Why not have the gears inside the bottom bracket?

Looking into an IGH it seems the biggest disadvantage is having a huge amount of rotating mass. I've seen one handmade bike where the IGH sits above the bracket, inside a carbon fibre shroud around the place some bikes have rear suspension. There are two chains, one driving the IGH and another to drive the rear wheel.

Why not have an internally geared bottom bracket where the cranks drive the input shaft and the crank ring rotates at a ratio selectable by the gear lever?

Then the rear wheel could have a single drive wheel driven by a chain or belt and a lightweight hub just solid enough to mount the spokes and disc. Then you could have mud-free gearing while reducing rotating mass even below what you have with a conventional cassette... and given that you have thrown out the bottom bracket the overall weight gain would be less.

Quite a job for the home builder to adapt an existing hub I guess but it should be possible.

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The design you saw is probably a G-Boxx or derivative. – lantius Aug 5 '11 at 6:29
Thanks - the bikes/history page there is very interesting. Amazing to think this was thought of at least as early as the 1930s. – Adam Eberbach Aug 5 '11 at 6:33
The rotation of mass is a minor problem, but the actual weight and balance of the bike might make it worthwhile in a niche market kind of way. – zenbike Aug 5 '11 at 7:13
Also, the weight differential should be about equal, since you move the IGH hub to the BB, but must add a rear hub to the wheel, which will be approximate to the weight of the BB you remove. – zenbike Aug 5 '11 at 8:18
The effect of a rotating mass is proportional to the radius around which it rotates. A hub is quite compact, and the resulting gyroscopic effect is negligible compared to the wheel. – Daniel R Hicks Aug 5 '11 at 11:33

The lack of popularity is partially because it's a solution in need of a problem. The effect of rotating mass in general is generally overstated when considering the total energy that goes into cycling, and mass near the axle has even less of an effect. You can work out the math here.

The main advantage comes when considering unsprung mass on full-suspension bikes, where having weight on the rider's side of the suspension can improve the handling and response of the bike. This is why inroads have been made in downhill circles.

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Agreed. I like the idea for all the same reasons I like an IGH. Low maintenance, sealed unit, and consistent "always on" shifting. If we could move the weight of the hub, especially for something like the Rohloff, to a more balanced location on the bike (BB instead of rear hub), that would be cool. It is not necessary, however, and would require a frame built for it, so it won't likely ever happen on a commercial level. – zenbike Aug 5 '11 at 7:10
Since I bought a Spot Acme I don't think I could go back to a chain and conventional derailleur, but I do wish the weight could be more toward the front of the bike. With its carbon fork it feels like a light bike up front, and a heavy bike in back - especially with a rack and panniers. – Adam Eberbach Apr 19 '13 at 1:49

There are a couple of crank designs that use this principle. Off hand, the HammerSchmidt crank from SRAM is the most practical current application of it because it doesn't require deviation from the current frame specifications.

While it is only 2 gears, and the shifting is built into the crank, rather than inside the BB shell, that is a limitation of the currently marketed frame specifications, and it is only likely to change if consumers show a massive interest in BB Gearbox, enough to convince a manufacturer or 2 to stick their neck out on a new design.

Another factor to consider in modifying or building your own frame is How to keep the BB spindle relatively placed where it is in relation to the saddle position, for correct fit applications, while simultaneously increasing the BB shell by an approximate factor of 3, and not decreasing ether tire clearance or ground clearance.

Suntour, Hayes, Honda and Nicolai were all rumored to be building something like this for the DH market, but nothing has currently come available for sale that I can find. These photos are of the Suntour Gearbox offered up at the 2006 Interbike Show.

All of these are issues that could be worked out, and I think overall, this is an excellent idea if we could apply it to the belt drive, commuter/touring market. Commercially, probably not feasible, but maybe a niche market for a custom builder?

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Zerode has one for sale currently: zerodebikes.com/page/Bikes. Super expensive IIRC, but the design looks solid and I could see it trickling down eventually. – AlexCuse Nov 27 '12 at 17:51
Apparently there was a B-Boxx announced back in 2008 too, but nothing seems to have come of it. bikerumor.com/2008/09/04/… – armb Feb 5 at 10:46

There's also the Schlumpf drives, which fit inside a standard bottom bracket. http://www.haberstock-mobility.com/en/products/schlumpf-drive.html

Only two speeds though, like the HammerSchmidt.

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There actually is a 18-Speed IG drive that is on the market: the pinion drive.

Only a few framebuilders incorporate this in their frames, but they do exist...

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This, sir, is going to change my life. – Dakatine Oct 7 '15 at 13:52
Also, there's this one ongoing. nuseti.com – Dakatine Oct 7 '15 at 20:36

Poland-based company "efneo" is developing a three speed crankshaft gearbox: http://www.efneo.com/. This product will allegedly ship next year.

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Currently scheduled for November 2015: indiegogo.com/projects/3-speed-front-bicycle-gearbox-efneo#/… – armb Aug 12 '15 at 10:22
And in January 2016, it still hasn't shipped, but "we have come back from our latest trip to our partner's factory in Taiwan and have some good news. We are just starting final tests of fully manufactured (not prototyped) gearboxes." indiegogo.com/projects/3-speed-front-bicycle-gearbox-efneo#/… – armb Feb 5 at 10:42