Construction of an analog Wheel-Pedals-Combination
From the technical point of view, a wheel-pedals-combination is nothing else than two joysticks in a slightly different form. Therefore, at first some information about analog joysticks: A simple analog joystick consists of the stick that can be moved in two dimensions (forth/back and left/right) and two buttons. The buttons are simple closing switches with one side connected to ground and the other side connected to their respective pin (2, 7, 10, 14, see pin chart). The analog axes of the stick are implemented as potentiometers between Vcc and their respective pin. The resistance range is normally from 0 Ohm to 100 kOhm. The minimal values (0 Ohm) represent the joystick position "left" for the X-axis and "forth" for the Y-axis. According to this, both resistances are 100 kOhm if the position is "back right". Slightly different values are not a real problem because the joysticks always have to be calibrated. It has to be said that linear potentiometers should to be used (the effect of the use of logarithmic potentiometers can be imagined quite easily).
To transfer this to wheel and pedals is now not very difficult (at least in theory). The wheel is the X-axis of the first joystick, the accelerator the Y-axis of the first joystick, the brake is the X-axis of the second joystick and an optional clutch the Y-axis of the second joystick. How this is done mechanically is everyone's own choice. For some inspiration, have a look for example at Lew's Wheels.
If the clutch is left out, the pin for the Y-axis of the second joystick still has to have a defined potential (otherwise there are problems detecting the "second joystick" which can lead to a non working brake). For defining this potential, the pin can be connected directly or via a resistor to Vcc.
Tip for use in some games: If you let Windows detect your wheel and pedals as two normal joysticks (2 axes, 2 buttons), the use in the games might be easier than if you set up a user-defined gamecontroller for it, because the games know simple joysticks, but might not know user-defined ones.