Originally Posted by
wphamilton
The bike is self-stable if it naturally turns into the lean.
I think trail is the main thing. The wheel contacts the ground behind the steering axis - how far behind, that's the trail. When the bike is leaning a bit to one side, the ground pushing on the wheel will push the back part of the wheel up, which makes the front part of the wheel turn down toward the ground, i.e. into the turn.