This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Mon, 23 Dec 2002 15:04:39 PST
Sturmey-Archer hubs have been in service for many years (1887):
Yet they have had a design flaw from the beginning that escaped
scrutiny through most of the popular life of the common 3-speed AW
hub. This flaw has consistently been "swept under the rug" or laid at
the feet of the mechanic so completely that few have questioned why it
jumps into free wheeling when ridden forcefully in top gear. I think
the symptom and cause should be explained to prevent injuries.
Drive is transmitted from the sprocket through a four slot driver,
through which a small movable cross (clutch) protrudes to transmit
drive in, low, direct,and high gear positions.
In low gear, the clutch is moved to the right to lift the (high gear)
pawls, driving the ring gear directly so that the ring gear drives the
planets with output going through the planet cage and its (low gear)
pawls at 3/4 the input speed. With one pair of pawls raised and the
other pair driving, the typical clicking sound of AW hubs is absent.
In second gear, (direct drive), the clutch drives the hub body
directly through the right hand pawls, making the planet carrier (low
gear) pawls click as they lag behind.
Top gear engages when the clutch is fully extended to the left against
the planet cage, between the four protruding planet (pinion) pins, to
drive the planets that turn the ring gear 4/3 the input speed. In
this position, the low gear pawls in the planet cage click as they lag
behind. It is the inverse of low gear and hence the reciprocal
relationship between low 1/3 down and high 1/4 up with respect to
direct drive on AW hubs.
Because the pinion pins are free fit in the housing, they are slightly
skewed to the hub axis due to canting within the clearance when loaded
by the clutch, a "tilt" that has a disengaging bias to the driving
clutch. However, because the axle bends slightly from chain tension,
depth of engagement between clutch and pinion pins varies during
rotation. These two effects can disengage the clutch and pins under
high torque, dropping the mechanism into free wheeling forward. The
result is that the rider, if standing, dives over the bars, with the
This condition is apparent upon examining the clutch and pins that
both show wear, slanting to enhance disengagement. However, replacing
these parts does not resolve the condition. SA has always maintained
that the shift cable was misadjusted, something that is easily
disproven by disengaging the shift chain entirely. With the cable
disconnected, the clutch is free to make perfect contact with the face
of the planet cage, the best adjustment possible for top gear, and
still disengagement occurs.
This problem could have been resolved by putting a slight flare to the
ends of the planet pins and a similar matching slant on the clutch
faces, giving their engagement a preferential retaining force instead
of the opposite. Most motorcycle gear boxes use such features,
especially in older non-synchronized sliding gear boxes... the classic
clunk of BMW boxes for instance.
Similarly, the spring-less ratchet of the SW (Silent) hub was sensitive
to lubricant viscosity and with anything more than 10W oil could
freewheel forward, the pawls clinging to the ramps by oil viscosity
while not engaging. This hub was discontinued after a short run
probably because one could not place blame on user error.