added angular limits to the Generic D6 constraint. Works for small angles. Will add a check for different combinations, and use different extraction of ordering of rotation from the diff quaternion.

Improved vehicle interpolation of wheels, and added Z-up axis option for the Demo
made 'getWorldTransform' const method in btMotionState
added future 'deactivationCallback'(not used yet)

src/LinearMath/btDefaultMotionState.h

@@ -19,16 +19,25 @@ struct	btDefaultMotionState : public btMotionState
 	}
 
 	///synchronizes world transform from user to physics
-	virtual void	getWorldTransform(btTransform& centerOfMassWorldTrans )
+	virtual void	getWorldTransform(btTransform& centerOfMassWorldTrans ) const 
 	{
 			centerOfMassWorldTrans = 	m_centerOfMassOffset.inverse() * m_graphicsWorldTrans ;
 	}
 
 	///synchronizes world transform from physics to user
+	///Bullet only calls the update of worldtransform for active objects
 	virtual void	setWorldTransform(const btTransform& centerOfMassWorldTrans)
 	{
 			m_graphicsWorldTrans = centerOfMassWorldTrans * m_centerOfMassOffset ;
 	}
+
+	///Bullet gives a callback for objects that are about to be deactivated (put asleep)
+	/// You can intercept this callback for your own bookkeeping. 
+	///Also you can return false to disable deactivation for this object this frame.
+	virtual bool deactivationCallback(void*	userPointer) {
+		return true;
+	}
+
 };
 
 #endif //DEFAULT_MOTION_STATE_H

src/LinearMath/btMotionState.h

@@ -29,10 +29,15 @@ class	btMotionState
 			
 		}
 		
-		virtual void	getWorldTransform(btTransform& worldTrans )=0;
+		virtual void	getWorldTransform(btTransform& worldTrans ) const =0;
 
+		//Bullet only calls the update of worldtransform for active objects
 		virtual void	setWorldTransform(const btTransform& worldTrans)=0;
-
+		
+		//future: when Bullet makes attempt to deactivate object, you can intercept this callback (return false to disable deactivation for this object this frame)
+		virtual bool deactivationCallback(void*	userPointer) {
+			return true;
+		}
 };
 
 #endif //BT_MOTIONSTATE_H

src/LinearMath/btTransformUtil.h

@@ -108,7 +108,16 @@ public:
 	static void	calculateVelocity(const btTransform& transform0,const btTransform& transform1,btScalar timeStep,btVector3& linVel,btVector3& angVel)
 	{
 		linVel = (transform1.getOrigin() - transform0.getOrigin()) / timeStep;
-#ifdef USE_QUATERNION_DIFF
+		btVector3 axis;
+		btScalar  angle;
+		calculateDiffAxisAngle(transform0,transform1,axis,angle);
+		angVel = axis * angle / timeStep;
+	}
+
+	static void calculateDiffAxisAngle(const btTransform& transform0,const btTransform& transform1,btVector3& axis,btScalar& angle)
+	{
+	
+	#ifdef USE_QUATERNION_DIFF
 		btQuaternion orn0 = transform0.getRotation();
 		btQuaternion orn1a = transform1.getRotation();
 		btQuaternion orn1 = orn0.farthest(orn1a);
@@ -118,9 +127,7 @@ public:
 		btQuaternion dorn;
 		dmat.getRotation(dorn);
 #endif//USE_QUATERNION_DIFF
-
-		btVector3 axis;
-		btScalar  angle;
+	
 		angle = dorn.getAngle();
 		axis = btVector3(dorn.x(),dorn.y(),dorn.z());
 		axis[3] = 0.f;
@@ -130,13 +137,8 @@ public:
 			axis = btVector3(1.f,0.f,0.f);
 		else
 			axis /= btSqrt(len);
-
-		
-		angVel = axis * angle / timeStep;
-
 	}
 
-
 };
 
 #endif //SIMD_TRANSFORM_UTIL_H