There are foldable helmets, even smaller than this one: https://www.amazon.com/Bicycle-Foldable-Closca-Electric-Patented/dp/B01ENTUWG0/ https://www.amazon.com/B

There are foldable helmets, even smaller than this one: https://www.amazon.com/Bicycle-Foldable-Closca-Electric-Patented/dp/B01ENTUWG0/ https://www.amazon.com/Bicycle-Foldable-Closca-Electric-Pate... Also if it's locked to the bike you would break the lock and steal the bike. The helmet is the cheaper part. Helmets are often designed so locks can go through them.