The word bōsōzoku is also applied to motorcycle subculture with an interest in motorcycle customizing, often illegal, and making noise by removing the mufflers on their vehicles so that more noise is produced. These bōsōzoku groups also engage in dangerous or reckless driving, such as weaving in traffic, not wearing motorcycle helmets, and running red lights. Another activity is speeding in city streets, not usually for street racing but more for thrills. With many bikes involved, the leading one is driven by the leader, who is responsible for the event and is not allowed to be overtaken. Japanese police call them Maru-Sō (police code マル走 or 丸走), and dispatch a police vehicle to trail any groups of bikes to prevent any possible incidents, which can include riding very slowly through suburbs at speeds of 5–10 mph, creating a loud disturbance and waving imperial Japanese flags, to start fights which can include weapons such as wooden swords, metal pipes, baseball bats and Molotov cocktails. These bōsōzoku gangs are generally composed of people under the legal adult age, which in Japan is 20 years old.
They were first seen in the 1950s as the Japanese automobile industry expanded rapidly. The precursors to the bōsōzoku were known as kaminari zoku(雷族?, "Thunder Tribe"), urban motorcyclist more akin to the British rockers. Many, if not most, bōsōzoku members came from a lower socioeconomic class and may have used the motorcycle gang activities as a way to express disaffection and dissatisfaction with Japanese mainstream society. Many of the most hard-core bōsōzoku would become lower-ranking members of the yakuza after turning 20 years of age.
In the 1980s and 1990s, bōsōzoku would often embark on massed rides, in which up to 100 bikers would cruise together slowly en masse down an expressway or major highway. The motorcyclists would run toll booths without stopping and would ignore police attempts to detain them. New Year's Eve was a popular occasion for the massed rides. The bikers would sometimes smash the cars and threaten or beat up any motorists or bystanders who got in the way or expressed disapproval with the bikers' behavior. Participation in the gangs peaked at 42,510 members in 1982.
In 2004, the Japanese government passed a revised road traffic law which gave the police more power to arrest bikers riding recklessly in groups. With increased arrests and prosecutions, bōsōzoku participation went into decline. As of 2010, police reported that the new trend among bōsōzoku was to ride together in much smaller groups and to ride scooters instead of heavily modified motorcycles. Aichi prefecture was reported to have the highest number of riders, followed by Tokyo, Osaka, Ibaraki, and Fukuoka.
In February 2011, the Japanese National Police announced that membership in the gangs had fallen to 9,064, the lowest number since the collection of data on the gangs began in 1975. The police put the total number of gangs nationwide at 507, down 76 from 2009. Their number in the Tokyo area had fallen from 5,300 in 1980 to 119 in 2012.
Model of a bōsōzoku-style customized Suzuki Gsx400
Bōsōzoku are known to modify their motorcycles in peculiar and often showy ways. A typical customized bōsōzoku bike usually consists of an average Japanese road bike that appears to combine elements of an American chopper style bike and a British cafe racer, for example: over-sized fairings like those found on cafe racers, raised handle bars like those on a chopper. Loud paint schemes on the fenders or the gas tanks with motifs such as flames or kamikaze style "rising sun" designs are also quite common. The bikes will often be adorned with stickers and/or flags depicting the gang's symbol or logo. There are also marked regional differences in motorcycle modifications. For example, Ibarakibōsōzoku are known to modify their motorcycles in an extensively colorful, flashy way. They will often have three or four over-sized fairings in a tower like way in a motorcycle painted in bright yellow or pink with Christmas light–like adornments.
The stereotypical bōsōzoku look is often portrayed, and even caricatured, in many forms of Japanese media such as anime, manga, and films. The typical bōsōzoku member is often depicted in a uniform consisting of a jumpsuit like those worn by manual laborers or a tokkō-fuku(特攻服?), a type of military issued overcoat with kanji slogans usually worn open with no shirt underneath showing off their bandaged torsos and baggy matching pants tucked inside tall boots. Tokkō-fuku in Japanese means "Special Attack Uniform", which is the uniform of the Kamikaze pilots, which in Japanese were called the Tokkō-tai(特攻隊?, "Special Attack Battalion"). The uniforms will most likely be adorned with militaristic slogans, patriotic rising sun patches, ancient Chinese characters, or manji. They will also often wear a tasuki, a sash tied in an X around the torso, a look inspired by Japanese World War II fighter pilots. Leather jackets, often embroidered with club/gang logos, and even full leather suits are also seen as common elements of the bōsōzoku look. Among other items in the bōsōzoku attire are usually round sunglasses, long hachimaki headbands also with battle slogans and a pompadour hairstyle most likely akin to the greaser/rocker look or perhaps because of the hairstyle's association with yakuza thugs. The punch perm is considered a common bōsōzoku hairstyle as well. Surgical masks are also stereotypically worn by bōsōzoku, perhaps to conceal their identities. Females are also shown dressed in a similar style, but dress in a more feminine manner with long and often dyed hair, high-heeled boots and excessive make-up.