Fixed #550: zippers have much less effect.

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@10909 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/physics/btKart.cpp

@@ -700,7 +700,8 @@ void btKart::updateFriction(btScalar timeStep)
             }
 
         }
-
+        else
+        {
             btScalar rollingFriction = 0.f;
 
             if (wheelInfo.m_engineForce != 0.f)
@@ -728,6 +729,7 @@ void btKart::updateFriction(btScalar timeStep)
             }
 
             m_forwardImpulse[wheel] = rollingFriction;
+        }
         if(m_time_additional_impulse>0)
         {
             sliding = true;