GreenPlease wrote:
If you have expanding chainrings for both the crank and the rear hub you don't need a chain tensioner.
This ^^^. You could probably operate the expanding gears with servo in the bottom bracket and rear hub. It would have virtually stepless and uninterrupted gear ratios so you could simply set a target RPM and have the system constantly trim the gear reduction to within 1-2 RPM based on rear wheel rotational speed. Huge advantage on a mountain bike. No hand controls needed for shifting.
Now, to get the chainrings and cog to be mechanically efficient despite the gaps.
You'd still need a very small tensioner because of the lag in shifting. But it could just be mounted to the chain stay and also act as a guide to prevent dropped chains and make wheel changes easier. The system would also allow changes in wheelbase with horizontal dropouts and much easier wheel removal.
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