designed to absorb the impact during a fall from a bike. Brain injury and damage occurs when the head incurs sudden deceleration. The padding reduces the rate of this deceleration. Highly ventilated designs may use carbon or alloy strengtheners to boost the overall structural strength required for safety.
Some helmets have 'bee mesh' inside the front set of holes to prevent stinging insects from getting trapped inside.
The thin plastic outer shell keeps the inner shell together in the event of a crash, helping it absorb shock rather than simply splitting into several pieces. Some outer shells are placed into the main shell mould so they bond together forming a more cohesive helmet, while on other models they're glued on afterwards.
Common extras on the outer shell include reflective stickers, while a few helmets have provision for a rear LED light. For 2007 one helmet manufacturer introduced a mount for helmet cams and lights. With the popularity of DIY mountain bike videos and night riding this is an idea we expect to see more of.
Head retention system
Many helmets supplement the straps with a rear harness that grabs the occipital bone at the back of the skull to stabilise the helmet. These head retainers are adjustable which means that some helmets come in one, universal, size only. A ratchet or a winder is used to adjust a chord or a grooved strap to allow for varying head sizes and accommodates skullcaps, etc, in the winter.
Manufacturers have different names for these devices - such as RocLoc 4, ProFit 2 and Safe-T X - but they all do the same job.
The straps are designed to keep the helmet on the head and in the correct position (without letting it rotate back to expose the vulnerable forehead). All straps are adjustable and excess can be cut off and flamed to prevent fraying.
Some helmets have locks under the ear 'Y' piece to keep straps in order while others have a protective chin sleeve, designed to prevent skin irritation.
The chin strap should be worn so that it's firm, without flapping or pinching, and fits snugly around the ears. Ideally the fastener should be easy to do up and undo even when wearing gloves.
Vents are there to keep your head cool even as you get hot. Airflow is designed to enter through the front of the helmet, pass over the head and then pass out the rear. More vents don't necessarily make a helmet cooler; how the air flows through the helmets is what matters. While a very well-ventilated helmet might be a bit chilly in the winter, it's hard to complain about good airflow; you can always wear a skullcap or the like when it gets chilly.
A visor is useful for keeping the sun out of your eyes and for deflecting rain and snow in poor conditions. Because visors are associated with mountain biking, road helmets tend not to have visors, or if they do they're detachable while mountain bike lids do have visors - although they are also sometimes detachable. From this you can deduce that some helmets are designed with all-round use in mind.