More than 50,000 bicyclists suffered injuries in motor vehicle crashes in 2009, says a 2010 report on traffic safety facts from the National Highway Traffic Safety Administration. While bicycle helmets don't prevent crashes, they can absorb some of the impact that would have otherwise directly hit your skull. Your bike helmet loses some of its effectiveness, however, if you wear a hat or visor beneath it.
It's not a safe idea to wear a baseball cap, visor or anything else for that matter under your bicycle helmet. The Bicycle Helmet Safety Institute, a non-profit organization for bicycle helmet safety information, warns that hats and visors compromise the helmet’s fit and safety. Helmets are meant to fit snugly against your skull without anything between your head and the helmet’s inner harness. The cap and visor’s sweatbands, bills, and thickness make the helmet sit higher on your head than it's meant to and can leave parts of your head without proper protection. They pose additional injury risks with the hard metal fastener at the top of the cap and any hard fasteners on the visor. The same warnings apply to beaded braids, head scarves, or other bulky hairstyles and accessories.
If the baseball cap is meant to protect your scalp from the sun, you have a few other options that are safer beneath a bicycle helmet. Bald bicycle riders can opt to apply a sports formula, sweat-resistant sunscreen directly on their naked scalps, although even sunscreens that resist water and sweat need to be reapplied every two hours. If you want scalp protection through light or thin hair, go for a gel sunscreen that works well in hairier areas. Although the bike helmet covers much of your head, damaging sun rays still filter through the helmet’s vents and even in cloudy weather.
If the visor is meant to shield your eyes and face from the sun, consider some safer options. Some bicycle helmets come with built-in visors, which are often removable by snapping on and off. Snap-on visors sold separately or even bicycle helmet face shields are other options, although they pose a few of their own risks. The Bicycle Helmet Safety Institute says visor-related injuries are few and far between, but still include the visor’s hard plastic shattering, the edges cutting the rider, or the visor snagging on something during impact rather than flipping off as they are meant to do. Sunglasses help with sun protection.
A properly fitted bicycle helmet sits level on your head and is snug enough to stay securely in place without making you uncomfortable. An adjustable inner harness and different helmet sizes help you fine-tune the fit. Bicycle helmets come in children and adult sizes and sometimes have additional small, medium and large options. Opting for a larger helmet just to fit a baseball cap or visor is not a tricky way to get away with wearing a cap or visor beneath your helmet. Any size helmet will still sit too high atop your head with the additional bulk of the cap or visor and a helmet that is too large will be too loose and roomy to properly protect your skull.