How to cook beef shank so that it is fall-off-the-bone tender?
If your beef shank is chewy, it's undercooked. Period. Whatever the amount of time is, even if it seems like a lot to you, it's not enough. Overcooking will cause meat cuts with a lot of connective tissue (like shank) to dry, and even fall apart because it is too tender. But it will not still be chewy.