float yRotation = inputDirection.x * Time.fixedDeltaTime * speedMult * rotaRatio;
Quaternion turnRotation = Quaternion.Euler(0f, yRotation, 0f);
rb.MoveRotation(rb.rotation * turnRotation);

float turnAmount = inputDirection.x;
float maxAngle = 20f;
float tiltAngle = Mathf.Clamp(turnAmount, -maxAngle, maxAngle);

Vector3 newEulerAngles = new Vector3(rb.rotation.eulerAngles.x, rb.rotation.eulerAngles.y, tiltAngle);
rb.MoveRotation(Quaternion.Euler(newEulerAngles));