Hi Nabila,
Your suspension is missing (at least) two important components - the track rod and the shock (spring damper). Without those, gravity is enough to get the entire suspension to pivot around wildly.
I added a track rod just using Simscape Multibody geometry which limits the oscillations a little, but the suspension can get itself into configurations that would never happen in real life. You could add a spring-damper to resist gravity. You could also limit the movement of the joints, but I doubt you would want those limits to be active all the time.
Thanks,
--Steve
