diff --git a/Engine.cc b/Engine.cc
index f2d9451..34e3a91 100644
--- a/Engine.cc
+++ b/Engine.cc
@@ -89,7 +89,6 @@ Engine::Engine(const string & path, AV & av)
     : m_av(av)
 {
     m_next_object_index = 1;
-    m_next_joint_index = 1;
     m_eye[0] = 0;
     m_eye[1] = -1;
     m_eye[2] = 0;
@@ -311,9 +310,7 @@ int Engine::addAMotor(Object * o1, Object * o2)
     if (o2 != NULL)
         b2 = o2->getBody();
     dJointID jid = m_world.createAMotor(b1, b2);
-    int id = m_next_joint_index++;
-    m_joints[id] = jid;
-    return id;
+    return m_joints.add(jid);
 }
 
 int Engine::addHinge(Object * o1, Object * o2,
@@ -329,15 +326,13 @@ int Engine::addHinge(Object * o1, Object * o2,
     dJointID jid = m_world.createHinge(b1, b2,
             anchor_x, anchor_y, anchor_z,
             axis_x, axis_y, axis_z);
-    int id = m_next_joint_index++;
-    m_joints[id] = jid;
-    return id;
+    return m_joints.add(jid);
 }
 
 void Engine::setAMotorAxis(int jid, int anum, int rel,
         dReal x, dReal y, dReal z)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorAxis(m_joints[jid], anum, rel, x, y, z);
     }
@@ -345,7 +340,7 @@ void Engine::setAMotorAxis(int jid, int anum, int rel,
 
 void Engine::setAMotorNumAxes(int jid, int num_axes)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorNumAxes(m_joints[jid], num_axes);
     }
@@ -353,7 +348,7 @@ void Engine::setAMotorNumAxes(int jid, int num_axes)
 
 void Engine::setAMotorAngle(int jid, int anum, dReal val)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorAngle(m_joints[jid], anum, val);
     }
@@ -361,7 +356,7 @@ void Engine::setAMotorAngle(int jid, int anum, dReal val)
 
 void Engine::setAMotorLoStop(int jid, dReal val)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorLoStop(m_joints[jid], val);
     }
@@ -369,7 +364,7 @@ void Engine::setAMotorLoStop(int jid, dReal val)
 
 void Engine::setAMotorHiStop(int jid, dReal val)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorHiStop(m_joints[jid], val);
     }
@@ -377,7 +372,7 @@ void Engine::setAMotorHiStop(int jid, dReal val)
 
 void Engine::setAMotorVel(int jid, dReal val)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorVel(m_joints[jid], val);
     }
@@ -385,7 +380,7 @@ void Engine::setAMotorVel(int jid, dReal val)
 
 void Engine::setAMotorFMax(int jid, dReal val)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorFMax(m_joints[jid], val);
     }
@@ -393,7 +388,7 @@ void Engine::setAMotorFMax(int jid, dReal val)
 
 void Engine::setAMotorBounce(int jid, dReal val)
 {
-    if (m_joints.find(jid) != m_joints.end())
+    if (m_joints.contains(jid))
     {
         m_world.setAMotorBounce(m_joints[jid], val);
     }
diff --git a/Engine.h b/Engine.h
index 2b2ddc9..6898730 100644
--- a/Engine.h
+++ b/Engine.h
@@ -266,9 +266,8 @@ class Engine
         OdeWorld        m_world;
         std::map<int, Object *> m_objects;
         IDSet< refptr<AV::Sound> > m_sounds;
-        std::map<int, dJointID> m_joints;
+        IDSet<dJointID> m_joints;
         int             m_next_object_index;
-        int             m_next_joint_index;
         GLdouble        m_eye[3];
         GLdouble        m_center[3];
         GLdouble        m_up[3];