diff --git a/urdf/include/urdf/joint.h b/urdf/include/urdf/joint.h
index 54064e6..551e2c6 100644
--- a/urdf/include/urdf/joint.h
+++ b/urdf/include/urdf/joint.h
@@ -140,7 +140,8 @@ class JointCalibration
 {
 public:
   JointCalibration() { this->clear(); };
-  double reference_position, rising, falling;
+  double reference_position;
+  boost::shared_ptr<double> rising, falling;
 
   void clear()
   {
diff --git a/urdf/src/joint.cpp b/urdf/src/joint.cpp
index 0f2fecc..5a1adab 100644
--- a/urdf/src/joint.cpp
+++ b/urdf/src/joint.cpp
@@ -180,20 +180,20 @@ bool JointCalibration::initXml(TiXmlElement* config)
   if (rising_position_str == NULL)
   {
     ROS_DEBUG("joint calibration: no rising, using default value");
-    this->rising = 0;
+    this->rising.reset();
   }
   else
-    this->rising = atof(rising_position_str);
+    this->rising.reset(new double(atof(rising_position_str)));
 
   // Get falling edge position
   const char* falling_position_str = config->Attribute("falling");
   if (falling_position_str == NULL)
   {
     ROS_DEBUG("joint calibration: no falling, using default value");
-    this->falling = 0;
+    this->falling.reset();
   }
   else
-    this->falling = atof(falling_position_str);
+    this->falling.reset(new double(atof(falling_position_str)));
 
   return true;
 }