steerAngle keeps the wheel spinning

This is the code I used to control the steering and collision of my basic vehicle. When I try it out my front wheels are spinning round and round, why?

thanks for the respons

 var a = 50;
 var RL : Transform;
 var RR : Transform;
 var FL : Transform;
 var FR : Transform;
 var rl : WheelCollider;
 var rr : WheelCollider;
 var fl : WheelCollider;
 var fr : WheelCollider;
 var ground : boolean;
 
 
 function Update () {
 
     if(FL.collider.isGrounded || FL.collider.isGrounded || RL.collider.isGrounded || RR.collider.isGrounded) {
         GetComponent("movement").grounded = true;
         ground = true;
     }
     else {
         GetComponent("movement").grounded = false;
         ground = false;
     }
     var steer = Input.GetAxis("Horizontal")*a/rigidbody.velocity.magnitude;
 
     fr.steerAngle = steer;
     fl.steerAngle = steer;    
     
 }