From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: 7 speed internal gear rear hub (Sach or Nexus)
Date: 21 Jan 1998 19:02:10 GMT

David Sowden writes:

> Has anyone got details on how these work?

The Sachs has three sets of planets, each giving an "over", "under"
and direct ratio.  There fore there are nine speeds, three of which
are 1:1 and do not appear separately.  Thus there are seven.  Shimano
uses two planets and no direct (1:1) drive is available.  This means
that at least three of the gears must be compound.  These ratios have
lower efficiency than the simple ratios.

> Is the gear friction less than the friction of a chain when bet to
> reach the inner or outer rear sprocket of a 7 sprocket rear cluster
> derailleur?

The planetary gear is less efficient than direct chain drive but that
is assuming other things are in good condition.  This means the chain
is lubricated and the idler wheels of the derailleur are running
freely.

Jobst Brandt      <jbrandt@hpl.hp.com>