8f.4 Cassette or Freewheel Hubs
Description
This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
8f.4 Cassette or Freewheel Hubs
From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
All cassette hubs are not nearly alike. That is apparent from the
outside by their appearance and by the sprockets that fit on them.
More important to their longevity is how their insides are designed.
Among the mainline brands, some are a response not only to the choice
and interchangeability of sprockets but to the problem of broken rear
axles and right rear dropouts. These failures are caused by bending
loads at the middle of the rear axle that arise from bearing support
that is not at the ends of the axle. The following diagrams attempt
to categorize the freewheel and hub combination, and two cassette
designs with respect to these loads.
|
H H | |
H H Io-- |
/-------------------\ -o\
O O------
===X==================wX========= Axle has weak spot at "w"
O O------ (Freewheel & hub)
\-------------------/ -o/
H H Io-- |
H H | |
|
|
H H | |
H H | | |
/------------------\ /----\
O O O----O
===X==================XwX====X=== Axle has weak spot at "w"
O O O----O (Hugi and Campagnolo)
\------------------/ \----/
H H | | |
H H | |
|
|
H H | |
H H | | |
/------------------\/o---o\
O \-----O
===X=========================X=== Axle is loaded only at ends
O /-----O (Shimano and SunTour)
\------------------/\o---o/
H H | | |
H H | |
|
For clarity only three sprocket gear clusters are shown.
Strong cyclists put the greatest load on the axle by the pull of the
chain because there is a 2:1 or greater lever ratio from pedal to
chainwheel. The freewheel in the first diagram has the greatest
overhung load when in the rightmost sprocket. The second design has
the greatest bending moment on the axle when in the leftmost sprocket
and the third design is independent (in the first order) of chain
position. This third design carries its loads on bearings at the ends
of the axle for minimum axle stress while the other two put a large
bending moment on the middle of the axle.
Common freewheel hubs have not only the highest bending stress but the
smallest axle at 10mm diameter with threads that help initiate
cracking. The second design type generally uses a larger diameter
axle to avoid failure. However, these axles still have significant
flex that can adversely affect the dropout.
There are other important considerations in selecting a hub.
Among these are:
1. Durability of the escapement and its angular backlash (t/rev).
2. Flange spacing, offset, and diameter.
3. Type of bearings (cone / cartridge) and environmental immunity.
4. Ease of sprocket replacement and cost.
Currently the best solution for sprocket retention is a splined body
that allows individual sprockets to be slipped on and be secured by an
independent retainer. Screwing sprockets onto the body is
indefensible, considering the difficulty of removal. The same goes
for freewheels. No longer needing to unscrew tight freewheels is
another advantage for cassette hubs.
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