1. In the case of impact
The helmet must be replaced immediately after an impact. The purpose of wearing a helmet is to protect the head from one impact and only once. After a fall, even though the helmet appears to be intact on the surface, it may have been damaged or cracked where it cannot be seen. It's very dangerous to ride with a helmet that has been hit once. After all, it can't protect you.
2. Obvious damage
If you can clearly see the damage of the helmet (small cracks, loose belts, parts falling off...), it is very necessary to replace the helmet. Only a good helmet can effectively protect your safety.
3. After several years of use
Depending on your activity and the number of times you travel to wear a helmet, it is recommended that you should replace the helmet before it becomes too old (if the helmet looks good, the basic use time is 5 years). For a helmet that is too old, whose parts and materials will age and will not be as good as other helmets, especially if you place the helmet in a cold or humid place.
4. The athletic project changed
If you want to engage in a bicycle project that is different from the activity you are doing now, you need to change a helmet that is more suitable for the new athletic project, such as custom mountain bike helmets, children's bike helmets.
5. Other possible reasons
There are many other reasons: you may simply want to replace it and want a lighter, more design, more breathable custom bike helmets, and so on.
6. Which size helmet to choose
In general, the head circumference of an adult is 55-59cm, in order to better understand their size, consumers need to measure their own head circumference: use a soft ruler around the forehead to measure the maximum size of the head.