I need help making wheels.

I have a 2d bike using wheel joints but using motors doesnt give me a smooth ride. I need it to accelerate on press and be neutral on release and only slow down by gravity and such. Can anyone help? My current code decelerates and brakes terribly.

 using UnityEngine; 
 using System.Collections; 
  
  
 public class WheelCarJointMovement : MonoBehaviour { 
  //reference to the wheel joints
  WheelJoint2D[] wheelJoints; 
  //center of mass of the car
  public Transform centerOfMass;
  //reference tot he motor joint
  JointMotor2D motorBack;  
  //horizontal movement keyboard input
  float dir = 0f; 
  //input for rotation of the car
  float torqueDir = 0f;
  //max fwd speed which the car can move at
  float maxFwdSpeed = -5000;
  //max bwd speed
  float maxBwdSpeed = 2000f;
  //the rate at which the car accelerates
  float accelerationRate = 500;
  //the rate at which car decelerates
  float decelerationRate = -100;
  //how soon the car stops on braking
  float brakeSpeed = 2500f;
  //acceleration due to gravity
  float gravity = 9.81f;
  //reference to the wheels
  public Transform rearWheel;
  public Transform frontWheel;
  
  // Use this for initialization 
  void Start () { 
   //set the center of mass of the car
   rigidbody2D.centerOfMass = centerOfMass.transform.localPosition;
   //get the wheeljoint components
   wheelJoints = gameObject.GetComponents<WheelJoint2D>(); 
   //get the reference to the motor of rear wheels joint
   motorBack = wheelJoints[0].motor; 
  }  
   
  //all physics based assignment done here
  void FixedUpdate(){
   //add ability to rotate the car around its axis
   torqueDir = Input.GetAxis("Horizontal"); 
   if(torqueDir!=0){ 
    rigidbody2D.AddTorque(3*Mathf.PI*torqueDir, ForceMode2D.Force);
   } 
   else{
    rigidbody2D.AddTorque(0);
   }
 
   //horizontal movement input. same as torqueDir. Could have avoided it, but decided to 
   //use it since some of you might want to use the Vertical axis for the torqueDir
   dir = Input.GetAxis("Horizontal"); 
  
   //explained in the post in detail
   //check if there is any input from the user
   if(dir!=0)
    //add speed accordingly
    motorBack.motorSpeed = Mathf.Clamp(motorBack.motorSpeed -(dir*accelerationRate - gravity*Mathf.Sin((Mathf.PI)/180)*80 )*Time.deltaTime, maxFwdSpeed, maxBwdSpeed);
   //if no input and car is moving forward or no input and car is stagnant and is on an inclined plane with negative slope
   if((dir==0 && motorBack.motorSpeed < 0 ) ||(dir==0 && motorBack.motorSpeed==0)){
    //decelerate the car while adding the speed if the car is on an inclined plane
    motorBack.motorSpeed = Mathf.Clamp(motorBack.motorSpeed - (decelerationRate - gravity*Mathf.Sin((Mathf.PI)/180)*80)*Time.deltaTime, maxFwdSpeed, 0);
   }
   //if no input and car is moving backward or no input and car is stagnant and is on an inclined plane with positive slope
   else if((dir==0 && motorBack.motorSpeed > 0 )||(dir==0 && motorBack.motorSpeed==0)){
    //decelerate the car while adding the speed if the car is on an inclined plane
    motorBack.motorSpeed = Mathf.Clamp(motorBack.motorSpeed -(-decelerationRate - gravity*Mathf.Sin((Mathf.PI)/180)*80)*Time.deltaTime, 0, maxBwdSpeed);
   }
    
    
    
   //apply brakes to the car
   if (Input.GetKey(KeyCode.Space) && motorBack.motorSpeed > 0){
    motorBack.motorSpeed = Mathf.Clamp(motorBack.motorSpeed - brakeSpeed*Time.deltaTime, 0, maxBwdSpeed); 
   }
   else if(Input.GetKey(KeyCode.Space) && motorBack.motorSpeed < 0){ 
    motorBack.motorSpeed = Mathf.Clamp(motorBack.motorSpeed + brakeSpeed*Time.deltaTime, maxFwdSpeed, 0);
   }
   //connect the motor to the joint
   wheelJoints[0].motor = motorBack; 
  
  }
  
 }