There's a plethora of options and features in bike helmets these days -- some better than others -- but they all perform the same job: to protect your head from damage. Here we'll go over some of the features and options to choose from and try to help you narrow down what might be right for you.
Bike Helmet Types
One-size-fits-all/recreation is the most basic helmet available. They're designed to fit most head sizes with a very adjustable retention system. They use a one-size shell reduces the need for different size shells for different size heads that the more expensive models have. These are the cheapest models available and are very common among beginner riders or riders that don't care to spend big bucks on a lot of features that may not add more protection. These are just as safe as many other models, just made in a cheaper manner.
Road helmets are designed around having a lot of airflow to help keep the rider cool and also keeping the helmet lightweight. Road helmets do not typically have visors, but some allow them as an optional accessory. As road helmets increase in price, usually weight drops, more exotic materials are used, and more venting is implemented.
XC/mountain helmets are often very much like road helmets but usually have a visor. Many XC riders will just use road helmets, as many of the needs are the same. Some of the more trail-specific designs will start having a more rounded exterior to reduce hangups on branches and have longer rear and sides for additional coverage. Visors tend to be a little sturdier, as well, on trail-oriented models.
Enduro/aggressive trail helmets are the next level of protection in MTB helmets, with longer rears for even more head coverage. These helmets are still pretty well ventilated but are more designed for protection than comfort. As price goes up, so does the comfort level in terms of weight and venting.
Full Face Helmets
Full-face helmets are pretty self explanatory; they cover the entire head. These are nearly identical to helmets used for dirt biking and have many of the same features. These are used mostly for downhill or freeride, where the dangers of crashing are a bit higher and where less pedaling is required. Some aggressive enduro races have required full-face helmets, as well, if the trail is especially gnarly. Just like other helmets, the more you pay, the lighter and more breathable the helmets. But some will start adding more protective features as well.
Additional Helmet Features
So what are some of the additional features we keep talking about? Most, if not all, are designed around adding more comfort to your riding experience. Adding safety is one of these aspects which I would say adds to comfort, since if you feel like you're more likely to fare well in a crash you're more comfortable pushing the limits while riding.
Venting is one of the features most commonly added to helmets. The reason this costs more is because it takes additional shaping of the shell and addition reinforcing in specific areas in order to keep the helmet safe while reducing overall surface area. This usually involves some more expensive materials with a higher strength-to-weight ratio to keep weight down, as well.
Low helmet weight increases cost for the same reason, with the use of more expensive materials such as carbon fiber.
Antimicrobial helmet linings are also a common feature among higher dollar helmets. This helps reduce helmet funk from a lot hard days of riding. One of the materials used is X-Static fabric, which uses small amounts of silver to discourage microbial growth that cause bad odors.
Retention systems are usually nicer on higher dollar helmets as well. This usually means that they are easier to adjust and have smaller notches between adjustment clicks, making for a more customizable fit. Some may also move from a half-retention system, tightening just from the rear half, to a full circumference system that tightens from all directions of your head for a nicer fit.
Some of the additional safety features that are included in helmets include variable density foam, inner bracing in the shell and foam, and multi-thickness padding for a better fit to keep the helmet in place when it needs to be there.
All bicycle helmets are required to pass certification testing in order to be sold, which means all helmets are tested to the same level of safety among a given standard (different countries may require different safety standards and testing methods). What varies mostly is the amount of coverage a helmet offers at the cost of how much airflow is allowed for cooling. Find what fits your needs best and start trying some helmets on to see what feels comfortable.