hi lobuz,

You're going to want to read the section in the docs about constraints (Havok Physics/Havok Dynamics/Constraints).  It will explain what a constraint instance is.  Essentially, they are instances created from a, possibly shared, hkpConstraintData.  The hkpConstraintInstance you are seeing may indeed be from the hkpWheelConstraintData.  You can figure this out by calling hkpConstraintInstance->getData()->getType().  You'll notice that CONSTRAINT_TYPE_WHEEL is one of the types listed in hkpConstraintData.h

Let me know if that helps,
Jason G.