Proper fit is a must
Your helmet should neither dig into your scalp nor flop around your head. The more snug the fit, the better chance your lid has of staying put on your head when you don't stay put on your bike.
To ensure proper adjustment, start with the interior pads. Most are held in place with adhesive or Velcro, which can be added or removed to customise the fit, or replaced when they become grubby.
Next, adjust the straps so that each v-shaped buckle is positioned directly below your ears; then tighten the main buckle so that two fingers can barely fit between your chin and the strap.
Most helmets also have an adjustment device at the back - fiddle with yours until the helmet is comfortably tight. To check the fit, push up the front of your helmet. If it moves more than an inch, tighten the straps and adjust the back.
The Styrofoam layer between the shell and your head will be your best friend in a crash.
This expanded polystyrene interior crushes under direct impact, absorbing some of
the energy and giving you about 6/100ths of a second extra stopping time, which may be the difference between walking away relatively unscathed and receiving a serious head injury.
The foam also distributes the point of impact over a larger area, if the helmet is worn properly. One hit is usually all you get: if you whack your head and crush the foam, the helmet will be useless.
Since the late 1990s, helmets sold in the UK have been required to meet European standard EN1078. Some manufacturers also submit to more rigorous additional testing by the US nonprofit Snell Foundation.
Despite the uniform safety standard, some helmets sold in the UK are far more expensive than others. The main reason for this is vents. The greater the size and the quantity, the better the ventilation. It takes a lot of engineering to design a helmet that's riddled with holes but is also safe and durable.
In addition to the helmet, your hard-earned cash is also paying for the research and development that went into it.
So is that £100 helmet worth the money? Those bigger holes let more air through, so your head stays cooler. Better helmets also use a rigid plastic shell that's moulded to the foam core, rather than being taped on, as with less expensive models.
But the shell's purpose is to keep the foam core intact in a crash, not to absorb the impact itself, so either type is fine. Of course, if you're looking for a shell with more flash, you'll probably be happy to shell out more cash.