Comparative form of adjectives

[Aisylu Karimova]

The rule says: If the one-syllable adjective ends with a single consonant with a vowel before it, double the consonant and add –er for the comparative form; and double the consonant and add –est for the superlative form.
Now question: The words clean and cheap apply to that rule as well, as long as they also have one syllable, end with consonant with vowel in front. Why don't we double the last consonant in these words? Is there any other rule for that?

[bhaisahab]

The rule says: If the one-syllable adjective ends with a single consonant with a vowel before it, double the consonant and add –er for the comparative form; and double the consonant and add –est for the superlative form.
Now question: The words clean and cheap apply to that rule as well, as long as they also have one syllable, end with consonant with vowel in front. Why don't we double the last consonant in these words? Is there any other rule for that?

The rule is that if the last three letters are, consonant / vowel / consonant then you double the final consonant. For example, "red/redder/reddest; flat/flatter/flattest.