Disc brakes and bottle dynamos

[cygnet]

Just idly wondering is the hub dynamo so much better than a bottle that there is no scope for disc wheels and bottles?

My thoughts ran like this

Hubs:
Always 'on' (even if the drag is low)
Needs to be disconnected if you need to remove the wheel

Bottles:
Suffer in the wet because the track is the braking surface which obviously is smooth
Some types which used a special tyre, with a ridged track which limits your tyre choice

But with a disc brake there is potential for a permanent rim track, so overcome the wet problem, any tyre can be used, it can be 'turned off' by moving away from the rim and doesn't need disconnecting.

Does such a thing solve a problem that no longer exists?

[LittleWheelsandBig]

Some dynamos run on the rim, rather than the tyre.
http://www.velogical-engineering.com/velogical-rim-dynamo---standard-bicycle-dynamo---smooth-running-lightweight-efficient

Ignore this following statement as I completely misunderstood your comments. Running on the disc would reduce the rate the dynamo turns, which would probably require more mass to get the same output. A hot disc would tend to degrade any rubber/ elastomer drive wheel for a dynamo.

[Russell]

What you are referring to is basically a spoke dynamo.  I have two types:

One which can be disconnected by moving a lever away from the spokes but this model is no more efficient than a bottle although it has no slippage issues. Aufa FER2002  No longer available.

The other is permanently fixed to the rear hub and produces power all the time.  This is not an issue as it is as efficient as a hub dynamo (if not more so) and much smoother. Sunup Eco EcoDyn

[Cudzoziemiec]

A rim with a special track would limit you to that one rim. Not quite as limiting as being restricted to one particular tyre, since you wouldn't need to replace it, but nevertheless a restriction. And most of all, it would require that special rim to be made! Plus, it's not really a problem that needs solving; the drag from a dynamo hub is so small you won't notice it while riding and disconnection is easy enough, very easy with a lego-block connector as used by SP and Shimano.

[cygnet]

Yes, I was imagining the generator at the rim, not the disc itself or near the hub. Like the look of that sunup though, and the velogical one is probably what I was imagining without needing changes to the rim.

[Kim]

There are also those eddy-current dynamos that use the rim without physical contact.  Doesn't need a special rim, but does need some fairly tight clearances.