pronunciation - Why is "bicycle" pronounced differently from other obviously related words?

The word bicycle is pronounced /'baɪsɪkəl/ (bahy-si-kuhl), like sickle. However, the words unicycle and motorcycle both have the -cycle pronounced as /-'saɪkəl/ (sahy-kuhl). Is there some sort of reason for this, or is this just a vagary of English pronunciation?

Answer

Although such variation could be the result of things like when the word was borrowed into the language, this variation is probably due to the prosodic structure of the words; we get different vowel sounds because of the way that stress influences vowel quality in English.

In English, unstressed vowels are generally reduced. Take the word record for example.

• 
  

  The noun form takes first syllable stress: [ˈrɛ kɚd]. If you aren't familiar with IPA, note in particular the [ɛ] vowel and the [ɚ] r-colored schwa vowel. Schwa (and sometimes [ɪ]) is what often shows up in reduced, unstressed vowels in English. Since stress is on the first syllable, we get the r-colored schwa in the second syllable.

  
  


• 
  

  The verb form takes second syllable stress: [rə ˈkoɹd]. Note that the now-unstressed [ɛ] was reduced to [ə], schwa, while the [ɚ] now retains the full [o] sound (with an [ɹ] "r" after it).

  
  

Now for bicycle and unicycle:

• 
  

  bicycle [ˈbaɪ sɪ kəl] has primary stress on the first syllable. The "cy" syllable is unstressed, and so it is pronounced as a mere [sɪ] (or even [sə] by some). Note that tricycle has the same stress pattern, and has the same vowel.

  
  


• 
  

  unicycle [ˈju nɪ ˌsaɪ kəl], being a longer word, has multiple stressed syllables. Primary stress, again, falls on syllable #1, but the important thing is that secondary stress falls on syllable #3, the "cy" syllable. This means that "cy" is pronounced as a full [saɪ] instead of a reduced [sɪ]. Note that motorcycle has the same stress pattern and, again, same vowel.