It is obvious that both 'recycle' and 'bicycle' have a common element 'cycle'. 'Cycle' on its own is pronounced with a long 'I' sound /aɪ/. 'Cycle' in 'recycle' is pronounced how 'cycle' is normally pronounced. That is, with a long I /aɪ/. But the 'cycle' in 'bicycle' is pronounced with a short 'I' sound /ɪ/ (like kit, hit, sit). In phonetic notation:
Why does this happen? Can someone give some information?
This has to do with English stress patterns and prosody. English stress patterns are enormously complex and have many, many, many exceptions.
'Bicycle' has a short vowel in its second syllable because the prefix bi- is a stress-bearing prefix and can take primary stress (prominence). So when the primary stress moves from the cy to bi-, the cy gets unstressed and the diphthong [aɪ] gets shortened to [ɪ].
The diphthong in the second syllable of 'recycle' doesn't get reduced because the prefix re- doesn't often take primary stress, so the primary stress remains on the first syllable of -cycle.
There are two main kinds of syllables on the basis of loudness; stressed and unstressed. A stressed syllable is the prominent/loudest syllable in a word. Not all syllables in English words receive the same level of loudness (i.e. all syllables in a single word cannot have primary stress).
There are three levels of stress in English:
In the IPA, primary stress is represented by a preceding superscript vertical line [ˈ] while secondary stress by a preceding subscript vertical line [ˌ]. So the word pronunciation can be transcribed as [prəˌnʌnsiˈjeɪʃn]: the second syllable has secondary stress and the second last syllabl has primary stress.
The word 'cycle' is stressed on the first syllable and is pronounced /ˈsaɪkəl/ (with the diphthong [aɪ]).
The diphthong [aɪ] (as in bite) rarely occurs in an unstressed syllable. It has a systematic relationship with the short vowel [ɪ] (as in kit). This relationship is reflected in many words such as f[aɪ]nite - inf[ɪ]nite and many other remnants of Trisyllabic Laxing; div[aɪ]ne-div[ɪ]nity, der[aɪ]ve-der[ɪ]vative, and in dr[aɪ]ve-dr[ɪ]ven etc.
From this relationship, we can infer a general rule of thumb that [aɪ] will only occur in a stressed syllable and when that syllable gets unstressed, [aɪ] will get reduced to [ɪ].
The reason why 'bicycle' is pronounced /ˈbaɪsɪkəl/ is that the prefix bi- is a stress-bearing prefix and can take primary stress. Also note that English has a strong tendency towards antepenultimate stress. It means that English words tend to be stressed on the antepenult (third-last syllable).
'Cycle' has primary stress on its first syllable and when the prefix bi- is prepended to it, the primary stress moves from cy to bi-, leaving the cy unstressed. And as I said above, when a syllable having [aɪ] gets unstressed, the diphthong [aɪ] usually reduces to the short vowel [ɪ]. That's what happens in bicycle.
Re- on the other hand does not often take primary stress1, so when it's prepended to cycle, the primary stress remains on the cy. That's why the re- remains unstressed and the cy becomes the prominent syllable.
However, the cycle in unicycle, quadricycle, motorcycle is pronounced with the diphthong [aɪ]. It has been explained in this answer on ELU [Modified]:
motorcycle, quadricycle and unicycle consist of a pair of trochees: MO-tor-CY-cle, QUAD-ri-CY-cle, U-ni-CY-cle. The primary stress in these words falls on the first syllable, but the secondary stress falls on the first syllable of -cycle. In this context the vowel retains its full pronunciation i.e. it still receives stress, so the diphthong doesn't get reduced.
Notes:
There's often not much point in asking why a word has a particular pronunciation. There's rarely any logic or pattern.
Recycle is a word recently created (1920s, but only being used in its modern meaning in the 1960s). It means "cycle again" and the suffix "-cycle" has the same meaning as the word (meaning to go around).
Bicycle is created on a pattern created by the earlier "tricycle" in the sense of a three-wheeled carriage. Its pronunciation was influenced by French, or rather tricycle was a word created in French and borrowed into English, and then anglicised. Bicycle has a different stress pattern, and the suffix "-cycle" means "wheel", so a bicycle was originally an adjective "a bicycle velocipede" not a noun (the verb "to cycle" meaning to ride a bicycle is a back-formation from bicycle). So "cycle" is not really a common element. "Bicycle" is also 100 years older than recycle, so has had 100 more years for the pronunciation to be modified. The difference in stress results in different vowels.
So the reasons are partly random, partly historical. Essentially, they are different words which are pronounced differently, despite the similarity in spelling.
Short Answer: In English, we often stress the first syllable of nouns; the second syllable for verbs. "I went to the store and bought a RECord." "I went to the studio to reCORD my new album."
BIcycle is a noun; reCYCLE is a verb.
