diff --git a/src/karts/kart.cpp b/src/karts/kart.cpp
index eec80719e..a3451e1e1 100644
--- a/src/karts/kart.cpp
+++ b/src/karts/kart.cpp
@@ -630,13 +630,18 @@ void Kart::handleZipper()
     // Ignore a zipper that's activated while braking
     if(m_controls.m_brake) return;
     m_zipper_time_left  = stk_config->m_zipper_time;
-    const btVector3& v  = m_body->getLinearVelocity();
+    btVector3 v         = m_body->getLinearVelocity();
     float current_speed = v.length();
     float speed         = std::min(current_speed+stk_config->m_zipper_speed_gain, 
                                    getMaxSpeed());
-    // Avoid NAN problems, which can happen if e.g. a kart is rescued on
-    // top of zipper, and then dropped.
-    if(current_speed>0.00001) m_body->setLinearVelocity(v*(speed/current_speed));
+    // If the speed is too low, very minor components of the velocity vector
+    // can become too big. E.g. each kart has a z component (to offset gravity
+    // I assume, around 0.16) --> if the karts are (nearly) at standstill,
+    // the z component is exaggerated, resulting in a jump. Even if Z
+    // is set to 0, minor left/right movements are then too strong.
+    // Therefore a zipper only adds the speed if the speed is at least 1
+    // (experimentally found valud).  It also avoids NAN problems (if v=0).
+    if(current_speed>1.0f) m_body->setLinearVelocity(v*(speed/current_speed));
 }   // handleZipper
 //-----------------------------------------------------------------------------
 #define sgn(x) ((x<0)?-1.0f:((x>0)?1.0f:0.0f))
diff --git a/src/robots/default_robot.cpp b/src/robots/default_robot.cpp
index ec42f6322..8bb732bd6 100755
--- a/src/robots/default_robot.cpp
+++ b/src/robots/default_robot.cpp
@@ -174,10 +174,18 @@ void DefaultRobot::update(float dt)
     handleNitroAndZipper();
     // If we are supposed to use nitro, but have a zipper, 
     // use the zipper instead
-    if(m_controls.m_nitro && m_powerup.getType()==POWERUP_ZIPPER)
+    if(m_controls.m_nitro && m_powerup.getType()==POWERUP_ZIPPER && 
+        getSpeed()>1.0f && m_zipper_time_left<=0)
     {
-        m_controls.m_nitro = false;
-        m_controls.m_fire  = true;
+        // Make sure that not all AI karts use the zipper at the same
+        // time in time trial at start up, so during the first 5 seconds
+        // this is done at random only.
+        if(race_manager->getMinorMode()!=RaceManager::MINOR_MODE_TIME_TRIAL ||
+            (m_world->getTime()<3.0f && rand()%50==1) )
+        {
+            m_controls.m_nitro = false;
+            m_controls.m_fire  = true;
+        }
     }
 
     /*And obviously general kart stuff*/