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two cranks and two freewheels

Old 01-10-08, 10:23 AM
  #1  
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two cranks and two freewheels

Could use some insight on whether this would work:

--take a singlespeed bike with a flip-flop rear wheel
--put singlespeed freewheels on both sides
--put drive-side cranks and chains on both sides of the BB
--have the "forward" (right) freewheel use a much higher gear, while the "backward" (left) freewheel use a much smaller gear
--result: a de-facto fixed gear, with an easy ratio for backpedaling, and a hard ratio for forward pedaling

can anybody think of any reasons why this wouldn't work? I know that I'll have to swap pedal axles, but other than that?

this is just a ****s and giggles project, so no lectures on safety or practicality, please.
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Old 01-10-08, 10:35 AM
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You mean for braking on backpedal?
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Old 01-10-08, 10:37 AM
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Originally Posted by Stacey View Post
You mean for braking on backpedal?
right. braking/slowing down.
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Old 01-10-08, 10:42 AM
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To simplify, you'd want to have chains with an equal number of links. And to do that, you'd have to have an equal number of teeth. So for every tooth added to the bigger cog, you'd have to subtract a tooth from that side's chainwheel. For example if the drive side is 44x15, the left side would have to be 40x19.
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Old 01-10-08, 10:44 AM
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Originally Posted by caloso View Post
To simplify, you'd want to have chains with an equal number of links. And to do that, you'd have to have an equal number of teeth. So for every tooth added to the bigger cog, you'd have to subtract a tooth from that side's chainwheel. For example if the drive side is 44x15, the left side would have to be 40x19.
right, chain tension is one factor.

though I think that you could have something like a 39x13 on one side and a 51x17 on the other side -- not that that would make any sense.
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Old 01-10-08, 10:45 AM
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Right. Mathematically it should work, but it seems more complicated than necessary.
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Old 01-10-08, 10:46 AM
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I just wonder if there are any factors that I haven't thought of, or any reason why the drivetrain would bind. I think there was a topic on this a while ago, but I can't locate it.
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Old 01-10-08, 10:50 AM
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Cool idea. A two sided retro direct gearing--https://en.wikipedia.org/wiki/Retro-Direct

https://www.bikeforums.net/alt-bike-culture/131632-anyone-ever-build-2speed-hirondelle-style.html

Let us know how it turns out. Should eliminate the the chainline issues that traditional retro-direct gearing has.
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Old 01-10-08, 10:51 AM
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One drawback I can think of: you'll now have to roll up both pant cuffs!
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Old 01-10-08, 10:52 AM
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totally different from retro-direct. with this one, if you pedal backwards, you go backwards. only one freewheel is engaged at a time.
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Old 01-10-08, 10:55 AM
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Just so many things seem wrong in this idea... but as I think about it, it does seem reasonable.

When I think about it, I don't see anything wrong with the idea... except that no matter what you are doing you will hear freewheel clicking.

Also clearance on the left chainstay may be an issue (the right chainstay is sometimes dimpled for chain ring clearance).

But thinking about it while typing, I see that it would possibly work... so I guess the only real issue would be a slight delay while the freewheel engages when switching force.

OK, a real significant issue here... It might also be hard to get used to the mechanism for riding the bike. While riding forward one would apply rearward pressure to slow down, but at the same time the cadence would increase. This could actually be dangerous if one is riding at near their maximum cadence.

But wait, that doesn't make sense... If the "reverse" freewheel side is at a lower gear, then that freewheel would always be engaged and attempt to force the cranks faster than the "forward" side...

Oh, I see... that doesn't make sense since the freewheels would actually work in opposite directions... therefore you would have to pedal forward, or reverse, and have a neutral if not pedalling.

I really should sit down with a paper and pencil before rambling on, since I have just proven that I have no idea what will happen without some doodling...

Summary:

I don't know which would be the problem, but I can say confidently that it won't work in a traditional fixed gear way.

That is my final answer!

(Thanks for letting me think out loud)
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Old 01-10-08, 10:58 AM
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You'll eventually strip the pedal out of the left side crank. If you really want to do this try to find a tandem crank.

This chain tension will be the biggest issue. You'll have to find gearing that gives you the same chain wrap on both sides.
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Old 01-10-08, 11:04 AM
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I would really like to hear what Sheldon Brown would have to say.
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Old 01-10-08, 11:10 AM
  #14  
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Originally Posted by caloso View Post
I would really like to hear what Sheldon Brown would have to say.
candyman candyman candyman

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Old 01-10-08, 11:15 AM
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Originally Posted by Little Darwin View Post
...
When I think about it, I don't see anything wrong with the idea... except that no matter what you are doing you will hear freewheel clicking.

)
I like it, you'll be coasting up the hills!
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Old 01-10-08, 11:16 AM
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OK... Ignoring chain tension etc, and using numbers easy to calculate...

Drive side 48 x 12, a 4:1 ratio

Reverse side 48 x 24 a 2:1 ratio

Pedalling at 100 rpm forward would on the drive side give you a rear wheel speed of 400 rpm.

With the rear wheel spinning at 400 rpm, the non-drive side would work like a typical geared bike rolling backwards, so the spinning wheel would move the cranks.

At a 2:1 ratio, this would attempt to move the cranks at 200 rpm.

Typical cranks can not turn 100 rpm on one side, and 200 rpm on the other, so it will not work, unless the gearing on both sides is identical... which is not what was suggested.

This configuration will not work.

THAT is my final answer.
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Old 01-10-08, 11:18 AM
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Originally Posted by Little Darwin View Post
OK... Ignoring chain tension etc, and using numbers easy to calculate...

Drive side 48 x 12, a 4:1 ratio

Reverse side 48 x 24 a 2:1 ratio

Pedalling at 100 rpm forward would on the drive side give you a rear wheel speed of 400 rpm.

With the rear wheel spinning at 400 rpm, the non-drive side would work like a typical geared bike rolling backwards, so the spinning wheel would move the cranks.

At a 2:1 ratio, this would attempt to move the cranks at 200 rpm.

Typical cranks can not turn 100 rpm on one side, and 200 rpm on the other, so it will not work, unless the gearing on both sides is identical... which is not what was suggested.

This configuration will not work.

THAT is my final answer.


that makes sense

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Old 01-10-08, 11:22 AM
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Isn't the 2:1 ratio (NDS) freewheeling at this point?
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Old 01-10-08, 11:29 AM
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Originally Posted by Bob Barker View Post
Isn't the 2:1 ratio (NDS) freewheeling at this point?
As stated, it would not, since when viewed from the freewheel's perspective, the wheel is actually rolling backwards...

Just think about what happens when you push your bike backwards. The crank moves, and the speed it moves at is determined by the gear ratio and wheel speed... When we see it, it is usually slow because we are just pushing our bikes backwards... but at riding speed, it would be significant.

For additional effect, who hear hasn't hit a shin against the pedals when pushing a bike backwards... The force is real.

Last edited by Little Darwin; 01-10-08 at 12:14 PM.
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