Planetary Granny Gear for Bicycles

Out here in Oregon long bike rides often mean climbing big steep hills, which means shifting down into granny gear and slogging away for an hour or so till you get to the top. Once you get over the top and start going down, you want to shift to top gear to pick up speed in order to put a big stupid grin on your face. Or if you want to be practical, so the breeze will cool you off after you put in all that work climbing the hill.

The way things are out here is that soon after you get to the bottom, you face climbing another big stinking hill, which means shifting into low-low gear again. After doing this for a while you begin to wonder why you even have the other twenty zillion gears, the only ones you are using are top and bottom.

This wouldn't be a problem except for the way bicycle gear trains are built. You can't shift directly from low to high, you have to shift through all the intermediate gears as well, or a substantial number of them. And using the front derailleur to change chainwheels requires that you ease off on pedalling until the chain has been moved. This can be a real trick if you have misjudged how quickly the hill starts going up. You may even have to stop or turn around in order to be able to change to a smaller chainwheel.

Now there a number of internal gearboxes available for the hub of the rear wheel, but there really isn't anything available for the bottom bracket, and if you want big changes, this is where they need to be made. The reason is you don't want eight inch diameter gears whirling around at wheel speeds on the back axle. You really want to minimize your rotating momentum.

How about a planetary gearset for the bottom bracket using large diameter, very thin gears? You would only need three speeds, which means one of the elements could be locked to the frame. The crankshaft would alternately engage one of the other two elements, as would the chainwheel. I think a device like this could be made as light and durable as the current multiple chainring scheme, plus it would have the added benefit of being able to shift much more quickly. If would be virtually instantaneous, especially when compared to a derailleur.

It would take some figuring and no doubt some fiddling to be able to come up with a design that would fit in the space currently being used by current derailleur designs, but if it worked, and worked reliably, it could be a big hit in the bicycle market. Yeah, and if wishes were horses, beggars would ride. Anyway that's my big idea for the day. Here's a spreadsheet that illustrates the gear ratios you can get from planetary gear sets.