Should I use cassette or freewheel hubs?
From: David Keppel
People often ask "should I use a freewheel or a freehub?" The answer
is usually "yes."
The hub is the center of a wheel and is composed of an axle, bolted
to the bike frame, a hub shell or hub body, where the spokes attatch, and
bearings to let the shell rotate around the axle.
Freewheels screw onto threads on the rear hub's shell, and cogs attatch
to the freewheel. The freewheel's job is to provide a ratchet between the
cogs and the hub shell, so that you can coast. Freehubs are similar but
combine parts of the freewheel with parts of the hub shell. Freehubs are
also sometimes called "cassettes"
The usual problem with rear hubs is that axles bend and break. This
is because the axle diameter was chosen when single cogs were used and
the hub bearing was positioned close to the frame. Since then, wider cog clusters
have become the norm, the bearings and frame have moved further apart and
leverage on the axle has increased. But since the axle has not gotten
any stronger, it now has a tendency to fail.
Cassettes fix the problem by incorporating one hub bearing in to the
freewheel mechanism, so that the bearing is once again outboard and the
axle is carrying its load under less leverage. Some freewheel hubs solve the
problem by using fatter axles. Since increasing the axle diameter dramatically
improves axle strength, this is an effective solution and it is possible
to use a fat axle that is aluminum and thus lighter than a standard skinny
(weaker) steel axle.
Neither solution is perfect -- cassette hubs let you use standard replacement
axles, cones, washers, etc., but force you to use cogs and spacers and
whatnot by a particular manufacturer (and possibly derailleurs and
shifters -- e.g. XTR uses 4.9mm cog-to-cog spacing instead of the normal
5.0mm). On the other hand, fat axles are nonstandard as are some other replacement
As an aside, the cassette solution leaves a fairly long unsupported
axle stub on the left side, and this is sometimes a source of more bending
problems. Fatter axles solve the problem on both sides.
Note also that many cassette systems allow you to remove the cogs using
a lightweight tool and thus give you ready access to the spokes in case
of breakage. Freewheels attatch with a fine thread (another historical
artifact, I believe) and are thus more difficult to remove on the road,
making spoke replacement harder.
In principle, freehubs have all cogs attatch using the same size and
shape of spline, so, e.g., a 20T cog can be used as both a large cog for
a corncob cluster and as a middle cog for wide-range cluster. However, Shimano's
marketing is just the opposite and is directed at selling whole clusters,
without letting you replace individual cogs. (Shimano's policy is relevant
here since they sell 90+% of such hubs.) Freewheels have several spline
diameters in order to clear the bearings and ratchet. Further, small cogs typically
screw on to the freewheel body or special cogs with extra threads. This
introduces stocking problems and may make it hard to build some cog combinations.
I'm not a fan of freehubs for the simple reason that they lock me in
to one maker's choices about cogs and cog spacing. For example, I had a
1988 Shimano 6-speed freehub and by 1991 Shimano had, according to my local
bike store, discontinued 6-speed replacement cogs. Thus, simply replacing
one worn cog meant upgrading to a 7-speed system, which in turn requires
all new cogs, a new freehub body (lucky me -- for some it requires a new
hub and thus new wheel), and, if I wanted to keep index shifting, new
thumbshifters. Had this been a freewheel-equipped bicycle, I could have
easily switched to another maker's 6-speed freewheels.
Fortunately, the market is stablizing, with a growing number of makers
producing hubs and cogs using a spline pattern like the more recent Shimano
7-speed freehubs. However, it hasn't settled entirely, yet.
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