Relatively speaking, the lighter the bike helmet you choose, the better. But the lighter the cycling helmet, the less chance it has to pass the national safety certification test standard, and the higher the price. The premise of choosing a helmet is that it must pass the test standard of national safety certification, and most helmets around 200 yuan can do about 260G. This weight won't tire you out even over long distances, so don't worry too much about the weight of the helmet. Unless you're trying to minimize your weight for a bicycle race, it's not worth spending the extra money on a super light helmet.
Breathability is the key to choosing custom made bike helmets that will keep your head dry on long rides. The more or larger the vents on your helmet, the more airflow around your head and the cooler you'll feel. However, please keep in mind that the more or larger the ventilation holes in your helmet, the more your head is exposed, and therefore the less protection you will get. A road bike helmet will have more or larger ventilation holes than a mountain bike helmet.
You may only know that bike riding helmet is subjected to some impact testing, but the buckles and straps are also subjected to the appropriate tension test, and if they don't pass the appropriate tension test, they can't be used on helmets. Therefore, a qualified riding helmet, which a small buckle and strap must pass the corresponding test, is a helmet that truly meets national safety certification testing standards.
If the helmet cracks, don't use it, it's just a consolation to wear it. It doesn't have much substantive use. It doesn't matter if it's a small bump, but once you fall, the force is so great that it won't protect you. Don't underestimate a crack that has physically structurally altered a helmet's force and force distribution. To put it in a simpler way, you pull a brand new piece of A4 paper hard from both ends, it may take a bit of effort to break it, but once you tear a little slit in the middle of the paper, it's easy to tear the paper.