How to make a derailleur work in opposite directions?

Question

I am making a bike for a school project that can switch between upright and pseudo-prone position. It has two sets of crank assembly; one on one side of the rear gear set and one on the other; the task is to be able to switch positions. However, the problem is that when you use drive train for one position, it works fine and the derailleur can change gears easily. But the same can't be done for the other position because the derailleur only works in one direction. One potential solution is to fix one gear(the biggest or the smallest) for one position and switch the other gears for the other position.

Could someone please help me fix it such that all the gears can be used in both positions?

Here's a picture of the bike's design to explain:

Answer 1

When you switch to the crankset behind cogs, you need to move the derailleur above the cogset.

Think of it as rotating the entire drive train 180 degrees forward or flipping the entire bike over the bars and then moving wheels down until they touch the ground.

Note that the different crank positions require different chain length.

Answer 2

One thing that comes to mind is by employing a variation of the Retro Direct concept, you could have both cranks/chains on at the same time, and there wouldn't have to be any mechanical mode switching, just get in the different position. The problem there is it would be a two or one speed unless you found a way to get the freewheels on an internally geared hub. There are some links on that page that show ways of doing Retro Direct with primarily normal parts.

The only way I can think of two do what you describe using a normal rear derailer is have both the front and rear cranks go to turn a spindle (idler?) with another freewheel on it that is then connected to the rear wheel and derailer. The back cranks would then be turning that spindle with a retro direct type setup to reverse the direction (or maybe there's some other mechanical trick to do so, not sure.)

Answer 3

To expand on my comment from last night:

I suggest using a similar setup to a tandem (crossover synchronisation chain). A tandem rear crankset has chainring(s) on the right and another gear on the left for the timing chain. Install this in the normal upright position. A tandem front crankset just has the timing chainring on the left, nothing on the right. Install this in the prone position, with a very long chain on the left -- you might need an idler gear or two to keep the chainline clear of obstructions.

You might want to start by reading Sheldon Brown's page on building tandems especially the section about front drive, as well as John S Allen's page on tandem drivertrains

Unless you plan on getting from one riding position to the other while moving (YouTube or it didn't happen), you might want to remove the unused front pedals rather than having them spinning near your body parts. Shielding the rest of the drivetrain from contacting you or your clothing would probably be a good idea during the development/testing phase.