are the DH ones, not just mechs in boxes, like in the honda.

when i say make it fit, i dont mean make the gears smaller, i mean, make a frame to fit it.

sliding dropouts for tension, like on the orange P7, and a BB that is essentially the size of an Alfine or rholoff hub.

cranks go on the spindle, the chain wheel goes on the sprocket mount, and the whole thing is threaded so it screws into the frame, like a big BB

i dont know why no one has put an Alfine or Rholoff system into a BB yet.

Making small things that last long in harsh conditions is very very expensive, and bordering on impossible. Just look at the number of HTII failures people seem to report (if you believe they are not just a noisy few).

How do you make a bike with a huge mutha of a BB shell, and another shell inside it, without adding to weight, losing lots of ratios, or completely messing up the handling and ground clearance of your bike?

A couple of engineering student mates were on about developing a hydraulic drive train with continuously variable resistance. By changing the flow rate of the fluid (or something), you could have the equivalent of a higher or lower gear. Problem is, hydraulic transmissions are currently about 40% efficient, compared to about 95% for chain drives.

Empire were on about this – make the frame bolt onto the gearbox like the do with motorbikes.

If you can take the cassette, derailleur etc. off the wheel and put it low down in the middle of the bike, that helps suspension performance and handling.

Hydraulic transmission is a nice idea, but as you say inefficient.

When you are the engine you notice these things more. But if you have fossil fuels that you don't mind wasting they're fine.

had a look at this in the shop. it's **** heavy!

Here is the Nicolai one.
You can see it is part of the frame and they use the gboxx idea.

The ground clearance argument about a BB based gearbox is boloney – your BB clearance is not defined by the diameter of the BB shell, it's the diameter of your largest chainring. So long as the BB shell is no larger than whatever chainring you'd chose to drive a hub mounted g-box then it makes no difference, and more significantly makes the weight of the drivetrain unsprung (assuming a the BB shell isn't rigidly attached to the stays)

With a constant chainline you have more ability to control the suspension behaviour under load as the point of force will always be the same and therefore could be compensated for.

Not sure I like the idea of including the freewheel mechanism into the BB shell, as that would mean that when freewheeling down hill it'd be like straddling a chainsaw!

how is putting a geared hub where the BB is, instead of at the back, any heavier at all?
youve just moved it forward, which can only be a good thing anyway?

how is putting a geared hub where the BB is, instead of at the back, any heavier at all?

cos the bb shell is integral to the frame making it bigger is heavier than just a big hub

anyway, the answer is no – they have not had their day, but they are moving into middle age.

Padowan, but if you had something the size of a G-Boxx, for arguments' sake, but centred on the BB, you'd lose any ground clearance advantages?

If you were comparing a standard BB with no chainrings, to a G-Boxx with the input drive at the same point (BB) then yes, the G-Boxx would reduce your ground clearance (between ground and "BB" shell) because it's taking the drive that goes to the hub upwards meaning that the drive sprocket (however big it is) shouldn't protrude below the BB shell.

But I was looking at it from the point of having something like a Alfine or Rolhoff where the input and output shafts are on the same axis, where the drive gears surround the input shaft (BB) and drive a chainring around the same rotational point. In that instance, for a given chainring that becomes your clearance limitation over a hub mounted gearing.

Home made gearbox bike in the same vain as the Honda!

http://www.pinkbike.com/video/20950/

An Alfine can only give you the range you'd get from one of your chainrings on a derailleured setup. How's that going to work for moderately fit somewhat overweight biffers like me somewhere with some steepish ups like the Qs?

Not really true about the Alfine, as you get a lot more gear spread than just one front chainring (on normal setup).

There are a few compromises, in terms of jumps between gears, but the Alfine will cover enough ratio's for most situations.

I'm a biffer too, and the Alfine is great IMHO

My understanding is the alfine is a bit more than a 1×9 but not as much as a 3×9 in terms of gear spread – sort of equivalent to a 11-38 rear casseette