On Oct 31, 8:00 pm, Hank Wirtz <
[email protected]> wrote:
> Anybody tried the Sturmey-Archer X-FDD Dynamo Drum Brake hub? I'm
> thinking about one of these for my cruiser, with the idea that a hub
> brake might be good for winter commuting. Plus, it has 36H, whereas
> the Shimano has only been available as a 32H for several years.
I bought one about a year ago. Initially I was very pleased with it.
(The brake works well, and has needed no maintenance after the initial
month during which the pads wore in.) However the dynamo part of my
hub has already developed a problem. The dynamo worked OK for about 6
months of mainly summer usage (so very infrequent use of my lights)
but during the first winter it started to cut out, making a nasty
grating noise inside the hub. My first thoughts were that it was a
mechanical problem - some grit between brushes etc. But after
disassembling and reassembling the hum many times in an attempt to
diagnose and cure the problem, it became clear that there were no
brushes or sliding contacts. It's an ac dynamo! No signs inside of
wear and tear or arcing. And worse of all, there is a curious time
constant connected to the failures. Here is what I get on a typical
ride home in the night at present: the events are fairly repeatable
(1) Depart lab, ride at speed, dynamo works fine. Light works, dynamo
functions for about 6 or 7 minutes. Smooth, no problems.
(2) 6 or seven minutes into the ride, dynamo makes nasty grating noise
and light goes out.
(3) If I leave my light switch on for the rest of my ride home, the
hub makes intermittent grating noises for the rest of the journey, but
the light NEVER comes back on again.
(4) If instead I straight away hit the off switch on my lights, and
ride for about 10 seconds, I can then hit the on switch and get about
3 seconds of light before the grating noises start and the lights cut
out.
(5) The longer I turn my lights off for, the longer the lights stay
on again for when I turn them back on again. Typically in order to
guarantee 1 minute of on time, I need to have the lights off for 10
minutues.
(6) [ NOTE: I don't have any fancy batteries charging up -- my lights
are just two plain tungsten filament bulbs, with wires connecting them
in parallel to the dynamo, which has a simple switch between it and
the lights.
(7) It seems to be the case that the slower I ride, the further I can
get the lights to stay on for.
----
All the above leads me to believe (though I am by no means certain
that this is what is happening) that the coil inside the hub (which
remains still in the rest-frame of the bike) gets hot and undergoes
some kind of thermal change (an expansion) while it delivering power
into an external electrical load. After it has got hot enough, arcing
or insualtion breakdown, or shorting of part of the coil to ground
occurs somewhere inside the hub, and the sound of the sparking may
well be what I am hearing with the grating noise - though I see no
carbon deposits on any visible parts of the coil or internal casing.
If I do not switch the lights off, it looks like the hub continues to
deliver power into the defect (wherever it is) which keeps the coil
"hot" and prevents the system getting better. However when I switch
the lights off when they stop working, somehow (in a matter I don't
understand,), it appears that the hub stops delivering power into
the "defect" and so the coil slowly cools, at about 1/10 of the rate
at which it heated up, somehow remedying the problem and allowing me
to use the lights again later on after a sufficient delay. Perhaps
the coil contracts and causes sum short to release.
There are lots of parts of that diagnosis that are very much
conkectural. The hub body itself does not get hot to the touch at
all, but then it is not in contact with the coil inside which is the
part that is likely to be heating itself up, and it proabbly only gets
5W or something, and there's a lot of air cooling of the body.
It has not been possible to reproduce the problem by dismounting the
wheel and running it on a test rig, nor to dismantle a wheel fast
enough to determine if the coil itself is hot.
The coil is a user-non-servicable part, so essentially I am stuck.
I have a lemon of a wheel that only acts as a dynamo for about 6
minutes per ride, twice per day.
I was very disappointed with this hub, despite great hopes for it.
