> Years & years ago, you could get bicycles with automatic gear
> changes (derailleur type).  It worked on the tension of the the
> chain, high tension means it shifted down, low tension shifted up

I saw those bikes, but they didn't do what I wanted. More useful
than an automatic gearbox is one that knows what front and rear
sprockets are in use, and what is the next combination if you want
to go up or down a ratio. I haven't the tables I did to hand, but if
you're familiar with say a 21-speed, the sequence from granny
to top gear isn't front1-back[1234567] front2-back[1234567]
front3-back[1234567]. Up/down by one ratio is often a change
to both front and rear. I did figure out how to do it with a couple
of solenoids or steppers and a micro, but didn't (yet)

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist