It's a 4 speed retrodirect drive, never seen one with a double chainring like that. The 2nd tensioner on the chainstay is for picking up the slack when in the smaller ring. Oh and I guess you can only shift the front while pedaling forward. Not sure a front derailleur would work when pedaling in reverse.
this should help clear things up
https://hexbear.net/pictrs/image/7bed5750-3aeb-4b43-b43b-8a3563150abd.png
Front derailleur could work pedaling reverse honestly, it's just a bit of metal that shoves the chain to either side. I think the biggest obstacle would be that the teeth aren't designed that way around
50:50 chance somebody did it as a fun build because it's an interesting if obsolete technology or it's a cycling super crank utterly convinced this set up is the best thing to do and he posts about it a lot on bulletin boards
I won't let go of him until I understand what the fuck is going on in the picture
I can probably explain that actually (because 7bicycles has posted about it before coincidentally). I think the title "epicyclic gearing" is a misnomer as that seems to relate to planetary gearboxes which this doesn't appear to me to have.
What's happening here is actually retro-direct gearing, where two opposing freewheels (one engages when you pedal forwards, one when you pedal backwards) are used on two different size sprockets, so that you can have two different gears without any extra complexity like a derailleur. So to start riding you pedal forwards in your lower gear, once you get up to a good speed there, you start pedaling backwards to cruise at higher speeds with the higher gearing (you can set it up the other way around but this seems to be more common)
so that you can have two different gears without any extra complexity like a derailleur.
Predates the derailleur actually, just as a note.
Ok I understand the mechanics but I refuse to believe there are sickos out there pedaling backwards, what the fuck
https://www.youtube.com/watch?v=kCs_fAilDZI
shit, I love it, it's beaturiful
Just wanna note the more usual way to go on about this was to put the low gear pedalling forward, because it's easier to power down, and then put the high gear on the backwards pedal because that's just for flat cruising and don't matter so much.
