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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: https://fb-s-c-a.akamaihd.net/h-ak-xpa1/v/t1.0-0/s480x480/16830718_1834437703475584_285367870389000008_n.png?oh=c30063e42b9920c690e6a88d6cd2c4d6&oe=5946B72E&__gda__=1497063666_8c2eba10a3ec8f684fcf79a1c261ecc5

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  • I guess I don't understand. If you have two cranks, two chains, and two gear clusters, the "backwards" one should look normal to the "backwards" derailer.
    – Daniel R Hicks
    Commented Feb 26, 2017 at 13:47
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    Wait, two crank assemblies and one drivetrain?
    – ojs
    Commented Feb 26, 2017 at 13:58
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    Why not use a tandem timing chain to drive the normal crankset from the rear one? You might need to take the normal pedals off when riding prone to avoid them hitting anything.
    – Chris H
    Commented Feb 26, 2017 at 20:07
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    Muhammad: this question has been nominated for closure as Unclear what you are asking. At present I have voted to keep it open, but I think you should edit your question to provide additional information where people has pointed out issues. Well done for being upfront that it's a school project; help us by making it clearer.
    – andy256
    Commented Feb 27, 2017 at 0:22
  • @muhammad Agree with andy - add more info and I'll retract my close vote. It takes 5 votes to close a question.
    – Criggie ♦
    Commented Feb 27, 2017 at 1:23

3 Answers 3

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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.

enter image description here

Note that the different crank positions require different chain length.

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  • You're right about the derailleur position. Does this solution use one chain and move it manually to change modes?
    – Criggie ♦
    Commented Feb 26, 2017 at 18:34
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    Yes. The original picture shows that chain and crankset are removed and moved to other position. The question text is too ambiguous to guess if this was intended.
    – ojs
    Commented Feb 26, 2017 at 18:48
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    If you need to have derailleur drivetrains for both cranksets at the same time, you'll need either a mirrored drivetrain on the left side for the other crank or both cranksets driving a shared idler wheel that drives the rear wheel.
    – ojs
    Commented Feb 26, 2017 at 18:53
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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.)

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  • An IGH with two drive cogs would solve a lot of problems. Downside, the unused crank c/would be rotating at a different speed than the primary crank, which will risk pedal strike. Those old chainrings with freewheels in them could be perfect, allowing both chains to move but the non-driven crank to stay still.
    – Criggie ♦
    Commented Feb 26, 2017 at 18:37
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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.

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  • There are removable pedals that click into a socket that is permanently installed in the crank arm. Popular with folders and travel bikes. MKS make some, but they're not cheap. OP could have one pair of pedals and two sets of sockets.
    – Criggie ♦
    Commented Feb 27, 2017 at 19:37
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    @Criggie that's an interesting idea. I'd thought about folding pedals, but reckoned that as the whole thing sounds like a proof of principle it wouldn't be too much work to swap a set of normal pedals into the other cranks.
    – Chris H
    Commented Feb 27, 2017 at 19:40
  • Folding pedals would hang down, or you'd have to strap them to the crank arm. Either way its rotating weight. Remounting pedals frequently won't do the crank threads any favours.
    – Criggie ♦
    Commented Feb 27, 2017 at 19:43
  • @Criggie some of the folding pedals I've seen (not used) seem to have some means of holding them against the arm. I doubt it will be used enough for wear on the cranks to be an issue,but in the worst case a helicoil could be used, even pre-emptively, and that would give steel threads reducing further wear. Of course the spare pedals might be of of the way anyway. It would be nice if we heard back from the OP, otherwise I might add to the close votes
    – Chris H
    Commented Feb 27, 2017 at 19:51

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