Attack Bike's are single squad scout vehicles which are produced in Chapel-Barracks and the Orbital Relay. Early on in game, they are useful for scouting and hit and run attacks. Bike in, throw a grenade and then get away. After tier 1 they can be reinforced up to a squad size of 3 and have 3 weapon upgrades; a heavy bolter which adds a sidecar with a heavy bolter attached, a plasma gun or a flamer. This can make them a staple combat unit especially if you're playing as White Scars.