I've heard that one can install a bottom bracket that has what is essentially a built-in internal hub, giving one the ability to add a virtual double chainring where there's only one. This would be designed to be used in concert with whatever gear cluster or internal hub you have installed in the rear.
If this is true, does it exist in this form, or have I heard of a distorted version of the device? What are the advantages and disadvantages of such a bottom bracket?
Technically, no, there is no current commercially available gearbox bottom bracket. Most likely, the Truvativ Hammerschmidt is the crankset you are thinking of. There were also gearboxes designed by Honda, and a few other brands.
It allows a single chain ring, 2 x 9 or 2 x 10 gearing.
It is mainly aimed at the DH or freeride market, as it is a heavy, but very tough piece of kit. The gearing is not suited for XC or endurance rides, for that reason.
As you can see in the Anatomy View below, the gear change is accomplished not in the Bottom bracket, but in the chain guide.
I hope that helps.
The technical manual can be found here.
The Schlumpf drives have been around for several years and incorporate a two-speed planetary gear drive attached to the bottom bracket. With the heel of your shoe, you tap a small button centered on the end of the bottom bracket to engage and disengage the planetary gear. Three models are available: a "mountain gear" version that lowers whatever other gears you have by a factor of 2.5; a "speed gear" version that raises your other gears by 1.65x; and a "high-speed gear" version that raises your other gears by 2.5x. The Schlumpf drives have been marketed to specialty bicycles such as folding or recumbent bikes that use small diameter wheels, and thus can have difficulty getting "normal" gearing using off-the-shelf components. Schlumpfs have also been installed on unicycles which (obviously) are ordinarily a single fixed-gear.
An advantage of the Schlumpf drive is that with only one chain ring, there is no need for a front derailleur and the chain line is straight -- and one can tap the button to engage or disengage the drive even while stopped. However, despite the straight chain the Schlumpf drive appears to be less efficient in terms of power transmission. Measurements suggest that the Schlumpf drives "cost" about 5% extra in drive train losses. Since conventional derailleur systems range from around 92% to 97% in transmission efficiency (see for example, the work of Kyle and Berto here), an additional 5% loss can be a considerable cost.
Pinion has brought an 18-speed gearbox to the market, the P1.18. It isn't a "true" bottom-bracket with internal gears as is the Schlumpf -- rather, the gearbox is in a separate housing attached next to the bottom bracket, and it is not an "after-market" solution that can be easily added onto an existing bike. The Pinion is intended for downhill or freeride bicycles that are purpose-built around the device.
Wikipedia ("Bicycle Gearing") and Google eventually led me to this:
http://www.haberstock-mobility.com/en/products/schlumpf-drive.html
and this:
http://pinion.eu/discover-pinion/pinion-p1-18/
(I would be afraid to put my money on these... ;o)
FSA Metropolis. http://www.bikerumor.com/2010/04/26/fsa-unveils-metropolis-patterson-drive-internal-geared-crankset-for-commuters/
Advantages? No front derailer and better ground clearance at the crankset. Disadvantages? Expensive. Proprietary.
Kervelo uses some type of geared bottom bracket in its front wheel drive recumbent bikes. I don't know how it works but they have a website.
They make three models. A tourer, a racing model, and a tricycle.
