Note that the figure in the Java applet does not show quite the true story of reactant forces. The force Fa does not really act at the point where the front tire touches the ground. Actually, our truss model assumes that the truss is supported by a roller up near the handlebar joint, and this is where the vertical component Fa acts. (If you actually wanted to know the value of the force where the front tire meets the ground, you could use the approach outlined under Moment Equilibrium and Tire Reactions.)
Likewise, the force Fb in the Java applet does not necessarily act where the rear tire touches the ground. Instead, in our truss model, Fb acts at the rear support of the truss, which is assumed to be a fixed support at point b. Note that there can be two force components at a fixed support, but here the horizontal force component Fx will be zero because all of the applied forces on the bike are acting in the vertical (y) direction. Fx would not be zero if there were horizontal forces applied at the seat, pedals, or handlebars.