Riding a bike increases a Chaos Space Marine's Toughness by +1. The bike is generally fitted with twin-linked bolters, but these can be upgraded to a meltagun or a flamer at an additional cost of +10 pts each. Characters that have a Retinue or are otherwise part of a unit may only be given a bike if all of the models in the unit also have bikes. Chaos Space Marine bikes are fitted with scythes and blades which add +1 to the rider's Attacks characteristic.
Chaos Space Marine Bike
Source: Codex: Chaos Space Marines (1999), p. 20