1. The Three Parts of a Bicycle Helmet and the White Layer Inside the Shell
A bicycle helmet is made of an outer shell, an inner shock-absorbing liner, and a chin strap. A report from Tokyo explains that when you fall, the shell and liner dent or break. This absorbs the energy of the impact and reduces damage to your head [1]. The key point is that the helmet does not bounce the shock back. Instead, it changes shape to accept the hit. The part that changes shape the most is the liner. Expanded polystyrene, often used in liners, is made of many tiny bubbles. It is said that about 98% of its volume is air, and only 2% is material [1]. This makes it easy to crush and good for cushioning. However, this is a general value for polystyrene; the density of helmet liners differs for each product.
Safety Note: The Tokyo report states that the shell and liner dent or break to absorb impact energy and reduce head damage [1].
2. How the Helmet Liner Makes Your Head Take Longer to Stop
When your head hits hard ground, it stops very quickly. The shorter the time it takes to stop, the greater the force on your head. The helmet liner extends this stopping time slightly by crushing, which softens the force on your head. The Bicycle Helmet Safety Institute (BHSI) in the US explains that the liner crushes, turning some collision energy into heat, and extends the time until the head stops. In their example, the head stops in about 6 milliseconds [1]. 6 milliseconds is 0.006 seconds. This number is a guideline from that page and is not the same for every crash. Think of catching a baseball. If you pull your hand back as you catch the ball, it hurts less. The liner works like that, but unlike your hand, it cannot spring back. It buys time by crushing itself. How much the force is reduced depends on the foam's properties and thickness [1].
3. How Drop Tests Measure Impact Acceleration on a Model Head
We cannot test helmets on people. So, in tests, a helmet is put on a model human head and dropped onto a metal stand called an anvil. This measures the acceleration of the impact [1]. According to the Tokyo report, the SG standard for bicycles requires the impact acceleration to be 2,940 m/s² (300G) or less. It also requires that the time spent above 1,470 m/s² (150G) be 4 milliseconds or less [1]. 1G is the acceleration of gravity. 300G means an acceleration 300 times that of Earth's gravity. This means the standard looks at both the magnitude of the acceleration and whether the high acceleration lasts a short time. This matches the idea from the previous section: extending the stopping time. Note that this is an excerpt from the SG standard; JIS and EN standards have different test conditions. The same report says there was a big difference in shock absorption between helmets without bicycle standard labels and those with them [1]. Also, computer simulations show that when a head hits a curb, wearing a helmet lowers the pressure on the brain [1]. This is a calculation result, not a physical measurement.
4. Why a Helmet Protects Less When Hit Three Times in the Same Spot
In Tokyo's tests, a bicycle helmet meeting the SG standard was dropped three times from a height of about 1.1 m (about 3.6 feet) onto the same spot (the forehead). With each impact, the shock-absorbing performance dropped [1]. Reading the report's graph, the impact acceleration rose to about 119G, then about 297G, and finally about 590G. The third hit was about five times the first, and about twice the 300G limit of the SG standard. However, this is read from the graph of one sample and may not apply to all helmets. BHSI explains that many hard foams lose their ability to soften shock when the bubble walls crush. So, they recommend replacing the helmet after an impact [1]. This applies to "most hard foams," not all materials. Once you understand that the "almost all air" layer works by crushing, you see why it is recommended to replace a helmet after a big impact.
5. Why Hitting at an Angle Can Twist the Head and Brain
If your head hits at an angle, the brain may keep moving or stretch even after the head stops suddenly. Professor Svein Kleiven at KTH Royal Institute of Technology in Sweden explains that rotation stretches brain tissue the most and can lead to severe injury [1]. To reduce this rotation, there is MIPS. It places a low-friction slip layer inside the helmet. When a sideways impact happens, the helmet slides slightly against the head, reducing the rotational movement passed to the brain [1]. The three developers of MIPS received the Polhem Prize in 2019 [1]. Note that MIPS is not a certification system but the name of a feature in products. Since the numbers on its effectiveness come mostly from company explanations, we will stick to explaining how it works here.
6. Using Foam Food Containers to See That Foam Does Not Bounce Back
You can use clean, dry food trays or the edges of expanded polystyrene boxes. They are not the same as helmet liners, but they help you understand how "it doesn't bounce back when crushed." Slowly press the edge with your thumb to make a dent. Let go and see if it returns. Then, press the same spot again and compare if it feels different. Whether you feel a difference depends on the tray's material and thickness, so do not force a result. When choosing a helmet, look for a label showing it meets bicycle standards, and check whether the helmet has had a major impact. The primary source is the summary of the Tokyo report, which explains how to read the graphs [1].
Safety Note: Do not get cut by sharp edges. Look at the edges before using them, and do this with an adult. If you want to read more, start with the summary of the Tokyo Consumer Product Safety Council report [1].