I was talking about the impulse based method of handling rigid bodies.
As for constraints as springs and joints. They are easy to implement. You already have them. Springs are what keeps your verlets together. If you want flexible sticks, just make them contract over several frames. If you want hinge joint, just fuse a couple of non colliding verlets together by using a stiff spring with normal lenght of zero and voila!
Angular constraints are just springs working only one way.. they keep points from coming too close, but don't restrict movement to the other way.
In my 2d engine... these were easy to implement, but friction was hard, as I noticed you are strugling with it also. After a few days of trying I managed to come off with very good friction modelling, but in your case (the tires) it's much more difficult. Good luck.
As for constraints as springs and joints. They are easy to implement. You already have them. Springs are what keeps your verlets together. If you want flexible sticks, just make them contract over several frames. If you want hinge joint, just fuse a couple of non colliding verlets together by using a stiff spring with normal lenght of zero and voila!
Angular constraints are just springs working only one way.. they keep points from coming too close, but don't restrict movement to the other way.
In my 2d engine... these were easy to implement, but friction was hard, as I noticed you are strugling with it also. After a few days of trying I managed to come off with very good friction modelling, but in your case (the tires) it's much more difficult. Good luck.