From: karplus@cse.ucsc.edu (Kevin Karplus)
Newsgroups: rec.bicycles.tech
Subject: Re: Automatic transmissions for bikes
Date: 2 Jan 1993 06:01:42 GMT
Organization: University of California, Santa Cruz
Distribution: world
Reply-To: karplus@ce.ucsc.edu (Kevin Karplus)
NNTP-Posting-Host: ararat.ucsc.edu
Keywords: torque power cadence

If someone were to try to sell me an automatic transmission for a
bike, I think I would like the transmission to attempt to maintain
constant cadence, but to have an easily accessible shift lever (bar end or
thumb lever, depending on hadlebar style) that controls what the
setpoint for the cadence is.  

When I'm manually controlling the gears, I've found that I tend to be
most comfortable with a cadence a little less than 90rpm (1.5Hz), for
almost any power output, but when I'm very tired and putting out low
power, I'll sometimes prefer a lower cadence.  Although I've made
several conjectures about what my optimum cadence should be, as a
function of power, my attempts to measure it on an ergometer have not
been very conclusive---the optimum is fairly broad, and the
measurement error in determining efficiency far exceeds the effect of
even fairly large changes in cadence.

Given that I sit to ride up hills, I think I would be pretty happy
with a transmission that tried to maintain constant cadence, but
really annoyed at one that tried to maintain constant torque.
My biggest concern would be getting stuck in a high gear after a
stop---that happens to me too often already with manual shifting, and
an automatic transmission would probably have even more trouble
downshifting fast enough during a stop.  About the only time I stand
on my bike is starting up again when I've had to stop in a high gear.
(I'm thinking of getting a recumbent--what do recumbent riders do when
accidentally caught in too high a gear?)

I am not a racer, and I doubt that automatic transmissions would ever
be popular with racers, but they sure would be handy for city traffic,
where I already need both hands for signalling, steering, and braking.

Of course, I'm not willing to give up gear range for automatic shifting.
I currently have a range of about 4.16:1 from top to bottom gear, and
I could use a wider range, if it were available, since my speed range is
more like 10:1 (lots of 10% grades, and I often pull a heavy trailer).
I would be willing to have fairly coarse spacing (say 1.2:1 between
adjacent gears).  I don't see any way to get a sufficiently wide range
from just a rear derailleur (the widest range I've seen is only about
3:1), and I'm not sure I would trust an automatic derailleur to coordinate
double shifts.  

I've ridden Sturmey-Archer 3-speeds and 5-speeds, and consider them
superior to derailleurs for city riding in flat country--they are low
maintenance devices, and allow the use of sturdy chains that don't
need to be replaced every 5000 miles.  (With a chainguard that keeps
out road grit, chains for three-speeds last an extremely long time.)
Unfortunately, existing hub gears don't have a wide enough range for
the hills I now have to ride in.



-- 
Kevin Karplus		karplus@ce.ucsc.edu

Due to budgetary constraints the light at the end of the tunnel is
being turned off.