Force dissipation means that when a helmet bangs against something like the ground, that energy is managed by the helmet components to decrease the amount that is transmitted to your scalp, skull, brain and in part, spinal column and spinal cord. Moreover, a helmet cannot protect what it does not cover; seems simple and obvious but important in decision-making.
In a crash, the polystyrene is permanently crushed, so if you hit your head with your motorcycle helmet on, the helmet is no more use to you than a baseball cap. The helmet may look fine externally, save for a couple of small scratches and dents, but it probably no longer has impact-absorption in the particular area of impact.
Ok, now let’s talk about the brain.
Your brain has three main parts — the cerebrum, the cerebellum and the brain stem. The cerebrum is the top part of your brain. The uppermost layer (the cerebral cortex) is where all of the nerve cells have their cell bodies, with the rest of the cerebrum composed mostly of extensions from those cell bodies. Specialised groupings along the surface of your brain control different functions, like movement of your head, arms or legs, as well as different
parts of your personality, and how you interpret what you see. Your brain in encased in its own “helmet” — your skull.
Half-helmets generally cover the top part of your brain (cortex) but not the cerebellum and the brain that is essential in balance, while the brain stem keeps you alive. It is important to understand that the cell bodies in the cortex of both the cerebrum and the cerebellum pass their extensions (neurons or nerves) through the brain stem. Therefore, even very tiny injuries to the brain stem can damage huge areas of the rest of the brain as the nerves are very tightly packed.
Three-quarter helmets cover all of these areas but not your face below your forehead. Some have faceplates that snap on or rotate into place. These will at least protect you from bugs and road debris, as well as the cigarette flicked out of the car ahead of you. but their structural strength in terms of protecting the bony structures of your face are less than what you would have in a full-face helmet.
Full-face and to a somewhat (minimally) lesser extent, modular helmets, distribute this force throughout the helmet, reducing the amount that gets to your brain and keep you from tearing off parts of your face during a face-down slide.
Choose a helmet that fits your needs, your riding style and your personal risk-benefit ratio. If you choose to ride without a helmet, enjoy the breeze — and the bugs — but please, at least wear glasses or goggles with UV blockage and sunscreen and remember, regardless of how much you spent on it, replace your helmet every 5 years.