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@100
Fuck me, you are quite right, and fuck me this is a big reply, but you got me thinking . . . .
First up - thanks for saving me from wasting time and money playing around with this - I was going to pick up the pieces in the week as I was convinced it would work. It took me a while to visualise what you had said - and to confirm it I sketched the system out on my computer - and you are 100% right!
With a 16t freewheel and 24t freewheel (chosen for the easy maths) and two equal tooth count chain rings - so the system is pulling the same amount of links off each freewheel at the same rate (again for easy maths) - the following happens . . .
Assuming a certain starting position, you turn the cranks - which pulls the chain - which turns the rear small freewheel (left hand side in illustration below) - this freewheel's drive notches connect with and then pushes it's pawls - which are fixed to and so turn the hub - the hub then turns the the pawls on the bigger freewheel mounted on the other side.
But . . . the freewheel on the other side has it's centre (hub/inner freewheel/pawls) and it's outer (teeth) turning at different rates. . . .
Assuming a 16t small freewheel and a 24t big freewheel, when you turn the cranks and pull 24t worth of chain off the rear of the drive chain (half a revolution of the cranks on a 48t chain ring - for instance) then the 24t freewheel will turn one full revolution while the 16t will need to turn 1 and a half revolutions to 'supply' the chain ring with 24t worth of chain.
So the inside of the 24t freewheel (driven by the smaller 'drive' freewheel through the hub) will be turning faster than the outside - as the outside of the 24t will turing at 1 revolution per half crank turn (assuming a 48t chain ring) while the inside will be turing at 1.5 revolutions per half crank turn (assuming a 48t chain ring), this means the 24t freewheel's own pawls will quickly catch up with and then push it's drive notches (normally it is the notches that push the pawls) - and when they do the whole system will bind and come to a standstill as it's pawls can't move any further forward, which mean the hub can't continue to rotate.
Making the chain ring smaller on the 24 freewheel side (my initial intention to give two different ratios) only makes matters worse (grinds to a halt quicker) - making the chain ring bigger only transfers the problem to the other side.
It's totally fucked. :) The only way it would work would be as mechanical_vandal says, to have the same size freewheel on each side, which defeats my initial reason to try this.
In this illustration I have 'glued' the pawls down, out of the way of the drive notches on the 24t freewheel (right hand side freewheel), so it can turn and you can see the system moving smoothly, notice the inside of the 24t and specifically that it's pawls are turning faster that it's drive notches/outside/teeth.
Now imagine we unglue the pawls on the 24t and let them spring out into position, you can quickly see that they would meet the slow moving notches and the curtains would come down on this whole sorry mess. (ignore that the chain is not wrapped around the freewheels, it is just there to show that both sides are pulling the same amount of chain).
Anyhow, mechanical_vandal you are officially the winner of this thread !!! :) Cheers once again, you saved me a few quid and a week of oily hands - and well done for working this out in your head (!) - it's hardly intuitive stuff!
Right then, anyone ever tried two 650c wheels on the front on their bike?