Cloth keyframing


I am trying to animate a character jumping off a cliff wearing a robe - How do I get the robe to lift as soon as he jumps using keyframes.
I set the grav at -9 to frame 10 (for example) then I hit the I- key and add Force strength - and later the grav should change to 9 (@ frame 50 for example)
The problem is the grav does not change.
Am I forgetting something or just doing it wrong.
Can someone please help? :ba: