@Override
    public void write(JmeExporter ex) throws IOException {
        OutputCapsule capsule = ex.getCapsule(this);
        capsule.write(wheelSpatial, "wheelSpatial", null);
        capsule.write(frontWheel, "frontWheel", false);
        capsule.write(location, "wheelLocation", new Vector3f());
        capsule.write(direction, "wheelDirection", new Vector3f());
        capsule.write(axle, "wheelAxle", new Vector3f());
        capsule.write(suspensionStiffness, "suspensionStiffness", 20.0f);
        capsule.write(wheelsDampingRelaxation, "wheelsDampingRelaxation", 2.3f);
        capsule.write(wheelsDampingCompression, "wheelsDampingCompression", 4.4f);
        capsule.write(frictionSlip, "frictionSlip", 10.5f);
        capsule.write(rollInfluence, "rollInfluence", 1.0f);
        capsule.write(maxSuspensionTravelCm, "maxSuspensionTravelCm", 500f);
        capsule.write(maxSuspensionForce, "maxSuspensionForce", 6000f);
        capsule.write(radius, "wheelRadius", 0.5f);
        capsule.write(restLength, "restLength", 1f);
    }
returns how many degrees the wheel has turned since the last physics
step.

@return the rotation angle (in radians)
jme3-jbullet/src/main/java/com/jme3/bullet/objects/VehicleWheel.java