Wonky 2D Physics

I'm using the new 2D Wheel Joint and getting some strange results.

Example:

http://gfycat.com/InferiorNastyAdouri (boxes on wheels are just for visualization)

The code to drive all wheels:

     public class DriveControl : MonoBehaviour
     {
         public string axis = "Vertical";
         public float speed;
 
         public WheelJoint2D[] wheels;
 
         private void Update()
         {
             float rotation = Input.GetAxis(axis)*speed;
 
             foreach (WheelJoint2D wheel in wheels)
             {
                 JointMotor2D motor = wheel.motor;
                 motor.motorSpeed = rotation;
                 wheel.motor = motor;
             }
         }
     }

Wheel Components:

Some of the specific issues I noticed:

• Lots of jitter

• The cart sometimes floats above on its hinge - as if gravity was very low

• Strange acceleration

• Wheels move along seemingly random axis, instead of straight up and down as set in the Wheel Joint suspension settings

Any ideas why this is happening?