I'm charged with the maintenance of a Brox Compact that uses a SRAM
3x9 DualDrive. Basically it's a three-speed hub with a 9 speed
cassette stuck on the side, and a shifter that has a twist grip and a
thumbshift integrated in the same unit: thus, can shift both gears
with only one hand.
However, the hub is not part of a wheel. The Brox has two chains: one
running from the front of the boom where the chainring is, to the
middle under the seat, and the second running back to the rear axle.
The hub is under the seat too: it's driven by the front wheel and it
has a sprocket bolted to its side that drives the rear wheel. What's
the problem here? Ground clearance. It's very easy to ground the
jockey cage against speedhumps, kerbs etc. This has bent the
derailleur, and can't be doing the chain much good either.
The picture at http://www.lfns.co.uk/images/bike/thamesfestival.jpg
shows more or less how it's put together, if you can distinugish the
metal bits from the shadows.
So you're going to say "shorter derailleur" ... but wait, there's
more. We need a shifter for the hub. If we continue with the
integrated shifter (and it _is_ nice being able to do the whole thing
with one hand), we're stuck with the SRAM 1:1 cable pull ratio, and
they don't seem to make many mechs for that with a short cage. What
you see is a "long" X7, I also have looked at a "medium" X9, but it's
only about 10mm shorter. The only one they describe as having a
"short" variant is the X0, which is kinda pricey, and not having
actually seen it I don't know if it's _really_ short or if it's just
another half inch, The cassette ratio is fairly close, and we only use
one chainring, so there's no real need for a long cage.
So I'm thinking (always dangerous)
* is there a thumbshifter suitable for the hub that we could squeeze
onto the same handlebar as a separate 1:2 twist grip unit in a
reasonable ergonomic arrangement? Then we'd have a wider choice of
mechs. I don't know if the hub cable pull is the same for other SRAM
hubs or for others from other manufacturers
* or maybe we'll stick with the SRAM stable, and invest in disposable
gear hangers so that next time the cheap bit breaks before the
expensive bit bends. But unless the rider has necessary mechanical
aptitude to replace the hanger (on the road, in the dark) it lacks
somewhat as a "get you home" option, and even then it's not really
desirable.
* other ideas? Most of the time we're riding it we find that the hub
gear is far more useful than the derailleur anyway just because it
shifts faster: we use the hub for starting up and most low/medium
speed rinding, and the derailleur for fine-tuning on fast bits. Don't
think we can afford a Rohloff, but maybe a Nexus or similar? It is
nice to have a range of close ratios at around the 15km/h-20km/h
speed, though, otherwise it's hard work keeping up with the skaters.
Budget rather depends on whether we're talking about one-off costs or
disposable items: if it takes £100 or so to fix it properly that's
fine, but I don't want to be paying £60/pop for items that will get
bent on traffic calming features every couple of months.
What does the panel think?
-dan