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The success of the cassette Freehub is partly a technical success and partly a marketing success. Freehubs are superior to the older thread on design in many ways:
When Shimano started moving to 7-speed systems, in the mid 1980s, they used their marketing clout to push things along. They did this by making 7-speed an option only supported with their Freehubs, and not making 7-speed freewheels (except for a couple of expensive, high-end road models, Dura-Ace and Santé.) Thus, if you wanted 7-speed Shimano SIS shifting (and everybody did!) you had to buy into the cassette Freehub system.
Almost overnight, Freehubs became the norm for good-quality bikes, and thread-on freewheels were relegated to only the cheapest low-end bikes.
Shimano continued to make thread-on freewheels for the lower end market, in 5- and 6-speed versions. After a few years, once the cassette Freehub was well established for higher end bikes, they even started making 7-speed freewheels.
Starting in the early 1990s, Shimano cassettes and freewheels used the new Hyperglide design. Hyperglide made a huge improvement in shifting quality, but required that each sprocket be aligned precisely with its neighbors. This was not practicable with screw-on sprockets, so the new series of Hyperglide Freehubs and freewheels used all splined sprockets.
This presented a problem in freewheel design, however. The use of all splined sprockets meant that a separate lockring was required to hold the stack together. Due to the constraints of needing to provid access for a freewheel remover, and access to the bearing cone, it was impossible to make a Hyperglide freewheel with anything smaller than a 13 tooth sprocket.
This wouldn't have been a problem back in the days of 27 inch wheels and 52 tooth chainrings, but for mountain bikes with smaller wheels and smaller chainrings it resulted in a top gear that many people found unsatisfactory.
Of course, this wasn't a problem with the cassette Freehub, so most of the industry expected Shimano to ignore the problem...after all, anybody who wanted a higher gear could select a bike with a cassette Freehub and get an 11 tooth sprocket.
It was quite startling (to those of us who follow such things) when, in 2001, Shimano introduced the Mega 7 freewheel, a very nicely chosen* 11-13-15-18-21-24-34 setup!
To make this work, they had to completely redesign the venerable thread-on freewheel from the ground up. The solution they found is a technological tour de force, because the resulting freewheel is superior in several ways to all that came before. It is easier to service, freer turning and should prove to be more durable as well. This page is an attempt to explain this little known engineering feat. First, let's take a look at a more conventional design:
If there was some reason to remove the sprockets from their carrier, the outer lockring just above the 14 tooth sprocket could be unscrewed with a suitable tool (or a hammer and punch.)
The other sprockets are held by a small diameter lockring on the other end, just outboard of the 11 tooth sprocket.
The bearing races are larger in diameter, with more balls than conventional sysems, causing these freewheels to be unusually free running, as well as reducing play in the bearings.
The upper section of this in the photo is the part that screws on to the hub. The lower section, with the large diameter threads and the scalloped flange screws into the sprocket carrier. When it is out of the sprocket carrier, as shown, these two parts spin freely relative to one another in both directions.
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