Anyone who wears a cycle helmet tilted far enough back that they can slap the palm of one hand against their forehead should do just that – whilst saying 'd'oh!' Because they're doing it wrong. To work as intended in the event of a fall from your bike, a helmet needs to fit correctly, be adjusted correctly, and be worn correctly. The brim of the helmet should be no more than a couple of fingers' width above your eyebrows, so that it covers at least half of your forehead.
Find the right size
Adult cycle helmets come in only a handful of sizes, such as small, medium, and large. Some are one-size-fits-all – or at least one-size-fits-most. The fit can be fine-tuned using straps and internal padding, but it's worth measuring the circumference of your head anyway, if only to save time when you go to the shop to try on different models. Hold the tape measure horizontally around your head, above your ears and 2-3cm above your eyebrows. Your head is probably 50-something centimetres, with men having larger heads than women on average.
Try it on
Knowing that a helmet will fit on your head won't tell you whether it's comfortable. Go to your local bike shop and try on different helmets. You may be borderline between sizes. Some helmets fit some head shapes better than others. Or you may prefer the strapping arrangement of one helmet over another.
You may also prefer the styling of one helmet over another. Apart from the fact that women's helmets tend to be smaller, that's the principal difference with gender-specific designs: pastel colours. A few women's helmets have extra room at the back to thread a pony tail over the rear ratchet-strap but that's not critical even if you do have long hair. If the helmet fits and you like the look of it, wear your blokey or girly helmet with pride!
Fine-tune the fit
Make sure any foam pads inside the helmet are Velcroed firmed in place; there may be a choice of thicknesses. Most helmets have a ratchet strap of some kind that cradles that back of your head. Adjust this until the helmet feels snug but not uncomfortable. Leave the chin strap undone for now.
Now shake your head briskly from side to side, like you're indicating 'no'. The helmet should move with your head and not come loose. Then, without shaking your head, bend over so the top of your head is pointing at the floor. Even with the chinstrap undone, the helmet should fit snugly enough that it stays on. If it does fall off, the ratchet strap needs tightening or you need thicker helmet pads... or a smaller helmet. Repeat these tests until you find a helmet that stays where it's meant to and that still feels comfortable.
The straps at the side of the helmet should meet just under your earlobes. The chin strap should be loose enough that you can yawn without it digging in, but snug enough that you can get only a finger or two underneath. It can take a while to sort this out, as you faff with sliders and strap lengths. Take this time. You need to get it right so that the helmet won't move about on your head. Be aware that the straps can work loose over time, so you may need to readjust them later.
Insulation, perspiration, precipitation
A helmet with lots of ventilation is better for riding hard in the heat; a helmet with less ventilation is better for riding at a steady pace in the cold. You could get two helmets, or you could weatherproof a well-ventilated helmet.
For colder days, a Buff, bandana or traditional cotton cycling cap should fit fine underneath your helmet. You'll need to loosen the ratchet strap at the back a little. A cycling cap is especially useful in rain, hail or snow. As the peak sits low over your eyes – lower than most helmet peaks – you won't be constantly blinking away rainwater or ice or having your glasses go all blurry.
Wearing something under your helmet is a wise precaution in the sun if you're balding. Again, Buffs, bandanas and cycling caps work fine. If it's really hot, dampen the Buff/bandana/cap with water first, to give a better cooling effect as air passes over it.
Lights, cameras - caution
When a helmet hits the ground, the plastic outer layer has two jobs: one, to distribute the impact to a larger area of expand polystyrene beneath; and two, to skid over the ground rather than grab at it. If the helmet snags and spins your head around, this can make neck or rotational brain injuries worse. So it's safest if anything on the outer surface of the helmet can snap-free in a spill. Most visors are designed to do this. Some cameras and lights have mounts designed to do the same.
The foam pads in helmets absorb sweat. Take them out and hand-wash them if they start to niff. If the helmet itself gets dirty, you can hand-wash that too, even immersing it. Just be sure to let it to dry out naturally.
Exposure to sweat and sunlight slowly damages helmets, reducing their effectiveness. Manufacturers recommend getting a new one every three-to-five years. Any helmet that receives a significant blow should be replaced straight away. Some manufacturers even offer a crash replacement policy, which is worth asking about when you buy.
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