Subject: Cassette or Freewheel Hubs
From: Jobst Brandt
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 10 mm 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:
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 lockring. 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.
More Articles by Jobst Brandt
Next: Sturmey-Archer 3-Speed Hubs
Previous: Tubular Tire Repair
|Articles by Sheldon Brown and others|
Last Updated: by John Allen