does anybody remember

[gray198]

many years ago there was a programme on Tv called 'Tomorrows World'. I seem to remember an 'Invention' of a bike chainwheel which had a constantly changing gear ratio depending on the amount of effort the rider was putting in. It's a long time ago but I think this was achieved with a system of springs on the chainwheel which compressed as more effort was applied making the gearing constantly variable. Needless to say I have never seen anything like this on a bike. So I just wondered if anyone else remembers this, or am I just dreaming??

 

[flecc]

I only vaguely remember it, but it wasn't very practical as it meant a mismatch of chain link to teeth lengths as the shifting took place. It also wasted rider energy in loading up the springs.

Much better is the system in the adjoining thread

 

[rog_london]

Not QUITE what you're referring to - but I have a mountain bike which is probably close to 20 years old and that has a non-circular triple chain wheel. This was an idea of Shimano, and what it actually does is it varies the ratio as it turns - so you get more leverage and the chain wheel travels faster when your legs are at the bottom and top of their stroke respectively, and the opposite when your pedals are level with the bottom bracket. Put another way, the gear ratio varies throughout the rotation of the chain wheel. Of course the derailleur idler takes up the change in effective chain length as this is happening.

The idea as I recall was to give the rider some additional help by effectively varying the gear ratio so you are in a slightly lower gear when you don't have maximum push available - that being the first situation I mentioned above.

I don't know whether it works out in practice or not - I guess it didn't work out for Shimano because the idea fell out of favour fairly quickly. That is unless someone knows different. They called it the 'Biopace' system.

Rog.

PS - I see it's been mentioned in another thread - sorry to duplicate it. Apparently it's an idea still having a following.....

 

[flecc]

Yes, very much so Rog, Bradley Wiggins winning the Tour de France using one should be proof enough that they can work well. Some riders can't get on with them though, our former champion rider Chris Boardman among those.