Originally Posted by
DrIsotope
Syllables, and what syllable the stress falls on.
bi-sick-uhl
tri-sick-uhl
mo-tor-sigh-kuhl
That's not really a good explanation because one can stress or not stress a long "i" sound. There's definitely a vowel shift in "bicycle" and "tricycle". My guess is that the repeated long "i" sound just sounds awkward to most people.
On the other hand, the word "cyclical" is pronounced with a long i sound by the British, and a short i sound by Americans, but it doesn't come up in conversation very often.