Notifying change of a calculated databound property in WPF

As an exercise I decided to create a bicycle gear calculator in WPF. I created two private fields with setters that trigger OnPropertyChanged(), but I have one databound property, ratio, which behaves as "readonly", because it is dynamically calculated. When I run the program, the textboxes show, the initial values are correctly displayed, the "working" word in the property changed handler is displayed, but the ratio TextBlock doesn't update.

I suspect this is due to the way the property is "getted", I wonder if it would be absolutely necessary to add a private field for every, I wonder if there should be any DependencyProperty in this... But actually I have reached my knowledge limit on this and cannot get this trivial program to work.

This is my model:

class SingleGearsetModel : INotifyPropertyChanged
{
    public SingleGearsetModel()
    {
        crank = 44;
        cog = 16;
    }

    private int _crank;
    private int _cog;

    public int crank { 
        get{return _crank;}
        set{
            _crank = value;
            OnPropertyChanged("crank");
        }
    }

    public int cog {
        get{return _cog;}
        set{
            _cog = value;
            OnPropertyChanged("cog");
        } 
    }

    public double ratio
    {
        get {
            return (double)crank / (double)cog;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string arg)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(arg));
            Console.Writeline("working");
        }
    }

} // end class

This is my XAML:

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="CalculadorFix