Bi- and Di-

We walked around in the campus after lunch. I saw a bicycle parking rack with "Bicycles" sign. Hmmm, bicycles = bi + cycles, i.e. a thing with 2 cycles.

Then I am wondering why is bi-cycles, why not di-cycles? When will people use bi- or di- to describe "two", "twice"?
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Definition of bicycle from dictionary.com
1868, coined from bi- "two" + Gk. kyklos "circle, wheel" (see cycle), on the pattern of tricycle; both the word and the vehicle superseding earlier velocipede. Probably not from Fr., though often said to be. The confusion apparently is because Pierre Lallement, employee of a Fr. carriage works, improved Macmillan's 1839 pedal velocipede in 1865 and took the invention to America. See also penny-farthing.

I list 3 definitions from dictionary.com about bi-
(1) bi- 
a combining form meaning "twice", "two", used in the formation of compound words: bifacial; bifarious.

(2) bi- or bin-
pref.
1.
a. Two: biform.
b. Both: binaural.
c. Both sides, parts, or directions: biconcave.
d. Occurring at intervals of two: bicentennial.
e. Usage Problem Occurring twice during: biweekly.
f. Containing twice the proportion of a specified chemical element or group necessary for stability: bicarbonate.
g. Containing two chemical atoms, radicals, or groups: biphenyl.
2.
a. Occurring at intervals of two: bicentennial.
b. Usage Problem Occurring twice during: biweekly.
c. Containing twice the proportion of a specified chemical element or group necessary for stability: bicarbonate.
d. Containing two chemical atoms, radicals, or groups: biphenyl.
3.
a. Containing twice the proportion of a specified chemical element or group necessary for stability: bicarbonate.
b. Containing two chemical atoms, radicals, or groups: biphenyl.

(3) Bi-
Bi-\ [L. bis twice, which in composition drops the -s, akin to E. two. See Bis-, Two, and cf. Di-, Dis-.]
1. In most branches of science bi- in composition denotes two, twice, or doubly; as, bidentate, two-toothed; biternate, doubly ternate, etc.
2. (Chem.) In the composition of chemical names bi- denotes two atoms, parts, or equivalents of that constituent to the name of which it is prefixed, to one of the other component, or that such constituent is present in double the ordinary proportion; as, bichromate, bisulphide. Be- and di- are often used interchangeably.

I list 3 definitions from dictionary.com about di-
(1) di- 
a prefix occurring in loanwords from Greek, where it meant “two,” “twice,” “double” (diphthong); on this model, freely used in the formation of compound words (dicotyledon; dipolar) and in chemical terms (diatomic; disulfide).

(2) di-
pref.
Two; twice; double: dichromatic.
Containing two atoms, radicals, or groups: dichloride.

(3) Di-
Di-\ [Gr. di`s- twice; akin to two, L. bis twice. See Two, and cf. Bi-, Dia-. The L. pref. dis- sometimes assumes the form di-. See Dis-.] A prefix, signifying twofold, double, twice; (Chem.) denoting two atoms, radicals, groups, or equivalents, as the case may be. See Bi-, 2.