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The question title pretty much sums it up, but has a bike with an alternate drivetrain style like this been made?

This would mean a derailleur mounted essentially upside down with the tension pushing the chain upward. I could see where this would protect the derailleur and keep the chain out of the way from dust and debris more. However, I could also envision some issues with front shifting due the the chain coming up from the front chain ring as well.

Pictures would be much appreciated!

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You could potentially rig the bike with a idler wheel high on the seat stay, and feed the chain down to the cluster, then up to the derailer. But the idler would need to be rigged so it would move in and out with the derailer, so that chain angle was acceptable. Probably better to just design a more compact and less damage-prone derailer somehow. –  Daniel R Hicks Apr 6 at 13:04

3 Answers 3

up vote 10 down vote accepted

The derailleur needs to guide the chain into the sprocket - which means it goes on the bottom.

The tension pulley needs to go on the slack side of the chain - which again means it goes on the bottom (the top side of the chain loop has the drive tension).

If the drive train was reversed, you could do it. Put the drive wheel in front and steer with your butt - like this.

Hey! US Patent 6588786 has what you've described. Bonus: you don't have to steer with your butt - cables link the handle bars to the forks. I found this while googling for 'front drive bicycle'. enter image description here

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... that is some kind of weird. Nice find! –  dsalo Apr 7 at 0:42
    
Awesome, thanks for feeding my curiosity! I just happened to be reading an article that mentioned destroying a couple rear mechs and I thought about this. Probably wouldn't make the best MTB design though! –  Aaron Apr 7 at 13:47

This would never work due to fact that no force would be transmitted to the rear wheel until the derailleur cage was at maximum extension.

The derailleur has to be below the chainstay to allow it to take up the slack in the chain.

I suppose you could split the derailleur into to parts, one to keep the chain tension and the other to change gears, but that seems excessively complex.

The real solution to the derailleur problem is an internally geared hub.

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Or singlespeeding. –  Batman Apr 3 at 23:34
    
Or a IGH mounted above the crank like the Lahar DHV M9. –  DWGKNZ Apr 4 at 7:52
    
I know it violates the KISS principal, but a figure 8 routing of the chain? –  BPugh Apr 4 at 13:48
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@jimirings yep, didn't think of that, but wasn't there a two speed hub that used a backwards rotation for one gear? –  BPugh Apr 4 at 19:52
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@BPugh there's retro direct which uses a chain tensioner and two freewheels to give two gears, but it's not really a hub gear, it's all external. –  Mσᶎ Apr 5 at 4:47

Actually there is one such solution from Campagnolo which did not really stick:

Campio Paris-Roubaix

The general idea was to skip tensor altogether, and recompense change in chain length (distance it travels) by moving sprocket (whole wheel) back and forward.

The main disadvantage of such approach was reduced number of speeds you can have in our bike; mainly by disallowing you to have front gear changer. That wasn't too big problem on some races, just like Paris-Roubaix classic, but still lack of freedom was with speed selection (bracket size needed to correspond to distance by which wheel was moved) and wheel changing during race.

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