system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why isn't the -cycle in bicycle and tricycle pronounced the same as the -cycle in the unicycle and motorcycle? Aren't they all cycles and not "sickles"?
gpt
It's all about English foot structure. What's a foot? The short answer is a foot is a two-syllable unit where one of the syllables is stressed. English is by and large organized into trochees, which are feet with stress on the first syllable. (Think words like *happy* or *pickle*; they're stressed on the first syllable)

English is a "quantity-sensitive" language, which means we also organize syllables into "heavy" or "light". Heavy syllables attract stress, and unstressed syllables are likely to be reduced (made light). This brings us to the -cycle words.

Notice the first two you mention are 3-syllabke words, and the others are 4-syllable words. In both cases, we would probably analyze them as consisting of two feet (although the former are going to have a *degenerate* half-foot of only one syllable). The key difference is that in the 3-syllable word the first syllable of -cycle is the tail end of the previous foot (the unstressed part of the foot), but in the 4-syllable case it's the front of a new foot (the stressed part). 

* bicy-cle
* tricy-cle
* uni-cycle
* motor-cycle

Because English is quantity-sensitive, we don't like a heavy syllable in an unstressed position, so the heavy *cy* becomes light in the 3-syllable words. (Some more background: *cy*, pronounced /sai/ has a diphthong, which is like two vowels mashed into one unit, and this is considered heavy. Single lax vowrls are considered light, so this is what the diphthong turns into when it's unstressed in these words)

So it's kind of an interesting interaction between several features of English: trochaic feet parsed from the left edge, quantity-sensitivity, and vowrk reduction.