Huge under steer on ford escort!?

my escort has huge under steer when i do right turns. the left wheel will screech on the road when turning, it does it at low speeds as well. i feel it most on roundabouts it feels like hitting an ice patch and the noise it makes always gives me an audience lol. ive had 2 new wishbones, 2 new track road ends, new break pads. ive checked the wheel for any play to check to see if a baring has gone but it was all ok. the inner tread of my tyre is worn away quite badly. when going in a Straight line there is no pull on the wheels, it drives as straight as an arrow. what could be the problem?. i also get a knocking noise everytime i go over bumps. what could it be? suspension??. if so how does it explain the under steer and knocking.?