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-   -   Double Freewheel Fixie (https://www.bikeforums.net/bicycle-mechanics/390117-double-freewheel-fixie.html)

MnIceBiker 02-20-08 06:17 PM

Double Freewheel Fixie
 
Wonder if anyone can tell me why this doesn't work: I threaded a 16t freewheel onto both sides of a Surly flip-flop hub. On the right side, I had a 42t chainring. On the left, a 32t chainring. The idea was to effectively make a fixed-gear with a decent gain ratio that was really easy to skid/slow down on, i.e.- pedal forward, the 42/16 works and the 32/16 freewheels. Pedal backward and the 32/16 works, giving less resistence, and the 42/16 freewheels.

The problem is, whichever gear is harder binds up (no matter which side of the bike it is on.) The original set up only worked in reverse. So I switched to 52/16 on the left side and it pedaled forward, but bound up in reverse.

Any ideas why this brilliant scheme didn't work?

carpediemracing 02-20-08 06:28 PM

hm interesting.

Are you using the same cranks and a "mirror image" bottom bracket? I think chain line would be affected if you're not, and one side may bind based on how the wheel is lined up. In other words the chainring/cog combination that is better lined up works. Because the other one doesn't line up, the other direction binds.

Lining up both chainrings with both freewheels might be difficult. I'm sure it's possible, I just don't know how some of the newer BBs would work. Track racers occasionally tried using two chainrings and a double sided hub in order to have a more efficient drivetrain. The one I'm thinking of also had a "dump" feature so that once up to speed, the racer could dump one of the chains to make the bike "lighter" and more efficient. More a gadget than anything else but hey, at least they tried it.

good luck with the double freewheel fixie,
cdr

MnIceBiker 02-20-08 06:32 PM

Chainline was definitely acceptable on both sides. I was using an Octalink BB w/Ultegra crank on one side and 105 on the other. The binding felt sort of like a coaster brake engaging. What I found more interesting is why the harder gear bound up no matter which side of the bike it was one. Can't figure that out...

Ex Pres 02-20-08 06:43 PM

There was a thread on this about a month ago and IIRC Little Darwin declared that it will not work (until he's proven wrong). Interesting discussion, but it's nice to now have some empirical evidence.

MnIceBiker 02-20-08 06:48 PM

Do you have a link to the thread? I assume it was in this forum. It was a novel idea in the first place, but now I just want to know WHY it won't work. Its like the time I couldn't remember the name of that actor that was in that movie... you know, the one with all the nonstop action and witty cop banter... :)

Ex Pres 02-20-08 07:16 PM

http://www.bikeforums.net/showthread...reewheel+fixed

Boss Moniker 02-20-08 07:19 PM

It's not going to work. Think about it: You pedal forward (turning clockwise when viewing from right side of bike), and the wheel moves forward. Viewing from the other side, the wheel is turning counterclockwise, thus it's causing the freewheel to engage in the same way that it does on a normal bike when you walk it in reverse. Because the non-drive side freewheel is turning, the cranks also have to turn, and unless you have exactly the same gear ratio, there's going to be binding. If you do have the same ratio, it's exactly like a fixed gear.

Now, if the one on the other side of the wheel engaged in the opposite direction (and the threads were reverse.. or you had a lockring) then it would work no matter the ratio, and you could pedal backwards at an easier ratio. You'd hear clicking all the time from either side depending on which way you're going. And I don't even know if anyone sells reverse freewheels or if it's possible to modify them.

Look up retro-direct-drive, which is where you have two freewheels that move independently of each other ON THE SAME SIDE of the wheel, and the chain loops around a pulley so it goes over both.. meaning if you pedal forwards you engage one freewheel while driving the other one in reverse (like pedaling in reverse, you'll hear clicking), and if you pedal backwards you engage the other and drive the first in reverse.. but keep in mind that they're both being engaged in the forward direction, so it's like having two gear ratios, except you use one by pedaling in reverse. And it's not a fixed gear: you can coast because both freewheels engage in the same direction.

roadfix 02-20-08 07:31 PM


Originally Posted by Boss Moniker (Post 6202026)
And it's not a fixed gear: you can coast because both freewheels engage in the same direction.

I'm trying to visualize this and I don't think you can coast.

MnIceBiker 02-20-08 07:34 PM

Thanks guys, I just read the original thread (and your post, Boss Moniker). That makes sense now. Now if I could get a hub with a reverse-threaded left side and a southpaw freewheel... Or one of those BMX hubs that doesn't spin the crank in reverse... heh heh...

Roll-Monroe-Co 02-20-08 07:36 PM

Somebody always beats me to the punch.

Yeah: http://en.wikipedia.org/wiki/Retro-Direct

MnIceBiker 02-20-08 07:36 PM

Wait... then it wouldn't stop at all... Dammit...

Boss Moniker 02-20-08 07:41 PM


Originally Posted by roadfix (Post 6202120)
I'm trying to visualize this and I don't think you can coast.

You certainly can. Remember: both freewheels engage in the same direction and coast in the same direction. It's only how the chain wraps around them that's different. If it helps, imagine two chains driven by two chainrings on the same crank, and driving independent freewheels on the same side of the wheel. One chain is looped normally , while the other crosses in the middle like so: ( )><( ) So if you hold the cranks still, both freewheels will be still, thus they'll both be coasting.

carpediemracing 02-21-08 08:58 AM

My brain isn't working really well but what if you crossed the chain on one side, say the left one. So the chain would go from the top of the cog to the bottom of the chainring? Would that make the left freewheel a "southpaw"?

cdr

jgedwa 02-21-08 09:15 AM

You win big points for creativity anyway. Its too complicated for me, I have to mull it over for a while and think about what the problem could be.

jim

Boss Moniker 02-21-08 12:24 PM


Originally Posted by carpediemracing (Post 6205023)
My brain isn't working really well but what if you crossed the chain on one side, say the left one. So the chain would go from the top of the cog to the bottom of the chainring? Would that make the left freewheel a "southpaw"?

cdr

Naw, if you crossed the left one it'd bind while pedalling forwards and coast while pedallig backwards. Or if you crossed the right one it'd coast forwards and bind backwards.

I was thinking about that double-chain retro direct more and it seems like a really slick way to make the drivetrain more solid. The hardest thing would be finding enough room to cross the chain (the chainstay would get in the way) and keeping the crossed chain from rubbing against itself.

due ruote 02-21-08 12:35 PM

Wow - crossed chains, reverse freewheels - this makes my head spin. Wait - I have an idea: brakes!

roadfix 02-21-08 12:38 PM


Originally Posted by jbonamici (Post 6206444)
Wait - I have an idea: brakes!

That's already been done...:p

Boss Moniker 02-21-08 02:42 PM

I just found this site where a guy tried what you suggested, although I think he immediately jumped to having the same ratio on each side.

http://spencerwright.org/mybikes/doublechain.html


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