The Chicago Municipal Code states:
8-8-080 Indecent exposure or dress.
Any person who shall appear, bathe, sunbathe, walk or be in any public park, playground, beach or the waters adjacent thereto, or any school facility and the area adjacent thereto, or any municipal building and the areas adjacent thereto, or any public way within the City of Chicago in such a manner that the genitals, vulva, pubis, pubic hair, buttocks, perineum, anus, anal region, or pubic hair region of any person, or any portion of the breast at or below the upper edge of the areola thereof of any female person, is exposed to public view or is not covered by an opaque covering, shall be fined not less than $100.00 nor more than $500.00 for each offense.
The Illinois Criminal Code states:Sec. 11-9. Public indecency.
(a) Any person of the age of 17 years and upwards who performs any of the following acts in a public place commits a public indecency:
(1) An act of sexual penetration or sexual conduct
as defined in Section 12-12 of this Code; or
(2) A lewd exposure of the body done with intent to
arouse or to satisfy the sexual desire of the person.
Breast-feeding of infants is not an act of public indecency.
(b) "Public place" for purposes of this Section means any place where the conduct may reasonably be expected to be viewed by others.
Public indecency is a Class A misdemeanor. A person convicted of a third or subsequent violation for public indecency is guilty of a Class 4 felony.
While Chicago tends to be a "no harm, no foul" city, while biking in the buff you are exposed (pun intended) to the whim of the police who may choose to enforce the law.