My Interpretation of Bob Durst's Differential Steering System, as described by Elliot and adapted for the tracked bicycle project for BM2006 (there is no assurance this is even close to workable, so don't blame me if it doesn't!)

Differentials apparently sum the torque about the common axis of rotation; the "innards" spin however is needed to allow this to happen.

Green = Differential (D1=Steering, D2=Drive, D3=Right, D4=Left (note: D3 & D4 may not be needed)

Red = Free Hub from 10-speed bike (allows back pedaling w/o transfer of force)

The transmission (marked 3-2-1-R above) determines forward or backward motion

Two or One persons may pedal (if the 2nd person does not pedal, there pedals remain stationary) and each has their own 10-speed selector and hand-brake.

The Collective Axle has the brake ??? (or maybe the actual wheels)

When the steering crank is turned, the D1 output goes to the Right and Left Drive Sprockets (one of these chains only is twisted 180 degrees out-of-phase resulting in opposite rotations). This torque is added to the Right and Left axels (and sums in either D3 and D4 or in D2 if D3 and D4 are not needed). The torque is thus added to on side and subtracted from the other, resulting in a different speed of motion from one side to the other, thus a turn.

If the main drive is not turning (no one pedaling) and the Steering Crank is turned, the two main drive sprockets turn in opposite directions (the torque summing in either D3 & D4 or in D2 if D3 and D4 are not needed), and the vehicle slowly twists in place like a top within it's own length…