You do not need to assume zero preliminary knowledge of the reader. You ask a question to get it answered - you need to be helped in the first place. This means that you can assume that the reader has at least as much knowledge on the topic as you do (otherwise he cannot answer).
However, it should be clear to the reader what knowledge you have, so that he can explain it on your level, adding or leaving out details that may (not) be needed for you, specifically. When asking a question, keep in mind that the person you're asking cannot look into your head - this is even more so on the internet.
Ideally, questions are useful for people searching later, but this is not the primary goal. (SE used to have a 'too localized' close reason for questions that were not going to help anyone else, but it has been removed a couple of years ago.)