Creating a custom plugin to add a discount to woocommerce products

I currently have a wordpress site setup using woocommerce and gravity forms plugins. I am selling bike wheels and on my product page I use gravity forms to display different customization options. the options are different if they select one wheel or two wheels.

What I’m trying to achieve

I want to add a 5% discount if two wheels are selected and 10% if four+ are selected. This discount is applied to the product as it is added to the cart.

I am attempting to create a custom plugin that uses javascript to get the value from the gravity forms input, and hooks into woocommerce to edit the total price before adding it to the cart.

What I have so far

custom.js

    jQuery(document).ready(function () {
  jQuery('.cart').submit(function () {
    var noOfWheels = jQuery(".rdoWheel input[type='radio']:checked").val();
    console.log(noOfWheels)
    var data = {
        action: 'my_discount',
        wheels: noOfWheels
    };

    jQuery.ajax({
        type: 'POST',
        url: discountAjax.ajax_url,
        data: data,
        success: function (data) {
            //do nothing

        },
    });

    return false;
});

});

discount.php

    add_action('wp_enqueue_scripts', 'load_script');
    function load_script() {
      wp_enqueue_script('discount', plugin_dir_url( __FILE__ ) . 'custom/custom.js', array( 'jquery' ) );
      wp_localize_script('discount', 'discountAjax', array('ajaxurl' => admin_url('admin-ajax.php')));
             }


      add_action('wp_ajax_woocommerce_discount', 'calculate', 10);
      add_action('wp_ajax_nopriv_woocommerce_discount', 'calculate', 10);

       function calculate() {
           if (isset($_POST['wheels'])) {
                global $woocommerce;
                $wheels = $_POST['wheels'];
                if ($wheels === "1") {
                          $val = 0;
                 } elseif ($wheels === "2"){
                           $val = 10;
                 }
                 session_start();
                 $_SESSION['val'] = $val;
                }
               }

    add_action('woocommerce_before_calculate_totals', 'add_discount');

        function add_discount( $cart_object) {
          @session_start();
          if (isset($_SESSION['val'])) {
             $wheels = $_SESSION['val'];
                foreach ( $cart_object->cart_contents as $key => $value ) {
                     $c_price = $value['data']->price;
                     $discountAmount = $c_price * $wheels/100; 
                     $value['data']->price = $value['data']->price - $discountAmount;

                   }
             //for testing purpose
             echo $_SESSION['val'];
             echo 'completed';
             unset($_SESSION['val']);
           }
            }

whats happening

It seems the woocommerce function is not firing at all. when i check with firebug I see my ajax request go through okay then nothing else. If i remove everything except for the add_discount function then it goes through and applies the discount. I cant seem to get it to work with javascript/ajax.