Use sqrt(speed) to scale the collision impulse, which results in

weaker impulses when hitting the terrain.

data/stk_config.xml

@@ -421,7 +421,7 @@
            outside of the chassis and results in more stable physical
            behaviour of the karts. -->
     <collision impulse-type="normal" 
-               impulse="3000" impulse-time="0.1" terrain-impulse="8000"
+               impulse="3000" impulse-time="0.1" terrain-impulse="1600"
                restitution="1.0" bevel-factor="0.5 0.0 0.7"
                physical-wheel-position="-1" />
 			 

src/karts/kart.cpp

@@ -1765,9 +1765,9 @@ void Kart::crashed(const Material *m, const Vec3 &normal)
             impulse.normalize();
         else
             impulse = Vec3(0, 0, -1); // Arbitrary
-        // impulse depends of kart speed
-        impulse *= 0.2f * m_body->getLinearVelocity().length();
-        impulse *= m_kart_properties->getCollisionTerrainImpulse();
+        // impulse depends of kart speed - and speed can be negative
+        impulse *= sqrt(fabsf(getSpeed()))
+                 * m_kart_properties->getCollisionTerrainImpulse();
         m_bounce_back_time = 0.2f;
         m_vehicle->setTimedCentralImpulse(0.1f, impulse);
     }