There's a very simple answer to the question about single pulley rear derailleurs. According to Frank Berto in his book "The Dancing Chain": "...there was a market prejudice against double pulleys, so Campagnolo, Simplex and Huret made products to meet the market demand." page 163.
According to Frank Berto there was a long running dispute about dual pulleys (and pulleys in general by the hub gear crowd) causing increased friction and reducing efficiency.
I imagine that 3 speed derailleur bikes were built in direct competition with 3 speed hub gear bikes???
Superstition and urban mythes!
Chas. Colerich Oakland, CA USA
Bicycle Specialties wrote:
> If it was such a stupid design why did other manufacturers make similar
> ones? The market for such a derailleur, designed to cover a relatively close
> range of gears was massive, probably far greater than the American market.
> Both of the Campag. models, two pulley or one pulley, had just one spring. In
> the Sport it was an upper one and the GS a lower one. The Sport was cheaper
> to make, had less friction and was perfectly adequate for the purpose it was
> intended. After they discontnued the Sport Campagnolo introduced the Nuovo Sport. They obviously must have had a good market for such a deralleur. The Nuovo Sport was a sort-of single roller version of the Valentino- pretty nasty.
> Of course the Italian "around town" bikes don't have Campag drop-outs they either have the derailleur fitted to a bolt-on hanger or cheap stamped out drop-outs with the necessary hole for the spring. I have a Legnano so equipped in front of me now.
> Mike Barry.
> Still messing about with derailleurs in snowy Toronto.