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"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."
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"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!
Mmmmm I read all that and don't agree with a lot of it (especially the first paragraph), the rest of the stuff I don't understand ! :)
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I think what you want is a coaster brake. As badtmy mentioned, it's been discussed before. Don't remember if it were free/free tho. Seems like a lot of trouble just so that you could skid. Two brakes is the simpler solution. KISS. On the other hand, would be fun to put theories to practice!
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your plan wont work, unless your figure a way to selectively lock and unlock either the driveside and non drive side so that only ONE crankarm is operating at any given time.
Otherwise the larger gearing will dominate the drivtrain, and besides to enable your system to work you would have to physically make the rear wheel spin in reverse. I know people like Chris Hoy have strong legs but i think thats out of the question even for people like him.
invent a rear hub, fixed- fixed, that you could selectively chose either driveside or non driveside. It probably wont be popular cos it will be heavy, but this is your idea.
A viable alternative is to get a back brake. :)
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your plan wont work, unless your figure a way to selectively lock and unlock either the driveside and non drive side so that only ONE crankarm is operating at any given time.
?
I think you have not read my original post, or read it and not understood what I am aiming to do.
A viable alternative is to get a back brake. :)
Agreed, always an option.
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One very important thing to bear in mind is the strength of the pawls inside the freewheel....the amount of force your legs can produce when driving a freewheel will be nothing compared to the force of effectively turning it into a fixed cog and completely locking it up every time you skid.
On a normal freewheel that kind of force is never there because the wheel will just keep spinning faster and the pressure on the pawls will never really get past a certain point.
The pressure on the pawls when forcing it to effectively 'lock up' ,especially from the kind of speeds you're hoping to achieve will be increased by a very significant amount. -
Dylan....two freewheels running opposite to each other will feel basically the same as a fixed wheel....when one is driving,the other is freewheeling....and it will do the same in both directions.
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His theory is sound...there's none of this 'having to overcome the big gear' because the transition from pedalling forwards to pedalling backwards instantly cancels out the big gear...only one freewheel is ever driving at once.
The ratios turning at different rates also makes no difference because the freewheel that isn't driving is simply coasting along.
In theory, the fact that he's using two freewheels does infact mean that he will have a big gear for driving and a smaller gear for resisting.The only problems i see are the strength of the freewheel pawls...finding freewheels with fast enough engagement to limit the spongy feel...being able to get both chains tight...and the constant freewheel noise (which can normally be eliminated by packing with grease.
As for the pedal thread loosening on a drive side crank run on the non drive side...i ran left hand drive profile cranks on the right side of my brooklyn race link for months with no problem....if you're worried, just use a bit of thread lock...although you will need to buy two sets of identical pedals so you can switch one right axle into one left body.
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Apart from stuff that's been mentioned already...
Why such a HUGE difference between ratios? 42/23 is too low. You can get major acceleration from a standing start closer to 60".
Surely having ratios with matching teeth counts will put you in the right ballpark for starters:
48 -> 42 = -6
15 -> 23 = +8 ... so using a 21 would match.However, **where are you going to find a freewheel with more than 18t?
**A 42/18 would give you a snappy gear on which to accelerate, and a smoother transition to your 'track' gear.
eg:
42 -> 46 = +4
18 -> 14 = -4Giving you approx 61"/87", with same number of chain links each side.
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ACS Claw freewheels come in 20t and are good quality.
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ACS Claw freewheels come in 20t and are good quality.
Oh well, that just gives a few more options :)
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Irrespective of the availability of freewheels up to 20t (or knackered old gear at 22t), those numbers are the fucking shit.
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i just tried to read this thread before my morning coffee and i have just put myself into a coma.
k, thanks. :)
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At 90+, there's definitely a high chance of dementia.
fatboyralph
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Brucy
turnpedal
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np, was suprised to find it tbh. Looks like it will be horrible, good luck! :)
I do hope that it's not possible for the pawls in both freewheels to engage at the same time, that would be messy due to the difference in ratios and pulling the wheel against a chainstay. Think it's pretty unlikely.