Lend me your brains

I mean, when I flip my wheel over to use a different sprocket, I slide the axle forward in the dropouts.

This is because you are using the same chain length for both ratios, my set up would have the chain length tuned specifically for each ratio.

I'm pretty sure it wouldn't work. Not quite sure why but I have a bad feeling about it. At the very least, the LHS tensioner would have to be upside down, no?

With a freewheel the tensioner only prevents the chain from jumping off, doesn't actually take the strain of the drivetrain (which is on the 'top' of a conventional chainline).
So a single tensioner on the right hand side, used with the leftie drivetrain set at correct tension would be okay.. right?

but you can't do this for both combinations at the same time. So you'll have to find two magic-gear ratios that will allow you to run the wheel in the same positon, or one chain will be slack.

Yeah I think you are right, unless I choose my ratios carefully I might have one chain a little slacker that the other.

But I will work the maths out and, if needed use a half link, to keep the chains fairly taught.

This is because you are using the same chain length for both ratios, my set up would have the chain length tuned specifically for each ratio.

Sir, I refer you to my previous point, and provenrad's comment.

Essentially what we're saying is that tuning chain length is not as easy as it sounds if you want to avoid having a slack chain (and you* do* want to avoid this).

By the way, this does sound like a fun idea and should be fairly cheap to try out, even if it doesn't work. I'm not trying to sound negative, hope my comments don't come across that way!

What'll you use to keep the reverse freewheel on the hub?

If I use front brake in a skid I normally (always) fall off.

You will have 2 different chain lengths, which, all things being equal, will mean one side will wear / stretch faster than the other. So if you do manage to get the rear wheel straight in the dropouts after a couple of months I would imagine you will struggle to. Perhaps the effects of this won't be noticeable.

I can't get my head around why having different ratio forwards to backwards won't work, seems possible but perhaps not practical. I do wonder if it's possible that the pawls in your freehubs could lock to oppose each other and not allow the cranks to be moved in either direction. I'm not too sure. {edit - this is bollocks}

Try it! Tell us what happens!

Once the wheel is locked the gear ratio is irrelevant. Have you tried leaning further forwards?

I'm pretty sure it wouldn't work. Not quite sure why but I have a bad feeling about it. At the very least, the LHS tensioner would have to be upside down, no?

+1

No real need for tensioners.

ooh a chain tensioner on one side! that would work. Do it anyway!

I suspect that the larger freewheel will unscrew itself, jam itself against the rear chainstay and chuck you over the handlebars.

The larger (23t) freewheel is mounted on the non drive side of a fixed/fixed (or free/free) hub - therefore it is being tightened when driven and freewheeling in the 'unscrew' direction.

With a freewheel the tensioner only prevents the chain from jumping off, doesn't actually take the strain of the drivetrain (which is on the 'top' of a conventional chainline).
So a single tensioner on the right hand side, used with the leftie drivetrain set at correct tension would be okay.. right?

Yeah I reckon so, although it would look gash and (in theory) it would be possible to figure out the magic gears anyway...

I'm thinking that the real issue will be in the time it takes for the freewheels to engage, making the braking side feel like mush, and possibly something odd going on with inertia as Brucy said.

Imagine having a loose chain.

Any reason ?

Any reason ?

as mentioned earlier, you cannot just add chain length corresponding to the extra teeth in your drivetrain. if you could, there would be no need for long horizontal dropouts.
Good luck with this idea - if you assemble it I'd love to have a go! :-)

Any reason ?

Inefficiency; higher risk of dropping the chain; people on the forum will see pictures and make snide remarks.

I hope this works! But I know how much I've fiddled with different gearings and half links just to get one chain line sweet. I'm buying a freewheel tomorrow. The race is on to have London's first dual-chain fixed. I slap you with my glove.

This would be hard to do, the chainline on both sides will be extreme. Unless you shorten the BB shell and use a threadless BB it might be possible, but then the crank arms will probably hit the chainstays.

I commend you for trying and I'm not trying to discourage you as it sounds like a lot of fun, but is it really worth it just to be able to skid?

Take lots of measurements of the whole rear end of the bike first of all...

MAGIC FUCKING GEAR!

jeezus!

Sir, I refer you to my previous point, and provenrad's comment.

Essentially what we're saying is that tuning chain length is not as easy as it sounds if you want to avoid having a slack chain (and you* do* want to avoid this).

The maths for matching ratios is fairly simple, hopefully I can get the chain lengths within reason.

If I did use a tensioner, I would of course only have to use one, the non-drive side would be set up taught and untensioned as you would any fixed drive chain (so no need for tensioning) - the drive side would then be made the slack side and tensioned.

But . . . I so think I can set this up with acceptable (and safe) tolerances without tensioners. (??)

By the way, this does sound like a fun idea and should be fairly cheap to try out, even if it doesn't work. I'm not trying to sound negative, hope my comments don't come across that way!

No! This thread is for exactly that, come up with an idea, float it out on the water and invite people to sink it, if it sinks, refine it.

So throw your stones, that's the point. (just don't aim for my nuts)

What'll you use to keep the reverse freewheel on the hub?

Nothing would be needed.

Freewheels generally don't need lock rings, this one would not be an exception as it drives towards 'tighten' and freewheels towards 'loosen', like all other freewheels.

If I use front brake in a skid I normally (always) fall off.

You will have 2 different chain lengths, which, all things being equal, will mean one side will wear / stretch faster than the other. So if you do manage to get the rear wheel straight in the dropouts after a couple of months I would imagine you will struggle to. Perhaps the effects of this won't be noticeable.

Good point on the wear and tear. I might make the chain length equal if I can.

I can't get my head around why having different ratio forwards to backwards won't work, seems possible but perhaps not practical. I do wonder if it's possible that the pawls in your freehubs could lock to oppose each other and not allow the cranks to be moved in either direction. I'm not too sure. {edit - this is bollocks}

Definitely bollocks ! ;p

There is no information (in a technical sense) communicated between the freewheels, they are doing only what any other freewheel does, freewheeling or driving.

Try it! Tell us what happens!

Once the wheel is locked the gear ratio is irrelevant. Have you tried leaning further forwards?

Agreed, once the wheel is locked the ratio is largely redundant, but lower ratios make locking the wheel up easier and that would be the point.

Also simple modulation of the rear wheel speed would be easier on a lower gearing.

Tried leaning forward more, and it does help, but less than ideal and still not enough control for me.

Yeah I reckon so, although it would look gash and (in theory) it would be possible to figure out the magic gears anyway...

I'm thinking that the real issue will be in the time it takes for the freewheels to engage, making the braking side feel like mush, and possibly something odd going on with inertia as Brucy said.

Thought experiments say this is not a problem.

magic gear calculator: linky

Cheers fred, might come in useful.

I thought this was fixed both sides but it turns out not:

http://www.fixedgeargallery.com/2005/mar/TheBikeCo-op.htm

I thought this was fixed both sides but it turns out not:

http://www.fixedgeargallery.com/2005/mar/TheBikeCo-op.htm

Cool ! That's kind of half the idea right there !! :) See I am not mentally ill !!!!!

So I just extend this idea by having differing ratios.

Cheers for the link.

if you put two free wheels on a double fixed hub, both the conventional way round to work on the right hand side when you flip the wheel, your left hand side hub will now run backwards as you have flipped it over. so if you put a chain on both sides, when pedaling forwards the right hand side freewheel will engage and the left hand freewheel will clickety clickety free wheel (causing a bit of drag) if you pedal backwards or try and coast your right hand freewheel will not freewheel but the left hand one will now engage and give you drive in the reverse direction. no reason why you couldn't have different ratios on both sides but chain tension could be fun (although not that important because being freewheels you can use a chain tensioner on either side).

this would allow you to have a high forward gear and low reverse gear making for easy skidding/braking, you will also have a lot more weight and frictional losses to deal with.

IMHO it will work, but not very efficiently, and whatever you do while you are moving you have a bloody freewheel clicking away which would drive me insane!!!!

http://www.londonfgss.com/post170556-47.html

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

"This is not the case. When the rider exerts normal forward force on the cranks, the right side drivetrain makes the rear hub spin forward. The rear hub, in turn, spins the inside, threaded half of the left side freewheel, which causes the left side freewheel to engage, spinning the left side chain. So while the top half of the right side chain is in tension, the bottom half of the left side chain is also. The former is pulling forward, the latter is pulling backwards. In other words, the left side chainring is being pulled around by the left side freewheel. The left side freewheel is being twisted by the hub, which is being twisted by the right side freewheel, which is being twisted by the right side chainring."

...

"Furthermore, breaking-in a dual-drivetrain bike is distinctly different than breaking in a normal fixie. Specifically, as either freewheel winds slightly tighter on the hub (which happens with any threaded, ie non-freehub, system), the amount of pedal lag changes slightly. This is because as a freewheel threads onto a hub, its engagement point move slightly relative to the hub, causing them to go into or out of sync with the engagement points on the other freewheel. This makes for a much different first few rides on these bikes, as the drivetrain becomes, somewhat sporadically, more or less responsive. Again, though, this process is much safer than the break-in process on a normal fixie, where a cog can thread on far enough that it is no longer touching the lockring. In these cases the lockring is prone to completely unthreading, allowing the cog to do the same. This is not the case with the dual-drivetrain system, in which niether freewheel will ever get any looser because of any pedaling force. That's because backwards pedaling forces don't affect the threaded (inner) portion of a freewheel - only a freewheel tool can do that.
Regardless, breaking-in a dual-drivetrain bike isn't especially fun. When one freewheel threads onto the hub more (for example, as a result of a hard skid), the result is often a drivetrain which is twisting itself at both pivot points. When this happens, the top half of the right-side chain and the lower half of the left-side chain are both very tight. As a result, both the bottom bracket spindle and the rear hub are under significant torsion. This makes riding the bike very * slow; * it seems to resist any force the rider puts on it."

etc etc.

so apparently it does work, after a fashion, but it binds a bit.

edit - snap!

BillB got there first !

:)

Still will give it a go.