From ibikeu Wiki

Bike propaganda is a term which refers to any material, printed or otherwise, which promotes cycling, speaks to the needs of cyclists, or conveys any positive message about cycling. Bike Propaganda can come in the form of fliers, postcards, spokecards, or even bumper stickers (although this has an element of irony to it.)

Forms

As the bicycle engages the human body in a certain manner, is also offers semiotic opportunities.

The bicycle is a sign for a bicycle (i.e. not a car.) It can be a sign for a specific bicycle (Racing, Mountain, Fixed, Female), or it can signify its (high/low) monetary value, or its technological standard, its age, its make, etc. Customizing the message of the bike with stickers is widespread, and a communicative practice also common with motorized traffic (bumper stickers). Spoke cards (laminated paper, painted laminate, sometimes with route details) issued by certain group rides are inserted between the spokes of the wheels. In some circles spoke cards denote a rider who has participated in certain rides. They might also denote a rider who has mastered the higher levels of managing his bike as a semiotic object.

Event fliers are often of a small format (4 or more to a letter sized sheet), and occasionally made available on the web, which can then be printed in a decentralized manner. The distribution of event fliers is of great interest because it by hand from cyclist to cyclist, or, more often, from cyclist to bicycle. That is, the flier is being attached in some manner to a bike parked anywhere. This form of leafleting is a highly effective manner of marketing, and, in some cultures, is a crucial structure in growing the community of cyclists. In this activity the bicycle becomes the mailbox of the cyclist.

Bicycle hangers which feature a hole to hang the message on the bike (pizza advertisement) have also been used.