Irrlicht Engine

You're not kidding about the tweaking, takes hours to get everything just right.

Newton seems like a good option, but like Rogerborg pointed out, IrrNewt is no longer updated. I have recently started working with it, very easy to use and it auto calculates a lot for you.

Recompiling the IrrNewt source for Irrlicht 1.3.1 is easy, just one edit required. I've only had one issue ...