qr_size = 45;
qr_height = 2;

wall_width = 3;

wheel_width = 9;
wheel_radius = sqrt(2 * pow(qr_size / 2, 2)) + wall_width / 2;

motor_shaft_radius = 3.7;
motor_shaft_flat_width = 4.8;

tread_radius = 4 / 2;