Sibling comment to yours points to a relatively recent (this century) article with a mostly complete theoretical model for bike self-stability. There are other

Sibling comment to yours points to a relatively recent (this century) article with a mostly complete theoretical model for bike self-stability. There are other theories though, some more or less developed than others. It turns out to be a fiendishly hard control-theory problem, and at least one aspect is chaotic. Which theory is correct has not, to my knowledge, been definitively determined by experiment. Until it is, I think it is fair to say that it is unsolved. Unlike lift, which is very well understood but often poorly explained.